Chapter 59: Antiretroviral Agents and Treatment of HIV Infection

History of Antiretroviral Therapy. The first effective antiretroviral agent, zidovudine, was synthesized by Horwitz in 1964 as a false nucleoside with disappointing anticancer activity. The drug was shown by Osterag in 1972 to inhibit the in vitro replication of a murine type D retrovirus (McLeod and Hammer, 1992). Mitsuya and Broder, working in Bethesda in 1985, reported that this drug had potent in vitro anti-HIV activity (Mitsuya et al., 1985). Clinical studies of zidovudine began that same year, and by 1987 this drug was approved and marketed for the control of HIV infection based on the results of a small but definitive randomized clinical trial (Fischl et al., 1987). Large numbers of nucleoside analogs already had been synthesized as potential anticancer and immunomodulatory drugs, and this made it possible for similar nucleoside reverse transcriptase inhibitors (NRTIs) to be tested efficiently and approved.

Selective non-nucleoside reverse transcriptase inhibitors (NNRTIs) were identified by iterative screening using purified viral enzyme (Pauwels et al., 1990). The clinical development of these drugs was hindered by the rapid emergence of drug resistance (Wei et al., 1995). However, three drugs in this category were approved by 1998 (Table 59–1). HIV protease inhibitors (PIs) were the products of rational drug design, relying on technology developed to identify transition-state peptidomimetic antagonists of proteases in the renin-angiotensin cascade (Flexner, 1998). Highly selective antagonists of the HIV protease were reported as early as 1987. Phase 1 trials of the first of these drugs, saquinavir, began in 1989, and this drug was approved for prescription use in 1995. Two additional protease inhibitors, ritonavir and indinavir, were approved within the next 4 months.

Innovation in drug approval and regulation facilitated the availability of multiple agents capable of fighting this infection. In 1989, the FDA agreed to make promising agents available to patients with advanced disease through an expanded access program. The capacity to measure plasma HIV RNA concentrations and CD4 cell counts, and proof of the predictive value of these surrogate end points (Mellors et al., 1996) made it possible to collapse the time frame for clinical drug development. The most promising drug combinations were identified by their effect on these end points in clinical trials as short as 6 months.

A detailed understanding of the molecular basis of drug resistance guides the search for new agents and informs the selection of combination strategies for existing drugs. The large number of possible drug combinations has given patients more chances at virus control but also complicates the practice of HIV medicine. Current challenges include access to effective long-term treatment in resource-poor countries and the continuing need for identification of new drugs for treatment-experienced patients with resistance to approved drugs (Flexner, 2007).

Randomized controlled trials found that a combination of two potent drugs (e.g., an NNRTI plus a PI) had equivalent virologic efficacy to either agent plus two NRTIs (Riddler et al., 2008). In treatment-naive patients, a regimen containing a non-nucleoside plus two nucleoside reverse transcriptase inhibitors was as effective as a regimen containing an additional nucleoside (Shafer et al., 2003), indicating the equivalence of these three-drug and four-drug regimens. Four or more drugs may be used simultaneously in pretreated patients harboring drug-resistant virus, but the number of agents a patient can take is limited by toxicity and inconvenience. Even for heavily treatment-experienced patients, a three-drug regimen containing at least two potent active agents is often as effective as regimens containing additional active agents (Steigbigel et al., 2008).

Several small studies have now shown that patients who are fully suppressed (HIV plasma RNA <50 copies/mL) for months on a triple drug combination may be switched to a single "boosted" protease inhibitor (e.g., lopinavir/ritonavir or atazanavir/ritonavir) and maintain full suppression of viral replication for years on antiretroviral monotherapy. Such simplification strategies are investigational and should only be used in patients who are known to be highly adherent and are closely monitored, because randomized trials have found higher failure rates in patients maintained on only a single active agent (Wilkin et al., 2009).

Pharmacodynamic synergy is probably not an important consideration in regimen selection, although most prescribers prefer to use drugs that attack at least two different molecular sites. This could include NRTIs that target the active site of the enzyme combined with an NNRTI that binds to a different site on the same enzyme, or an inhibitor of a different enzyme, such as HIV protease or integrase. Regimens containing an NNRTI or PI plus two NRTIs have similar long-term efficacy.

As antiretroviral therapy becomes more effective and easier to take, long-term toxicity of these drugs is of greater concern. An important consequence of long-term therapy is the development of a metabolic syndrome characterized by insulin resistance, fat redistribution, and hyperlipidemia and known as the HIV lipodystrophy syndrome. Lipodystrophy occurs in 10-40% of treated patients and has been seen with most drug combinations used in clinical trials. Symptomatic manifestations have been most strongly linked to the older generation of NRTIs, especially stavudine, which had more substantial mitochondrial toxicities; these drugs are now less commonly used in the developed world. The pathogenesis is still somewhat mysterious but involves phenotypic and metabolic changes similar to those seen with other human lipodystrophy syndromes (Garg, 2004). Clinical features include peripheral fat wasting (lipoatrophy), central fat accumulation including enlarged breasts and buffalo hump, insulin resistance and hyperglycemia, and elevations in serum cholesterol and triglycerides. Switching from one drug regimen to another may not reverse the symptoms, emphasizing its ubiquitous nature and possible role of HIV infection per se. Treatment is symptom directed and should include management of hyperlipidemias as recommended by the American Heart Association (Chapter 31). Lipodystrophy has been associated with an increased risk of myocardial infarction in virologically controlled patients, emphasizing the importance of cardiovascular risk factor reduction.

There is evidence that chronic HIV infection per se increases long-term cardiovascular risks, but the quantitative contribution of drug therapy to this risk is not well defined. Metabolic abnormalities associated with chronic HIV infection and possibly exacerbated by some drugs include insulin resistance, hyperglycemia, and increased risk of diabetes mellitus, as well as osteopenia and its attendant complications (Calmy et al., 2009).

A potential concern that applies to all protease inhibitors and NNRTIs is clinically significant pharmacokinetic drug interactions (Piscitelli and Gallicano, 2001). All agents in these two drug classes can act as inhibitors and/or inducers of hepatic CYPs and other drug metabolizing enzymes, as well as drug transport proteins. Prescribing practices should be guided by up-to-date knowledge of these potential effects. Internet-based educational resources are updated frequently (see, e.g., Flexner and Pham, 2009) and are an excellent way to track evolving knowledge about the undesired effects of drugs used in combination with antiretrovirals.

An increasingly recognized complication of initiating antiretroviral therapy is accelerated inflammatory reaction to overt or subclinical opportunistic infections or malignancies. This is thought to reflect reversal of immunodeficiency, resulting in new antimicrobial host defenses. This immune reconstitution inflammatory syndrome (IRIS) is most commonly seen when initiating therapy in individuals with low CD4 counts and/or advanced HIV disease, and it is associated with a better virologic response to therapy (Manabe et al., 2007). Not surprisingly, this is now most prevalent in resource-poor countries, where it may occur in >10% of newly treated patients. Infections most commonly associated with IRIS include tuberculosis and other mycobacterial diseases, cryptococcosis, hepatitis virus infections, and Pneumocystis pneumonia. Duration of symptoms ranges from a few days to more than a year. Symptomatic relief can be obtained with anti-inflammatory drugs, but systemic corticosteroids do not appear to shorten the course of symptoms (Manabe et al., 2007).

All but one of the drugs in this class are nucleosides that must be triphosphorylated at the 5′-hydroxyl to exert activity. The sole exception, tenofovir, is a nucleotide monophosphate analog that requires two additional phosphates to acquire full activity. These compounds inhibit both HIV-1 and HIV-2, and several have broad-spectrum activity against other human and animal retroviruses; emtricitabine, lamivudine, and tenofovir are active against hepatitis B virus (HBV), and tenofovir also has activity against herpesviruses (Chapter 58; De Clercq, 2003).

High-level resistance to nucleoside/nucleotide reverse transcriptase inhibitors, especially thymidine analogs, occurs slowly by comparison to NNRTIs and first-generation protease inhibitors. For example, zidovudine resistance was noted in only one-third of treated subjects after 1 year of monotherapy (Fischl et al., 1995). High-level resistance can occur rapidly with lamivudine and emtricitabine. In most cases, high-level resistance requires accumulation of a minimum of three to four codon substitutions, although a two-amino-acid insertion is associated with resistance to all drugs in this class (Gallant et al., 2003). Cross-resistance is common but often confined to drugs having similar chemical structures; zidovudine is a thymidine analog, and a zidovudine-resistant isolate is much more likely to be cross-resistant to the thymidine analog stavudine than to the cytidine analog lamivudine.

Nucleoside/nucleotide analogs are generally less active as single agents than other antiretroviral drugs. When used investigationally as monotherapy, most of these drugs produced only a 30-90% mean peak decrease in plasma concentrations of HIV RNA; abacavir monotherapy, however, produced up to a 99% decrease (Hervey and Perry, 2000). CD4 lymphocyte count increases were also modest with nucleoside monotherapy (mean increases of 50-100 cells/mm³, depending on disease stage). Nonetheless, these drugs remain a critical component of therapy, and nearly all patients starting antiretroviral treatment do so with at least one agent from this class. Although modest in their own antiviral potency, several nucleoside analogs have favorable safety and tolerability profiles and are useful in suppressing the emergence of HIV isolates resistant to the more potent drugs in combination regimens.

Zidovudine is active in lymphoblastic and monocytic cell lines but is substantially less active in chronically infected cells (Geleziunas et al., 1993), probably because it has no impact on cells already infected with HIV. Zidovudine appears to be more active in lymphocytes than in monocyte-macrophage cells because of enhanced phosphorylation in the former. For the same reason, the drug is more potent in activated than in resting lymphocytes because the phosphorylating enzyme, thymidine kinase, is S-phase-specific (Gao et al., 1994).

Because the conversion of zidovudine 5′-monophosphate to diphosphate is very inefficient, high concentrations of the monophosphate accumulate inside cells (Dudley, 1995) and may serve as a precursor depot for formation of triphosphate. As a consequence, there is little correlation between extracellular concentrations of parent drug and intracellular concentrations of triphosphate, and higher plasma concentrations of zidovudine do not increase intracellular triphosphate concentrations proportionately.

Resistance to zidovudine is associated with mutations at reverse transcriptase codons 41, 44, 67, 70, 210, 215, and 219 (Gallant et al., 2003). These mutations are referred to as thymidine analog mutations (TAMs) because of their ability to confer cross-resistance to other thymidine analogs such as stavudine. Two clusters of resistance mutations occur commonly. The pattern of 41L, 210W, and 215Y is associated with high-level resistance to zidovudine, as well as cross-resistance to other drugs in this class, including tenofovir and abacavir. The pattern 67N, 70R, 215F, and 219Q is less common and also associated with lower levels of resistance and cross-resistance. TAMs associated with resistance to zidovudine and stavudine promote excision of the incorporated nucleotide anabolites through pyrophosphorolysis (Naeger et al., 2002). Mutations accumulated gradually when zidovudine was used as the sole antiretroviral agent, and clinical resistance developed in only 31% of patients after 1 year of zidovudine monotherapy (Fischl et al., 1995). Cross-resistance to multiple nucleoside analogs has been reported following prolonged therapy and has been associated with a mutation cluster involving codons 62, 75, 77, 116, and 151. A mutation at codon 69 (typically T69S) followed by a two-amino-acid insertion produces cross-resistance to all available nucleoside and nucleotide analogs (Gallant et al., 2003).

The M184V substitution in the reverse transcriptase gene associated with the use of lamivudine or emtricitabine greatly restores sensitivity to zidovudine (Gallant et al., 2003). The combination of zidovudine and lamivudine produces greater long-term suppression of plasma HIV RNA than does zidovudine alone (Eron et al., 1995). As a result, this agent is usually combined with lamivudine in clinical practice.

Absorption, Distribution, and Elimination. Zidovudine is absorbed rapidly and reaches peak plasma concentrations within 1 hour (Dudley, 1995). Like other nucleoside analogs, the elimination t_1/2 of the parent compound (~1 hour) is considerably shorter than that of the intracellular triphosphate, which is 3-4 hours (Table 59–2). Failure to recognize this led to serious overdosing of the drug when it was first approved; the recommended dose was 250 mg every 4 hours in 1987, compared with 300 mg twice a day presently.

Zidovudine undergoes rapid first-pass hepatic metabolism by conversion to 5-glucuronyl zidovudine, which limits systemic bioavailability to ~64%. Food may slow absorption but does not alter the AUC (area under the plasma concentration-time curve) (Table 59–2), and the drug can be administered regardless of food intake (Dudley, 1995). The pharmacokinetic profile of zidovudine is not altered significantly during pregnancy, and drug concentrations in the newborn approach those of the mother. Parent drug crosses the blood-brain barrier relatively well and achieves a cerebrospinal fluid (CSF)-to-plasma ratio of ~0.6. Zidovudine also is detectable in breast milk, semen, and fetal tissue. Zidovudine concentrations are higher in the male genital tract than in the peripheral circulation, suggesting active transport or trapping.

Untoward Effects. Patients initiating zidovudine treatment often complain of fatigue, malaise, myalgia, nausea, anorexia, headache, and insomnia. These symptoms usually resolve within the first few weeks of treatment. Bone marrow suppression, mainly anemia and granulocytopenia, occurs most often in individuals with advanced HIV disease and very low CD4 counts and also was more common with the higher doses used when the drug was first approved. Erythrocytic macrocytosis is seen in ~90% of all patients but usually is not associated with anemia.

