Several anticancer agents act through the microtubules, either causing disorganized stabilization of microtubules in areas away from the centriole or causing destabilization of the mitotic spindle, interfering with mitosis. The vinca alkaloids are effective in the treatment of hematological malignancies, breast, germ cell, and lung cancers, whereas the taxanes have become leading agents in the treatment of ovarian, breast, head and neck, and lung cancers. A new class of agents, the epothilones, resembles the taxanes in their action but has limited cross-resistance with taxanes; the only approved epothilone in the U.S., ixabepilone, is indicated for metastatic breast cancer.
History. The beneficial properties of the Madagascar periwinkle plant, Catharanthus roseus (formerly called Vinca rosea), a species of myrtle, have been described in medicinal folklore. Periwinkle extracts attracted interest because of their hypoglycemic effects in diabetes. Purified alkaloids, including vinblastine and vincristine, caused regression of an acute lymphocytic leukemia in mice and were among the earliest clinical agents for treatment of leukemias, lymphomas, and testicular cancer. A closely related derivative, vinorelbine, has important activity against lung and breast cancer.
Chemistry. The vinca alkaloids are asymmetrical dimeric compounds formed by condensation of the vindoline and catharanthine subunits.
Mechanism of Action. The vinca alkaloids are cell-cycle–specific agents and, in common with other drugs such as colchicine, podophyllotoxin, the taxanes, and the epothilones, block cells in mitosis. The biological activities of the vincas can be explained by their ability to bind specifically to β tubulin and to block its polymerization with α tubulin into microtubules.
When cells are incubated with vinblastine, the microtubules dissolve and highly regular crystals form, containing 1 mole of bound vinblastine per mole of tubulin. Cell division arrests in metaphase. In the absence of an intact mitotic spindle, duplicated chromosomes cannot align along the division plate. They disperse throughout the cytoplasm (exploded mitosis) or may clump in unusual groupings, such as balls or stars. Cells blocked in mitosis undergo changes characteristic of apoptosis.
In addition to their key role in the formation of mitotic spindles, microtubules are found in high concentration in the brain and contribute to other cellular functions such as movement, phagocytosis, and axonal transport. Side effects of the vinca alkaloids, such as their neurotoxicity, may relate to disruption of these functions.
Drug Resistance. Despite their structural similarity, the vinca alkaloids have unique individual patterns of clinical effectiveness (see the individual vinca alkaloid sections). However, in most experimental systems, they share cross-resistance. Their antitumor effects are blocked by multidrug resistance mediated by the mdr gene and its glycoprotein. Tumor cells become cross-resistant to a wide range of chemically dissimilar agents (the vinca alkaloids, epipodophyllotoxins, anthracyclines, and taxanes). Chromosomal abnormalities consistent with gene amplification have been observed in resistant cells in culture, and the cells contain markedly increased levels of the P-glycoprotein, a membrane efflux transporter (Endicott and Ling, 1989). Ca2+ channel blockers such as verapamil can reverse resistance of this type in vitro; however, clinical trials of resistance-reversing agents have been disappointing. Other membrane transporters, such as the MRP and the closely related breast cancer resistance protein, may mediate multidrug resistance. Still other forms of resistance to vinca alkaloids stem from mutations in β tubulin or in the relative expression of isoforms of β tubulin; both changes prevent the inhibitors from effectively binding to their target.
Cytotoxic Actions. The very limited myelosuppressive action of vincristine makes it a valuable component of several combination therapy regimens for leukemia and lymphoma, while the lack of severe neurotoxicity of vinblastine is a decided advantage in lymphomas and in combination with cisplatin against testicular cancer. Vinorelbine, which causes a mild neurotoxicity as well as myelosuppression, has an intermediate toxicity profile. Vincristine is a standard component of regimens for treating pediatric leukemias, lymphomas, and solid tumors, such as Wilms tumor, neuroblastoma, and rhabdomyosarcoma. In large-cell non-Hodgkin's lymphomas, vincristine remains an important agent, particularly when used in the CHOP regimen with cyclophosphamide, doxorubicin, and prednisone. Vinblastine is employed in treating bladder cancer, testicular carcinomas, and Hodgkin's disease. Vinorelbine has activity against non–small cell lung cancer and breast cancer.
Absorption, Fate, and Excretion. The liver cytochromes extensively metabolize all three agents, and the metabolites are excreted in the bile (Robieux et al., 1996). Only a small fraction of a dose (<15%) is found in the urine unchanged. In patients with hepatic dysfunction (bilirubin >3 mg/dL), a 50-75% reduction in dose of any of the vinca alkaloids is advisable, although firm guidelines for dose adjustment have not been established. The pharmacokinetics of each of the three drugs are similar, with an elimination t1/2 of 20 hours for vincristine, 23 hours for vinblastine, and 24 hours for vinorelbine.
Therapeutic Uses. Vinblastine sulfate (velban, others) is given intravenously; special precautions must be taken against subcutaneous extravasation, because this may cause painful irritation and ulceration. The drug should not be injected into an extremity with impaired circulation. After a single dose of 0.3 mg/kg of body weight, myelosuppression reaches its maximum in 7-10 days. If a moderate level of leukopenia (~3000 cells/mm3) is not attained, the weekly dose may be increased gradually by increments of 0.05 mg/kg of body weight. In regimens designed to cure testicular cancer, vinblastine is used in doses of 0.3 mg/kg every 3 weeks. Doses should be reduced by 50% for patients with plasma bilirubin >1.5 mg/dL.
One important clinical use of vinblastine is with bleomycin and cisplatin (see "Therapeutic Uses" under "Bleomycin") in the curative therapy of metastatic testicular tumors, although it has been supplanted by etoposide or ifosfamide in this disease. It is a component of the standard curative regimen for Hodgkin's disease [doxorubicin (adriamycin), bleomycin, vinblastine, and dacarbazine (ABVD)]. It also is active in Kaposi sarcoma, neuroblastoma, Langerhans cell histiocytosis, carcinoma of the breast, and choriocarcinoma.
Clinical Toxicities. The nadir of the leukopenia that follows the administration of vinblastine usually occurs within 7-10 days, after which recovery ensues within 7 days. Other toxic effects of vinblastine include mild neurological manifestations. GI disturbances including nausea, vomiting, anorexia, and diarrhea may be encountered. The syndrome of inappropriate secretion of antidiuretic hormone has been reported. Loss of hair, stomatitis, and dermatitis occur infrequently. Extravasation during injection may lead to cellulitis and phlebitis.
