Chapter 50: Chemotherapy of Protozoal Infections: Amebiasis, Giardiasis, Trichomoniasis, Trypanosomiasis, Leishmaniasis, and Other Protozoal Infections

The organisms can be differentiated by antigen-detection enzyme-linked immunosorbent assays or by polymerase chain reaction (PCR)–based diagnostics. Humans are the only known hosts for these protozoa, which are transmitted almost exclusively by the fecal-oral route. Ingested E. histolytica cysts from contaminated food or water survive acid gastric contents and transform into trophozoites that reside in the large intestine. The outcome of E. histolytica infection is variable. Many individuals remain asymptomatic but excrete the infectious cyst form, making them a source for further infections. In other individuals, E. histolytica trophozoites invade into the colonic mucosa with resulting colitis and bloody diarrhea (amebic dysentery). In a smaller proportion of patients, E. histolytica trophozoites invade through the colonic mucosa, reach the portal circulation, and travel to the liver, where they establish an amebic liver abscess.

Infection with Giardia results in one of three syndromes: an asymptomatic carrier state, acute self-limited diarrhea, or chronic diarrhea. Asymptomatic infection is most common; these individuals excrete Giardia cysts and serve as a source for new infections. Most adults with symptomatic infection develop an acute self-limited illness with watery, foul-smelling stools, abdominal distension, and flatus. However, a significant proportion of these individuals go on to develop a chronic diarrhea syndrome (>2 weeks of illness) with signs of malabsorption (steatorrhea) and weight loss.

Primary infection with T. gondii produces clinical symptoms in ~10% of immunocompetent individuals. The acute illness is usually self-limiting, and treatment rarely is required. Individuals who are immunocompromised, however, are at risk of developing toxoplasmic encephalitis from reactivation of tissue cysts deposited in the brain. The vast majority of cases of toxoplasmic encephalitis are seen in patients with AIDS, in whom the disease is fatal if it is not recognized and treated appropriately. Clinical manifestations of congenital toxoplasmosis vary widely, but chorioretinitis, which may present decades after perinatal exposure, is the most commonly recognized finding.

Infectious oocysts in feces may be spread either by direct human-to-human contact or by contaminated water supplies, the latter being recognized as an established route of epidemic infection. Groups at risk include travelers, children in day-care facilities, male homosexuals, animal handlers, veterinarians, and other healthcare personnel. Immunocompromised individuals are especially vulnerable. After ingestion, the mature oocyte is digested, releasing sporozoites that invade host epithelial cells, penetrating the cell membrane but not actually entering the cytoplasm. In most individuals, infection is self-limited. However, in AIDS patients and other immunocompromised individuals, the severity of voluminous, secretory diarrhea may require hospitalization and supportive therapy to prevent severe electrolyte imbalance and dehydration.

The parasite is entirely extracellular, and early human infection is characterized by the finding of replicating parasites in the bloodstream or lymph without CNS involvement (stage 1); stage 2 disease is characterized by CNS involvement (Blum et al., 2006). Symptoms of early-stage disease include febrile illness, lymphadenopathy, splenomegaly, and occasional myocarditis that result from systemic dissemination of the parasites. There are two types of African trypanosomiasis, the East African (Rhodesian) and West African (Gambian), caused by T. brucei rhodesiense and T. brucei gambiense, respectively. T. brucei rhodesiense produces a progressive and rapidly fatal form of disease marked by early involvement of the CNS and frequent terminal cardiac failure; T. brucei gambiense causes illness that is characterized by later involvement of the CNS and a more long-term course that progresses to the classical symptoms of sleeping sickness over months to years. Neurological symptoms include confusion, poor coordination, sensory deficits, an array of psychiatric signs, disruption of the sleep cycle, and eventual progression into coma and death.

Recent efforts by not-for-profit agencies have fostered significant new efforts to develop new therapies for the neglected tropical diseases including African sleeping sickness, Chagas' disease, and Leishmaniasis (Croft, 2007; Frearson et al., 2007; Nwaka and Hudson, 2006; Pink et al., 2005; Stuart et al., 2008). These efforts involve partnerships with academic and industrial groups. The recent increase in high throughput screening centers within academic groups with interest in developing drugs for these applications is also helping to fuel discovery of lead compounds. The genome data is being exploited to develop publicly accessible databases that organize data relevant to drug discovery projects (Aguero et al., 2008). From these efforts an orally available novel diamidine, pafuramidine maleate (DB289), was recently advanced to phase IV trials for the treatment of early-stage (stage 1) sleeping sickness, representing the only new clinical candidate since the discovery of eflornithine (Mdachi et al., 2009). Although the trials were stopped due to toxicity issues (Kennedy, 2008), the discovery of pafuramidine provides hope that the current approaches and increased effort will yield new clinical entities for these diseases in the near future.

Reactivation of disease also may occur in patients who are immunosuppressed after organ transplantation or because of infection (e.g., AIDS, leukemia, and other neoplasias). Occurrences of T. cruzi infection in transplant patients or through blood transfusions have been reported in the U.S., and as a consequence the FDA-approved diagnostic blood screening for the parasite (Bern et al., 2008). Reports of autochthonous Chagas cases are also on the rise in the U.S. Acute infection is evidenced by a raised tender skin nodule (chagoma) at the site of inoculation; other signs may be absent or range from fever, adenitis, skin rash, and hepatosplenomegaly to, albeit rarely, acute myocarditis, and death. Invading metacyclic trypomastigotes penetrate host cells, especially macrophages, where they proliferate as amasti-gotes. These then differentiate into trypomastigotes that enter the bloodstream. Circulating trypomastigotes do not multiply until they invade other cells or are ingested by an insect vector during a blood meal. After recovery from the acute infection that lasts a few weeks to months, individuals usually remain asymptomatic for years despite sporadic parasitemia. During this period, their blood can transmit the parasites to transfusion recipients and accidentally to laboratory workers. Approximately 20-30% of infected individuals eventually progress to clinically evident disease that is characterized by chronic disease of the heart and GI tract as they age. Progressive destruction of myocardial cells and neurons of the myenteric plexus results from the special tropism of T. cruzi for muscle cells. Recent studies with improved techniques indicate the presence of T. cruzi at sites of cardiac lesions, and it is now well recognized that parasite persistence is linked directly to pathology and disease outcome (Tarleton, 2003). Whether an undefined autoimmune response also contributes to the pathogenesis of Chagas' disease is unknown; however, the data indicate that immunological defenses, especially cell-mediated immunity, do modulate the course of disease.

Clearly, new drugs with better potency against parasites in the chronic phase and with reduced toxicities are needed. A key factor in new drug development for Chagas' disease is the need for better methods to assess parasitological cure. Several important reports documenting significant advances in this area have recently been published (Bustamante et al., 2008; Cooley et al., 2008). Additionally, some promising strides toward the development of new agents have been made. Inhibitors of an essential protease, cruzipain, have anti-parasite activity, and one (K777) is in preclinical development for the treatment of Chagas' disease (Doyle et al., 2007). In the absence of new drugs, however, alternative measures such as improved vector control and housing accommodations have been used to reduce the transmission of Chagas' disease substantially in Brazil, Chile, and Venezuela (World Health Organization, 1999).

Various forms of leishmaniasis affect people in southern Europe and many tropical and subtropical regions throughout the world. Flagellated extracellular free promastigotes, regurgitated by feeding flies, enter the host, where they attach to and become phagocytized by tissue macrophages. These transform into amastigotes, which reside and multiply within phagolysosomes until the cell bursts. Released amastigotes then propagate the infection by invading more macrophages. Amastigotes taken up by feeding sandflies transform back into promastigotes, thereby completing the transformation cycle. The particular localized or systemic disease syndrome caused by Leishmania depends on the species or subspecies of infecting parasite, the distribution of infected macrophages, and especially the host's immune response. In increasing order of systemic involvement and potential clinical severity, major syndromes of human leishmaniasis have been classified into cutaneous, mucocutaneous, diffuse cutaneous, and visceral (kala azar) forms. Leishmaniasis increasingly is becoming recognized as an AIDS-associated opportunistic infection.

The classification, clinical features, course, and chemotherapy of human leishmaniasis syndromes as well as the biochemistry and immunology of the parasite and host germane to chemotherapy have been recently reviewed (Chappuis et al., 2007; Croft and Coombs, 2003; Stuart et al., 2008). Cutaneous forms of leishmaniasis generally are self-limiting, with cures occurring in 3-18 months after infection. However, this form of the disease can leave disfiguring scars. The mucocutaneous, diffuse cutaneous, and visceral forms of the disease do not resolve without therapy. Visceral leishmaniasis caused by L. donovani is fatal unless treated.

Babesiosis, caused by either Babesia microcoti or B. divergens, is a tick-borne zoonosis that superficially resembles malaria in that the parasites invade erythrocytes, producing a febrile illness, hemolysis, and hemoglobinuria. This infection usually is mild and self-limiting but can be severe or even fatal in asplenic or severely immunocompromised individuals. Currently recommended therapy is with a combination of clindamycin and quinine for severe disease, and the combination of azithromycin and atovaquone for mild or moderate infections.

Balantidiasis, caused by the ciliated protozoan Balantidium coli, is an infection of the large intestine that may be confused with amebiasis. Unlike amebiasis, however, this infection usually responds to tetracycline therapy.

Isospora belli, a coccidian parasite, causes diarrhea in AIDS patients and responds to treatment with trimethoprim-sulfamethoxazole. Cyclospora cayetanensis, another coccidian parasite, causes self-limited diarrhea in normal hosts and can cause prolonged diarrhea in individuals with AIDS. It is susceptible to trimethoprim-sulfamethoxazole.

Microsporidia are spore-forming unicellular eukaryotic fungal parasites that can cause a number of disease syndromes, including diarrhea in immunocompromised individuals. Infections with microsporidia of the Encephalitozoon genus, including E. hellum, E. intestinalis, and E. cuniculi, have been treated successfully with albendazole, a benzimidazole derivative and inhibitor of β-tubulin polymerization (Gross, 2003) (Chapter 51). Immunocompromised individuals with intestinal microsporidiosis due to E. bieneusi (which does not respond as well to albendazole) have been treated successfully with the antibiotic fumagillin (Molina et al., 2002).

Amphotericin B

The pharmacology, formulation, and toxicology of amphotericin B are presented in Chapter 57. Only those features of the drug pertinent to its use in leishmaniasis are described here.