Chronic zidovudine administration has been associated with nail hyperpigmentation. Skeletal muscle myopathy can occur and is associated with depletion of mitochondrial DNA, most likely as a consequence of inhibition of DNA polymerase-γ. Serious hepatic toxicity, with or without steatosis and lactic acidosis, is rare but can be fatal. Risk factors for the lactic acidosis-steatosis syndrome include female sex, obesity, and prolonged exposure to the drug (Tripuraneni et al., 2004).

Precautions and Interactions. Zidovudine is not a substrate or inhibitor of CYPs. However, probenecid, fluconazole, atovaquone, and valproic acid may increase plasma concentrations of zidovudine probably through inhibition of glucuronosyl transferase (Dudley, 1995). The clinical significance of these interactions is unknown because intracellular triphosphate levels may be unchanged despite higher plasma concentrations. Zidovudine can cause bone marrow suppression and should be used cautiously in patients with preexisting anemia or granulocytopenia and in those taking other marrow-suppressive drugs. Stavudine and zidovudine compete for intracellular phosphorylation and should not be used concomitantly. Three clinical trials found a significantly worse virologic outcome in patients taking these two drugs together as compared with either agent used alone (Havlir et al., 2000).

Despite being the oldest antiretroviral drug, zidovudine is still in widespread use, especially in resource-poor settings. This is a consequence of broad experience with the drug and its well-known tolerability, toxicity, and efficacy profiles. Zidovudine (retrovir, others) is marketed in oral tablets, capsules, and solution as well as a solution for intravenous injection. Zidovudine is available in coformulated tablets with lamivudine (combivir) or with lamivudine and abacavir (trizivir). Zidovudine monotherapy reduced the risk of perinatal transmission of HIV by 67% (Connor et al., 1994), and combining zidovudine with other antiretroviral drugs is even more efficacious in this setting.

Stavudine resistance is seen most frequently with mutations at reverse transcriptase codons 41, 44, 67, 70, 210, 215, and 219 (Gallant et al., 2003), which are the same mutations associated with zidovudine resistance. Clusters of resistance mutations that include M41L, K70R, and T215Y are associated with a lower level of in vitro resistance than seen with zidovudine but are found in up to 38% of patients who fail to respond to stavudine. TAMs associated with resistance to zidovudine and stavudine promote excision of the incorporated triphosphate anabolites through pyrophosphorolysis (Naeger et al., 2002). As with zidovudine, resistance mutations for stavudine appear to accumulate slowly. Cross-resistance to multiple nucleoside analogs has been reported following prolonged therapy and has been associated with a mutation cluster involving codons 62, 75, 77, 116, and 151. In addition, a mutation at codon 69 (typically T69S) followed by a 2-amino acid insertion produces cross-resistance to all current nucleoside and nucleotide analogs (Gallant et al., 2003).

Absorption, Distribution, and Elimination. Stavudine is well absorbed and reaches peak plasma concentrations within 1 hour (Hurst and Noble, 1999). Bioavailability is not affected by food. The drug undergoes active tubular secretion, and renal elimination accounts for ~40% of parent drug.

Stavudine concentrations are higher in patients with low body weight, and the dose should be decreased from 40 to 30 mg twice daily in patients weighing <60 kg, although WHO recommends 30 mg twice daily in all patients. Dose also should be adjusted in patients with renal insufficiency (Jayasekara et al., 1999).

Plasma protein binding is <5%. The drug penetrates well into the CSF, achieving concentrations that are ~40% of those in plasma. Placental concentrations of stavudine are about half those of zidovudine, possibly reflecting stavudine's lower lipid solubility.

Untoward Effects. The most common serious toxicity of stavudine is peripheral neuropathy.

Neuropathy occurred in up to 71% of patients in initial monotherapy trials with a dose of 4 mg/kg per day. With the current recommended dose of 40 mg twice daily, the neuropathy incidence is ~12% (Hurst and Noble, 1999). Although this is thought to reflect mitochondrial toxicity, stavudine is a less potent inhibitor of DNA polymerase-γ than either didanosine or zalcitabine, suggesting that other mechanisms may be involved. Peripheral neuropathy is more common with higher doses or concentrations of stavudine and is more prevalent in patients with underlying HIV-related neuropathy or in those receiving other neurotoxic drugs. Stavudine is also associated with a progressive motor neuropathy characterized by weakness and in some cases respiratory failure, similar to Guillain-Barré syndrome (HIV Neuromuscular Syndrome Study Group, 2004).

Lactic acidosis and hepatic steatosis have been associated with stavudine use. This may be more common when stavudine and didanosine are combined. Elevated serum lactate is more common with stavudine than with zidovudine or abacavir (Tripuraneni et al., 2004), but the comparative risk of hepatic steatosis is unknown. Acute pancreatitis is not highly associated with stavudine but is more common when stavudine is combined with didanosine than when didanosine is given alone (Havlir et al., 2001).

Of all nucleoside analogs, stavudine use is associated most strongly with fat wasting, or lipoatrophy (Calmy et al., 2009). Whether this is a consequence of the extensive use of this agent combined with its mitochondrial toxicity or reflects a pathogenetic mechanism that has yet to be discovered remains to be determined. Stavudine has fallen out of favor in the developed world largely because of this toxicity. Other reported adverse effects include elevated hepatic transaminases, headache, nausea, and rash; however, these side effects are almost never severe enough to cause discontinuation of the drug.

Precautions and Interactions. Stavudine is mainly renally cleared and is not subject to metabolic drug interactions. The incidence and severity of peripheral neuropathy may be increased when stavudine is combined with other neuropathic medications, and therefore drugs such as ethambutol, isoniazid, phenytoin, and vincristine should be avoided.

Combining stavudine with didanosine leads to increased risk and severity of peripheral neuropathy and potentially fatal pancreatitis; therefore, these two drugs should not be used together (Havlir et al., 2001). Stavudine and zidovudine compete for intracellular phosphorylation and should not be used concomitantly. Three clinical trials found a significantly worse virologic outcome in patients taking these two drugs together as compared with either agent used alone (Havlir et al., 2000).

In early monotherapy trials, stavudine reduced plasma HIV RNA by 70-90% and delayed disease progression compared with continued zidovudine therapy. Lamivudine improves the long-term virologic response to stavudine, possibly reflecting the benefits of the M184V mutation (Kuritzkes et al., 1999). Many large prospective clinical trials have demonstrated potent and durable suppression of viremia and sustained increases in CD4+ cell counts when stavudine is combined with other nucleoside analogs plus NNRTIs or protease inhibitors (Hurst and Noble, 1999). Stavudine is no longer a popular drug in the developed world because of toxicity. However, it continues to be widely used in resource-poor settings because of its availability as an inexpensive generic version, often co-formulated with nevirapine and lamivudine.

The molecule has two chiral centers and is manufactured as the pure 2R, cis(−)-enantiomer (Figure 59–2). The racemic mixture from which lamivudine originates has antiretroviral activity but is less potent and substantially more toxic than the pure (−)-enantiomer. Compared with the (+)-enantiomer, the phosphorylated (−)-enantiomer is more resistant to cleavage from nascent RNA/DNA duplexes by cellular 3′-5′ exonucleases, which may contribute to its greater potency. The IC₅₀ of lamivudine against laboratory strains of HIV-1 ranges from 2 to 670 nM, although the IC₅₀ in primary human peripheral blood mononuclear cells is as high as 15 μM (Perry and Faulds, 1997).

High-level resistance to lamivudine occurs with single-amino-acid substitutions, M184V or M184I. These mutations can reduce in vitro sensitivity to lamivudine by up to 1000-fold (Perry and Faulds, 1997). The same mutations confer high-level cross-resistance to emtricitabine and a lesser degree of resistance to abacavir (Gallant et al., 2003). The M184V mutation restores zidovudine susceptibility in zidovudine-resistant HIV (Larder et al., 1995) and also partially restores tenofovir susceptibility in tenofovir-resistant HIV harboring the K65R mutation (Wainberg et al., 1999). The same K65R mutation confers resistance to lamivudine, emtricitabine, didanosine, stavudine, and abacavir.

HIV-1 isolates harboring the M184V mutation have increased transcriptional fidelity in vitro (Wainberg et al., 1996) and decreased replication capacity (Miller et al., 1999). Variants with the M184I mutation are even more impaired with regard to in vitro replication (Larder et al., 1995) and usually are replaced in lamivudine-treated patients by the M184V mutation. The reduced fitness of lamivudine-resistant viruses harboring these mutations, and their ability to prevent or partially reverse the effect of thymidine analog mutations, may contribute to the sustained virologic benefits of zidovudine and lamivudine combination therapy (Eron et al., 1995).

Lamivudine is used to treat HBV infection (Chapter 58), and some parallels in drug resistance are worth noting. High-level resistance to lamivudine occurs with a single mutation in the HBV DNA polymerase gene; as with HIV, this consists of a methionine-to-valine substitution (M2041V) in the enzyme active site. Resistance to lamivudine occurs in up to 90% of HIV/HBV co-infected patients after 4 years of treatment. However, virologic benefits persist in some treated patients harboring lamivudine-resistant HBV possibly because the mutated virus has substantially reduced replicative capacity (Leung et al., 2001).

Absorption, Distribution, and Elimination. The oral bioavailability of lamivudine is >80% and is not affected by food. Although lamivudine was marketed originally with a recommended dose of 150 mg twice daily based on the short plasma t_1/2 of the parent compound, the intracellular t_1/2 of lamivudine 5′-triphosphate is 12-18 hours, and the drug is now approved for use once daily at 300 mg (Moore et al., 1999).

Lamivudine is excreted primarily unchanged in the urine, and dose adjustment is recommended for patients with a creatinine clearance <50 mL/minute (Jayasekara et al., 1999). Lamivudine does not bind significantly to plasma proteins and freely crosses the placenta into the fetal circulation. Like zidovudine, lamivudine concentrations are higher in the male genital tract than in the peripheral circulation, suggesting active transport or trapping. Penetration to the CNS appears to be moderate, with a CSF-to-plasma concentration ratio of ≤0.15 (Perry and Faulds, 1997). The clinical significance of the low CSF penetration is unknown.

Untoward Effects. Lamivudine is one of the least toxic antiretroviral drugs and has few significant adverse effects.

Neutropenia, headache, and nausea have been reported at higher than recommended doses. Pancreatitis has been reported in pediatric patients, but this has not been confirmed in controlled trials of adults or children. Because lamivudine also has activity against HBV and substantially lowers plasma HBV DNA concentrations, caution is warranted in using this drug in patients co-infected with HBV or in HBV-endemic areas; discontinuation of lamivudine may be associated with a rebound of HBV replication and exacerbation of hepatitis.

Precautions and Interactions. Because lamivudine and emtricitabine have nearly identical resistance and activity patterns, there is no rationale for their combined use. Lamivudine is synergistic with most other nucleoside analogs in vitro (Perry and Faulds, 1997).

In early monotherapy studies, initial declines in plasma HIV-1 RNA concentrations of up to 90% occurred within 14 days but rebounded rapidly with emergence of lamivudine-resistant HIV (Perry and Faulds, 1997). Patients randomized to the combination of lamivudine plus zidovudine had substantially better mean decreases in plasma HIV-1 RNA at 52 weeks (97% vs. 70% decrease in copies/mL) and increases in CD4+ lymphocyte counts (+61 versus –53 cells/mm³) compared with those receiving zidovudine alone (Eron et al., 1995). In a large randomized double-blind trial, combining lamivudine with zidovudine or stavudine caused ~12-fold further decline in viral load at 24 weeks compared with zidovudine or stavudine monotherapy (Kuritzkes et al., 1999); in the same trial, combining lamivudine with didanosine conferred no additional benefits. Lamivudine has been effective in combination with other antiretroviral drugs in both treatment-naive and experienced patients (Perry and Faulds, 1997) and is a common component of therapy, given its safety, convenience, and efficacy.

Lamivudine (epivir-hbv) also is approved for treatment of chronic hepatitis B.

Carbovir, a related guanine analog, was withdrawn from clinical development owing to poor oral bioavailability (Hervey and Perry, 2000). The IC₅₀ of abacavir for primary clinical HIV-1 isolates is 0.26 μM, and its IC₅₀ for laboratory strains ranges from 0.07 to 5.8 μM.

Clinical resistance to abacavir is associated with four specific codon substitutions: K65R, L74V, Y115F, and M184V (Gallant et al., 2003). Individually, these substitutions produce only modest (2- to 4-fold) resistance to abacavir, but in combination can reduce susceptibility by up to 10-fold. The Y115F mutation is seen uniquely with abacavir and causes low-level resistance. The L74V mutation is associated with cross-resistance to the purine analog didanosine. K65R confers cross-resistance to all nucleosides except zidovudine. An alternate pathway for abacavir resistance involves mutations at codons 41, 210, and 215, which have been associated with a reduced likelihood of virologic response. Abacavir sensitivity is greatly reduced by the multinucleoside resistance clusters, including that associated with the Q151M, as well as the 2-amino acid insertion following codon 69 (Gallant et al., 2003).