Therapeutic Uses. Vincristine sulfate (vincasar pfs, others) used together with glucocorticoids is the treatment of choice to induce remissions in childhood leukemia and in combination with alkylating agents and anthracycline for pediatric sarcomas; the common intravenous dosage for vincristine is 2 mg/m2 of body surface area at weekly or longer intervals. Vincristine seems to be tolerated better by children than by adults, who may experience severe, progressive neurological toxicity and require a lower dose of 1.4 mg/m2. Administration of the drug more frequently than every 7 days or at higher doses increases the toxic manifestations without proportional improvement in the response rate. Precautions also should be used to avoid extravasation during intravenous administration of vincristine. Doses should be reduced by 50% or 75% for patients with plasma bilirubin >1.5 mg/dL or >3 mg/dL, respectively.
Clinical Toxicities. The clinical toxicity of vincristine is mostly neurological. Early sensory changes do not warrant dose reduction. The more severe neurological manifestations may be avoided or reversed by either suspending therapy or reducing the dosage upon first evidence of motor dysfunction. Severe constipation, sometimes resulting in colicky abdominal pain and obstruction, may be prevented by a prophylactic program of laxatives and hydrophilic (bulk-forming) agents and usually is a problem only with doses >2 mg/m2.
Alopecia occurs in ~20% of patients given vincristine; however, the alopecia is always reversible, frequently without cessation of therapy. Modest leukopenia may follow vincristine administration. Thrombocytopenia, anemia, GI cholic and obstipation, and the syndrome of inappropriate secretion of antidiuretic hormone are less common adverse effects. Inadvertent injection of vincristine into the CSF causes a devastating and often fatal irreversible coma and seizures (Williams et al., 1983). CSF exchange has averted a fatal outcome in anecdotal reports.
Vinorelbine (navelbine, others) is administered in normal saline as an intravenous infusion over 6-10 minutes. When used alone, it is given at doses of 30 mg/m2 either weekly or for 2 out of every 3 weeks. When used with cisplatin for the treatment of non–small cell lung cancer, it is given at doses of 25 mg/m2 either weekly or for 3 out of every 4 weeks. A lower dose (20-25 mg/m2) may be required for patients who have received prior chemotherapy, and dosage adjustment is necessary for hematological toxicity. Its primary toxicity is granulocytopenia, with only modest thrombocytopenia and less neurotoxicity than other vinca alkaloids. Vinorelbine may cause allergic reactions and mild, reversible changes in liver enzymes. An oral formulation of vinorelbine is active in non–small cell lung carcinoma, and phase III studies are ongoing (Krzakowski et al., 2008). Similar to the other vincas, doses should be reduced by 50% or 75% in patients with plasma bilirubin 2.1-3 mg/dL or >3 mg/dL, respectively.
The first compound of this series, paclitaxel (taxol, others), was isolated from the bark of the Western yew tree in 1971 and presented significant problems in formulation because of its poor aqueous solubility. It was reformulated and approved by the U.S. Food and Drug Administration (FDA) in 2005 as an albumin-bound nanoparticle solution for infusion (nab-paclitaxel, abraxane). Paclitaxel and its congenic, the semisynthetic docetaxel (taxotere), exhibit unique pharmacological properties as inhibitors of mitosis, differing from the vinca alkaloids and colchicine derivatives in that they bind to a different β-tubulin site and promote rather than inhibit microtubule formation. The taxanes have a central role in the therapy of ovarian, breast, lung, GI, genitourinary, and head and neck cancers (Rowinsky and Donehower, 1995).
Chemistry. Paclitaxel is a diterpenoid compound that contains a complex eight-member taxane ring as its nucleus (Figure 61–12). The side chain linked to the taxane ring at C13 is essential for its antitumor activity. Modification of the side chain has led to identification of the more potent analog, docetaxel (Figure 61–12), which shares the same spectrum of clinical activity as paclitaxel, but differs in its toxicity. Originally purified as the parent molecule from yew bark, paclitaxel now can be obtained for commercial purposes by semisynthesis from 10-desacetylbaccatin, a precursor found in yew needles. It also has been successfully synthesized (Nicolaou et al., 1994) in a complex series of reactions. Paclitaxel has very limited water solubility and is administered in a vehicle of 50% ethanol and 50% polyethoxylated castor oil (cremophor el); this vehicle likely is responsible for a high rate of hypersensitivity reactions. Patients receiving this formulation are protected by pretreatment with a histamine H1-receptor antagonist such as diphenhydramine, an H2-receptor antagonist such as cimetidine (see Chapter 32), and a glucocorticoid such as dexamethasone (see Chapter 42).
Nab-paclitaxel is soluble in aqueous solutions and can be administered safely without prophylactic antihistamines or steroids. This form of paclitaxel has increased cellular uptake via an albumin-specific mechanism.
Docetaxel, somewhat more soluble than paclitaxel, is administered in polysorbate 80 and is associated with a lower incidence of hypersensitivity reactions than paclitaxel dissolved in cremophor. However, pretreatment with dexamethasone for 3 days starting 1 day prior to therapy is required to prevent progressive fluid retention and minimize the severity of hypersensitivity reactions.
Mechanism of Action. Interest in paclitaxel was stimulated by the drug's unique ability to promote microtubule formation at cold temperatures and in the absence of GTP. It binds specifically to the β-tubulin subunit of microtubules and antagonizes the disassembly of this key cytoskeletal protein, with the result that bundles of microtubules and aberrant structures derived from microtubules appear in the mitotic phase of the cell cycle. Arrest in mitosis follows. Cell killing is dependent on both drug concentration and duration of cell exposure. Drugs that block cell-cycle progression prior to mitosis antagonize the toxic effects of taxanes.
Drug interactions have been noted; the sequence of cisplatin preceding paclitaxel decreases paclitaxel clearance and produces greater toxicity than the opposite schedule (Rowinsky and Donehower, 1995). Paclitaxel decreases doxorubicin clearance and enhances cardiotoxicity, while docetaxel has no apparent effect on anthracycline pharmacokinetics.