Antiprotozoal Effects. In 1997, the FDA approved liposomal amphotericin B (ambisome) for the treatment of visceral leishmaniasis. Amphotericin B is a highly effective antileishmanial agent that cures >90% of the cases of visceral leishmaniasis in clinical studies, and it has become the drug of choice for antimonial-resistant cases (Bern et al., 2006; Croft, 2008, Olliaro et al., 2005). It is considered a second-line drug for cutaneous or mucosal leishmaniasis, where it has been shown effective for the treatment of immunocompromised patients (Alvar et al., 2006). However, because cutaneous leishmaniasis is typically self-limiting, the drug has not been evaluated for treatment of a broader range of patient populations. The lipid preparations of the drug have reduced toxicity, but the cost of the drug and the difficulty of administration remain a problem in endemic regions.

The mechanism of action of amphotericin B against leishmania is similar to the basis for the drug's antifungal activities (Chapter 57). Amphotericin complexes with ergosterol precursors in the cell membrane, forming pores that allow ions to enter the cell. Leishmania has similar sterol composition to fungal pathogens, and the drug binds to these sterols preferentially over the host cholesterol. No significant resistance to the drug has been encountered after nearly 30 years of use as an antifungal agent.

Therapeutic Uses. Numerous dosing schedules have been reported for the treatment of visceral leishmaniasis, with most achieving high cure rates and good safety (Alvar et al., 2006, Bern et al., 2006; Olliaro et al., 2005). Typical schemes of 10-20 mg/kg total dose given in divided doses over 10-20 days by intravenous infusion have yielded >95% cure rates. In the U.S., the FDA recommends 3 mg/kg intravenously on days 1-5, 14, and 21 for a total dose of 21 mg/kg (Meyerhoff, 1999). Shorter courses of the drug have been tested with good efficacy and provide a potential alternative with lower cost, although only a limited number of patients have been tested: Cure rates of 89-91% were observed for single doses of 3.75, 5, and 7.5 mg/kg. Recent data suggest that a single dose of 5 mg/kg followed by 7-14 days treatment with oral miltefosine was effective at curing visceral leishmaniasis, and this dosing scheme warrants additional study (Sundar et al., 2008).

Chloroquine

The pharmacology and toxicology of chloroquine are presented in Chapter 49 (anti-malarials). Chloroquine does have an FDA-approved use for extra-intestinal amebiasis at a dose of 1 g (600 mg base) daily for 2 days, followed by 500 mg daily for at least 2-3 weeks. Treatment is usually combined with an effective intestinal amebicide. The interested reader should consult the 11th edition of this book for further details.

Diloxanide Furoate

Diloxanide furoate (furamide, others) is the furoate ester of diloxanide, a derivative of dichloroacetamide. Diloxanide furoate is a very effective luminal agent for the treatment of E. histolytica infection but is no longer available in the U.S. The interested reader should consult the 10th edition of this book for further details on this agent.

Both in animal models and in vitro, eflornithine arrests the growth of several types of tumor cells, providing the basis for its clinical evaluation as an antitumor agent. The discovery that eflornithine cured rodent infections with T. brucei first focused attention on protozoal polyamine biosynthesis as a potential target for chemotherapeutic attack (Bacchi et al., 1980). Eflornithine currently is used to treat West African (Gambian) trypanosomiasis caused by T. brucei gambiense (Balasegaram et al., 2006a, 2009; Chappuis, 2007, Chappuis et al., 2005; Priotto et al., 2006, 2008; Robays et al., 2008a). In contrast, the drug is largely ineffective for East African trypanosomiasis. Eflornithine's difficult treatment regimen is the primary limitation to its use in the field. However, the negotiation of the World Health Organization (WHO) of a stable supply of eflornithine through 2011 has led to the widespread availability of the drug and to its use in control and research programs run by world health agencies.

Eflornithine is both safer and more efficacious than melarsoprol for late-stage Gambiense sleeping sickness, and it is now the recommended first-line treatment for this disease when adequate care can be provided for its administration. Eflornithine is no longer available for systemic use in the U.S. but is available for treatment of Gambian trypanosomiasis by special request from the CDC.

Antitrypanosomal Effects. The effects of eflornithine have been evaluated both on drug-susceptible and drug-resistant T. brucei in vitro and in infections with these parasites in rodent models (reviewed in: Barrett et al., 2007). Eflornithine is a cytostatic agent that has multiple biochemical effects on trypanosomes, all of which are a consequence of polyamine depletion. The polyamine putrescine is depleted to undetectable levels, intracellular levels of spermidine and trypanothine are reduced by 25-50%, and methionine metabolism is altered. Depletion of the polyamines, or of trypanothione, would be expected to be lethal to the cells based on genetic studies that have disrupted the biosynthetic genes in the pathway. As a consequence, macromolecular biosynthesis is depressed, and the parasites transform from the long, slender dividing forms into the short, stumpy nonreplicating forms. These latter parasites are unable to synthesize variable cell surface glycoprotein and eventually are cleared by the immune system.

The molecular mechanism of eflornithine action clearly is inhibition of ornithine decarboxylase. Eflornithine irreversibly inhibits both mammalian and trypanosomal ornithine decarboxylases, thereby preventing the synthesis of putrescine, a precursor of polyamines needed for cell division. Eflornithine inactivates the enzyme through covalent labeling of an active-site cysteine residue, and an X-ray structure of the T. brucei enzyme bound to the drug has been reported (Grishin et al., 1999). A number of studies demonstrate conclusively that ornithine decarboxylase is the target of eflornithine action that leads to cell death. Mutant bloodstream trypanosomes lacking ornithine decarboxylase or wild-type trypanosomes treated with eflornithine cannot replicate, and mice inoculated with these null parasites become resistant to infection by wild-type parasites (Mutomba et al., 1999). The product of the ornithine decarboxylase reaction, putrescine, rescues the growth deficit in both the mutant parasites and eflornithine-treated cells. The null mutant parasites grown with putrescine are not affected by eflornithine, demonstrating the selectivity of the drug for the target enzyme.

The mechanisms of selective toxicity between the host and parasite, or between the different species of T. brucei, are less clear (Barrett et al., 2007). The parasite and human enzymes are equally susceptible to inhibition by eflornithine; however, the mammalian enzyme is turned over rapidly, whereas the parasite enzyme is stable, and this difference likely plays a role in the selective toxicity. In addition, mammalian cells may be able to replenish polyamine pools through uptake of extracellular polyamines, whereas the slender bloodstream forms of human trypanosomes divide within human blood, which contains only very low levels of these essential compounds. Further, the parasites lack efficient transport mechanisms.

T. brucei rhodesiense cells are less sensitive to eflornithine inhibition than T. brucei gambiense cells, and studies in vitro suggest that the effective doses are increased by 10-20 times in the refractory cells. The molecular basis for the higher dose requirement in T. brucei rhodesiense is still poorly understood; however, it has been postulated to involve differences both in enzyme stability and in the metabolism of S-adenosylmethionine compared with the sensitive T. brucei gambiense cell lines.

Absorption, Fate, and Excretion. Eflornithine is given by intravenous infusion. The drug does not bind to plasma proteins but is well distributed and penetrates into the CSF, where it is estimated that concentrations of at least 50 μM must be reached to clear parasites (Burri and Brun, 2003). Despite the ability of the drug to penetrate the blood-brain barrier, studies in mice suggest that eflornithine crosses the healthy blood-CNS interface poorly, and that it is the breakdown of this barrier caused by T. brucei infection that allows greater penetration to occur (Sanderson et al., 2008). In these studies, suramin enhanced eflornithine uptake into the CNS, suggesting that this combination might lead to lower dose requirements for eflornithine. Renal clearance after intravenous administration is rapid (2 mL/minute per kilogram), with >80% of the drug cleared by the kidney largely in unchanged form (Burri and Brun, 2003). There is some evidence that eflornithine displays dose-dependent pharmacokinetics at the highest doses used clinically.

Therapeutic Uses. Eflornithine is used for the treatment of late-stage West African trypanosomiasis caused by T. brucei gambiense (Balasegaram et al., 2006a, 2008; Chappuis, 2007; Chappuis et al., 2005; Priotto et al., 2006, 2008; Robays et al., 2008a). Most patients in the reported studies had advanced disease with CNS complications. The preferred regimen for adult patients was found to be 100 mg/kg given intravenously every 6 hours as a 2-hour infusion for 14 days. Virtually all patients improved on this regimen unless they were extremely ill, and the WHO and Médecins Sans Frontières report improved rates >90%. The probability of disease-free survival 2 years after treatment was calculated to be 0.88 in one study (Priotto et al., 2008) and the case-fatality rate of 1% for eflornithine was the lowest reported for second-stage sleeping sickness in any study (Chappuis et al., 2005). Children (<12 years of age) received higher doses of eflornithine (150 mg/kg given intravenously every 6 hours for 14 days) based on prior findings that eflornithine trough concentrations in both the CSF and blood were significantly lower among children than in adults (Milord et al., 1993). Patients who failed therapy in this study tended to have trough CSF concentrations <50 μM. Equal doses of eflornithine were less effective when given by the oral route probably because of limited bioavailability. The problem cannot be overcome simply by increasing the oral dose because of ensuing osmotic diarrhea.

Eflornithine has proven to be less successful for treating AIDS patients with West African trypanosomiasis, presumably because host defenses play a critical role in clearing drug-treated T. brucei gambiense from the bloodstream.

The standard course eflornithine treatment regime is very challenging to administer in rural settings in Africa. A randomized phase III trial testing nifurtimox-eflornithine combination therapy for second-stage T. brucel gambiense suggested that the treatment course for eflornithine could be reduced to 7 days in combination with nifurtimox. A recently concluded clinical trial confirms the efficacy of nifurtimox used in combination with eflornithine (NECT) (Priotto, 2009). This new protocol uses a shortened course of eflornithine in combination with oral nifurtimox, with dosing as follows: 400 mg/kg per day given intravenously every 12 h by 2 h infusion for 7 days plus nifurtimox (orally at 15 mg/kg per day in 3 divided doses [every 8 h]) for 10 days. This combination is logistically easier to administer and it requires less eflornithine, which is expensive to synthesize. Importantly, compared to eflornithine alone, NECT achieves a higher cure rate (96.5% vs. 91.5%). NECT has been added to the WHO essentials medicines list and is likely to become the front line treatment for the indication. Drug combination may also reduce the potential for eflornithine resistance to develop in the field. Given the paucity of drugs available for the treatment of late stage disease, the development of eflornithine resistance would represent a very serious problem.