Absorption, Distribution, and Elimination. Abacavir's oral bioavailability is >80% regardless of food intake (Table 59–2). Abacavir is eliminated by metabolism to the 5′-carboxylic acid derivative catalyzed by alcohol dehydrogenase, and by glucuronidation to the 5′-glucuronide (Figure 59–3). These metabolites account for 30% and 36% of elimination, respectively (Hervey and Perry, 2000). Abacavir is not a substrate or inhibitor of CYPs. Abacavir is 50% bound to plasma proteins, and the CSF/plasma AUC ratio is ~0.3. Although the introduced dose of abacavir was 300 mg twice daily, carbovir triphosphate accumulates inside the cell and has a reported elimination t_1/2 of up to 21 hours (Hervey and Perry, 2000); thus a regimen of 600 mg once daily is approved now.

Untoward Effects. The most important adverse effect of abacavir is a unique and potentially fatal hypersensitivity syndrome. This syndrome is characterized by fever, abdominal pain, and other gastrointestinal (GI) complaints; a mild maculopapular rash; and malaise or fatigue. Respiratory complaints (cough, pharyngitis, dyspnea), musculoskeletal complaints, headache, and paresthesias are reported less commonly. Median time to onset of symptoms is 11 days, and 93% of cases occur within 6 weeks of initiating therapy (Hetherington et al., 2002). The presence of concurrent fever, abdominal pain, and rash within 6 weeks of starting abacavir is diagnostic and necessitates immediate discontinuation of the drug. Patients having only one of these symptoms may be observed to see if additional symptoms appear. Unlike many hypersensitivity syndromes, this condition worsens with continued treatment. Abacavir can never be restarted once discontinued for hypersensitivity because reintroduction of the drug leads to rapid recurrence of severe symptoms, accompanied by hypotension, a shocklike state, and possibly death. The reported mortality rate of restarting abacavir in sensitive individuals is 4% (Hervey and Perry, 2000).

Abacavir hypersensitivity occurs in 2-9% of patients depending on the population studied. The cause is a genetically mediated immune response linked to both the HLA-B*5701 locus and the M493T allele in the heat-shock locus Hsp70-Hom (Mallal et al., 2008). The latter gene is implicated in antigen presentation, and this haplotype is associated with aberrant tumor necrosis factor-α release after exposure of human lymphocytes to abacavir ex vivo. This is one of the strongest pharmacogenetic associations ever described. In one white population, the combination of these two markers occurred in 94.4% of cases and <0.5% of controls for a positive predictive value of 93.8% and a negative predictive value of 99.5% (Mallal et al., 2008). Abacavir should not be given to those with the HLA-B*5701 genotype; in all others, the risk of true hypersensitivity is essentially zero (Mallal et al., 2008). Aside from hypersensitivity, abacavir is a well-tolerated drug. Carbovir 5′-triphosphate is a weak inhibitor of human DNA polymerases, including DNA polymerase-γ (Hervey and Perry, 2000). Abacavir therefore has not been associated with adverse events thought to be due to mitochondrial toxicity. Epidemiological associations link abacavir use and increased risk of myocardial infarction (D:A:D Study Group et al., 2008).

Precautions and Interactions. Abacavir is not associated with any clinically significant pharmacokinetic drug interactions. However, a large dose of ethanol (0.7 g/kg) increased the abacavir plasma AUC by 41% and prolonged the elimination t_1/2 by 26% (McDowell et al., 2000) possibly owing to competition for alcohol dehydrogenase.

Abacavir is effective in combination with other nucleoside analogs, NNRTIs, and protease inhibitors. Adding abacavir to zidovudine and lamivudine resulted in a substantially greater decrease in plasma HIV-l RNA than seen with the two-drug regimen of zidovudine plus lamivudine in adults or children (Hervey and Perry, 2000). Abacavir is available in a co-formulation with zidovudine and lamivudine (trizivir) for twice-daily dosing. However, the combination of abacavir, zidovudine, and lamivudine was less effective in a randomized, double-blind, placebo-controlled trial in treatment-naive patients than was the combination of zidovudine, lamivudine, and efavirenz or the four-drug regimen of zidovudine, lamivudine, abacavir, and efavirenz; 79% of patients in the abacavir, zidovudine, lamivudine group had undetectable plasma HIV RNA at 32 weeks as compared with 89% with the other regimens (Gulick et al., 2004).

Abacavir is available in a co-formulation with lamivudine (epzicom) for once-daily dosing, which is how it is most commonly used. Abacavir is approved for use in adult and pediatric patients ≥3 months of age, with dosing in the latter based on body weight.

Specific resistance occurs with a single substitution at codon 65 of reverse transcriptase (K65R). This mutation reduces in vitro sensitivity by only 3- to 4-fold but has been associated with clinical failure of tenofovir-containing regimens (Chapman et al., 2003). Tenofovir sensitivity and virologic efficacy also are reduced in patients harboring HIV isolates with high-level resistance to zidovudine or stavudine, specifically those having three or more TAMs, including M41L or L120W. However, HIV variants that are resistant to zidovudine show only partial resistance to tenofovir, possibly reflecting less efficient excision of tenofovir diphosphate by pyrophosphorolysis (Naeger et al., 2002). The M184V mutation associated with lamivudine or emtricitabine resistance partially restores susceptibility in tenofovir-resistant HIV harboring the K65R mutation (Wainberg et al., 1999).

The K65R mutation was reported in only 2-3% of tenofovir-treated patients in initial clinical studies, and this mutation usually was not associated with treatment failure. Patients failing most tenofovir-containing regimens are more likely to harbor genotypic resistance to the other drugs in the regimen. Notable exceptions are once-daily combination regimens of three nucleosides, specifically tenofovir plus didanosine and lamivudine and tenofovir plus abacavir and lamivudine. Both of these regimens were associated with very high early rates of virologic failure or nonresponse, and at the time of failure, the K65R mutation was present in 36-64% of virus isolated from patients (Department of Health and Human Services, 2010); these combinations should be avoided.

Absorption, Distribution, and Elimination. Tenofovir disoproxil has an oral bioavailability of 25%. A high-fat meal increases the bioavailability to 39%, but the drug can be taken without regard to food (Chapman et al., 2003). Tenofovir is not bound significantly to plasma proteins. The plasma elimination t_1/2 ranges from 14 to 17 hours. The reported t_1/2 of intracellular tenofovir diphosphate is 11 hours in activated peripheral blood mononuclear cells and ≥49 hours in resting cells (Chapman et al., 2003). The drug therefore can be dosed once daily. Tenofovir undergoes both glomerular filtration and active tubular secretion. Following an intravenous dose, 70-80% of the drug is recovered unchanged in the urine. Doses should be decreased in those with renal insufficiency (Chapman et al., 2003).

Untoward Effects. Tenofovir generally is well tolerated, with few significant adverse effects reported except for flatulence.

In placebo-controlled double-blinded trials, the drug had no other adverse effects reported more frequently than with placebo after treatment for up to 24 weeks. Unlike the antiviral nucleotides adefovir and cidofovir (Chapter 58), tenofovir is not toxic to human renal tubular cells in vitro (Chapman et al., 2003). However, rare episodes of acute renal failure and Fanconi's syndrome have been reported with tenofovir, and this drug should be used with caution in patients with preexisting renal disease. Tenofovir use is associated with small declines in estimated creatinine clearance after months of treatment in some patients (Gallant and Moore, 2009), and because the dose needs to be reduced if renal insufficiency is present, renal function (creatinine and phosphorus) should be monitored regularly in patient taking this drug. Because tenofovir also has activity against HBV and may lower plasma HBV DNA concentrations, caution is warranted in using this drug in patients co-infected with HBV and in regions with high HBV seroprevalence because discontinuation of tenofovir may be associated with a rebound of HBV replication and exacerbation of hepatitis.

Precautions and Interactions. Tenofovir is not metabolized to a significant extent by CYPs and is not known to inhibit or induce these enzymes. However, tenofovir has been associated with a few potentially important pharmacokinetic drug interactions.

A 300-mg dose of tenofovir increased the didanosine AUC by 44-60%, probably as a consequence of inhibition of purine nucleoside phosphorylase by both tenofovir and tenofovir monophosphate (Robbins et al., 2003). These two drugs probably should not be used together; if the combination is essential, the dose of didanosine should be reduced from 400 to 250 mg/day (Chapman et al., 2003). Although tenofovir is not known to induce CYPs, it has been reported to reduce the atazanavir AUC by ~26%. In addition, low-dose ritonavir (100 mg twice daily) increases the mean tenofovir AUC by 34%, lopinavir/ritonavir increases the AUC by 32%, and atazanavir increases the tenofovir AUC by 25%. These interactions are most likely mediated by tenofovir's interaction with drug transport proteins.

The use of tenofovir in antiretroviral-experienced patients resulted in a further sustained decrease in HIV plasma RNA concentrations of 4.5-7.4-fold relative to placebo after 48 weeks of treatment (Chapman et al., 2003). Several large trials have confirmed the antiretroviral activity of tenofovir in three-drug regimens with other agents, including other nucleoside analogs, protease inhibitors, and/or NNRTIs. In a randomized double-blind comparison trial in which treatment-naive patients also received lamivudine and efavirenz, tenofovir 300 mg once daily was as effective and less toxic than stavudine 40 mg twice daily (Gallant et al., 2004). Tenofovir is also being investigated as a component of prophylactic regimens, including in the prevention of mother-to-child transmission, and it may have advantages over zidovudine in these settings (Foster et al., 2009).

Tenofovir is also approved for the treatment of chronic hepatitis B in adults.

Like lamivudine, it has two chiral centers and is manufactured as the enantiomerically pure (2R,5S)-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine (FTC) (Figure 59–2). Emtricitabine is active against HIV-1, HIV-2, and HBV. The IC₅₀ of emtricitabine against laboratory strains of HIV-1 ranges from 2 to 530 nM, although, on average, the drug is ~10 times more active in vitro than lamivudine (Saag, 2006).

High-level resistance to emtricitabine occurs with the same mutations affecting lamivudine (mainly the methionine-to-valine substitution at codon 184), although these appear to occur less frequently with emtricitabine. In three studies, M184V/I occurred about half as frequently with emtricitabine-containing regimens as with lamivudine, and patients presenting with virologic failure were two to three times as likely to have wild-type virus at the time of failure as compared with lamivudine (Saag, 2006). The M184V mutation restores zidovudine susceptibility to zidovudine-resistant HIV and also partially restores tenofovir susceptibility to tenofovir-resistant HIV harboring the K65R mutation (Wainberg et al., 1999). The same K65R mutation confers resistance to emtricitabine and the other cytidine analog lamivudine, as well as didanosine, stavudine, and abacavir.

Absorption, Distribution, and Elimination. Emtricitabine is absorbed rapidly and has an oral bioavailability of 93%. Food reduces the C_max but does not affect the AUC, and the drug can be taken without regard to meals. Emtricitabine is not bound significantly to plasma proteins. Compared with other nucleoside analogs, the drug has a slow systemic clearance and long elimination t_1/2 of 8-10 hours (Saag, 2006). In addition, the estimated t_1/2 of the intracellular triphosphate is long, possibly up to 39 hours, providing the pharmacokinetic rationale for once-daily dosing. Emtricitabine is excreted primarily unchanged in the urine, undergoing glomerular filtration and active tubular secretion. The dose should be reduced in those with estimated creatinine clearances of <50 mL/minute.

Untoward Effects. Emtricitabine is one of the least toxic antiretroviral drugs and, like its chemical relative lamivudine, has few significant adverse effects and no effect on mitochondrial DNA in vitro (Saag, 2006).

Prolonged exposure has been associated with hyperpigmentation of the skin, especially in sun-exposed areas. Elevated hepatic transaminases, hepatitis, and pancreatitis have been reported, but these have occurred in association with other drugs known to cause these toxicities. Because emtricitabine also has in vitro activity against HBV, caution is warranted in using this drug in patients co-infected with HBV and in regions with high HBV seroprevalence; discontinuation of lamivudine, which is closely related to emtricitabine, has been associated with a rebound of HBV replication and exacerbation of hepatitis.

Precautions and Interactions. Emtricitabine is not metabolized to a significant extent by CYPs, and it is not susceptible to any known metabolic drug interactions. The possibility of a pharmacokinetic interaction involving renal tubular secretion, such as that between trimethoprim and lamivudine, has not been investigated for emtricitabine; the drug does not alter the pharmacokinetics of tenofovir.

Two small monotherapy trials showed that the maximal antiviral effect of emtricitabine (mean 1.9 log unit decrease in plasma HIV RNA concentration) was achieved with a dose of 200 mg/day. Several large trials have confirmed the antiretroviral activity of emtricitabine in three-drug regimens with other agents, including nucleoside or nucleotide analogs, protease inhibitors, and/or NNRTIs. In two randomized comparison studies, emtricitabine and lamivudine-based triple-combination regimens had similar efficacy (Saag, 2006).

Resistance to didanosine is associated with mutations at reverse transcriptase codons 65 and 74. The L74V substitution, which reduces susceptibility 5- to 26-fold in vitro, is seen most commonly in patients failing to respond to didanosine. Other nucleoside analog mutations, including thymidine analog mutations, can contribute to didanosine resistance even though the drug does not appear to select for these mutations de novo. The reverse transcriptase insertion mutations at codon 69 produce cross-resistance to all current nucleoside analogs, including didanosine (Gallant et al., 2003). The M184V mutation seen in response to emtricitabine and lamivudine reduces didanosine susceptibility in vitro but probably plays no role in clinical resistance to this drug.