In cultured tumor cells, resistance to taxanes is associated in some lines with increased expression of the mdr-1 gene and its product, P-glycoprotein; other resistant cells have β-tubulin mutations, and these latter cells may display heightened sensitivity to vinca alkaloids (Cabral, 1983). Other resistant cell lines display an increase in survivin, an anti-apoptotic factor, α aurora kinase, an enzyme that promotes completion of mitosis. The taxanes preferentially bind to the βII-tubulin subunit of microtubules; therefore, cells may become resistant by upregulating the βIII-isoform of tubulin (Ranganathan et al., 1998). The basis of clinical drug resistance is not known. Cell death occurs by apoptosis, but the effectiveness of paclitaxel against experimental tumors does not depend on an intact p53 gene product.
Preclinical studies have suggested that nab-paclitaxel has an increased antitumor effect in breast cancer and a higher intratumoral drug concentration compared to cremophor-delivered paclitaxel. The reasons are not clear but may relate to maintenance of the drug in the nanoparticle micellar system or to increased expression of SPARC [Secreted Protein, Acidic and Rich in Cysteine; aka osteonectin, a matri-cellular linkage protein expressed in pro-fibrotic states and linked to myriad pathologies [Kos and Wilding, 2010; Chlenski and Cohn, 2010]) on tumor cells, leading to an increased drug uptake.
Absorption, Fate, and Excretion. Paclitaxel is administered as a 3-hour infusion of 135-175 mg/m2 every 3 weeks or as a weekly 1-hour infusion of 80-100 mg/m2. Prolonged infusions (96 hours) also have been evaluated in different tumor histologies and are active. The drug undergoes extensive metabolism by hepatic CYPs (primarily CYP2C8 with a contribution of CYP3A4); <10% of a dose is excreted in the urine intact. The primary metabolite identified thus far is 6-OH paclitaxel, which is inactive, but multiple additional hydroxylation products are found in plasma (Cresteil et al., 1994).
Paclitaxel clearance is nonlinear and decreases with increasing dose or dose rate. In studies of 96-hour infusions of 140 mg/m2 (35 mg/m2/day), the presence of hepatic metastases >2 cm in diameter decreased clearance and led to high drug concentrations in plasma and greater myelosuppression. Paclitaxel disappears from the plasma compartment with a t1/2 of 10-14 hours and a clearance of 15-18 L/hr/m2. The critical plasma concentration for inhibiting bone marrow elements depends on duration of exposure but likely lies at ~50-100 nM (Huizing et al., 1993).
Nab-paclitaxel achieves a higher serum concentration of paclitaxel compared to cremophor-solubilized paclitaxel, but the increased clearance of nab-paclitaxel results in a similar drug exposure (Gardner et al., 2008). Nab-paclitaxel is most often administered intravenously over 30 minutes at 260 mg/m2 every 3 weeks; however, alternate dosing regimens are being evaluated. Like the other taxanes, nab-paclitaxel should not be given to patients with an absolute neutrophil count <1500 cells/mm3. Docetaxel pharmacokinetics are similar to those of paclitaxel, with an elimination t1/2 of ~12 hours. Clearance is primarily through CYP3A4- and CYP3A5-mediated hydroxylation, leading to inactive metabolites (Clarke and Rivory, 1999). In contrast to paclitaxel, the pharmacokinetics of docetaxel are linear for doses ≤115 mg/m2.
Dose reductions in patients with abnormal hepatic function have been suggested, and 50-75% doses of taxanes should be used in the presence of hepatic metastases >2 cm in size or in patients with abnormal serum bilirubin. Drugs that induce CYP2C8 or CYP3A4, such as phenytoin and phenobarbital, or those that inhibit the same cytochromes, such as antifungal imidazoles, significantly alter drug clearance and toxicity.
Paclitaxel clearance is markedly delayed by cyclosporine A and a number of other drugs employed experimentally as inhibitors of the P-glycoprotein. This inhibition may be due to a block of CYP-mediated metabolism or effects on biliary excretion of the parent drug or metabolites.
Therapeutic Uses. The taxanes have become central components of regimens for treating metastatic ovarian, breast, lung, GI, genitourinary, and head and neck cancers. In current regimens, these drugs are administered once weekly or once every 3 weeks. The appropriate use of the steroid-sparing nab-paclitaxel still is being evaluated in clinical trials; in a randomized phase III study comparing 175 mg/m2 of paclitaxel to 260 mg/m2 of nab-paclitaxel in women with metastatic breast cancer, the nab-paclitaxel arm had a higher response rate and longer time to progression compared to the paclitaxel arm (Gradishar et al., 2005).
Clinical Toxicities. Paclitaxel exerts its primary toxic effects on the bone marrow. Neutropenia usually occurs 8-11 days after a dose and reverses rapidly by days 15-21. Used with filgrastim [granulocyte-colony stimulating factor (G-CSF)], doses as high as 250 mg/m2 over 24 hours are well tolerated, and peripheral neuropathy becomes dose limiting. Many patients experience myalgias for several days after receiving paclitaxel. In high-dose schedules, or with prolonged use, a stocking-glove sensory neuropathy can be disabling, particularly in patients with underlying diabetic neuropathy or concurrent cisplatin therapy. Mucositis is prominent in 72- or 96-hour infusions and in the weekly schedule.
Hypersensitivity reactions occurred in patients receiving paclitaxel infusions of short duration (1-6 hours) but have largely been averted by pretreatment with dexamethasone, diphenhydramine, and histamine H2-receptor antagonists, as noted above. Premedication is not necessary with 96-hour infusions. Many patients experience asymptomatic bradycardia, and occasional episodes of silent ventricular tachycardia also occur and resolve spontaneously during 3- or 24-hour infusions.
Nab-paclitaxel produces increased rates of peripheral neuropathy compared to cremophor-delivered paclitaxel but rarely causes hypersensitivity reactions.
Docetaxel causes greater degrees of neutropenia than paclitaxel but less peripheral neuropathy and asthenia and less frequent hypersensitivity. Fluid retention is a progressive problem with multiple cycles of docetaxel therapy, leading to peripheral edema, pleural and peritoneal fluid, and pulmonary edema in extreme cases. Oral dexamethasone, 8 mg/day, begun 1 day prior to drug infusion and continuing for 3 days, greatly ameliorates fluid retention. In rare cases, docetaxel may cause a progressive interstitial pneumonitis, with respiratory failure supervening if the drug is not discontinued (Read et al., 2002).