Toxicity and Side Effects. Eflornithine is reported to cause adverse reactions in most of the treated patients (Balasegaram et al., 2008; Burri and Brun, 2003; Chappuis, 2007; Chappuis et al., 2005; Priotto et al., 2006, 2008); however, they are generally reversible on withdrawal of the drug. Abdominal pain, both mild and moderate, and headache were the predominant complaints followed by reactions at the injection sites. Tissue infections and pneumonia were also observed. The most severe reactions reported for one study, with numbers given for events classified as major, included fever peaks (6%), seizures (4%), and diarrhea (2%) (Priotto et al., 2008). The case fatality rate for eflornithine (~1.2%) is significantly lower than for melarsoprol (4.9%), and overall eflornithine is superior to melarsoprol with respect to both safety and efficacy. Reversible hearing loss can occur after prolonged therapy with oral doses. Although this has been a problem in the use of eflornithine for cancer chemotherapy, hearing loss has not been observed during the treatment of sleeping sickness.

Therapeutic doses of eflornithine are large and require co-administration of substantial volumes of intravenous fluid. This poses significant practical limitations in remote settings and can cause fluid overload in susceptible patients. For NECT, the severe adverse events were reduced compared to eflornithine alone (14% vs. 29%), and treatment related deaths were also fewer (0.7% vs. 2%) (Priotto, 2009). Both NECT and eflornithine alone showed significantly fewer treatment-related deaths than what has been reported for melarsoprol. With eflornithine alone, most reported deaths were due to septic shock. The typical side effects of eflornithine (diarrhea, fever, infection, hypertension, and skin rash) were reduced in the NECT arm of the study, however more patients experienced tremors, nausea and vomiting.

Emetine and Dehydroemetine.

The use of emetine, an alkaloid derived from ipecac ("Brazil root"), as a direct-acting systemic amebicide dates from the early part of the 20th century. Dehydroemetine (MEBADIN) has similar pharmacological properties but is considered to be less toxic. Although both drugs were once used widely to treat severe invasive intestinal amebiasis and extraintestinal amebiasis, they have been replaced by metronidazole, which is as effective and far safer. Thus, emetine and dehydroemetine should not be used unless metronidazole is contraindicated. In the U.S., dehydroemetine is available under an investigational new drug protocol from the CDC drug service. Details of the pharmacology and toxicology of emetine and dehydroemetine are presented in the fifth and earlier editions of this book.

Fumagillin

Fumagillin (fumidil b, others) is an acyclic polyene macrolide produced by the fungus Aspergillus fumigatus.

Both fumagillin and its synthetic analog TNP-470 are toxic to microsporidia, and fumagillin is used widely to treat the microsporidian Nosema apis, a pathogen of honey bees. Fumagillin and TNP-470 also inhibit angiogenesis and suppress tumor growth, and TNP-470 is undergoing clinical trials as an anticancer agent (Chapter 61). Human methionine-aminopeptidase-2 (MetAP2) has been identified as the target for the drugs' antitumor activity, and a gene encoding MetAP2 has been identified in the genome of the microsporidian parasite E. cuniculi.

Fumagillin is used topically to treat keratoconjunctivitis caused by E. hellem at a dose of 3-10 mg/mL in a balanced salt suspension. For the treatment of intestinal microsporidiosis caused by E. bieneusi, fumagillin was used at a dose of 20 mg orally three times daily for 2 weeks (Molina et al., 2002). Adverse effects of fumagillin may include abdominal cramps, nausea, vomiting, and diarrhea. Reversible thrombocytopenia and neutropenia also have been reported (Molina et al., 2002). Fumagillin has not been approved for the systemic treatment of microsporidia infection in the U.S.

8-Hydroxyquinolines

The halogenated 8-hydroxyquinolines iodoquinol (diiodohydroxyquin) and clioquinol (iodochlorhydroxyquin) have been used as luminal agents to eliminate intestinal colonization with E. histolytica. Iodoquinol (yodoxin) is the safer of the two agents and is the only one available for use as an oral agent in the U.S. When used at appropriate doses (never to exceed 2 g/day and duration of therapy not greater than 20 days in adults), adverse effects are unusual. However, the use of these drugs, especially at doses exceeding 2 g/day, for long periods is associated with significant risk. The most important toxic reaction, which has been ascribed primarily to clioquinol, is subacute myelo-optic neuropathy. This disease is a myelitis-like illness that was first described in epidemic form (thousands of afflicted patients) in Japan; only sporadic cases have been reported elsewhere, but the actual prevalence is undoubtedly higher. Peripheral neuropathy is a less severe manifestation of neurotoxicity owing to these drugs. Administration of iodoquinol in high doses to children with chronic diarrhea has been associated with optic atrophy and permanent loss of vision.

Because of its superior adverse-event profile, paromomycin is preferred by many authorities as the luminal agent used to treat amebiasis; however, iodoquinol is a reasonable alternative. Iodoquinol is used in combination with metronidazole to treat individuals with amebic colitis or amebic liver abscess but may be used as a single agent for asymptomatic individuals found to be infected with E. histolytica. For adults, the recommended dose of iodoquinol is 650 mg orally three times daily for 20 days, whereas children receive 10 mg/kg of body weight orally three times a day (not to exceed 1.95 g/day) for 20 days.

The pharmacology and toxicology of the 8-hydroxyquinolines are described in greater detail in the fifth and earlier editions of this book.

Melarsoprol (mel b; arsobal), consisting of two arsenic-containing stereoisomers in a 3:1 ratio, is insoluble in water and is supplied as a 3.6% (w/v) solution in propylene glycol for intravenous administration. It is available in the U.S. only from the CDC.

Antiprotozoal Effects. Melarsoprol has many wide-ranging nonspecific effects on both the trypanosome and host cells (Fries and Fairlamb, 2003). Melarsoprol is a prodrug and is metabolized rapidly (t_1/2 = 30 minutes) to melarsen oxide, the active form of the drug. Arsen-oxides react avidly and reversibly with vicinal sulfhydryl groups, including those of proteins, and thereby inactivate a great number and variety of enzymes. The same nonspecific mechanisms by which melarsoprol is lethal to parasites are probably responsible for its toxicity to host tissues. The basis for the trypanocidal action of melarsoprol is not understood, probably owing to its high reactivity with many biomolecules. Disruption of energy metabolism by inhibition of glycolytic enzyme was long thought to explain its trypanocidal activity. Other evidence suggests, however, that this is not a primary effect. Melarsoprol reacts with trypanothione, the spermidine-glutathione adduct that substitutes for glutathione in these parasites. Binding of melarsoprol to trypanothione results in formation of melarsen oxide-trypanothione adduct (Mel T), a compound that is a potent competitive inhibitor of trypanothione reductase, the enzyme responsible for maintaining trypanothione in its reduced form. Both the sequestering of trypanothione and the inhibition of trypanothione reductase would be expected to have lethal consequences to the cell. However, critical evidence directly linking melarsoprol's action on the trypanothione system to parasite death is still lacking.

The number of treatment failures owing to increasing resistance of trypanosomes to melarsoprol has risen sharply in recent years. Failure rates of 20-27% in Uganda, Angola, and the Congo have been reported (Burri and Keiser, 2001; Pepin and Mpia, 2005; Robays et al., 2008b). In vitro analysis of field isolates indicates that resistant strains are an order of magnitude less sensitive to the drug than sensitive strains (Brun et al., 2001). However, not all treatment failures can be linked to drug resistance, suggesting that additional factors contribute to lack of efficacy (Maina et al., 2007).

Resistance to melarsoprol is likely to involve transport defects, although the situation is complicated by evidence for multiple transport mechanisms of the drug (de Koning, 2008). The best characterized transporter is an unusual adenine-adenosine transporter termed the P2 transporter that has activity on melarsoprol as well as pentamidine and berenil. Cross-resistance between these compounds is observed frequently. Point mutations in this transporter are found in melarsoprol-resistant field isolates. However, null mutant cell lines that lack this transporter were resistant to berenil but had only two-fold resistance to melarsoprol and pentamidine. Another transporter, HAPT1, has been identified, and the concomitant loss of both the P2 and HAPT transporters led to high-level cross-resistance to both melarsen and pentamidine (de Koning, 2008).

Absorption, Fate, and Excretion. Melarsoprol is always administered intravenously. A small but therapeutically significant amount of the drug enters the CSF and has a lethal effect on trypanosomes infecting the CNS. The compound is excreted rapidly, with 70-80% of the arsenic appearing in the feces.

Therapeutic Uses. Melarsoprol is the only effective drug available for treatment of the late meningoencephalitic stage of East African (Rhodesian) trypanosomiasis, which is 100% fatal if untreated. The drug is also effective in the early hemolymphatic stage of these infections, but because of its toxicity, it is reserved for therapy of late-stage infections. Patients infected with T. brucei rhodesiense who relapse after a course of melarsoprol usually respond to a second course of the drug. In contrast, patients infected with T. brucei gambiense who are not cured with melarsoprol rarely benefit from repeated treatment with this drug. Such patients often respond well to eflornithine.

The original treatment schedules for melarsoprol were derived empirically >40 years ago; new clinical trials have been undertaken with dosage schemes based on a better knowledge of melarsoprol pharmacokinetics, and these have yielded new recommendations for treatment protocols (Pepin and Mpia, 2006; Schmid et al., 2005). Melarsoprol is administered by slow intravenous injection, with care to avoid leakage into the surrounding tissues because the drug is intensely irritating. For T. brucei gambiense a continuous 10-day course of 2.2 mg/kg per day is equivalent to the longer course treatment and is now recommended (Pepin and Mpia, 2006; Schmid et al., 2005). The 10-day therapy has not reduced the incidence of encephalopathy (the treatment-related death rate was still 5.9% for this protocol), but it has the advantage of easier administration, and in an area where hospitalization is difficult, this is particularly important. These studies further demonstrated that historically used graded dosing schemes led to increased incidence of convulsions and reduced efficacy; thus these schedules should not be used for the treatment of T. brucei gambiense (Pepin and Mpia, 2006).