Absorption, Distribution, and Elimination. Didanosine is acid labile and is degraded at low gastric pH (Dudley, 1995). An antacid buffer is used in some formulations to improve bioavailability. Chewable tablets contain calcium carbonate and magnesium hydroxide, whereas the powder form contains citrate–phosphate buffer. The pediatric powder formulation lacks buffer and is reconstituted with purified water and mixed with a liquid antacid preparation. Food decreases didanosine bioavailability (AUC) by ~55%, so all formulations of didanosine must be administered at least 30 minutes before or 2 hours after eating (Moreno et al., 2007). This complicates dosing of didanosine in combination with antiretroviral drugs that must be given with food, as is the case for most HIV protease inhibitors. The enzyme purine nucleoside phosphorylase (PNP) probably contributes to the presystemic clearance of didanosine because tenofovir, which inhibits PNP, greatly increases concentrations of orally administered didanosine (Robbins et al., 2003). PNP converts didanosine to hypoxanthine, which is ultimately converted to uric acid.

Peak plasma concentrations of didanosine are seen ~1 hour after oral administration of the chewable tablets or powder formulations and 2 hours after delayed-release capsules. The plasma elimination t_1/2 of parent drug is ~1.5 hours, but the estimated intracellular t_1/2 of dideoxyadenosine 5′-triphosphate is substantially longer, 25-40 hours (Moreno et al., 2007). As a result, didanosine can be administered once daily. Didanosine is excreted both by glomerular filtration and by tubular secretion and does not undergo metabolism to a significant degree. Drug doses therefore must be adjusted in patients with renal insufficiency or renal failure (Jayasekara et al., 1999). Didanosine is not protein bound to a significant degree. The cerebrospinal penetration of didanosine is less than that of zidovudine, with a CSF-to-plasma ratio of 0.2, but the clinical significance of this is unclear.

Untoward Effects. The most serious toxicities associated with didanosine include peripheral neuropathy and pancreatitis, both of which are thought to be a consequence of mitochondrial toxicity. Up to 20% of patients reported peripheral neuropathy in early clinical trials (Moreno et al., 2007). As with other dideoxynucleosides, peripheral neuropathy is more common with higher doses or concentrations of didanosine and is more prevalent in patients with underlying HIV-related neuropathy, low CD4 count, or in those receiving other neurotoxic drugs. Typically, this is a symmetrical distal sensory neuropathy that begins in the feet and lower extremities but may involve the hands as it progresses (stocking/glove distribution). Patients complain of pain, numbness, and tingling in the affected extremities. If the drug is stopped as soon as symptoms appear, the neuropathy will stabilize and should improve or resolve. However, irreversible neuropathy can occur with continued use. Retinal changes and optic neuritis also have been reported with didanosine, and patients should undergo periodic retinal examinations.

Acute pancreatitis is a rare but potentially fatal complication of didanosine. Acute pancreatitis is associated with higher doses and concentrations of didanosine but has occurred in up to 7% of patients using the recommended dose of 200 mg twice daily (Moreno et al., 2007). Pancreatitis is more common with advanced HIV disease, and other risk factors include a previous history of pancreatitis, alcohol or illicit drug use, and hypertriglyceridemia. Combining didanosine with stavudine, which is also associated with peripheral neuropathy and pancreatitis, increases the risk and severity of both toxicities (Havlir et al., 2001).

As with other dideoxynucleosides and zidovudine, serious hepatic toxicity—with or without steatosis, hepatomegaly, and lactic acidosis—occurs very rarely but can be fatal. Risk factors for the lactic acidosis-steatosis syndrome include female sex, obesity, and prolonged exposure to the drug (Tripuraneni et al., 2004).

Other reported adverse effects include elevated hepatic transaminases, headache, and asymptomatic hyperuricemia and portal hypertension. The impact of didanosine on heart attack risk is under review. Diarrhea is reported more frequently with didanosine than with other nucleoside analogs and has been attributed to the antacid in the buffered oral preparations (Moreno et al., 2007). Didanosine chewable tablets contain 36.5 mg phenylalanine and should be avoided in those with phenylketonuria. Buffered powder for oral solution contains 1.4 g sodium per packet and should be used cautiously in those on sodium-restricted diets.

Precautions and Interactions. Buffering agents included in didanosine formulations can interfere with the bioavailability of some co-administered drugs because of altered pH or chelation with cations in the buffer. For example, the ciprofloxacin AUC is decreased by up to 98% when given with didanosine, and concentrations of ketoconazole and itraconazole, whose absorption is pH dependent, also are diminished (Piscitelli and Gallicano, 2001). A 200-mg dose of buffered didanosine reduced the indinavir AUC by 84%. These interactions generally can be avoided by separating administration of didanosine from that of other agents by at least 2 hours after or 6 hours before the interacting drug. The enteric-coated formulation of didanosine does not alter ciprofloxacin or indinavir absorption.

Didanosine is excreted renally, and shared renal excretory mechanisms provide a basis for drug interactions. Oral ganciclovir can increase plasma didanosine concentrations approximately 2-fold and may be associated with an increase in didanosine toxicity. Allopurinol can increase the didanosine AUC more than 4-fold and is contraindicated. Tenofovir increases the didanosine AUC by 44-60% and also may increase the risk of didanosine toxicity. If these two drugs must be given together, it is recommended to decrease the didanosine dose from 400 to 250 mg once daily for patients >60 kg (Chapman et al., 2003). Methadone decreases the didanosine AUC by 57-63% (Rainey et al., 2000) possibly as a consequence of altered GI motility and delayed absorption, although this has not been associated with a higher risk of failing didanosine treatment.

Didanosine should be avoided in patients with a history of pancreatitis or neuropathy because the risk and severity of both complications increase. Co-administration of other drugs that cause pancreatitis or neuropathy also will increase the risk and severity of these symptoms. Ethambutol, isoniazid, vincristine, cisplatin, and pentamidine also should be avoided.

The combination of didanosine and hydroxyurea was used to exploit a beneficial interaction that creates a favorable intracellular ratio of concentrations of dideoxyadenosine 5′-triphosphate to deoxythymidine 5′-triphosphate (Frank et al., 2004). Although this combination may boost didanosine antiviral activity modestly, it also increases toxicity, producing peripheral neuropathy and fatal pancreatitis, and should be avoided (Havlir et al., 2001).

Zalcitabine.

Zalcitabine (2′,3′-dideoxycytidine; ddC) is a synthetic cytidine analog reverse transcriptase inhibitor that is designated as an orphan drug for the treatment of advanced HIV infection. It is no longer marketed because of toxicity (mainly peripheral neuropathy) and the need for thrice daily dosing. It is active against HIV-1, HIV-2, and hepatitis B virus (HBV). For additional information on this drug, see the 11th edition of this book.

The use of efavirenz or nevirapine in combination with other antiretroviral drugs is associated with favorable long-term suppression of viremia and elevation of CD4+ lymphocyte counts (Sheran, 2005). Efavirenz in particular is a common component of first regimens for treatment-naive patients in recognition of its convenience, tolerability, and potency. Rashes occur frequently with all NNRTIs, usually during the first 4 weeks of therapy. These generally are mild and self-limited, although rare cases of potentially fatal Stevens-Johnson syndrome have been reported with nevirapine, efavirenz, and etravirine. Fat accumulation can be seen after long-term use of NNRTIs (Calmy et al., 2009), and fatal hepatitis has been associated with nevirapine use.

The in vitro IC₅₀ of this drug ranges from 10 to 100 nM. Like other compounds in this class, nevirapine does not have significant activity against HIV-2 or other retroviruses (Sheran, 2005).

Absorption, Distribution, and Elimination. Nevirapine is well absorbed, and its bioavailability is not altered by food or antacids. The drug readily crosses the placenta and has been found in breast milk, a feature that has encouraged use of nevirapine for prevention of mother-to-child transmission of HIV (Mirochnick et al., 1998).

Nevirapine is eliminated mainly by oxidative metabolism involving CYP3A4 and CYP2B6. Less than 3% of the parent drug is eliminated unchanged in the urine. Nevirapine has a long elimination t_1/2 of 25-30 hours at steady state, but t_1/2 may be longer in some individuals, especially those of African descent (Sheran, 2005). The drug is a moderate inducer of CYPs, including CYP3A4; thus the drug induces its own metabolism, which decreases the t_1/2 from 45 hours following the first dose to 25-30 hours after 2 weeks. To compensate for this, it is recommended that the drug be initiated at a dose of 200 mg once daily for 14 days, with the dose then increased to 200 mg twice daily if no adverse reactions have occurred. Clinical studies of nevirapine have investigated once-daily use, but this dosing regimen is not approved.

Untoward Effects. The most frequent adverse event associated with nevirapine is rash, which occurs in ~16% of patients. Mild macular or papular eruptions commonly involve the trunk, face, and extremities and generally occur within the first 6 weeks of therapy. Pruritus is also common. In most patients the rash resolves with continued administration of drug. Up to 7% of patients discontinue therapy owing to rash; administration of glucocorticoids may cause a more severe rash. Life-threatening Stevens-Johnson syndrome is rare but occurs in up to 0.3% of recipients (Sheran, 2005).

Elevated hepatic transaminases occur in up to 14% of patients. Clinical hepatitis occurs in up to 1% of patients. Severe and fatal hepatitis has been associated with nevirapine use, and this may be more common in women with CD4 counts >250 cells/mm³, especially during pregnancy (Sheran, 2005). Other reported side effects include fever, fatigue, headache, somnolence, and nausea.

Precautions and Interactions. Because nevirapine induces CYP3A4, this drug may lower plasma concentrations of co-administered CYP3A4 substrates. Methadone withdrawal has been reported in patients receiving nevirapine, presumably as a consequence of enhanced methadone clearance. Plasma ethinyl estradiol and norethindrone concentrations decrease by 20% with nevirapine, and alternative methods of birth control are advised.

In original monotherapy studies, a rapid fall in plasma HIV RNA concentrations of ≥99% was followed by a return toward baseline within 8 weeks because of rapid emergence of resistance (Havlir et al., 1995; Wei et al., 1995). Nevirapine should never be used as a single agent or as the sole addition to a failing regimen. Nevirapine is approved for use in infants and children ≥15 days old, with dosing based on body surface area.

Single-dose nevirapine has been used commonly in pregnant HIV-infected women to prevent mother-to-child transmission. A single oral intrapartum dose of 200 mg nevirapine followed by a single dose given to the newborn reduced neonatal HIV infection to 13% compared with 21.5% infection with a more complicated zidovudine regimen (Sheran, 2005). Although this regimen is very inexpensive and generally well tolerated, the high prevalence of nevirapine resistance following the single oral dose (Eshleman et al., 2004), coupled with the recent recognition of fatal nevirapine hepatitis, has prompted a reexamination of the role this regimen should play in the prevention of vertical transmission (Department of Health and Human Services, 2010).

Absorption, Distribution, and Elimination. Efavirenz is well absorbed from the GI tract and reaches peak plasma concentrations within 5 hours. There is diminished absorption of the drug with increasing doses. Bioavailability (AUC) is increased by 22% with a high-fat meal.

Efavirenz is >99% bound to plasma proteins and, as a consequence, has a low CSF-to-plasma ratio of 0.01. The clinical significance of this low CNS penetration is unclear, especially because the major toxicities of efavirenz involve the CNS. The drug should be taken initially on an empty stomach at bedtime to reduce side effects (Sheran, 2005).

Efavirenz is cleared via oxidative metabolism, mainly by CYP2B6 and to a lesser extent by CYP3A4. The parent drug is not excreted renally to a significant degree. Efavirenz is cleared slowly, with an elimination t_1/2 of 40-55 hours at steady state. This safely allows once-daily dosing. Clearance is even slower in recipients with the 516G→T and 983C→T CYP 2B6 genotype (Haas et al., 2009), a common polymorphism in those of Japanese and African ancestry.

Untoward Effects. The most important adverse effects of efavirenz involve the CNS. Up to 53% of patients report some CNS or psychiatric side effects, but <5% discontinue the drug for this reason. CNS symptoms may occur with the first dose and last for hours; more severe symptoms may require weeks to resolve. Patients commonly report dizziness, impaired concentration, dysphoria, vivid or disturbing dreams, and insomnia. Episodes of frank psychosis (depression, hallucinations, and/or mania) have been associated with initiating efavirenz. Fortunately, CNS side effects generally become more tolerable and resolve within the first 4 weeks of therapy.

Rash occurs frequently with efavirenz, in up to 27% of adult patients (Sheran, 2005). Rash usually occurs within the first few weeks of treatment but resolves spontaneously and rarely requires drug discontinuation. Life-threatening skin eruptions such as Stevens-Johnson syndrome have been reported during postmarketing experience with efavirenz but are rare.

Other side effects reported with efavirenz include headache, increased hepatic transaminases, and elevated serum cholesterol. False-positive urine screening tests for marijuana metabolites also can occur depending on the assay used (Sheran, 2005).

Efavirenz is the only antiretroviral drug that is unequivocally teratogenic in primates. When efavirenz was administered to pregnant cynomolgus monkeys, 25% of fetuses developed malformations. In 13 known cases where women were exposed to efavirenz during the first trimester of pregnancy, fetuses or infants had significant malformations, mainly of the brain and spinal cord. Women of childbearing potential therefore should use two methods of birth control and avoid pregnancy while taking efavirenz.

Precautions and Interactions. Efavirenz is a moderate inducer of hepatic enzymes, especially CYP3A4, but also a weak to moderate CYP inhibitor; because of the drug's long t_1/2, there is no need to alter drug dose during the first few weeks of treatment.