Estramustine (emcyt) is a combination of estradiol coupled to normustine (nornitrogen mustard) through a carbamate link. Estramustine has weaker estrogenic and antineoplastic activity than estradiol and other alkylating agents. Although the intent of the combination was to enhance the uptake of the alkylating agent into estradiol-sensitive prostate cancer cells, estramustine does not function in vivo as an alkylating agent but rather binds to β tubulin and microtubule-associated proteins, causing microtubule disassembly and antimitotic actions.
Estramustine is used solely for the treatment of metastatic or locally advanced hormone refractory prostate cancer (Kitamura, 2001) at an initial dosage of 14 mg/kg/day in three or four divided doses.
Absorption, Fate, and Excretion. Following oral administration, at least 75% of a dose of estramustine is absorbed from the GI tract and rapidly dephosphorylated. Estramustine undergoes extensive first-pass metabolism by CYP1A2 and CYP3A4 to an active oxidized 17-keto derivative, estramustine, and to multiple inactive products; both active forms accumulate in the prostate. Some hydrolysis of the carbamate linkage occurs in the liver, releasing estradiol, estrone, and the normustine group. Estramustine and estromustine have a plasma t1/2 of 10 and 14 hours, respectively, and are excreted as inactive metabolites, mainly in the feces (Bergenheim and Henriksson, 1998). Estramustine inhibits the clearance of taxanes.
Clinical Toxicities. In addition to myelosuppression, estramustine also possesses estrogenic side effects (gynecomastia, impotence, elevated risk of thrombosis, and fluid retention), hypercalcemia, acute attacks of porphyria, impaired glucose tolerance, and hypersensitivity reactions, including angioedema.
Microtubule-damaging compounds are limited by difficulties in formulation, drug delivery, and susceptibility to multidrug resistance. A new group of microtubule-targeting drugs, the epothilones, overcomes these problems in experimental systems. Several epothilones currently are in various stages of clinical development. Ixabepilone (ixempra) is approved for breast cancer treatment. Others in the pipeline include the epothilone B analogs patupilone (EPO906) and 21-aminoepothilone B (BMS-310705), the epothilone D analog KOS-1584 (R1645), and the synthetic sagopilone.
Chemistry. The epothilones are 16-membered polyketides discovered as cytotoxic metabolites from a strain of Sorangium cellulosum, a myxobacterium originally isolated from soil on the bank of the Zambezi River in southern Africa (Gerth et al., 1996).
Six natural epothilones (A-F), and synthetic and semisynthetic analogs are in various stages of development (Lee and Swain, 2008). Most trials of epothilones to date have evaluated compounds of the subtypes A, B, and D, which differ in their functional groups at carbon 12.
Initial studies of the natural epothilone compounds A, B, and D showed good in vitro cytotoxic activity at nanomolar concentrations, epothilone B having roughly twice the potency as epothilones A and D (Lee and Swain, 2008). The early in vivo activity in animals, however, was disappointing due to instability of their lactone ring. Modification of epothilone B by substituting a nitrogen for the lactone oxygen yielded ixabepilone, which is not susceptible to esterases.
Mechanism of Action. The epothilones resemble taxanes in that they bind to β tubulin and trigger microtubule nucleation at multiple sites away from the centriole. This chaotic microtubule stabilization triggers cell-cycle arrest at the G2-M interface and apoptosis. Epothilones bind to a site distinct from that of taxanes. In colon cancer cell lines, p53 and Bax trigger apoptosis in ixabepilone-treated cells.
In vitro studies suggest that ixabepilone is less susceptible to P-glycoprotein-mediated multidrug resistance when compared to taxanes. Other mechanisms implicated in epothilone resistance include mutation of the β-tubulin binding site and upregulation of isoforms of β tubulin.
Absorption, Distribution, and Excretion. Ixabepilone is administered intravenously. Because of its minimal aqueous solubility, it is delivered in the solubilizing agent, polyoxyethylated castor oil/ethanol (cremophor el). cremophor has been implicated as the cause of infusion reactions associated with paclitaxel and with other drug formulations, but such reactions are infrequent when administration is preceded by premedication with H1 and H2 antagonists. The drug is cleared by hepatic CYPs and has a plasma t1/2 of 52 hours.
Therapeutic Uses. In a phase III study for registration, patients with metastatic breast cancer resistant to or pretreated with anthracyclines and resistant to taxanes had an improved progression-free survival of 1.6 months with ixabepilone plus capecitabine compared to capecitabine alone (p = .0003) (Thomas et al., 2007).
Ixabepilone also is indicated as monotherapy for metastatic breast cancer in patients who have previously progressed through treatment with anthracyclines, taxanes, and capecitabine.
The recommended dose of ixabepilone as monotherapy or in combination with capecitabine is 40 mg/m2 administered over 3 hours every 3 weeks. Note that because of additive myelosuppression, the phase III trial used an attenuated dose of capecitabine (2000 mg/m2) administered with ixabepilone compared to 2500 mg/m2 when capecitabine was administered alone. Patients should be premedicated with both an H1 and H2 antagonist before receiving ixabepilone to minimize hypersensitivity reactions.
The combination of ixabepilone and capecitabine is contraindicated in patients with a baseline neutrophil count <1500 cells/mm3, a platelet count <100,000 cells/mm3, serum transaminases >2.5 × ULN or bilirubin above normal. In patients receiving ixabepilone monotherapy with mild to moderate hepatic dysfunction (bilirubin <1.5 × ULN or 1.5-3 × ULN, respectively), starting doses of 32 and 20 mg/m2 are recommended due to delayed drug clearance.
Toxicities. Epothilones have toxicities similar to those of the taxanes, namely neutropenia peripheral sensory neuropathy, fatigue, diarrhea, and asthenia. Grade 3/4 peripheral sensory neuropathy was seen in 21% of patients receiving combined therapy with ixabepilone and capecitabine and in 14% of patients receiving monotherapy. Ixabepilone in combination with capecitabine causes a 68% rate of grade 3/4 neutropenia; it causes a 54% rate of grade 3/4 neutropenia when given as monotherapy.
The camptothecins are potent, cytotoxic antineoplastic agents that target the nuclear enzyme topoisomerase I. The lead compound in this class, camptothecin, was isolated from the Chinese tree Camptotheca acuminata in 1966. Initial efforts to develop the compound as a sodium salt were compromised by severe and unpredictable toxicity, principally myelosuppression and hemorrhagic cystitis. Elucidation of the mechanism of action and a better understanding of its physicochemical properties during the 1980s led to the development of more soluble and less toxic analogs. Irinotecan and topotecan, currently the only camptothecin analogs approved for clinical use, have activity in colorectal, ovarian, and small cell lung cancer.