The 10-day therapy is currently undergoing clinical testing for T. brucei rhodesiense. However, until clinical data are available to support a dosing change, patients with Rhodesian sleeping sickness should instead receive one of several older schedules developed for this disease. A number of dosing schemes are in use or recommended by various agencies, including the WHO (1998) and Médecins Sans Frontières (2007). In the U.S., the CDC recommends the following treatment schedule: three series of three daily doses with a 7-day rest period between series. The first series gives 1.8, 2.7, and 3.6 mg/kg on days 1, 2, and 3, respectively. The subsequent series are 3.6 mg/kg daily. Encephalopathy develops more frequently in patients with T. brucei rhodesiense compared to T. brucei gambiense, possibly related to this graded dosing regimen. Concurrent administration of prednisolone is frequently employed throughout the treatment course; prednisolone it is not proven to reduce the frequency of reactive encephalopathy (Schmid et al., 2005) but it does help to control hypersensitivity reactions that occur most often during the second or subsequent courses of melarsoprol therapy.

Side Effects. Treatment with melarsoprol is associated with significant toxicity and morbidity (Balasegaram et al., 2006b; Burri and Brun, 2003; Kennedy, 2008; Schmid et al., 2005). A febrile reaction often occurs soon after drug injection, especially if parasitemia is high. The most serious complications involve the nervous system. A reactive encephalopathy occurs in ~5-10% of patients, leading to death in about half of these. The cause is unknown, but encephalopathy, when it occurs, typically develops 9-11 days after treatment starts (Schmid et al., 2005). Peripheral neuropathy, noted in ~10% of patients receiving melarsoprol, probably is due to a direct toxic effect of the drug. Hypertension and myocardial damage are not uncommon, although shock is rare. Albuminuria occurs frequently, and occasionally the appearance of numerous casts in the urine or evidence of hepatic disturbances may necessitate modification of treatment. Vomiting and abdominal colic also are common, but their incidence can be reduced by injecting melarsoprol slowly into the supine, fasting patient. The patient should remain in bed and not eat for several hours after the injection is given.

Precautions and Contraindications. Melarsoprol should be given only to patients under hospital supervision so that the dosage regimen may be modified if necessary. Initiation of therapy during a febrile episode has been associated with an increased incidence of reactive encephalopathy. Administration of melarsoprol to leprous patients may precipitate erythema nodosum. Use of the drug is contraindicated during epidemics of influenza. Severe hemolytic reactions have been reported in patients with deficiency of glucose-6-phosphate dehydrogenase. Pregnancy is not a contraindication for treatment with melarsoprol.

Antiparasitic and Antimicrobial Effects. Metronidazole and related nitroimidazoles are active in vitro against a wide variety of anaerobic protozoal parasites and anaerobic bacteria (Freeman et al., 1997). The compound is directly trichomonacidal. Sensitive isolates of T. vaginalis are killed by <0.05 μg/mL of the drug under anaerobic conditions; higher concentrations are required when 1% oxygen is present or to affect isolates from patients who display poor therapeutic responses to metronidazole. The drug also has potent amebicidal activity against E. histolytica. Trophozoites of G. lamblia are affected by metronidazole at concentrations of 1-50 μg/mL in vitro. In vitro studies on drug-sensitive and drug-resistant protozoan parasites indicate that the nitro group on C5 of metronidazole is essential for activity and that substitutions at the 2 position of the imidazole ring that enhance the resonance conjugation of the chemical structure increase antiprotozoal activity. In contrast, substitution of an acyl group at the 2 position ablates such conjugation and reduces antiprotozoal activity (Upcroft et al., 1999).

Metronidazole manifests antibacterial activity against all anaerobic cocci and both anaerobic gram-negative bacilli, including Bacteroides spp., and anaerobic spore-forming gram-positive bacilli. Nonsporulating gram-positive bacilli often are resistant, as are aerobic and facultatively anaerobic bacteria.

Metronidazole is clinically effective in trichomoniasis, amebiasis, and giardiasis, as well as in a variety of infections caused by obligate anaerobic bacteria, including Bacteroides, Clostridium, and microaerophilic bacteria such as Helicobacter and Campylobacter spp. Metronidazole may facilitate extraction of adult guinea worms in dracunculiasis even though it has no direct effect on the parasite (Chapter 51).

Mechanism of Action and Resistance. Metronidazole is a prodrug; it requires reductive activation of the nitro group by susceptible organisms. Its selective toxicity toward anaerobic and microaerophilic pathogens such as the amitochondriate protozoa T. vaginalis, E. histolytica, and G. lamblia and various anaerobic bacteria derives from their energy metabolism, which differs from that of aerobic cells (Land and Johnson, 1997; Samuelson, 1999; Upcroft and Upcroft, 1999). These organisms, unlike their aerobic counterparts, contain electron transport components such as ferredoxins, small Fe–S proteins that have a sufficiently negative redox potential to donate electrons to metronidazole. The single electron transfer forms a highly reactive nitro radical anion that kills susceptible organisms by radical-mediated mechanisms that target DNA and possibly other vital biomolecules. Metronidazole is catalytically recycled; loss of the active metabolite's electron regenerates the parent compound. Increasing levels of O₂ inhibit metronidazole-induced cytotoxicity because O₂ competes with metronidazole for electrons generated by energy metabolism. Thus, O₂ can both decrease reductive activation of metronidazole and increase recycling of the activated drug. Anaerobic or microaerophilic organisms susceptible to metronidazole derive energy from the oxidative fermentation of ketoacids such as pyruvate. Pyruvate decarboxylation, catalyzed by pyruvate:ferredoxin oxidoreductase (PFOR), produces electrons that reduce ferredoxin, which, in turn, catalytically donates its electrons to biological electron acceptors or to metronidazole.

Clinical resistance to metronidazole is well documented for T. vaginalis, G. lamblia, and a variety of anaerobic and microaerophilic bacteria but has yet to be shown for E. histolytica. Resistant strains of T. vaginalis derived from nonresponsive patients have shown two major types of abnormalities when tested under aerobic conditions. The first correlates with impaired oxygen-scavenging capabilities, leading to higher local O₂ concentrations, decreased activation of metronidazole, and futile recycling of the activated drug. The second type is associated with lowered levels of PFOR and ferredoxin, the latter owing to reduced transcription of the ferredoxin gene. That PFOR and ferredoxin are not completely absent may explain why infections with such strains usually respond to higher doses of metronidazole or more prolonged therapy. Studies on metronidazole-resistant isolates of G. intestinalis indicate that similar mechanisms may be operating, with PFOR levels reduced 5-fold compared with susceptible strains (Upcroft and Upcroft, 2001). Metronidazole resistance has not been found in clinical isolates of E. histolytica but has been induced in vitro by culturing trophozoites in gradually increasing concentrations of the drug. Interestingly, although some decrease in PFOR levels was reported, metronidazole resistance was mediated primarily by increased expression of superoxide dismutase and peroxiredoxin in amebic trophozoites (Wassmann et al., 1999). Resistance of anaerobic bacteria to metronidazole is being recognized increasingly and has important clinical consequences. In the case of Bacteroides spp., metronidazole resistance has been linked to a family of nitroimidazole (nim) resistance genes, nimA, -B, -C, -D, -E, and -F, that can be encoded chromosomally or episomally (Gal and Brazier, 2004). The exact mechanisms underlying resistance are not known, but nim genes appear to encode a nitroimidazole reductase capable of converting a 5-nitroimidazole to a 5-aminoimidazole, thus stopping the formation of the reactive nitroso group responsible for microbial killing. Metronidazole has been used widely for the treatment of the microaerophilic organism Helicobacter pylori, the major cause of ulcer disease and gastritis worldwide. However, Helicobacter can develop resistance to metronidazole rapidly. Multiple mechanisms probably are operating, but there are data associating loss-of-function mutations in an oxygen-independent NADPH nitroreductase (rdxA gene) with resistance to metronidazole (Mendz and Mégraud, 2002).

Absorption, Fate, and Excretion. The pharmacokinetic properties of metronidazole and its two major metabolites have been investigated intensively (Lamp et al., 1999). Preparations of metronidazole are available for oral, intravenous, intravaginal, and topical administration. The drug usually is absorbed completely and promptly after oral intake, reaching concentrations in plasma of 8-13 μg/mL within 0.25-4 hours after a single 500-mg dose. (Mean effective concentrations of the compound are ≤8 μg/mL for most susceptible protozoa and bacteria.) A linear relationship between dose and plasma concentration pertains for doses of 200-2000 mg. Repeated doses every 6-8 hours result in some accumulation of the drug; systemic clearance exhibits dose dependence. The t_1/2 of metronidazole in plasma is ~8 hours; its volume of distribution approximates total body water. Less than 20% of the drug is bound to plasma proteins. With the exception of the placenta, metronidazole penetrates well into body tissues and fluids, including vaginal secretions, seminal fluid, saliva, breast milk, and CSF.

After an oral dose, >75% of labeled metronidazole is eliminated in the urine largely as metabolites; ~10% is recovered as unchanged drug. The liver is the main site of metabolism, and this accounts for >50% of the systemic clearance of metronidazole. The two principal metabolites result from oxidation of side chains, a hydroxy derivative and an acid. The hydroxy metabolite has a longer t_1/2 (~12 hours) and has ~50% of the antitrichomonal activity of metronidazole. Formation of glucuronides also is observed. Small quantities of reduced metabolites, including ring-cleavage products, are formed by the gut flora. The urine of some patients may be reddish brown owing to the presence of unidentified pigments derived from the drug. Oxidative metabolism of metronidazole is induced by phenobarbital, prednisone, rifampin, and possibly ethanol. Cimetidine appears to inhibit hepatic metabolism of the drug.

Therapeutic Uses. The uses of metronidazole for anti-protozoal therapy have been reviewed extensively (Freeman et al., 1997; Nash, 2001; Stanley, 2003). Metronidazole cures genital infections with T. vaginalis in both females and males in >90% of cases. The preferred treatment regimen is 2 g metronidazole as a single oral dose for both males and females. Tinidazole, which has a longer t_1/2 than metronidazole, is also used at a 2-g single dose and appears to provide equivalent or better responses than metronidazole. For patients who cannot tolerate a single 2-g dose of metronidazole (or 1 g twice daily in the same day), an alternative regimen is a 250-mg dose given three times daily or a 375-mg dose given twice daily for 7 days. When repeated courses or higher doses of the drug are required for uncured or recurrent infections, it is recommended that intervals of 4-6 weeks elapse between courses. In such cases, leukocyte counts should be carried out before, during, and after each course of treatment.