Efavirenz decreases concentrations of phenobarbital, phenytoin, and carbamazepine; the methadone AUC is reduced by 33-66% at steady state. Rifampin concentrations are unchanged by concurrent efavirenz, but rifampin may reduce efavirenz concentrations slightly. Efavirenz reduces the rifabutin AUC by 38% on average. Efavirenz has a variable effect on HIV protease inhibitors. Indinavir, saquinavir, and amprenavir concentrations are reduced, but ritonavir and nelfinavir concentrations are increased. Drugs that induce CYPs 2B6 or 3A4 (e.g., phenobarbital, phenytoin, and carbamazepine) would be expected to increase the clearance of efavirenz and should be avoided (Sheran, 2005).

Efavirenz is used widely in the developed world because of its convenience, effectiveness, and long-term tolerability. Especially popular is the once-daily single pill co-formulation of efavirenz, tenofovir, and emtrictabine (atripla). To date, no antiretroviral regimen has produced better long-term treatment responses than efavirenz-containing regimens in randomized prospective clinical trials. As a result, efavirenz plus two nucleoside reverse transcriptase inhibitors remains a preferred regimen for treatment-naive patients. Generic versions of efavirenz are increasingly used in treatment regimens in resource-poor countries because of this drug's better toxicity profile compared to nevirapine. Efavirenz can be safely combined with rifampin and is useful in patients also being treated for tuberculosis.

Efavirenz is approved for adult and pediatric patients ≥3 years of age and weighing at least 10 kg. Efavirenz is only available as tablets and capsules; pediatric dosing is based on weight range.

Etravirine appears to have conformational and positional flexibility in the NNRTI binding pocket that allow it to inhibit the function of the HIV-1 reverse transcriptase in the presence of common NNRTI resistance mutations (Deeks and Keating, 2008). Clinically significant drug resistance and treatment failure require the presence of multiple mutations, including V90I, A98G, L100I, K101E/P, V106I, V179D/F, Y181C/I/V, and G190A/S. Best response to an etravirine-containing regimen is seen in patients harboring three or fewer resistance mutations, although likelihood of virologic benefit decreases with each additional mutation. One limitation of etravirine resistance data is that most information comes from patients also receiving darunavir, and few resistance and response data are available for other regimens (Fulco and McNicholl, 2009).

Absorption, Distribution, and Elimination. Etravirine is absorbed rapidly after oral administration with peak concentrations occurring 2.5-4 hours after dosing. Food increases the etravirine AUC by 50% (Deeks and Keating, 2008), and it is therefore recommended that the drug be administered with food. Methyl- and dimethyl-hydroxylated metabolites are produced in the liver primarily by CYPs 3A4, 2C9, and 2C19, accounting for most of the elimination of this drug. No unchanged drug is detected in the urine. The terminal elimination t_1/2 is ~41 hours; twice-daily dosing of this drug is the historical consequence of enormous pill burdens from older formulations that are no longer in use, and it is likely that the drug could be given once daily (Deeks and Keating, 2008). Etravirine is 99% bound to plasma proteins, mainly to albumin and α₁-acid glycoprotein.

Untoward Effects. In randomized placebo-controlled trials combining etravirine with darunavir in treatment-experienced patients, the only side effect occurring more commonly with etravirine than with placebo was rash (17% versus 9%), usually occurring within a few weeks of starting therapy and resolving within 1-3 weeks. Overall, 2% of patients in these trials discontinued etravirine because of rash. Severe rash including Stevens-Johnson syndrome and toxic epidermal necrolysis have been reported. Etravirine was not associated with more neuropsychiatric or hepatic adverse effects than placebo (Deeks and Keating, 2008).

Precautions and Interactions. Etravirine is an inducer of CYP3A4 and glucuronosyl transferases, and an inhibitor of CYPs 2C9 and 2C19, and can therefore be involved in a number of clinically significant pharmacokinetic drug interactions.

Etravirine can be combined with darunavir/ritonavir, lopinavir/ritonavir, and saquinavir/ritonavir without the need for dose adjustments. The dose of maraviroc should be doubled to 600 mg twice daily when these two drugs are combined. Etravirine should not be administered with tipranavir/ritonavir, fosamprenavir/ritonavir, or atazanavir/ritonavir in the absence of better data to guide dosing. Etravirine should not be combined with efavirenz, nevirapine, or delavirdine. Unlike other NNRTIs, etravirine does not appear to alter the clearance of methadone (Fulco and McNicholl, 2009).

In a pooled analysis of >1200 treatment-experienced patients taking darunavir/ritonavir, 61% of patients randomized to etravirine achieved a plasma HIV RNA <50 copies/mL at 48 weeks compared to 40% on placebo (p < 0.0001). Etravirine-treated patients also had a moderately better mean CD4 cell count increase at week 48 (98 vs. 73 cells/mm³). Week 48 virologic responses were dependent on the number of baseline etravirine-resistance mutations, with a differential versus placebo of 75% versus 44% if no mutations were present, falling to 25% versus 17% if four or more were present (Fulco and McNicholl, 2009). In small studies, virologic response with etravirine was poor unless the regimen contained at least one other active antiretroviral.

Delavirdine.

Delavirdine is a bisheteroarylpiperazine NNRTI that selectively inhibits HIV-1 (Figure 59–4). The in vitro IC₅₀ ranges from 6 to 30 nM for laboratory HIV-1 isolates to 1 to 700 nM for clinical isolates (Scott and Perry, 2000). Delavirdine (rescriptor) is no longer used widely because of its short t_1/2 and requirement for thrice-daily dosing.

Delavirdine does not have significant activity against HIV-2 or other retroviruses. Delavirdine shares resistance mutations with efavirenz and nevirapine, and any patient who fails treatment with this NNRTI should not be treated with those drugs.

Delavirdine is well absorbed, especially at pH<2. Antacids, histamine H₂-receptor antagonists, proton pump inhibitors, and achlorhydria may decrease its absorption. Standard meals do not alter the delavirdine AUC, and the drug can be administered irrespective of food. The drug may have nonlinear pharmacokinetics because the plasma t_1/2 increases with increasing doses (Scott and Perry, 2000). Delavirdine clearance is primarily through oxidative metabolism by CYP3A4, with <5% of a dose recovered unchanged in the urine. At the recommended dose of 400 mg three times daily, the mean elimination t_1/2 is 5.8 hours (range: 2-11 hours because of the considerable interpatient variability in clearance).

As with all drugs in this class, the most common side effect of delavirdine is rash, which occurs in 18-36% of subjects. Rash usually is seen in the first few weeks of treatment and often resolves despite continued therapy. Severe dermatitis, including erythema multiforme and Stevens-Johnson syndrome, has been reported but is rare. Elevated hepatic transaminases and hepatic failure also have been reported. Neutropenia also may occur rarely (Scott and Perry, 2000).

Delavirdine is both a substrate for and an inhibitor of CYP3A4 and can alter the metabolism of other CYP3A4 substrates. Potent inducers of CYP3A4, such as carbamazepine, phenobarbital, phenytoin, rifabutin, and rifampin, may decrease delavirdine concentrations and should be avoided. Delavirdine increases the plasma concentrations of most HIV protease inhibitors (Scott and Perry, 2000).

Initial monotherapy studies with delavirdine produced only transient decreases in plasma HIV RNA concentrations owing to rapid emergence of resistance. Later studies of delavirdine in combination with nucleoside analogs showed sustained decreases in HIV-1 RNA.

These drugs prevent proteolytic cleavage of HIV gag and pol precursor polypeptides that include essential structural (p17, p24, p9, and p7) and enzymatic (reverse transcriptase, protease, and integrase) components of the virus. This prevents the metamorphosis of HIV virus particles into their mature infectious form (Flexner, 1998). Infected patients treated with HIV protease inhibitors as sole agents experienced a 100- to 1000-fold mean decrease in plasma HIV RNA concentrations within 12 weeks, an effect similar in magnitude to that produced by NNRTIs (Ho et al., 1995).

Although the approved dose of ritonavir for antiretroviral treatment is 600 mg twice daily, doses of 100 or 200 mg once or twice daily are sufficient to inhibit CYP3A4 and increase ("boost") the concentrations of most concurrently administered CYP3A4 substrates. Lower doses of ritonavir are much better tolerated. The enhanced pharmacokinetic profile of HIV protease inhibitors administered with ritonavir reflects inhibition of both first-pass and systemic clearance, resulting in improved oral bioavailability and a longer elimination t_1/2 of the co-administered drug. This allows a reduction in both drug dose and dosing frequency while increasing systemic concentrations (Flexner, 2000). Combinations of darunavir, lopinavir, fosamprenavir, and atazanavir with ritonavir are approved for once-daily administration.

P-gp in capillary endothelial cells of the blood-brain barrier limits the penetration of HIV protease inhibitors into the brain (Kim et al., 1998), although the low CSF-to-plasma drug concentration ratio characteristic of these drugs also may reflect extensive binding to plasma proteins. Most HIV protease inhibitors penetrate less well into semen than do nucleoside reverse transcriptase inhibitors and NNRTIs. Virologic responses in plasma, CSF, and semen usually are concordant (Taylor et al., 1999), and the clinical significance of P-gp and protein-binding effects is unclear.

Initial (primary) resistance mutations in the enzymatic active site confer only a 3- to 5-fold drop in sensitivity to most drugs; these are followed by secondary mutations often distant from the active site that compensate for the reduction in proteolytic efficiency. Accumulation of secondary resistance mutations increases the likelihood of cross-resistance to other PIs (Flexner, 1998; Kuritzkes, 2004). Patients failing boosted PI-based combination regimens are more likely to have resistance mutations to the other drugs in the regimen, especially NRTIs (Kuritzkes, 2004), suggesting a high genetic barrier to resistance.

The primary saquinavir resistance mutation occurs at HIV protease codon 90 (a leucine-to-methionine substitution), although primary resistance also has been reported with a glycine-to-valine substitution at codon 48. Secondary resistance mutations occur at codons 36, 46, 82, 84, and others, and these are associated with clinical saquinavir resistance as well as cross-resistance to most other HIV protease inhibitors (Noble and Faulds, 1996).

Absorption, Distribution, and Elimination. Fractional oral bioavailability is low (~4%) owing mainly to extensive first-pass metabolism (Flexner, 1998), and so this drug should always be given in combination with ritonavir. Low doses of ritonavir increase the saquinavir steady-state AUC by 20- to 30-fold (Flexner, 2000), allowing administration once or twice daily.

Substances that inhibit intestinal but not hepatic CYP3A4, such as grapefruit juice, increase the saquinavir AUC by 3-fold at most (Flexner, 2000). Saquinavir is metabolized primarily by intestinal and hepatic CYP3A4 (Fitzsimmons and Collins, 1997); its metabolites are not known to be active against HIV-1. The parent drug and its metabolites are eliminated through the biliary system and feces (>95% of drug), with minimal urinary excretion (<3%).

Untoward Effects. The most frequent side effects of saquinavir are GI: nausea, vomiting, diarrhea, and abdominal discomfort. Most side effects of saquinavir are mild and short lived, although long-term use is associated with lipodystrophy.

Precautions and Interactions. Saquinavir clearance is increased with CYP3A4 induction; thus, co-administration of inducers of CYP3A4 such as rifampin, phenytoin, or carbamazepine lowers saquinavir concentrations and should be avoided (Flexner, 1998). The effect of nevirapine or efavirenz on saquinavir may be partially or completely reversed with ritonavir. Most drug interactions seen with saquinavir/ritonavir reflect the effect of the boosting agent.

The primary ritonavir resistance mutation is usually at protease codon 82 (several possible substitutions for valine) or codon 84 (isoleucine-to-valine substitution). Additional mutations associated with increasing resistance occur at codons 20, 32, 46, 54, 63, 71, 84, and 90. High-level resistance requires accumulation of multiple mutations.

Absorption, Distribution, and Elimination. Absorption of ritonavir is rapid and is only slightly affected by food, depending on the formulation. The overall absorption of ritonavir from the capsule formulation increases by 13% when the capsule is taken with meals, but the bioavailability of the oral solution decreases by 7% (Flexner, 1998). A heat-stable 100-mg tablet formulation is now available. Interindividual variability in pharmacokinetics is high, with a variability exceeding 6-fold in trough concentrations among patients given 600 mg ritonavir every 12 hours as capsules (Hsu et al., 1998).

Ritonavir is metabolized primarily by CYP3A4 and to a lesser extent by CYP2D6. Ritonavir and its metabolites are mainly eliminated in feces (86% of parent drug and metabolites), with only 3% of drug eliminated unchanged in the urine. Ritonavir induces its own metabolism; steady-state concentrations are reached within 2 weeks. Ritonavir is 98-99% bound to plasma proteins, mainly to α₁-acid glycoprotein. Physiological concentrations of α₁-acid glycoprotein increase the in vitro IC₅₀ by a factor of 10, whereas albumin increases the IC₅₀ by a factor of 4 (Molla et al., 1998).

Untoward Effects. The major side effects of ritonavir are GI and include dose-dependent nausea, vomiting, diarrhea, anorexia, abdominal pain, and taste perversion. GI toxicity may be reduced if the drug is taken with meals. Peripheral and perioral paresthesias can occur at the therapeutic dose of 600 mg twice daily. These side effects generally abate within a few weeks of starting therapy. Ritonavir also causes dose-dependent elevations in serum total cholesterol and triglycerides, as well as other signs of lipodystrophy, and it could increase the long-term risk of atherosclerosis in some patients.