Chemistry. All camptothecins have a fused five-ring backbone that includes a labile lactone ring (Figure 61–13). The hydroxyl group and S-conformation of the chiral center at C20 in the lactone ring are required for biological activity. Appropriate substitutions on the A and B rings of the quinoline subunit enhance water solubility and increase potency for inhibiting topoisomerase I. Topotecan [(S)-9-dimethylaminoethyl-10-hydroxycamptothecin hydrochloride] is a semisynthetic molecule with a basic dimethylamino group that increases its water solubility. Irinotecan (7-ethyl-10-[4-(1-piperidino)-1-piperidino] carbonyloxycamptothecin, or CPT-11) differs from topotecan in that it is a prodrug. The carbamate bond between the camptothecin moiety and the dibasic bispiperidine side chain at position C10 (which makes the molecule water soluble) is cleaved by a carboxylesterase to form the active metabolite, SN-38 (see Chapter 6).
Mechanism of Action. The DNA topoisomerases are nuclear enzymes that reduce torsional stress in supercoiled DNA, allowing selected regions of DNA to become sufficiently untangled and relaxed to permit replication, repair, and transcription. Two classes of topoisomerase (I and II) mediate DNA strand breakage and resealing, and both have become the target of cancer chemotherapies. Camptothecin analogs inhibit the function of topoisomerase I, while a number of different chemical entities (e.g., anthracyclines, epipodophyllotoxins, acridines) inhibit topoisomerase II. Topoisomerase I binds covalently to double-stranded DNA through a reversible transesterification reaction. This reaction yields an intermediate complex in which the tyrosine of the enzyme is bound to the 3′-phosphate end of the DNA strand, creating a single-strand DNA break. This "cleavable complex" allows for relaxation of the DNA torsional strain, either by passage of the intact single-strand through the nick or by free rotation of the DNA about the noncleaved strand. Once the DNA torsional strain has been relieved, the topoisomerase I reseals the cleavage and dissociates from the newly relaxed double helix.
The camptothecins bind to and stabilize the normally transient DNA-topoisomerase I cleavable complex. Although the initial cleavage action of topoisomerase I is not affected, the re-ligation step is inhibited, leading to the accumulation of single-stranded breaks in DNA. These lesions are reversible and not by themselves toxic to the cell. However, the collision of a DNA replication fork with this cleaved strand of DNA causes an irreversible double-strand DNA break, ultimately leading to cell death (Tsao et al., 1993). Camptothecins are therefore S phase–specific drugs, because ongoing DNA synthesis is necessary for cytotoxicity. This has important clinical implications. S phase–specific cytotoxic agents generally require prolonged exposures of tumor cells to drug concentrations above a minimum threshold to optimize therapeutic efficacy. In fact, preclinical studies of low-dose, protracted administration of camptothecin analogs have less toxicity, and equal or greater antitumor activity, than shorter, more intense courses.
The precise sequence of events that leads from drug-induced DNA damage to cell death has not been fully elucidated. In vitro, camptothecin-induced DNA damage abolishes the activation of the p34cdc2/cyclin B complex, leading to cell-cycle arrest at the G2 phase (Tsao et al., 1993). Treatment with camptothecins can induce the transcription of c-fos and c-jun early-response genes, and this occurs in association with internucleosomal DNA fragmentation, a characteristic of programmed cell death.
Mechanisms of Resistance. A variety of mechanisms of resistance to topoisomerase I–targeted agents have been characterized in vitro, although little is known about their significance in the clinical setting. Decreased intracellular drug accumulation may underlie resistance in cell lines. Topotecan, but not SN-38 or irinotecan, is a substrate for P-glycoprotein. However, the clinical relevance of P-glycoprotein-mediated efflux as a mechanism of resistance against topotecan remains unclear, as the magnitude of the effect in preclinical studies was found to be substantially lower than that observed with other MDR substrates, such as etoposide or doxorubicin. Other reports have associated topotecan and irinotecan resistance with the MRP class of transporters (Miyake et al., 1999). Cell lines that lack carboxylesterase activity demonstrate resistance to irinotecan (Van Ark-Otte et al., 1998), but in patients, the liver and red blood cells may have sufficient carboxylesterase activity to convert irinotecan to SN-38. Camptothecin resistance also may result from decreased expression or mutation of topoisomerase I. Although a good correlation has been found in certain tumor cell lines between sensitivity to camptothecin analogs and topoisomerase I levels (Sugimoto et al., 1990), clinical studies have not confirmed this association. Chromosomal deletions or hypermethylation of the topoisomerase I gene are possible mechanisms of decreased topoisomerase I expression in resistant cells. A transient downregulation of topoisomerase I has been demonstrated following prolonged exposure to camptothecins in vitro and in vivo. Mutations leading to reduced topoisomerase I enzyme catalytic activity or DNA-binding affinity have been associated with experimental camptothecin resistance (Tamura et al., 1991). In addition, enzyme phosphorylation or polyADP ribosylation may reduce the activity of topoisomerase I and its susceptibility to inhibition. Finally, exposure of cells to topoisomerase I–targeted agents upregulates topoisomerase II, an alternative enzyme for DNA strand passage.
Very little is known about how the cell deals with the stabilized DNA-topoisomerase complexes. Cellular repair processes may not readily recognize the drug-enzyme-DNA complex. However, an enzyme with specific tyrosyl-DNA phosphodiesterase activity may be involved in the disassembly of topoisomerase I–DNA complexes (Yang et al., 1996).
Absorption, Fate, and Excretion.
Topotecan. Topotecan is approved for intravenous administration. However, there has been interest in developing an oral dosage form for the drug, which has a bioavailability of 30-40% in cancer patients. Topotecan exhibits linear pharmacokinetics, and it is rapidly eliminated from systemic circulation. The biological t1/2 of total topotecan, which ranges from 3.5-4.1 hours, is relatively short compared to that of other camptothecins. Only 20-35% of the total drug in plasma is found to be in the active lactone form. Within 24 hours, 30-40% of the administered dose appears in the urine. Doses should be reduced in proportion to reductions in CrCl. Although several oxidative metabolites have been identified, hepatic metabolism appears to be a relatively minor route of drug elimination. Unlike most other camptothecins considered for clinical development, plasma protein binding of topotecan is low, at only 7-35%, which may explain its relatively greater CNS penetration.