Treatment failures owing to the presence of metronidazole-resistant strains of T. vaginalis are becoming increasingly common. Most of these cases can be treated successfully by giving a second 2-g dose to both patient and sexual partner. In addition to oral therapy, the use of a topical gel containing 0.75% metronidazole or a 500- to 1000-mg vaginal suppository will increase the local concentration of drug and may be beneficial in refractory cases. Metronidazole is an effective amebicide and is the agent of choice for the treatment of all symptomatic forms of amebiasis, including amebic colitis and amebic liver abscess. The recommended dose is 500-750 mg metronidazole taken orally three times daily for 7-10 days, or for children, 35-50 mg/kg/day given in three divided doses for 7-10 days.

Although standard recommendations are for 7-10 days' duration of therapy, amebic liver abscess has been treated successfully by short courses (2.4 g daily as a single oral dose for 2 days) of metronidazole or tinidazole (Stanley, 2003). E. histolytica persist in most patients who recover from acute amebiasis after metronidazole therapy, so it is recommended that all such individuals also be treated with a luminal amebicide.

Although effective for the therapy of giardiasis, metronidazole has yet to be approved for treatment of this infection in the U.S. However, tinidazole is approved for the treatment of giardiasis as a single 2-g dose and is appropriate first-line therapy. Metronidazole is a relatively inexpensive, highly versatile drug with clinical efficacy against a broad spectrum of anaerobic and microaerophilic bacteria. It is used for the treatment of serious infections owing to susceptible anaerobic bacteria, including Bacteroides, Clostridium, Fusobacterium, Peptococcus, Peptostreptococcus, Eubacterium, and Helicobacter. The drug is also given in combination with other antimicrobial agents to treat polymicrobial infections with aerobic and anaerobic bacteria. Metronidazole achieves clinically effective levels in bones, joints, and the CNS. Metronidazole can be given intravenously when oral administration is not possible. A loading dose of 15 mg/kg is followed 6 hours later by a maintenance dose of 7.5 mg/kg every 6 hours, usually for 7-10 days. Metronidazole is used as a component of prophylaxis for colorectal surgery and is employed as a single agent to treat bacterial vaginosis. It is used in combination with other antibiotics and a proton pump inhibitor in regimens to treat infection with H. pylori (Chapter 45).

Metronidazole is used as primary therapy for Clostridium difficile infection, the major cause of pseudomembranous colitis. Given at doses of 250-500 mg orally three times daily for 7-14 days (or even longer), metronidazole is an effective and cost-saving alternative to oral vancomycin therapy. However, a reported increase in treatment failures and higher rates of disease recurrence with metronidazole (as compared to oral vancomycin) are raising questions about the role of metronidazole as first-line therapy (McFarland, 2008). Finally, metronidazole is also used in the treatment of patients with Crohn's disease who have perianal fistulas, and it can help control colonic (but not small bowel) Crohn's disease. However, high doses (750 mg three times daily) for prolonged periods may be necessary, and neurotoxicity may be limiting (Podolsky, 2002). Metronidazole and other nitroimidazoles can sensitize hypoxic tumor cells to the effects of ionizing radiation, but these drugs are not used clinically for this purpose.

Toxicity, Contraindications, and Drug Interactions. The toxicity of metronidazole has been reviewed (Raether and Hanel, 2003). Side effects only rarely are severe enough to discontinue therapy. The most common are headache, nausea, dry mouth, and a metallic taste. Vomiting, diarrhea, and abdominal distress are experienced occasionally. Furry tongue, glossitis, and stomatitis occurring during therapy may be associated with an exacerbation of candidiasis. Dizziness, vertigo, and very rarely, encephalopathy, convulsions, incoordination, and ataxia are neurotoxic effects that warrant discontinuation of metronidazole. The drug also should be withdrawn if numbness or paresthesias of the extremities occur. Reversal of serious sensory neuropathies may be slow or incomplete. Urticaria, flushing, and pruritus are indicative of drug sensitivity that can require withdrawal of metronidazole. Metronidazole is a rare cause of Stevens-Johnson syndrome (toxic epidermal necrolysis); one report described a high rate of this syndrome among individuals receiving high doses of metronidazole and concurrent therapy with the antihelminthic mebendazole (Chen et al., 2003).

Dysuria, cystitis, and a sense of pelvic pressure have been reported. Metronidazole has a well-documented disulfiram-like effect, and some patients experience abdominal distress, vomiting, flushing, or headache if they drink alcoholic beverages during or within 3 days of therapy with this drug. Patients should be cautioned to avoid consuming alcohol during metronidazole treatment even though the risk of a severe reaction is low. By the same token, metronidazole and disulfiram or any disulfiram-like drug should not be taken together because confusional and psychotic states may occur. Although related chemicals have caused blood dyscrasias, only a temporary neutropenia, reversible after discontinuation of therapy, occurs with metronidazole.

Metronidazole should be used with caution in patients with active disease of the CNS because of its potential neurotoxicity. The drug also may precipitate CNS signs of lithium toxicity in patients receiving high doses of lithium. Plasma levels of metronidazole can be elevated by drugs such as cimetidine that inhibit hepatic microsomal metabolism. Moreover, metronidazole can prolong the prothrombin time of patients receiving therapy with coumadin anticoagulants. The dosage of metronidazole should be reduced in patients with severe hepatic disease.

Given in high doses for prolonged periods, metronidazole is carcinogenic in rodents; it also is mutagenic in bacteria (Raether and Hanel, 2003). Mutagenic activity is associated with metronidazole and several of its metabolites found in the urine of patients treated with therapeutic doses of the drug. However, there is no evidence that therapeutic doses of metronidazole pose any significant increased risk of cancer to human patients. There is conflicting evidence about the teratogenicity of metronidazole in animals. Although metronidazole has been taken during all stages of pregnancy with no apparent adverse effects, its use during the first trimester generally is not advised.

Antiprotozoal Effects. In vitro analysis of a large number of alkylglycerophosphoethanolamines (AGPEs), alkylglycerophosphocholines (AGPCs), and APCs, originally developed as anticancer agents, demonstrated that a number of these compounds had potent antileishmanial activity against cultured promastigote or amastigote parasites (Croft et al., 2003). Of the APCs, miltefosine demonstrated the best activity against both stages of parasites. The potency of miltefosine varies for different Leishmania spp., with L. donovani being the most sensitive (1.2<ED₅₀<5 μM against amastigotes) and L. major being the least sensitive (8<ED₅₀<40 μM against amastigotes). Several phospholipid analogs also had good potency against cultured T. cruzi parasites, whereas they were relatively inactive against T. brucei. In vivo, miltefosine had the best activities of the tested APCs, and by 5 days of treatment with 20 mg/kg orally, it gave >95% suppression of L. donovani and L. infantum amastigotes in the affected organs of mice. It also had good activity when administered as a 6% ointment to skin lesions of L. mexicana or L. major on BALB/c mice.

Against visceral leishmaniasis, 95-98% cure rates were observed with a 100 mg/day oral dose for 28-42 days (Croft et al., 2003) and 95% with a 28-day regimen of 100 mg/day orally for adults and 2.5 mg/kg daily for children (Bhattacharya et al., 2007). Thus, oral miltefosine appears to be a safe and effective treatment for visceral leishmaniasis (Berman, 2008). Recent studies of oral miltefosine also have shown >90% efficacy against some species of cutaneous leishmaniasis (registered in Colombia for this use in 2005). However, significant strain variation has been observed in the clinical trials (Soto and Berman, 2006; Soto et al., 2007): For L. panamensis in Colombia, cure rates were from 84-91% (versus the placebo cure rate of 37%), and it was considerably less effective for L. brasiliensis in Guatemala.

The mechanism of action of miltefosine is not yet understood. Potential targets in mammalian cells include PKC; CTP:phosphocholine-cytidylyltransferase inhibition, which disrupts phosphatidylcholine biosynthesis; and altered sphingomyelin biosynthesis, where increased levels of cellular ceramide may trigger apoptosis (Croft et al., 2003). Studies in Leishmania suggest that the drug may alter ether-lipid metabolism, cell signaling, or glycosylphosphatidylinosital anchor biosynthesis. A transporter for miltefosine has been cloned recently by functional rescue of a laboratory-generated resistant strain of L. donovani. The transporter is a P-type ATPase that belongs to the aminophospholipid translocase subfamily, and the basis for the drug resistance appears to be point mutation in the transporter leading to decreased drug uptake. Mutations in this transporter lead to miltefosine resistance in laboratory models (Croft et al., 2006; Seifert et al., 2007).

Absorption, Fate, and Excretion. Miltefosine is well absorbed orally and distributed throughout the human body. Detailed pharmacokinetic data are lacking, with the exception that miltefosine has a long t_1/2 in humans (Berman, 2008). Plasma concentrations are proportional to the dose, and maximum serum concentrations for dosing of 50-150 mg per day were 20-70 μg/mL with a t_1/2 of 1 week and a terminal t_1/2 of ~4 weeks. The C_max in children was ~30% less than in adults.

Therapeutic Uses. Oral miltefosine is registered for use in India for the treatment of visceral leishmaniasis with clinical trial study patients receiving the following oral doses (Bhattacharya, 2007): for adults >25 kg, 100 mg daily divided into two parts, and for adults <25 kg, 50 mg daily in 1 dose for 28 days; for children, 2.5 mg/kg/day in two divided doses. For cutaneous disease, the dose in clinical testing is 2.5 mg/kg per day orally (maximum, 150 mg/day) for 28 days. In the U.S., the recommended dose for both visceral and cutaneous disease is 2.5 mg/kg per day (maximum dose of 150 mg/day) for 28 days, given in two divided doses (Medical Letter, 2007). Miltefosine is also available in Afghanistan, Pakistan, and South America. Because of its teratogenic potential, miltefosine is contraindicated in pregnant women. Women should receive a negative pregnancy test prior to treatment, and birth control is required during and for at least 2 months after treatment. The compound cannot be given intravenously because it has hemolytic activity.

Toxicity and Side Effects. Vomiting and diarrhea have been reported as frequent side effects in up to 60% of the patients. Elevations in hepatic transaminases and serum creatinine also have been reported. These effects are typically mild and reversible, and they resolve quickly once the drug is withdrawn.

Antiprotozoal Effects and Mechanisms of Action. Nifurtimox and benznidazole are trypanocidal against both the trypomastigote and amastigote forms of T. cruzi. Nifurtimox also has activity against T. brucei and can be curative against both early- and late-stage disease (see earlier discussion on nifurtimox-eflornithine combination therapy.