Precautions and Interactions. Ritonavir is one of the most potent known inhibitors of CYP3A4, markedly increasing the plasma concentrations and prolonging the elimination of many drugs. Ritonavir should be used with caution in combination with any CYP3A4 substrate and should not be combined with drugs that have a narrow therapeutic index such as midazolam, triazolam, fentanyl, and ergot derivatives (Flexner, 1998). Ritonavir is a mixed competitive and irreversible inhibitor of CYP 3A4 and its effects can persist for 2-3 days after the drug is discontinued (Washington et al., 2003). Ritonavir is also a weak inhibitor of CYP2D6. Potent inducers of CYP3A4 activity such as rifampin may lower ritonavir concentrations and should be avoided or dosage adjustments considered. The capsule and solution formulations of ritonavir contain alcohol and should not be administered with disulfiram or metronidazole (Chapter 23).

Ritonavir is also a moderate inducer of CYP3A4, glucuronosyl S-transferase, and possibly other hepatic enzymes and drug transport proteins. The concentrations of some drugs therefore will be decreased in the presence of ritonavir. Ritonavir reduces the ethinyl estradiol AUC by 40%, and alternative forms of contraception should be used (Piscitelli and Gallicano, 2001).

Amprenavir is an N,N-disubstituted (hydroxyethyl) amino sulfonamide nonpeptide HIV protease inhibitor. Although developed using a sophisticated structure-based drug-design program, the same compound was identified previously using a more traditional high-throughput screen of an available chemical library (Werth, 1994). Amprenavir contains a sulfonamide moiety, which may play a role in its dermatologic side effects. The drug is active against both HIV-1 and HIV-2, with an IC₉₀ for wild-type HIV-1 of ~80 nM.

Absorption, Distribution, and Elimination. Fosamprenavir is dephosphorylated rapidly to amprenavir in the intestinal mucosa. The phosphorylated prodrug is ~2000 times more water soluble than amprenavir. Meals have no significant effect on fosamprenavir pharmacokinetics (Arvieux and Tribut, 2005). Amprenavir is 90% bound to plasma proteins, mostly α₁-acid glycoprotein. This binding is relatively weak, and physiological concentrations of α₁-acid glycoprotein increase the in vitro IC₅₀ only 3- to 5-fold. Amprenavir clearance is mainly by hepatic CYP3A4, and excretion is by the biliary route. Amprenavir is a moderate inhibitor and inducer of CYP3A4. Ritonavir increases amprenavir concentrations by inhibiting CYP3A4, allowing lower fosamprenavir doses. The daily fosamprenavir dose may be reduced from 1400 mg twice daily to 1400 mg plus 200 mg ritonavir when given once daily, or 700 mg (one tablet) plus 100 mg ritonavir twice daily.

Untoward Effects. The most common adverse effects associated with fosamprenavir are GI and include diarrhea, nausea, and vomiting. Hyperglycemia, fatigue, paresthesias, and headache also have been reported. Fosamprenavir can produce skin eruptions; moderate to severe rash is reported in up to 8% of recipients, and onset is usually within 2 weeks of starting therapy (Arvieux and Tribut, 2005). Fosamprenavir has fewer effects on plasma lipid profiles than lopinavir-based regimens.

Precautions and Interactions. Inducers of hepatic CYP3A4 activity (e.g., rifampin and efavirenz) may lower plasma amprenavir concentrations. Because amprenavir is both a CYP3A4 inhibitor and inducer, pharmacokinetic drug interactions can occur and may be unpredictable. For example, atorvastatin, ketoconazole, and rifabutin concentrations increase significantly when fosamprenavir is given without ritonavir, whereas methadone concentrations decrease.

Twice-daily fosamprenavir/ritonavir produces virologic outcomes equivalent to lopinavir/ritonavir in both treatment-naive and treatment-experienced patients (Arvieux and Tribut, 2005). However, once-daily fosamprenavir/ritonavir is inferior to lopinavir/ritonavir in protease-inhibitor experienced patients, and the drug should only be given twice daily in this patient population. Fosamprenavir is approved for use in treatment-naive pediatric patients ≥2 years of age and treatment-experienced patients ≥6 years of age, at a dose of 30 mg/kg twice daily or 18 mg/kg plus ritonavir 3 mg/kg twice daily.

Mutations most likely to be associated with resistance include I47A/V, V32I, and L76V. Additional mutations associated with lopinavir failure in treatment-experienced patients include those at HIV protease codons 10, 20, 24, 33, 46, 50, 53, 54, 63, 71, 73, 82, 84, and 90 (Oldfield and Plosker, 2006). There is no evidence that exposure to the low doses of ritonavir in the lopinavir/ritonavir co-formulation selects for ritonavir-specific resistance mutations.

Absorption, Distribution, and Elimination. Lopinavir is absorbed rapidly after oral administration. Food has a minimal effect on bioavailability of lopinavir/ritonavir tablets, and the drug can be taken with or without food. Although the tablets contain lopinavir/ritonavir in a fixed 4:1 ratio, the observed plasma concentration ratio for these two drugs following oral administration is nearly 20:1, reflecting the sensitivity of lopinavir to the inhibitory effect of ritonavir on CYP3A4. Lopinavir undergoes extensive hepatic oxidative metabolism by CYP3A4. Approximately 90% of total drug in plasma is the parent compound, and <3% of a dose is eliminated unchanged in the urine. Both lopinavir and ritonavir are highly bound to plasma proteins, mainly to α₁-acid glycoprotein, and have a low fractional penetration into CSF and semen.

When administered orally without ritonavir, lopinavir plasma concentrations were exceedingly low mainly owing to first-pass metabolism. Both the first-pass metabolism and systemic clearance of lopinavir are very sensitive to inhibition by ritonavir.

A single 50-mg dose of ritonavir increased the lopinavir AUC by 77-fold compared with 400 mg lopinavir alone; 100 mg ritonavir increased the lopinavir AUC by 155-fold. Lopinavir trough concentrations were increased 50- to 100-fold by co-administration of low doses of ritonavir (Oldfield and Plosker, 2006). Multiple-dose pharmacokinetic studies have not been conducted with lopinavir in the absence of ritonavir. Adding 100 mg ritonavir twice daily to the lopinavir/ritonavir co-formulation (a total of 200 mg twice daily of ritonavir) has only a modest further effect on lopinavir concentrations, increasing the mean steady-state AUC by 46%.

Untoward Effects. The most common adverse events reported with the lopinavir/ritonavir co-formulation have been GI: loose stools, diarrhea, nausea, and vomiting. These are less frequent and less severe than those reported with the 600 mg twice-daily standard dose of ritonavir but more common compared to those of boosted atazanavir and darunavir. The most common laboratory abnormalities include elevated total cholesterol and triglycerides. Because the same adverse effects occur with ritonavir, it is unclear whether these side effects are due to ritonavir, lopinavir, or both.

Precautions and Interactions. Because lopinavir metabolism is highly dependent on CYP3A4, concomitant administration of agents that induce CYP3A4, such as rifampin, may lower plasma lopinavir concentrations considerably. St. John's wort is a known inducer of CYP3A4, leading to lower concentrations of lopinavir and possible loss of antiviral effectiveness. Co-administration of other antiretrovirals that can induce CYP3A4, including amprenavir, nevirapine or efavirenz, may require increasing the dose of lopinavir (Oldfield and Plosker, 2006).

Although lopinavir is a weak inhibitor of CYP3A4 in vitro, the ritonavir in the co-formulated capsule strongly inhibits CYP3A4 activity and probably dwarfs any lopinavir effect. The liquid formulation of lopinavir contains 42% ethanol and should not be administered with disulfiram or metronidazole (Chapter 23). Ritonavir is also a moderate CYP inducer at the dose employed in the co-formulation and can adversely decrease concentrations of some co-administered drugs (e.g., oral contraceptives). There is no direct proof that lopinavir is a CYP inducer in vivo; however, concentrations of some co-administered drugs (e.g., amprenavir and phenytoin) are lower with the lopinavir/ritonavir co-formulation than would have been expected with low-dose ritonavir alone (Oldfield and Plosker, 2006).

In one study, 70 subjects who had failed therapy with one previous HIV protease inhibitor were treated for 2 weeks with lopinavir, followed by the addition of nevirapine. At 48 weeks, 60% of subjects had plasma HIV-1 RNA levels of <50 copies/mL despite substantial phenotypic resistance to other HIV protease inhibitors (Oldfield and Plosker, 2006). Because plasma concentrations of lopinavir generally are much higher than those required to suppress HIV replication in vitro, the drug may be capable of suppressing HIV isolates with low-level protease inhibitor resistance.

The adult lopinavir/ritonavir dose is 400/100 mg (two tablets) twice daily, or 800/200 mg (four tablets) once daily, with or without food. Lopinavir/ritonavir should not be dosed once daily in treatment-experienced patients. Lopinavir/ritonavir is approved for use in pediatric patients ≥14 days, with dosing based on either weight or body surface area. A pediatric tablet formulation is available for use in children >6 months of age.

This mutation was present in 100% of viruses isolated from patients failing therapy in one clinical trial (Croom et al., 2009). Isolates with only this mutation are still susceptible to inhibition by other protease inhibitors. Sensitivity to atazanavir is affected by various primary and secondary mutations that accumulate in patients who have failed other HIV protease inhibitors, with high-level resistance more likely if five or more additional mutations are present (Croom et al., 2009).

Absorption, Distribution, and Elimination. Atazanavir is absorbed rapidly after oral administration, with peak concentrations occurring ~2 hours after dosing. Atazanavir absorption is sensitive to food: a light meal increases the AUC by 70%, whereas a high-fat meal increases the AUC by 35% (Croom et al., 2009). It is therefore recommended that the drug be administered with food, which also decreases the interindividual variability in pharmacokinetics, unless given with ritonavir. Absorption is pH dependent, and proton pump inhibitors or other acid-reducing agents substantially reduce atazanavir concentrations after oral dosing; this effect is only partially reversed by concomitant ritonavir.

Atazanavir undergoes oxidative metabolism in the liver primarily by CYP3A4, which accounts for most of the elimination of this drug. Only 7% of the parent drug is excreted unchanged in the urine. The mean elimination t_1/2 of atazanavir at the standard 400-mg once-daily dose is ~7 hours; however, the drug has nonlinear pharmacokinetics, and the t_1/2 increases to nearly 10 hours at a dose of 600 mg (Croom et al., 2009). Atazanavir is 86% bound to plasma proteins, both to albumin and α₁-acid glycoprotein. It is present in CSF at <3% of plasma concentrations but has excellent penetration into seminal fluid (Croom et al., 2009).

Untoward Effects. Like indinavir, atazanavir frequently causes unconjugated hyperbilirubinemia, although this is mainly a cosmetic side effect and not associated with hepatotoxicity.

Approximately 40% of subjects receiving 400 mg atazanavir once daily in initial clinical trials developed a significant increase in total bilirubin (Croom et al., 2009), although only 5% developed jaundice. This is a consequence of inhibition of UDP-glucuronosyl transferase by atazanavir, and the side effect occurs more prominently in those who are genetically deficient in this enzyme, e.g., patients with Gilbert's syndrome (Rotger et al., 2005). Postmarketing reports include hepatic adverse reactions of cholecystitis, cholelithiasis, cholestasis, and other hepatic function abnormalities.

Other side effects reported with atazanavir include diarrhea and nausea, mainly during the first few weeks of therapy. Overall, 6% of patients discontinued atazanavir because of side effects during 48 weeks of treatment. Patients treated with atazanavir in randomized clinical trials had significantly lower fasting triglyceride and cholesterol concentrations than patients treated with nelfinavir, lopinavir, or efavirenz (Croom et al., 2009), suggesting a reduced propensity to cause these side effects. In addition, atazanavir is not known to cause glucose intolerance or changes in insulin sensitivity.

Precautions and Interactions. Because atazanavir is metabolized by CYP3A4, concomitant administration of agents that induce this enzyme (e.g., rifampin) is contraindicated. Efavirenz 600 mg once daily reduced the unboosted atazanavir AUC by 74%. Atazanavir is a moderate inhibitor of CYP3A4 and may alter plasma concentrations of other CYP3A4 substrates. Atazanavir inhibits CYP3A4 less than ritonavir and does not appear to inhibit other CYP isoforms. Atazanavir is a moderate UGT 1A1 inhibitor, and when given with or without ritonavir increases the raltegravir AUC 41-72% and increases the raltegravir C_12h approximately 2-fold. Atazanavir is not known to induce hepatic drug-metabolizing enzymes.

Ritonavir significantly increases the atazanavir AUC and reduces atazanavir systemic clearance. Ritonavir 100 mg once daily increases the atazanavir 300 mg once-daily steady-state AUC by 2.5-fold and increases the C_min 6.5-fold (Croom et al., 2009). Low-dose ritonavir also counters the effect of some inducers, for example efavirenz, on the atazanavir AUC.

Proton pump inhibitors reduce atazanavir concentrations substantially with concomitant administration. These drugs and H₂ blockers should be avoided in patients receiving atazanavir without ritonavir.

In addition to its use for treatment-naive and experienced adults, atazanavir, in combination with ritonavir, is approved for treatment of pediatric patients >6 years of age, with dosing based on weight.

Clinically significant drug resistance requires accumulation of multiple primary mutations. At least three darunavir-associated resistance mutations are required to confer resistance, and the most commonly seen are V32I, L33F, I47V, I54L and L89V (McKeage et al., 2009). Pretreatment resistance to darunavir is highly unlikely in treatment-naive patients and occurs in <10% of protease inhibitor-experienced patients.