Irinotecan. The conversion of irinotecan to SN-38 is mediated predominantly by carboxylesterases in the liver. Although SN-38 can be measured in plasma shortly after beginning an intravenous infusion of irinotecan, the AUC of SN-38 is only ~4% of the AUC of irinotecan, suggesting that only a relatively small fraction of the dose is ultimately converted to the active form of the drug. Irinotecan exhibits linear pharmacokinetics at doses evaluated in cancer patients. In comparison to topotecan, a relatively large fraction of both irinotecan and SN-38 are present in plasma as the biologically active intact lactone form. Another potential advantage of this analog is that the t1/2 of SN-38 is 11.5 hours, which is much longer than the t1/2 of topotecan. CSF penetration of SN-38 in humans has not been characterized yet, although in rhesus monkeys, it is only 14%, significantly lower than that observed for topotecan.
In contrast to topotecan, hepatic metabolism represents an important route of elimination for both irinotecan and SN-38. Oxidative metabolites have been identified in plasma, all of which result from CYP3A-mediated reactions directed at the bispiperidine side chain. These metabolites are not significantly converted to SN-38. The total body clearance of irinotecan was found to be two times greater in brain cancer patients taking antiseizure drugs that induce hepatic CYPs, further attesting to the importance of oxidative hepatic metabolism as a route of elimination for this drug (Gilbert et al., 2003).
Glucuronidation of the hydroxyl group at position C10 (resulting from cleavage of the bispiperidine promoiety) produces the only known metabolite of SN-38. Biliary excretion appears to be the primary elimination route of irinotecan, SN-38, and their metabolites, although urinary excretion also contributes significantly (14-37%). Uridine diphosphate-glucuronosyltransferase 1A1 (UGT1A1), converts SN-38 to its inactive derivative (Iyer et al., 1998). The extent of SN-38 glucuronidation inversely correlates with the risk of severe diarrhea after irinotecan therapy. UGT1A1 also glucuronidates bilirubin. Polymorphisms of this enzyme are associated with familial hyperbilirubinemia syndromes such as Crigler-Najjar syndrome and Gilbert syndrome. Crigler-Najjar syndrome is rare (one in a million births), but Gilbert syndrome occurs in up to 15% of the general population and results in a mild hyperbilirubinemia that may be clinically silent. The presence of UGT enzyme polymorphisms may have a major impact on the clinical use of irinotecan. A positive correlation has been found between baseline serum unconjugated bilirubin concentration and both severity of neutropenia and the AUC of irinotecan and SN-38 in patients treated with irinotecan. Moreover, severe irinotecan toxicity has been observed in cancer patients with Gilbert syndrome, presumably due to decreased glucuronidation of SN-38. The presence of bacterial glucuronidase in the intestinal lumen potentially can contribute to irinotecan's GI toxicity by releasing unconjugated SN-38 from the inactive glucuronide metabolite excreted in the bile.
Topotecan. Topotecan (hycamtin) is indicated for previously treated patients with ovarian and small cell lung cancer. Its significant hematological toxicity has limited its use in combination with other active agents in these diseases (e.g., cisplatin).
The recommended dosing regimen of topotecan for ovarian cancer and small cell lung cancer is a 30-minute infusion of 1.5 mg/m2/day for 5 consecutive days every 3 weeks. For cervical cancer in conjunction with cisplatin, the dose of topotecan is 0.75 mg/m2 on days 1, 2, and 3, repeated every 21 days. Because a significant fraction of the topotecan administered is excreted in the urine, patients with decreased CrCl may experience increased toxicity (O'Reilly et al., 1996). Therefore, the dose of topotecan should be reduced to 0.75 mg/m2/day in patients with moderate renal dysfunction (CrCl of 20-40 mL/min), and topotecan should not be administered to patients with severe renal impairment (CrCl <20 mL/min). Hepatic dysfunction does not alter topotecan clearance and toxicity. A baseline neutrophil count >1500 cells/mm3 and a platelet count >100,000 is necessary prior to topotecan administration. For small cell lung cancer, oral therapy can be used at a dosage of 2.3 mg/m2/day for 5 consecutive days repeated every 21 days. The oral dose is reduced to 1.8 mg/m2 for patients with a CrCl of 30-49 mL/min.
Irinotecan. Approved single-agent dosage schedules of irinotecan (camptosar, others) in the U.S. include 125 mg/m2 as a 90-minute infusion administered weekly (on days 1, 8, 15, and 22) for 4 out of 6 weeks, and 350 mg/m2 given every 3 weeks. In patients with advanced colorectal cancer, irinotecan is used as first-line therapy in combination with fluoropyrimidines or as a single agent or in combination with cetuximab following failure of a 5-FU/oxaliplatin regimen.
Topotecan. The dose-limiting toxicity with all dosing schedules is neutropenia, with or without thrombocytopenia. The incidence of severe neutropenia at the recommended phase II dose of 1.5 mg/m2 daily for 5 days every 3 weeks may be as high as 81%, with a 26% incidence of febrile neutropenia. In patients with hematological malignancies, GI side effects such as mucositis and diarrhea become dose limiting. Other less common and generally mild topotecan-related toxicities include nausea and vomiting, elevated liver transaminases, fever, fatigue, and rash.
Irinotecan. The dose-limiting toxicity with all dosing schedules is delayed diarrhea, with or without neutropenia. In the initial studies, up to 35% of patients experienced severe diarrhea. Adoption of an intensive regimen of loperamide (4 mg of loperamide starting at the onset of any loose stool beginning more than a few hours after receiving therapy, followed by 2 mg every 2 hours) (see Chapter 47) has effectively reduced this incidence by more than half. However, once severe diarrhea occurs, standard doses of antidiarrheal agents tend to be ineffective. Diarrhea generally resolves within a week and, unless associated with fever and neutropenia, rarely is fatal.