Both nifurtimox and benznidazole are activated by a NADH-dependent mitochondrial nitroreductase, leading to the intracellular generation of nitro radical anions that are thought to account for the trypanocidal effects. The generated nitro anion radicals form covalent attachments to macromolecules leading to cellular damage that kills the parasites. Transfer of electrons from the activated drug regenerates the native nitrofuran and forms superoxide radical anions and other reactive oxygen species such as hydrogen peroxide and hydroxyl radical. Reaction of free radicals with cellular macromolecules results in cellular damage that includes lipid peroxidation and membrane injury, enzyme inactivation, and damage to DNA. Reduced nitroreductase expression through single allele gene knockout experiments or through drug selection leads to drug resistance (Wilkinson et al., 2008).

Absorption, Fate, and Excretion Nifurtimox is well absorbed after oral administration, with peak plasma levels observed after ~3.5 hours (Paulos et al., 1989). Despite this, only low concentrations of the drug (10-20 μM) are present in plasma, and <0.5% of the dose is excreted in urine. The elimination t_1/2 is ~3 hours. High concentrations of several unidentified metabolites are found, however, and nifurtimox clearly undergoes rapid biotransformation, probably via a presystemic first-pass effect. Whether the metabolites have any trypanocidal activity is unknown.

hTerapeutic Uses. Nifurtimox and benznidazole are employed in the treatment of American trypanosomiasis (Chagas' disease) caused by T. cruzi (Bern et al., 2007; Tarleton et al., 2007). However, because of toxicity issues, benznidazole is the preferred treatment. Both drugs markedly reduce the parasitemia, morbidity, and mortality of acute Chagas' disease, with parasitological cures obtained in 80% of these cases. Parasitological cure is defined by a negative result in PCR and serological tests for the parasite. In the chronic form of the disease, parasitological cures are still possible in up to 50% of the patients, although the drug is less effective than in the acute stage. Treatment with nifurtimox or benznidazole has no effect on irreversible organ lesions. Although a component of tissue destruction may be autoimmune in nature, the continued presence of parasites in the infected organs of patients with chronic disease argues that Chagas' disease should be treated as a parasitic disease.

The clinical response of the acute illness to drug therapy varies with geographic region; parasite strains present in Argentina, southern Brazil, Chile, and Venezuela appear to be more susceptible than those in central Brazil. A large-scale clinical trial (BENEFIT) is currently underway to test the efficacy of benznidazole in the treatment of both acute and chronic Chagas' disease (http://clinicaltrials.gov/show/NCT00123916); (Marin-Neto et al., 2008); until the trial's results are available, the current recommendations are that patients <50 years of age with either acute- or recent chronic-phase disease, without advanced cardiomyopathy, should be treated (Bern et al., 2007); in patients >50 years of age, treatment is optional because of lowered drug tolerability in this group; dosing is 5 mg/kg per day for 60 days. Therapy is strongly encouraged for patients that will receive immunosuppressive therapy or who are HIV positive. For patients with late chronic-phase disease (>10 years) with advanced cardiomyopathy, parasitological cure is less likely, and the general consensus is not to treat these patients. However, the decision to treat should be made on an individual basis. Therapy with nifurtimox or benznidazole should start promptly after exposure for persons at risk of T. cruzi infection from laboratory accidents or from blood transfusions.

Both drugs are given orally. The treatment schedules recommended by the CDC in the U.S. are as follows: for nifurtimox, adults (>17 years of age) with acute infection should receive 8-10 mg/kg per day in three to four divided doses for 90 days; children 1-10 years of age should receive 15-20 mg/kg per day in three to four divided doses for 90 days; for individuals 11-16 years old, the daily dose is 12.5-15 mg/kg given according to the same schedule. For benznidazole, the recommended treatment for adults (>13 years) is 5-7 mg/kg per day in two divided doses for 60 days, with children up to 12 years receiving 10 mg/kg per day (Bern et al., 2007). Gastric upset and weight loss can occur during treatment. If the latter occurs, dosage should be reduced. The ingestion of alcohol should be avoided during treatment because the incidence of side effects may increase.

Nifurtimox is used in combination with eflornithine in treating late stage T.b. gambiense sleeping sickness (see "Eflornithine" earlier in this chapter).

Toxicity and Side Effects. Children tolerate nifurtimox and benznidazole better than adults. Nonetheless, drug-related side effects are common. They range from hypersensitivity reactions (e.g., dermatitis, fever, icterus, pulmonary infiltrates, and anaphylaxis) to dose- and age-dependent complications primarily referable to the GI tract and both the peripheral and central nervous systems. Nausea and vomiting are common, as are myalgia and weakness. Peripheral neuropathy and GI symptoms are especially common after prolonged treatment; the latter complication may lead to weight loss and preclude further therapy. Headache, psychic disturbances, paresthesias, polyneuritis, and CNS excitability are less frequent. Leukopenia and decreased sperm counts also have been reported. Because of the seriousness of Chagas' disease and the lack of superior drugs, there are few absolute contraindications to the use of these drugs.

Antimicrobial Effects. Nitazoxanide and its active metabolite, tizoxanide (desacetyl-nitazoxanide), inhibit the growth of sporozoites and oocytes of C. parvum and inhibit the growth of the trophozoites of G. intestinalis, E. histolytica, and T. vaginalis in vitro (Adagu et al., 2002). Activity against other protozoans, including Blastocystis hominis, Isospora belli, and Cyclospora cayetanensis also has been reported. Nitazoxanide also demonstrated activity against the intestinal helminths: Hymenolepis nana, Trichuris trichiura, Ascaris lumbricoides, Enterobius vermicularis, Ancylostoma duodenale, Strongyloides stercoralis, and the liver fluke Fasciolahepatica. Effects against some anaerobic or microaerophilic bacteria, including Clostridium spp. and H. pylori, also have been reported. Nitazoxanide reportedly displays antiviral activity and is now undergoing clinical trials for the treatment of hepatitis C (Rossignol and Keeffe, 2008).

Mechanism of Action and Resistance. Nitazoxanide interferes with the PFOR enzyme-dependent electron-transfer reaction, which is essential to anaerobic metabolism in protozoan and bacterial species. One proposed mechanism of nitazoxanide is blockade of the first step in the PFOR chain by inhibiting the binding of pyruvate to the thiamine pyrophosphate cofactor (Hoffman et al., 2007). Resistance to nitazoxanide has been induced in Giardia in vitro, but clinically resistant clinical isolates have not yet been reported.

Absorption, Fate, and Excretion. Following oral administration, nitazoxanide is hydrolyzed rapidly to its active metabolite, tizoxanide, which undergoes conjugation primarily to tizoxanide glucuronide. Bioavailability after an oral dose is excellent, and maximum plasma concentrations of the metabolites are detected within 1-4 hours following administration of the parent compound. Tizoxanide is >99.9% bound to plasma proteins. Tizoxanide is excreted in the urine, bile, and feces; tizoxanide glucuronide is excreted in the urine and bile. The pharmacokinetics of nitazoxanide in individuals with impaired hepatic or renal function have not been reported.

Therapeutic Uses. In the U.S., nitazoxanide is approved for the treatment of G. intestinalis infection (therapeutic efficacy of 85-90% for clinical response) and for the treatment of diarrhea caused by cryptosporidia (therapeutic efficacy, 56-88% for clinical response) in adults and children >1 year of age (Amadi et al., 2002; Rossignol et al., 2001). The efficacy of nitazoxanide in children (or adults) with cryptosporidia infection and AIDS has not been clearly established. For children ages 12-47 months, the recommended dose is 100 mg nitazoxanide every 12 hours for 3 days; for children ages 4-11 years, the dose is 200 mg nitazoxanide every 12 hours for 3 days. A 500-mg tablet, suitable for adult dosing (every 12 hours), is available.

Nitazoxanide has been used as a single agent to treat mixed infections with intestinal parasites (protozoa and helminths) in several trials. Effective parasite clearance (based on negative follow-up fecal samples) after nitazoxanide treatment was shown for G. intestinalis, E. histolytica/E. dispar, B. hominis, C. parvum, C. cayetanensis, I. belli, H. nana, T. trichiura, A. lumbricoides, and E. vermicularis (Romero-Cabello et al., 1997), although more than one course of therapy was required in some cases. Nitazoxanide may have some efficacy against Fasciola hepatica infections (Favennec et al., 2003) and has been used to treat infections with G. intestinalis that are resistant to metronidazole and albendazole (Abboud et al., 2001).

Toxicity and Side Effects. To date, adverse effects appear to be rare with nitazoxanide. Abdominal pain, diarrhea, vomiting, and headache have been reported, but rates were no different from those in patients receiving placebo. A greenish tint to the urine is seen in most individuals taking nitazoxanide. Nitazoxanide is a pregnancy Category B agent, based on animal teratogenicity and fertility studies, but there is no clinical experience with its use in pregnant women or nursing mothers.

Mechanism of Action. Paromomycin shares the same mechanism of action as neomycin and kanamycin (binding to the 30S ribosomal subunit) and has the same spectrum of antibacterial activity. Paromomycin is available only for oral use in the U.S. Following oral administration, 100% of the drug is recovered in the feces, and even in cases of compromised gut integrity, there is little evidence for clinically significant absorption of paromomycin. Parenteral administration carries the same risks of nephrotoxicity and ototoxicity seen with other aminoglycosides.

Antimicrobial Effects; Therapeutics Uses. Paromomycin is the drug of choice for treating intestinal colonization with E. histolytica. It is used in combination with metronidazole to treat amebic colitis and amebic liver abscess and can be used as a single agent for asymptomatic individuals found to have E. histolytica intestinal colonization. Recommended dosing for adults and children is 25-35 mg/kg per day orally in three divided doses. Adverse effects are rare with oral usage but include abdominal pain and cramping, epigastric pain, nausea and vomiting, steatorrhea, and diarrhea. Rarely, rash and headache have been reported. Paromomycin has been used to treat cryptosporidiosis in AIDS patients both as a single agent and in combination with azithromycin, but in a randomized controlled trial, paromomycin was no more effective than placebo for this indication (Hewitt et al., 2000). Paromomycin has been advocated as a treatment for giardiasis in pregnant women, especially during the first trimester, when metronidazole is contraindicated and as an alternative agent for metronidazole-resistant isolates of G. intestinalis. Although there is limited clinical experience, response rates of 55-90% have been reported (Gardner and Hill, 2001). Dosing in adults is 500 mg orally three times daily for 10 days, whereas children have been treated with 25-30 mg/kg per day in three divided oral doses. Paromomycin formulated as a 6.25% cream has been used to treat vaginal trichomoniasis in patients who had failed metronidazole therapy or could not receive metronidazole. Some cures have been reported, but vulvovaginal ulcerations and pain have complicated treatment and absorption via this route (and the possibility of aminoglycoside-related toxicities) must be anticipated.