Absorption, Distribution, and Elimination. Darunavir is absorbed rapidly after oral administration with ritonavir, with peak concentrations occurring 2-4 hours after dosing. Ritonavir increases darunavir bioavailability and increases the darunavir AUC by up to 14-fold. The drug should be taken with food because food increases the darunavir AUC by 30% (McKeage et al., 2009).

Darunavir undergoes oxidative metabolism in the liver primarily by CYP3A4, accounting for most of the elimination of this drug. About 8% of the parent drug is excreted unchanged in the urine. When combined with ritonavir, the mean elimination t_1/2 of darunavir is ~15 hours. In the presence of ritonavir 100 mg twice daily, the AUC after a 600-mg dose administered twice daily was increased 14-fold (McKeage et al., 2009). Darunavir is 95% bound to plasma proteins, mainly to α₁-acid glycoprotein.

Untoward Effects. Because darunavir must be combined with a low dose of ritonavir, drug administration can be accompanied by all of the side effects caused by ritonavir, including GI complaints in up to 20% of patients. Darunavir, like fosamprenavir, contains a sulfa moiety, and rash has been reported in up to 10% of recipients.

Overall, 3% of patients in randomized clinical trials discontinued darunavir because of side effects during 48 weeks of treatment, compared to 7% of patients treated with lopinavir/ritonavir. Darunavir/ritonavir is associated with increases in plasma triglycerides and cholesterol, although the magnitude of increase is lower than that seen with lopinavir/ritonavir (McKeage et al., 2009). Although causality is not firmly established, darunavir has been associated with episodes of hepatotoxicity.

Precautions and Interactions. Because darunavir is metabolized by CYP3A4, concomitant administration of agents that induce this enzyme (e.g., rifampin) is contraindicated. The drug interaction profile of darunavir/ritonavir is dominated by those expected with ritonavir.

Efavirenz 600 mg once daily reduced the darunavir AUC by 13% when combined with darunavir/ritonavir 300/100 mg twice daily. Darunavir/ritonavir 600/100 twice daily increased the maraviroc AUC by 344%, and the maraviroc dose should be reduced to 150 mg twice daily when combined with darunavir.

In treatment-naive patients receiving tenofovir plus emtricitabine randomized to darunavir/ritonavir 800/100 mg once daily, virologic outcomes after 48 weeks were comparable to those seen with lopinavir/ritonavir 800/200 mg once daily (84% vs. 78%, respectively, achieved plasma HIV RNA <50 copies/mL). In protease inhibitor–experienced patients receiving an optimized background regimen and randomized to darunavir/ritonavir 600/100 mg twice daily, virologic outcomes after 48 weeks were better than those seen with lopinavir/ritonavir 400/100 mg twice daily (71% vs. 60%, respectively, achieving plasma HIV RNA <50 copies/mL) (McKeage et al., 2009).

Darunavir/ritonavir can be used as a once-daily (800/100 mg) or twice-daily (600/100 mg) regimen with nucleosides in treatment-naive adults and as a twice-daily regimen in treatment-experienced adults, taken with food. Darunavir/ritonavir twice daily is approved for use in pediatric patients >6 years of age, with dosing based on weight.

Indinavir.

Chemistry and Antiviral Activity. Indinavir is a peptidomimetic hydroxyethylene HIV protease inhibitor (Figure 59–6) whose structure was based on a renin inhibitor with some similarity to the phenylalanine-proline cleavage site in the HIV gag polyprotein (Plosker and Noble, 1999), although indinavir is not itself a renin inhibitor. Indinavir is 10-fold more potent against the HIV-1 protease than that of HIV-2, and its 95% inhibitory concentration (IC₉₅) for wild-type HIV-1 ranges from 25 to 100 nM.

Mechanisms of Resistance. The primary indinavir resistance mutations occur at HIV protease codons 46 (a methionine-to-isoleucine or leucine), 82, and 84. However, secondary resistance mutations can accumulate at codons 10, 20, 24, 46, 54, 63, 71, 82, 84, and 90, and these are associated with clinical indinavir resistance as well as cross-resistance to other HIV protease inhibitors (Plosker and Noble, 1999).

Absorption, Distribution, and Elimination. Indinavir is absorbed rapidly after oral administration, with peak concentrations achieved in ~1 hour. Unlike other drugs in this class, food can adversely affect indinavir bioavailability; a high-calorie, high-fat meal reduces plasma concentrations by 75% (Plosker and Noble, 1999). Therefore, indinavir must be taken with ritonavir or while fasting or with a light low-fat meal.

Indinavir has the lowest protein binding of the HIV protease inhibitors, with only 60% of drug bound to plasma proteins (Plosker and Noble, 1999). As a consequence, indinavir has higher fractional CSF penetration than other drugs in this class, although the clinical significance of this is unknown.

The short t_1/2 of indinavir makes thrice-daily (every 8 hours) dosing necessary unless the drug is combined with ritonavir. Indinavir clearance is greatly reduced by low doses of ritonavir, which also overcome the deleterious effects of food on bioavailability (Flexner, 2000). This allows indinavir to be dosed twice daily regardless of meals.

Untoward Effects. A unique and common adverse effect of indinavir is crystalluria and nephrolithiasis. This stems from the poor solubility of the drug, which is lower at pH 7.4 than at pH 3.5 (Plosker and Noble, 1999). Precipitation of indinavir and its metabolites in urine can cause renal colic, and nephrolithiasis occurs in ~3% of patients. Patients must drink sufficient fluids to maintain dilute urine and prevent renal complications. Risk of nephrolithiasis is related to higher plasma drug concentrations, which presumably produce higher urine concentrations, regardless of whether or not the drug is combined with ritonavir (Dieleman et al., 1999).

Like atazanavir, indinavir frequently causes unconjugated hyperbilirubinemia, and 10% of patients develop an indirect serum bilirubin concentration >2.5 mg/dL (Plosker and Noble, 1999). This is generally asymptomatic and not associated with serious long-term sequelae. As with other HIV protease inhibitors, prolonged administration of indinavir is associated with the HIV lipodystrophy syndrome, especially fat accumulation. Indinavir has been associated with hyperglycemia and can induce a relative state of insulin resistance in healthy HIV-seronegative volunteers following a single 800-mg dose (Noor et al., 2002). Dermatologic complications have been reported, including hair loss, dry skin, dry and cracked lips, and ingrown toenails (Plosker and Noble, 1999).

Precautions and Interactions. Patients taking indinavir should drink at least 2 L of water daily to prevent renal complications. This is especially problematic for those who live in warm climates.

Because indinavir solubility decreases at higher pH, antacids or other buffering agents should not be taken at the same time. Didanosine formulations containing an antacid buffer should not be taken within 2 hours before or 1 hour after indinavir. Like most other HIV protease inhibitors, indinavir is metabolized by CYP3A4 and is a moderately potent CYP3A4 inhibitor. Indinavir should not be co-administered with other CYP3A4 substrates that have a narrow therapeutic index. Drugs that induce CYP3A4 may lower indinavir concentrations and should be avoided.

Therapeutic Use. Indinavir (crixivan) is no longer widely prescribed because of problems with nephrolithiasis and other nephrotoxicities, and it lacks significant advantages over other available HIV protease inhibitors. When combined with ritonavir and nucleoside analogs, twice-daily indinavir produces viral load reductions comparable with those of other HIV protease inhibitor regimens (Flexner, 2000).

Nelfinavir.

Chemistry and Antiviral Activity. Nelfinavir is a nonpeptidic protease inhibitor that is active against both HIV-1 and HIV-2 (Bardsley-Elliot and Plosker, 2000) (Figure 59–6). The mean IC₉₅ for HIV-1 in various in vitro assays is 59 nM.

Mechanisms of Action and Resistance. The primary nelfinavir resistance mutation is unique to this drug and occurs at HIV protease codon 30 (aspartic acid-to-asparagine substitution); this mutation results in a 7-fold decrease in susceptibility. Isolates with only this mutation retain full sensitivity to other HIV protease inhibitors (Bardsley-Elliot and Plosker, 2000). Less commonly, a primary resistance mutation occurs at position 90, which can confer cross-resistance. In addition, secondary resistance mutations can accumulate at codons 35, 36, 46, 71, 77, 88, and 90, and these are associated with further resistance to nelfinavir, as well as cross-resistance to other HIV protease inhibitors.

Absorption, Distribution, and Elimination. Nelfinavir absorption is very sensitive to food effects; a moderate-fat meal increases the AUC 2- to 3-fold, and higher concentrations are achieved with high-fat meals (Bardsley-Elliot and Plosker, 2000). Intraindividual and interindividual variabilities in plasma nelfinavir concentrations are large as a consequence of variable absorption. Originally approved at a dose of 750 mg three times daily, nelfinavir is now administered at a dose of 1250 mg twice daily using a reduced-volume 625-mg tablet. Nelfinavir's high dependence on fatty foods for optimal absorption, combined with the fact that nelfinavir is the only HIV protease inhibitor whose pharmacokinetics are not substantially improved with ritonavir, have reduced its popularity.

Nelfinavir undergoes oxidative metabolism primarily by CYP2C19 but also by CYP3A4 and CYP2D6. Its major hydroxy-t-butylamide metabolite, M8, is formed by CYP2C19 and has in vitro antiretroviral activity similar to that of the parent drug. This is the only known active metabolite of any HIV protease inhibitor. Nelfinavir induces its own metabolism, and average trough concentrations after 1 week of therapy are approximately one-half those at day 2 of therapy (Bardsley-Elliot and Plosker, 2000).

Untoward Effects. The most important side effect of nelfinavir is diarrhea or loose stools, which resolve in most patients within the first 4 weeks of therapy. Up to 20% of patients report chronic occasional diarrhea lasting >3 months, although <2% of patients discontinue the drug because of diarrhea. Nelfinavir augments intestinal calcium-dependent chloride channel secretory responses in vitro, and electrolyte analysis of stool is most consistent with a secretory diarrhea (Rufo et al., 2004). Otherwise, nelfinavir is generally well tolerated but has been associated with glucose intolerance, elevated cholesterol levels, and elevated triglycerides, like other drugs in this class.

Precautions and Interactions. Because nelfinavir is metabolized by CYPs 2C19 and 3A4, concomitant administration of agents that induce these enzymes may be contraindicated (as with rifampin) or may necessitate an increased nelfinavir dose (as with rifabutin). Nelfinavir is a moderate inhibitor of CYP3A4 and may alter plasma concentrations of other CYP3A4 substrates. Nelfinavir inhibits CYP3A4 much less than does ritonavir and does not appear to inhibit other CYP isoforms. Nelfinavir also induces hepatic drug-metabolizing enzymes, reducing the AUC of ethinyl estradiol by 47% and norethindrone by 18% (Flexner, 1998). Combination oral contraceptives therefore should not be used as the sole form of contraception in patients taking nelfinavir. Nelfinavir reduces the zidovudine AUC by 35%, suggesting induction of glucuronosyl S- transferase.

Therapeutic Use. Nelfinavir (viracept) is approved for the treatment of HIV infection in adults and children in combination with other antiretroviral drugs. In large randomized comparative trials, long-term virologic suppression with nelfinavir-based combination regimens is statistically significantly inferior to lopinavir/ritonavir, atazanavir, or efavirenz-based regimens. This possibly reflects the unpredictable nature of nelfinavir absorption but contributes to the decreasing popularity of this drug. Nelfinavir is well tolerated in pregnant HIV-infected women and shows no evidence of teratogenesis (Bardsley-Elliot and Plosker, 2000) but detection of a potentially carcinogenic containant in 2007 led to a recommendation that the drug not be used pregnant women.

Tipranavir.

Chemistry and Antiviral Activity. Tipranavir is a non-peptidic, dihydropyrone protease inhibitor that is active against both HIV-1 and HIV-2 (Figure 59–6). The IC₅₀ for HIV-1 in vitro ranges from 30 to 70 nM, although human plasma increases the IC₅₀ by 4-fold (Orman and Perry, 2008).

Mechanisms of Action and Resistance. Tipranavir binding to the active site of the HIV protease depends less on water-mediated hydrogen bonding than other molecules in this class, which may explain, in part, its unique resistance profile.

Clinically significant drug resistance requires accumulation of multiple mutations. The three mutations most commonly associated with treatment failure were L33F/I/V, V82T/L and I84V, although 21 mutations in 16 amino acids have been associated with reduced susceptibility to this drug. In general, tipranavir-resistant HIV retains sensitivity only to saquinavir and darunavir (Orman and Perry, 2008). Fewer than 3% of patients who have failed other protease inhibitors harbor virus with >10-fold resistance tipranavir. Because most HIV strains sensitive to tipranavir are also sensitive to darunavir, the latter drug is preferred for most treatment-experienced patients because of its better tolerability and toxicity profile.

Absorption, Distribution, and Elimination. Tipranavir must be administered with ritonavir because of poor oral bioavailability. The recommended regimen of tipranavir/ritonavir 500/200 mg twice daily includes a ritonavir dose higher than that of other boosted HIV protease inhibitors; lower doses of ritonavir should not be used. Food does not alter pharmacokinetics in the presence of ritonavir but may reduce GI side effects. Tipranavir is 99.9% bound in the presence of plasma proteins (Orman and Perry, 2008). Metabolism is mainly via CYP3A4; mean elimination t_1/2 is 5-6 hours in the presence of ritonavir.