The second most common irinotecan-associated toxicity is myelosuppression. Severe neutropenia occurs in 14-47% of the patients treated with the every-3-weeks schedule and is less frequently encountered among patients treated with the weekly schedule. Febrile neutropenia is observed in 3% of patients and may be fatal, particularly when associated with concomitant diarrhea. A cholinergic syndrome resulting from the inhibition of acetylcholinesterase activity by irinotecan may occur within the first 24 hours after irinotecan administration. Symptoms include acute diarrhea, diaphoresis, hypersalivation, abdominal cramps, visual accommodation disturbances, lacrimation, rhinorrhea, and less often, asymptomatic bradycardia. These effects are short lasting and respond within minutes to atropine. Atropine may be prophylactically administered to patients who have previously experienced a cholinergic reaction. Other common and generally manageable toxicities include nausea and vomiting, fatigue, vasodilation or skin flushing, mucositis, elevation in liver transaminases, and alopecia. Finally, there have been case reports of dyspnea and interstitial pneumonitis associated with irinotecan therapy in Japanese patients with lung cancer (Fukuoka et al., 1992).
Chemical structures of camptothecin and its analogs.
The first anticancer antibiotics were the series of actinomycins discovered by Waksman and colleagues in 1940. The most important of these, actinomycin D, has beneficial effects in the treatment of solid tumors in children and choriocarcinoma in adult women.
Chemistry and Structure-Activity Relationships. The actinomycins are chromopeptides. Most contain the same chromophore, the planar phenoxazone actinosin, which is responsible for their yellow-red color. The differences among naturally occurring actinomycins are confined to variations in the structure of the amino acids of the peptide side chains.
Mechanism of Action. The capacity of actinomycins to bind with double-helical DNA is responsible for their biological activity and cytotoxicity. X-ray studies of a crystalline complex between dactinomycin and deoxyguanosine permitted formulation of a model that explains the binding of the drug to DNA (Sobell, 1973). The planar phenoxazone ring intercalates between adjacent guanine–cytosine base pairs of DNA, while the polypeptide chains extend along the minor groove of the helix. The summation of these interactions provides great stability to the dactinomycin-DNA complex, and as a result of the binding of dactinomycin, the transcription of DNA by RNA polymerase is blocked. The DNA-dependent RNA polymerases are much more sensitive to the effects of dactinomycin than are the DNA polymerases. In addition, dactinomycin causes single-strand breaks in DNA, possibly through a free-radical intermediate or as a result of the action of topoisomerase II.
Cytotoxic Action. Dactinomycin inhibits rapidly proliferating cells of normal and neoplastic origin and, on a molar basis, is among the most potent antitumor agents known. The drug may produce alopecia and, when extravasated subcutaneously, causes marked local inflammation. Erythema, sometimes progressing to necrosis, has been noted in areas of the skin exposed to X-ray radiation before, during, or after administration of dactinomycin.
Absorption, Fate, and Excretion. Dactinomycin is administered by intravenous injection. Metabolism of the drug is minimal. The drug is excreted in both bile and urine and disappears from plasma with a terminal t1/2 of 36 hours. Dactinomycin does not cross the blood-brain barrier.
Therapeutic Uses. A wide variety of single-agent and combination chemotherapy regimens with dactinomycin (actinomycin D; cosmegen) are employed. The usual daily dose of dactinomycin is 10-15 μg/kg; this is given intravenously for 5 days; if no manifestations of toxicity are encountered, additional courses may be given at intervals of 2-4 weeks. In other regimens, 3-6 μg/kg/day, for a total of 125 μg/kg, and weekly maintenance doses of 7.5 μg/kg have been used. If infiltrated during administration, the drug is extremely corrosive to soft tissues.
The most important clinical use of dactinomycin is in the treatment of rhabdomyosarcoma and Wilms tumor in children, where it is curative in combination with primary surgery, radiotherapy, and other drugs, particularly vincristine and cyclophosphamide. Ewing, Kaposi, and soft-tissue sarcomas also respond. Dactinomycin and methotrexate form a curative therapy for choriocarcinoma.
Clinical Toxicities. Toxic manifestations include anorexia, nausea, and vomiting, usually beginning a few hours after administration. Hematopoietic suppression with pancytopenia may occur in the first week after completion of therapy. Proctitis, diarrhea, glossitis, cheilitis, and ulcerations of the oral mucosa are common; dermatological manifestations include alopecia, as well as erythema, desquamation, and increased inflammation and pigmentation in areas previously or concomitantly subjected to X-ray radiation. Severe injury may occur as a result of local drug extravasation.
Anthracyclines and Anthracenediones
Anthracyclines are derived from the fungus Streptomyces peucetius var. caesius. Idarubicin and epirubicin are analogs of the naturally produced anthracyclines doxorubicin and daunorubicin, differing only slightly in chemical structure, but having somewhat distinct patterns of clinical activity. Daunorubicin and idarubicin primarily have been used in the acute leukemias, whereas doxorubicin and epirubicin display broader activity against human solid tumors. These agents, which all possess potential for generating free radicals, cause an unusual and often irreversible cardiomyopathy, the occurrence of which is related to the total dose of the drug. The structurally similar agent mitoxantrone has useful activity against prostate cancer and AML, and is used in high-dose chemotherapy, but has less cardiotoxicity.
Chemistry. The anthracycline antibiotics have a tetracyclic ring structure attached to an unusual sugar, daunosamine. Cytotoxic agents of this class all have quinone and hydroquinone moieties on adjacent rings that permit the gain and loss of electrons. Although there are marked differences in the clinical uses of daunorubicin and doxorubicin, their chemical structures differ only by a single hydroxyl group on C-14 (substituent R4 on the diagram below). Idarubicin is 4-demethoxydaunorubicin (alteration in substituent R1), a synthetic derivative of daunorubicin; epirubicin is an epimer at the 4′ position of the sugar. Mitoxantrone, an anthracenedione, lacks a glycosidic side group.
Mechanism of Action. A number of important biochemical effects have been described for the anthracyclines and anthracenediones, all of which could contribute to their therapeutic and toxic effects. These compounds can intercalate with DNA, directly affecting transcription and replication. A more important action is the ability to form a tripartite complex with topoisomerase II and DNA. Topoisomerase II is an ATP-dependent enzyme that binds to DNA and produces double-strand breaks at the 3′-phosphate backbone, allowing strand passage and uncoiling of super-coiled DNA. Following strand passage, topoisomerase II re-ligates the DNA strands. This enzymatic function is essential for DNA replication and repair. Formation of the tripartite complex with anthracyclines or with etoposide inhibits the re-ligation of the broken DNA strands, leading to apoptosis. Defects in DNA double-strand break repair sensitize cells to damage by these drugs, while overexpression of transcription-linked DNA repair may contribute to resistance.