Paromomycin is also efficacious as a topical formulation containing 15% paromomycin in combination with 12% methylbenzonium chloride for the treatment of cutaneous leishmaniasis (Alvar et al., 2006). The drug has been administered parenterally alone or in combination with antimony to treat visceral leishmaniasis. Paromomycin (intramuscular injection, 11 mg/kg per day for 21 days) produced a cure rate for visceral leishmaniasis (94.6%) that was statistically equivalent to liposomal amphotericin B (Sundar et al., 2007). However, adverse events were more common with paromomycin than with liposomal amphotericin B.

Pentamidine as the di-isethionate salt is marketed for injection (pentam 300, others) or for use as an aerosol (nebupent). One milligram of pentamidine base is equivalent to 1.74 mg of the pentamidine isethionate. The di-isethionate salt is highly water soluble; however, solutions should be used promptly after preparation because pentamidine is unstable in solution.

Antiprotozoal and Antifungal Effects. The positively charged aromatic diamidines are toxic to a number of different protozoa yet show rather marked selectivity of action. Pentamidine is used for the treatment of early-stage T. brucei gambiense infection (Barrett et al., 2007; Kennedy, 2008) but is ineffective in the treatment of late-stage disease and has reduced efficacy against T. brucei rhodesiense. Its use is therefore limited to treatment of first-stage T. brucei gambiense sleeping sickness. Pentamidine is an alternative agent for the treatment of cutaneous leishmaniasis (Alvar et al., 2006; Croft, 2008). It has been used for the treatment of visceral disease but less toxic agents are preferred. Pentamidine is an alternative agent for the treatment and prophylaxis of Pneumocystis pneumonia caused by the ascomycetous fungus Pneumocystis jiroveci (formerly known as Pneumocystis carinii) (Thomas and Limper, 2004). Diminazene (berenil) is a related diamidine that is used as an inexpensive alternative to pentamidine for the treatment of early African trypanosomiasis and has been used outside the U.S. for the treatment of early-stage T. b. gambiense in periods of pentamidine shortage.

Mechanism of Action and Resistance. The mechanism of action of the diamidines is unknown. The dicationic compounds appear to display multiple effects on any given parasite and act by disparate mechanisms in different parasites (Barrett et al., 2007). In T. brucei, for example, the diamidines are concentrated via an energy-dependent high-affinity uptake system to millimolar concentrations in cells; this selective uptake is essential to their efficacy. The best-characterized diamidine transporter is the P2 purine transporter used by the melamine-based arsenicals, which explains the cross-resistance to diamidines exhibited by certain arsenical-resistant strains of T. brucei in vitro (de Koning, 2008). It is increasingly clear that multiple transporters are responsible for pentamidine uptake, and this may account for the fact that little resistance to this drug is observed in field isolates despite its years of use as a prophylactic agent. Although failure to concentrate diamidines is the usual cause of pentamidine resistance in vitro, other mechanisms also may be involved.

The positively charged hydrophobic diamidines may exert their trypanocidal effects by reacting with a variety of negatively charged intracellular targets. Indeed, ribosomal aggregation, inhibition of DNA and protein synthesis, inhibition of several enzymes, and loss of trypanosomal kinetoplast DNA have been reported. The loss of kinetoplast DNA may be mediated through inhibition of topoisomerase II. It is doubtful that an effect on topoisomerase accounts for the drug's broad antimicrobial activity or fully explains its mechanism of action. Inhibition of S-adenosyl-L-methionine decarboxylase in vitro (but no change in polyamine levels in vivo) and inhibition of a plasma Ca²⁺, Mg²⁺-ATPase also have been reported.

Absorption, Fate, and Excretion. The pharmacokinetics and bio-disposition of pentamidine isethionate have been studied most extensively in AIDS patients with P. jiroveci infections (Vöhringer and Arastéh, 1993); information from patients with Gambian trypanosomiasis is more limited (Bronner et al., 1995). Pentamidine isethionate is fairly well absorbed from parenteral sites of administration. Following a single intravenous dose, the drug disappears from plasma with an apparent t_1/2 of several minutes to a few hours and maximum plasma concentrations after intramuscular injection occurring at 1 hour. The t_1/2 of elimination is very slow, lasting from weeks to months, and the drug is 70% bound to plasma proteins. This highly charged compound is poorly absorbed orally and does not cross the blood-brain barrier, explaining why it is ineffective against late-stage trypanosomiasis.

After multiple parenteral doses, the liver, kidney, adrenal, and spleen contain the highest concentrations of drug, whereas only traces are found in the brain (Donnelly et al., 1988). The lungs contain intermediate but therapeutic concentrations after five daily doses of 4 mg/kg. Inhalation of pentamidine aerosols is used for prophylaxis of Pneumocystis pneumonia; delivery of drug by this route results in little systemic absorption and decreased toxicity compared with intravenous administration in both adults and children (Hand et al., 1994; Leoung et al., 1990). The actual dose delivered to the lungs depends on both the size of particles generated by the nebulizer and the patient's ventilatory patterns.

Therapeutic Uses. Pentamidine isethionate is used for the treatment of early-stage T. brucei gambiense and is given by intramuscular injection in single doses of 4 mg/kg per day for 7 days. Pentamidine has been used successfully in courses of 15-20 intramuscular doses of 4 mg/kg every other day to treat visceral leishmaniasis (Alvar et al., 2006). This compound provides an alternative to antimonials, lipid formulations of amphotericin B, or miltefosine but it is overall the least well tolerated of the available therapies and is not used routinely for this disease. Pentamidine isethionate given as 4-7 intramuscular doses of 4 mg/kg every other day has enjoyed some success in the treatment of cutaneous leishmaniasis (Oriental sore caused by L. tropica) but is not used routinely to treat this infection (Alvar et al., 2006).

Pentamidine is one of several drugs or drug combinations used to treat or prevent Pneumocystis infection. Pneumocystis pneumonia (PCP) is a major cause of mortality in individuals with HIV infection and AIDS and can occur in patients who are immunosuppressed by other mechanisms (e.g., high-dose corticosteroids or underlying malignancy). The availability of highly active antiretroviral therapy (HAART) has reduced the number of cases of PCP significantly within the U.S. and E.U. and greatly reduced the number of individuals requiring prophylaxis for PCP. Trimethoprim-sulfamethoxazole is the drug of choice for the treatment and prevention of PCP (Chapter 52). Pentamidine is reserved for two indications. Pentamidine given intravenously as a 4 mg/kg single daily dose for 21 days is used to treat severe PCP in individuals who cannot tolerate trimethoprim-sulfamethoxazole and are not candidates for alternative agents such as atovaquone or the combination of clindamycin and primaquine. Pentamidine has been recommended as a "salvage" agent for individuals with PCP who failed to respond to initial therapy (usually trimethoprim-sulfamethoxazole), but a meta-analysis suggested that pentamidine is less effective than other therapies (specifically the combination of clindamycin and primaquine or atovaquone) for this indication (Smego et al., 2001). Pentamidine administered as an aerosol preparation is used for the prevention of PCP in at-risk individuals who cannot tolerate trimethoprim-sulfamethoxazole and are not deemed candidates for either dapsone (alone or in combination with pyrimethamine) or atovaquone. Candidates for PCP prophylaxis are individuals with HIV infection and a CD4 count of <200/mm³ and individuals with HIV infection and persistent unexplained fever or oropharyngeal candidiasis. Secondary prophylaxis is recommended for everyone with a documented PCP episode. For prophylaxis, pentamidine isethionate is given monthly as a 300-mg dose in a 5-10% nebulized solution over 30-45 minutes. Although the monthly dosage regimen is convenient, aerosolized pentamidine has several disadvantages, including its failure to treat any extrapulmonary sites of Pneumocystis, the lack of efficacy against any other potential opportunistic pathogens (compared with trimethoprim-sulfamethoxazole), and a slightly increased risk for pneumothorax. Long-term aerosolized pentamidine prophylaxis (≥5 years) has not been associated with the development of any pulmonary disease (Obaji et al., 2003). For individuals who receive HAART and develop CD4 counts persistently above 200/mm³ for 3 months, primary or secondary PCP prophylaxis can be stopped.

Toxicity and Side Effects. Pentamidine is a toxic agent, and ~50% of individuals receiving the drug at recommended doses (4 mg/kg per day) show some adverse effect. Intravenous administration of pentamidine may be associated with hypotension, tachycardia, and headache. These effects are probably secondary to the ability of pentamidine to bind imidazoline receptors (Chapter 14) and can be ameliorated by slowing the rate of the intravenous infusion (Wood et al., 1998). As noted earlier, the diamidines were designed originally for use as hypoglycemics, and pentamidine retains that property. Hypoglycemia, which can be life threatening, may occur at any time during pentamidine treatment, even weeks into therapy. Careful monitoring of blood sugar is key. Paradoxically, pancreatitis, hyperglycemia, and the development of insulin-dependent diabetes have been seen in some patients. Pentamidine is nephrotoxic (~25% of treated patients show signs of renal dysfunction), and if the serum creatinine concentration rises >1.0-2.0 mg/dL, it may be necessary to withhold the drug temporarily or change to an alternative agent. Individuals developing pentamidine-induced renal dysfunction are at higher risk for hypoglycemia. Other adverse effects include skin rashes, thrombophlebitis, anemia, neutropenia, and elevation of hepatic enzymes. Intramuscular administration of pentamidine, while effective, is associated with the development of sterile abscesses at the injection site, which can become infected secondarily. For this reason, most authorities recommend intravenous administration. Aerosolized pentamidine is associated with few adverse events.

Quinacrine is an acridine derivative used widely during World War II as an antimalarial agent. Quinacrine hydrochloride is very effective against G. lamblia, producing cure rates of at least 90%. However, quinacrine is no longer available in the U.S. For a description of the pharmacology and toxicology of quinacrine, consult the fifth and earlier editions of this book.

Clinical formulations of sodium stibogluconate consist of multiple uncharacterized molecular forms, some of which have higher molecular masses than the compound shown. Typical preparations contain 30-34% pentavalent antimony by weight and m-chlorocresol (as a preservative). In the U.S., sodium stibogluconate can be obtained from the CDC.