Untoward Effects. Tipranavir use has been associated with rare fatal hepatotoxicity. Through 48 weeks of treatment, grade 3 or 4 elevation of hepatic transaminases occurred in 20% of treatment-naive and 10% of treatment-experienced patients. Tipranavir use has been associated with rare intracranial hemorrhage (including fatalities) and bleeding episodes in patients with hemophilia. The drug has anticoagulant properties in vitro and in animal models, and these effects are potentiated by vitamin E (Flexner et al., 2005). Tipranavir is more likely to cause elevation in lipids and triglycerides than other boosted PIs, possibly due to the higher dose of ritonavir. Tipranavir contains a sulfa moiety, and ~10% of treated patients report a transient rash. The current formulation contains a high amount of vitamin E, and patients should not take supplements containing this fat-soluble vitamin while taking tipranavir (Orman and Perry, 2008).

Precautions and Interactions. Like ritonavir, tipranavir is a substrate, inhibitor, and inducer of CYP enzymes. In combination, these two agents produce a broad array of clinically significant pharmacokinetic drug interactions. Tipranvir/ritonavir reduces the concentrations (AUC) of all co-administered protease inhibitors by 44-76% and should not be administered with any of these agents (Orman and Perry, 2008). This reflects the combined effect of the increased ritonavir dose, as well as tipranavir's unique capacity among PIs to induce expression of the P-glycoprotein drug transporter.

Therapeutic Use. Tipranavir (aptivus) is approved for use only in treatment-experienced adult and pediatric patients whose HIV is resistant to one or more protease inhibitors. In nearly 1500 patients with at least one major PI resistance mutation randomized to tipranavir/ritonavir or a comparator PI regimen, 34% had at least a one log sustained drop in plasma HIV RNA by week 48 compared to 15% of controls. However, 5.4% of tipranavir recipients discontinued treatment because of adverse events, compared to 1.6% of controls (Orman and Perry, 2008). Combining tipranavir with at least one other active antiretroviral drug, usually enfuvirtide, greatly improved virologic responses in these studies. Tipranavir/ritonavir is approved for use in adults and pediatric patients >2 years of age, with pediatric dosing based on weight or body surface area.

CXCR4, the co-receptor for T-lymphocyte tropic HIV, would seem to be an equally good target for antiretroviral drug development. However, two investigational CXCR4 antagonists produced no substantial drop in plasma HIV RNA in most treated patients, even though they efficiently eliminated CXCR4-tropic virus from the circulation (Stone et al., 2007), suggesting that these drugs cause rapid selection for HIV that is CCR5-tropic. Although these compounds are no longer in development for HIV infection, they do cause an increase in circulating leukocytes, presumably as a direct consequence of CXCR4 blockade. One of these compounds, plerixafor, is now approved as an adjunct to stem cell mobilization after cancer chemotherapy (Chapter 37).

HIV can develop resistance to this drug through two distinct pathways. A patient starting maraviroc therapy with HIV that is predominantly CCR5-tropic may experience a shift in tropism to CXCR4- or dual/mixed-tropism predominance. This is especially likely in patients harboring low-level but undetected CXCR4- or dual/mixed-tropic virus prior to initiation of maraviroc. Alternatively, HIV can retain its CCR5-tropism but gain resistance to the drug through specific mutations in the V3 loop of gp 120 that allow virus binding in the presence of inhibitor (MacArthur and Novak, 2008). This results in both an increase in IC₅₀ and a decrease in maximum percent inhibition of virus replication for such viruses in vitro.

Absorption, Distribution, and Elimination. The oral bioavailability of maraviroc, 23-33%, is dose dependent. Food decreases bioavailability (AUC) as much as 50%, but there are no food requirements for drug administration because clinical efficacy trials were conducted without food restrictions. Maraviroc is 76% protein bound in human plasma, with a volume of distribution of 194 L. Elimination is mainly via CYP3A4 with an elimination t_1/2 of 10.6 hours (MacArthur and Novak, 2008).

Untoward Effects. Maraviroc is generally well tolerated, with little significant toxicity.

One case of serious hepatotoxicity with allergic features has been reported, but in controlled trials significant (grade 3 or 4) hepatotoxicity was no more frequent with maraviroc than with placebo. There is a theoretical concern that CCR5 inhibition might interfere with immune function; for example, humans with the Δ-32 genetic deletion in CCR5 that prevents HIV-1 entry are also more likely to develop severe West Nile virus encephalopathy (MacArthur and Novak, 2008). However, to date there has been no obvious increase in serious infections or malignancies with maraviroc treatment.

Precautions and Interactions. Maraviroc is a CYP3A4 substrate and susceptible to pharmacokinetic drug interactions involving CYP3A4 inhibitors or inducers. Recommended dosing of this drug depends on concomitant medications. Maraviroc is not itself a CYP inhibitor or inducer in vivo, although high-dose maraviroc (600 mg daily) increased concentrations of the CYP2D6 substrate debrisoquine.

In phase 3 clinical trials involving heavily pretreated patients with documented multidrug-resistant HIV-1, the addition of maraviroc to optimized background therapy (OBT) resulted in an undetectable (<50 copies/mL) plasma HIV-1 RNA in 44% of patients after 24 weeks, compared to 17% with OBT alone; maraviroc-treated patients also had higher mean CD4 lymphocyte count increases (120 versus 61 cells/mL). A maraviroc-based regimen was inferior to an efavirenz-based regimen in one clinical trial in treatment-naive patients (MacArthur and Novak, 2008). Maraviroc has little to no efficacy in patients harboring CXCR4- or dual/mixed-tropic virus at baseline. In addition, the requirement for an expensive baseline phenotype test limits its cost effectiveness.

Chemistry and Antiviral Activity. Enfuvirtide is a 36-amino-acid synthetic peptide whose sequence is derived from a part of the transmembrane gp41 region of HIV-1 that is involved in fusion of the virus membrane lipid bilayer with that of the host cell membrane. Enfuvirtide is not active against HIV-2 but has a broad range of potencies against HIV-1 laboratory and clinical isolates. The reported in vitro IC₅₀ ranges from 0.1 nM to 1.7 μM depending on the HIV-1 strain and testing method employed (Dando and Perry, 2003).

Enfuvirtide was investigated originally as a possible vaccine component, in part because of a high degree of sequence conservation among HIV-1 strains. This peptide turned out to have potent anti-HIV activity in vitro, a property eventually attributed to selective inhibition of HIV-mediated membrane fusion (Jiang et al., 1993; Wild et al., 1994). Enfuvirtide is expensive to manufacture and must be administered by subcutaneous injection twice daily. Thus cost and route of administration limit its use to those with no other treatment options.

Mechanisms of Action and Resistance. Enfuvirtide has a unique mechanism of antiretroviral action. The peptide blocks the interaction between the N36 and C34 sequences of the gp41 glycoprotein by binding to a hydrophobic groove in the N36 coil. This prevents formation of a six-helix bundle critical for membrane fusion and viral entry into the host cell. Enfuvirtide inhibits infection of CD4+ cells by free virus particles, as well as cell-to-cell transmission of HIV in vitro. Enfuvirtide retains activity against viruses that have become resistant to antiretroviral agents of other classes because of its unique mechanism of action.

HIV can develop resistance to this drug through specific mutations in the enfuvirtide-binding domain of gp41. Of the patients experiencing virologic failure during enfuvirtide treatment, 94% had virus with mutations in the gp41 region associated with enfuvirtide resistance in vitro. The most common mutations involve a V38A or N43D substitution. Single-amino-acid substitutions can confer up to 450-fold resistance in vitro, although high-level clinical resistance is usually associated with two or more amino acid changes (Dando and Perry, 2003).

Absorption, Distribution, and Elimination. Enfuvirtide is the only approved antiretroviral drug that must be administered parenterally. The bioavailability of subcutaneous enfuvirtide is 84% compared with an intravenous dose (Dando and Perry, 2003). Pharmacokinetics of the subcutaneous drug are not affected by site of injection.

The major route of elimination for enfuvirtide has not been determined; only a deamidated metabolite at the C-terminal phenylalanine has been detected (Dando and Perry, 2003). The mean elimination t_1/2 of parenteral drug is 3.8 hours, necessitating twice-daily administration. Enfuvirtide is 98% bound to plasma proteins, mainly albumin.

Untoward Effects. The most prominent adverse effects of enfuvirtide are injection-site reactions. About 98% of patients develop local side effects including pain, erythema, and induration at the site of injection; 80% of patients develop nodules or cysts (Dando and Perry, 2003). Between 4% and 5% of patients discontinue treatment because of local reactions. Use of enfuvirtide has been associated with a higher incidence of lymphadenopathy and pneumonia in at least one study. Whether these are direct drug effects, a secondary consequence of drug-related immune dysfunction, or effects from another mechanism is the subject of investigation. Enfuvirtide suppresses interleukin-12 production in vitro by >90% at concentrations equal to or less than those required to inhibit HIV replication (Braun et al., 2001); the role this might play in clinical immunosuppression is unclear.

Precautions and Interactions. Enfuvirtide is not metabolized to a significant extent and not known to alter the concentrations of any co-administered drugs. Ritonavir, rifampin, or ritonavir plus saquinavir did not alter enfuvirtide concentrations (Dando and Perry, 2003).

Therapeutic Use. Enfuvirtide (fuzeon) is FDA-approved for use only in treatment-experienced adults who have evidence of HIV replication despite ongoing antiretroviral therapy.

In phase 3 clinical trials involving heavily pretreated patients with documented multidrug-resistant HIV-1, inclusion of enfuvirtide (90 mg subcutaneously twice daily) in combination with an optimized background regimen enhanced the fraction of patients with undetectable (<50 copies/mL) plasma HIV-1 RNA concentrations after 24 weeks of treatment (~16% on enfuvirtide versus ~6% without) (Dando and Perry, 2003). Treatment response is more likely in patients with at least two other active drugs in the regimen, based on history and HIV genotype. Given the cost, inconvenience, and cutaneous toxicity of this drug, enfuvirtide generally is reserved for patients who have failed all other feasible antiretroviral regimens.

The two major raltegravir resistance pathways involve primary mutations Q184R/H/K, or N155H in the integrase gene. Either mutation can confer 25- to 50-fold changes in drug sensitivity in vitro. The Y143C/H/R mutation has also been observed. In one phase 3 clinical trial, 64% of patients failing raltegravir combination therapy harbored at least one of these primary resistance mutations (Hicks and Gulick, 2009). Additional secondary mutations can accumulate and cause high-level resistance, including cross-resistance to investigational integrase inhibitors.

Absorption, Distribution, and Elimination. Peak concentrations of raltegravir occur ~1-3 hours after oral dosing. Elimination is biphasic, with a β-phase t_1/2 of ~1 hour and a terminal β-phase t_1/2 of 9 hours. The pharmacokinetics of raltegravir are highly variable. Moderate and high-fat meals increased raltegravir apparent bioavailability (AUC) by as much as 2-fold; a low-fat meal decreased AUC modestly (46%). There are no food requirements for raltegravir administration because clinical efficacy trials were conducted without food restrictions and because initial antiviral effects were maximal at all concentrations produced with the 400 mg twice-daily dose in clinical studies. Raltegravir is 83% protein bound in human plasma. Raltegravir is eliminated mainly via glucuronidation by UGT1A1.

Untoward Effects. Raltegravir is generally well tolerated, with remarkably little clinical toxicity. In clinical trials, the most common complaints occurring at a frequency higher than in placebo recipients were headache, nausea, asthenia, and fatigue. Creatine kinase elevations, myopathy, and rhabdomyolysis have been reported, although a causal relationship to drug exposure is unproven. Exacerbation of depression has also been reported (Hicks and Gulick, 2009).

Precautions and Interactions. As a UGT1A1 substrate, raltegravir is susceptible to pharmacokinetic drug interactions involving inhibitors or inducers of this enzyme. Atazanavir, a moderate UGT1A1 inhibitor, increases the raltegravir AUC 41-72%. Tenofovir increased the raltegravir AUC by 49%, but the mechanism for this interaction is unknown. When raltegravir is combined with the CYP inducer rifampin, the raltegravir dose should be doubled to 800 mg twice daily, although no clinical data on this combination have been published. Raltegravir is not a significant enzyme inhibitor or inducer in vitro and has little effect on the pharmacokinetics of co-administered drugs.

In phase 3 clinical trials involving heavily pretreated patients with documented multidrug-resistant HIV-1, the addition of raltegravir to optimized background therapy (OBT) resulted in an undetectable (<50 copies/mL) plasma HIV-1 RNA in 62% of patients after 48 weeks, compared to 33% with placebo (Hicks and Gulick, 2009). Response rates in this study ranged from 45-77%, depending on the number of additional active drugs in the regimen (0, 1, or 2). Raltegravir-treated patients also had higher mean CD4 lymphocyte count increases (109 versus 45 cells/mL).

Raltegravir has also been studied in treatment-naive patients and was recently approved for use in patients who have not taken prior antiretroviral therapy. In 10-day monotherapy studies, raltegravir decreased mean HIV-1 RNA by 1.7 log at a dose of 400 mg twice daily. In a prospective randomized comparison, raltegravir was as efficacious as efavirenz when combined with two nucleosides, with 83% and 84% of subjects, respectively, having undetectable HIV-1 RNA (<50 copies/mL) after 96 weeks of treatment. Raltegravir-treated subjects achieve an undetectable plasma HIV RNA faster than efavirenz-treated subjects, although the clinical significance of this effect is unknown (Hicks and Gulick, 2009).