Antiprotozoal Effects and Drug Resistance. The mechanism of the antileishmanial action of sodium stibogluconate remains an active area of research, and recent studies have provided interesting new insights (Croft et al., 2006). These studies support the old hypothesis that relatively nontoxic pentavalent antimonials act as prodrugs. These compounds are reduced to the more toxic Sb³⁺ species that kill amastigotes within the phagolysosomes of macrophages. This reduction preferentially occurs in the intracellular amastigote stage, and recently an enzyme, As⁵⁺ reductase, was characterized that is able to reduce Sb⁵⁺ to the active Sb³⁺ form. Furthermore, the overexpression of this enzyme in promastigotes increased their sensitivity to the drugs. Classically, it was thought that the antiparasitic effects of antimonials occurred through inhibition of glucose catabolism and fatty acid oxidation; however, recent studies suggest that the drugs operate by interfering with the trypanothione redox system. In drug-sensitive cell lines, Sb³⁺ induces a rapid efflux of trypanothione and glutathione from the cells, and also inhibits trypanothione reductase, thereby causing a significant loss of thiol reduction potential in the cells. This hypothesis is further supported by the observation that laboratory-generated resistance to antimonials leads to overexpression of glutathione and polyamine biosynthetic enzymes, resulting in increased trypanothione levels, which conjugate to the drug. Elevated levels of ABC efflux transporters (see Chapter 5) were observed, though these transporters seem to play only a minor role in drug resistance.

Absorption, Fate, and Excretion. The pentavalent antimonials attain much higher concentrations in plasma than do the trivalent compounds. Consequently, most of a single dose of sodium stibogluconate is excreted in the urine within 24 hours. Its pharmacokinetic behavior is similar whether the drug is given intravenously or intramuscularly; it is not active orally (Chulay et al., 1988). The agent is absorbed rapidly, distributed in an apparent volume of ~0.22 L/kg, and eliminated in two phases. The first has a t_1/2 of ~2 hours, and the second is much slower (t_1/2 = 33-76 hours). The prolonged terminal elimination phase may reflect conversion of the Sb⁵⁺ to the more toxic Sb³⁺ that is concentrated and slowly released from tissues. Indeed, ~20% of the plasma antimony is present in the trivalent form after pentavalent antimonial administration. Sequestration of antimony in macrophages also may contribute to the prolonged antileishmanial effect after plasma antimony levels have dropped below the minimal inhibitory concentration observed in vitro.

Therapeutic Uses. The changing use of sodium stibogluconate, meglumine antimonate, and other agents for the chemotherapy of leishmaniasis has been reviewed extensively (Alvar et al., 2006; Croft, 2008; Olliaro et al., 2005). Sodium stibogluconate is given parenterally, with the dosage regimen individualized depending on the local responsiveness of a particular form of leishmaniasis to this compound. The standard course is 20 mg/kg per day for 21 days for cutaneous disease and for 28 days for visceral disease. In some regions of the world prolonged dosage schedules with maximally tolerated doses are now needed for successful therapy of visceral, mucosal, and some cutaneous forms of leishmaniasis, in part to overcome increasing clinical resistance to antimonial drugs. Even high-dose regimens may no longer produce satisfactory results. Increased resistance has greatly compromised the effectiveness of these drugs, and the drug is now obsolete in India. This is in contrast to previous cure rates of >85% for both visceral and cutaneous disease. Liposomal amphotericin B is the recommended alternative for treatment of either visceral leishmaniasis (kala azar) in India or mucosal leishmaniasis in general; the orally effective compound miltefosine is likely to see much wider use in the coming years. Intralesional treatment has also been advocated as a safer, alternative method for treating cutaneous disease (Munir et al., 2008).

Children usually tolerate the drug well, and the dose per kilogram is the same as that given to adults. Patients who respond favorably show clinical improvement within 1-2 weeks of initiation of therapy. The drug may be given on alternate days or for longer intervals if unfavorable reactions occur in especially debilitated individuals. Patients infected with HIV present a challenge because they usually relapse after successful initial therapy with either pentavalent antimonials or amphotericin B.

Toxicity and Side Effects. The toxicity of the pentavalent antimonials is best evaluated in patients without systemic disease (i.e., visceral leishmaniasis). In general, high-dose regimens of sodium stibogluconate are fairly well tolerated; toxic reactions usually are reversible, and most subside despite continued therapy. Effects noted most commonly include pain at the injection site after intramuscular administration; chemical pancreatitis in nearly all patients; elevation of serum hepatic transaminase levels; bone marrow suppression manifested by decreased red cell, white cell, and platelet counts in the blood; muscle and joint pain; weakness and malaise; headache; nausea and abdominal pain; and skin rashes. Changes in the electrocardiogram that include T-wave flattening and inversion and prolongation of the QT interval found in patients with systemic disease are uncommon in other forms of leishmaniasis (Navin et al., 1992). Reversible polyneuropathy has been reported. Hemolytic anemia and renal damage are rare manifestations of antimonial toxicity, as are shock and sudden death.

Suramin sodium (bayer 205, formerly germanin, others) is soluble in water, but solutions deteriorate quickly in air; only freshly prepared solutions should be used. In the U.S., suramin is available only from the CDC.

Antiparasitic Effects. Suramin is a relatively slowly acting trypanocide (>6 hours in vitro) with high clinical activity against both T. b. gambiense and T. b. rhodesiense. Its mechanism of action is unknown (Barrett et al., 2007). Selective toxicity is likely to result from the ability of the parasite to take up the drug by receptor-mediated endocytosis of the protein-bound drug, with low-density lipoproteins the most important interacting proteins for this event. Suramin reacts reversibly with a variety of biomolecules in vitro, inhibiting many trypanosomal and mammalian enzymes and receptors unrelated to its antiparasitic effects. Compartmentation protects many vital molecules, such as the glycolytic enzymes inside trypanosomal glycosomes, from suramin because this compound does not cross membrane barriers by passive diffusion. Suramin-treated trypanosomes exhibit damage to intracellular membrane structures other than lysosomes, but whether or not this relates to the drug's primary action is unknown. Inhibition of a trypanosomal cytosolic serine oligopeptidase may account for part of the activity of suramin, but the drug is also an inhibitor of dihydrofolate reductase, thymidine kinase, and several glycolytic enzymes. No clear consensus for the mechanism of action has emerged, and the lack of any significant field resistance points to multiple potential targets.

Absorption, Fate, and Excretion. The clinical pharmacology of suramin was determined during clinical trials for its application as an antitumor agent. Because it is not absorbed after oral intake, suramin is given intravenously to avoid local inflammation and necrosis associated with subcutaneous or intramuscular injections. After its administration, the drug displays complex pharmacokinetics with marked interindividual variability. The drug is 99.7% serum protein bound and has a terminal elimination t_1/2 of 41-78 days. Suramin is not appreciably metabolized, and renal clearance accounts for elimination of ~80% of the compound from the body. This large polar anion does not enter cells readily, and tissue concentrations are uniformly lower than those in plasma. In experimental animals the kidneys contain considerably more suramin than do other organs. Such retention may account for the fairly frequent occurrence of albuminuria following injection of the drug in humans. Very little suramin penetrates the CSF, consistent with its lack of efficacy once the CNS has been invaded by trypanosomes.

Therapeutic Uses. Suramin is the first-line therapy for early-stage T. brucei rhodesiense infection (Barrett et al., 2007; Kennedy, 2008). Although it shows activity against T. brucei gambiense, it is not used for this application because other drugs are available for the treatment of this disease that are not complicated by the high activity of suramin against Onchocerca and Brugia, and possible immunological reactions resulting from killing of these parasitic worms. Because only small amounts of the drug enter the brain, suramin is used only for the treatment of early-stage African trypanosomiasis (before CNS involvement). Suramin will clear the hemolymphatic system of trypanosomes even in late-stage disease, so it is often administered before initiating melarsoprol to reduce the risk of reactive encephalopathy associated with the administration of that arsenical. Suramin is given by slow intravenous injection as a 10% aqueous solution. Treatment of active African trypanosomiasis should not be started until 24 hours after diagnostic lumbar puncture to ensure no CNS involvement, and caution is required if the patient has onchocerciasis (river blindness) because of the potential for eliciting a Mazzotti reaction (i.e., pruritic rash, fever, malaise, lymph node swelling, eosinophilia, arthralgias, tachycardia, hypotension, and possibly permanent blindness). The normal single dose for adults with T. brucei rhodesiense infection is 1 g. It is advisable to employ a test dose of 200 mg initially to detect sensitivity, after which the normal dose is given intravenously on days 1, 3, 7, 14, and 21, although a wide range of schedules are in use. The pediatric dose is 20 mg/kg, given according to the same schedule. Patients in poor condition should be treated with lower doses during the first week. Patients who relapse after suramin therapy should be treated with melarsoprol.

Toxicity and Side Effects. Suramin can cause a variety of untoward reactions that vary in intensity and frequency and tend to be more severe in debilitated patients. Fortunately, the most serious immediate reaction consisting of nausea, vomiting, shock, and loss of consciousness is rare (~1 in 2000 patients). Malaise, nausea, and fatigue are also common immediate reactions. Parasite destruction may cause febrile episodes and skin hypersensitivity rashes that are reduced by pretreatment with glucocorticoids; concomitant onchocerciasis optimally should be treated first with ivermectin to minimize these reactions (Chapter 51). The most common problem encountered after several doses of suramin is renal toxicity, manifested by albuminuria, and delayed neurological complications, including headache, metallic taste, paresthesias, and peripheral neuropathy. These complications usually disappear spontaneously despite continued therapy. Other, less prevalent reactions include vomiting, diarrhea, stomatitis, chills, abdominal pain, and edema. Laboratory abnormalities noted in 12-26% of patients with AIDS include leukopenia and occasional agranulocytosis, thrombocytopenia, proteinuria, and elevations of plasma creatinine, transaminases, and bilirubin, which are reversible. Unexpected findings in patients with AIDS include adrenal insufficiency and vortex keratopathy.

Precautions and Contraindications. Patients receiving suramin should be followed closely. Therapy should not be continued in patients who show intolerance to initial doses, and the drug should be employed with great caution in individuals with renal insufficiency. Moderate albuminuria is common during control of the acute phase, but persisting, heavy albuminuria calls for caution as well as modification of the treatment schedule. If casts appear, treatment with suramin should be discontinued. The occurrence of palmar-plantar hyperesthesia may presage peripheral neuritis.