Anthracyclines, by virtue of their quinone groups, also generate free radicals in solution and in both normal and malignant tissues (Myers, 1988). Anthracyclines can form semiquinone radical intermediates that can react with O2 to produce superoxide anion radicals. These can generate both hydrogen peroxide and hydroxyl radicals, which attack DNA (Serrano et al., 1999) and oxidize DNA bases. The production of free radicals is significantly stimulated by the interaction of doxorubicin with iron (Myers, 1988). Enzymatic defenses such as superoxide dismutase and catalase protect cells against the toxicity of the anthracyclines, and these defenses can be augmented by exogenous antioxidants such as alpha tocopherol or by an iron chelator, dexrazoxane (zinecard, others), which protects against cardiac toxicity (Swain et al., 1997).
Exposure of cells to anthracyclines leads to apoptosis; mediators of this process include the p53 DNA-damage sensor and activated caspases (proteases), although ceramide, a lipid breakdown product, and the Fas receptor-ligand system also have been implicated (Friesen et al., 1996).
As discussed in "Drug Resistance" under "Vinca Alkaloids," multidrug resistance is observed in tumor cell populations exposed to anthracyclines. Attempts to reverse or prevent the emergence of resistance through the simultaneous use of inhibitors of the P-glycoprotein (Ca++ channel blockers, steroidal compounds, and others) have yielded inconclusive results, primarily due to confounding effects of these inhibitors on anthracycline pharmacokinetics and metabolism. Anthracyclines also are exported from tumor cells by members of the MRP transporter family and by the breast cancer resistance protein, a "half" transporter (Doyle et al., 1998). Other biochemical changes in resistant cells include increased glutathione peroxidase activity, decreased activity or mutation of topoisomerase II, and enhanced ability to repair DNA strand breaks.
Absorption, Fate, and Excretion. Daunorubicin, doxorubicin, epirubicin, and idarubicin usually are administered intravenously and are cleared by a complex pattern of hepatic metabolism and biliary excretion. The plasma disappearance curves for doxorubicin and daunorubicin are multiphasic, with a terminal t1/2 of 30 hours. All anthracyclines are converted to an active alcohol intermediate that plays a variable role in their therapeutic activity. Idarubicin has a t1/2 of 15 hours, and its active metabolite, idarubicinol, has a t1/2 of 40 hours. The drugs rapidly enter the heart, kidneys, lungs, liver, and spleen. They do not cross the blood-brain barrier.
Daunorubicin and doxorubicin are eliminated by metabolic conversion to a variety of aglycones and other inactive products. Idarubicin is primarily metabolized to idarubicinol, which accumulates in plasma and likely contributes significantly to its activity. Clearance of anthracyclines and their active alcohol metabolites is delayed in the presence of hepatic dysfunction, and at least a 50% initial reduction in dose should be considered in patients with abnormal serum bilirubin levels (Twelves et al., 1998).
Idarubicin. The recommended dosage for idarubicin (idamycin pfs) is 12 mg/m2/day for 3 days by intravenous injection in combination with cytarabine. Slow injection over 10-15 minutes is recommended to avoid extravasation, as with other anthracyclines. It has less cardiotoxicity than the other anthracyclines.
Daunorubicin. Daunorubicin (daunomycin, rubidomycin; cerubidine, others) is available for intravenous use. The recommended dosage is 25-45 mg/m2/day for 3 days. The agent is administered with appropriate care to prevent extravasation, because severe local vesicant action may result. Total doses of >1000 mg/m2 are associated with a high risk of cardiotoxicity. Patients should be advised that daunorubicin may impart a red color to the urine.
Daunorubicin and idarubicin also are used in the treatment of AML in combination with Ara-C.
Clinical Toxicities. The toxic manifestations of daunorubicin as well as idarubicin include bone marrow depression, stomatitis, alopecia, GI disturbances, and rash. Cardiac toxicity is a peculiar adverse effect observed with these agents. It is characterized by tachycardia, arrhythmias, dyspnea, hypotension, pericardial effusion, and congestive heart failure poorly responsive to digitalis (see "Clinical Toxicities" under "Doxorubicin").
Therapeutic Uses. Doxorubicin is available for intravenous use. The recommended dose is 60-75 mg/m2, administered as a single rapid intravenous infusion that is repeated after 21 days. Care should be taken to avoid extravasation, because severe local vesicant action and tissue necrosis may result. A doxorubicin liposomal product (doxil) is available for treatment of AIDS-related Kaposi sarcoma and is given intravenously in a dose of 20 mg/m2 over 60 minutes and repeated every 3 weeks. The liposomal formulation also is approved for ovarian cancer at a dose of 50 mg/m2 every 4 weeks and as a treatment for multiple myeloma (in conjunction with bortezomib), where it is given as a 30-mg/m2 dose on day 4 of each 21-day cycle. Patients should be advised that the drug may impart a red color to the urine.
Doxorubicin is effective in malignant lymphomas. In combination with cyclophosphamide, vinca alkaloids, and other agents, it is an important ingredient for the successful treatment of lymphomas. It is a valuable component of various regimens of chemotherapy for adjuvant and metastatic carcinoma of the breast. The drug also is particularly beneficial in pediatric and adult sarcomas, including osteogenic, Ewing, and soft-tissue sarcomas.
Clinical Toxicities. The toxic manifestations of doxorubicin are similar to those of daunorubicin. Myelosuppression is a major dose-limiting complication, with leukopenia usually reaching a nadir during the second week of therapy and recovering by the fourth week; thrombocytopenia and anemia follow a similar pattern but usually are less pronounced. Stomatitis, mucositis, diarrhea, and alopecia are common but reversible. Erythematous streaking near the site of infusion ("adriamycin flare") is a benign local allergic reaction and should not be confused with extravasation. Facial flushing, conjunctivitis, and lacrimation may occur rarely. The drug may produce severe local toxicity in irradiated tissues (e.g., the skin, heart, lung, esophagus, and GI mucosa) even when the two therapies are not administered concomitantly.
Cardiomyopathy is the most important long-term toxicity. Two types of cardiomyopathies may occur:
An acute form is characterized by abnormal electrocardiographic changes, including ST and T-wave alterations and arrhythmias. This is brief and rarely a serious problem. An acute reversible reduction in ejection fraction is observed in some patients in the 24 hours after a single dose, and plasma troponin T, a cardiac enzyme released with myocardial damage, may increase in a minority of patients in the first few days following drug administration (Lipshultz et al., 2004). Acute myocardial damage, the "pericarditis–myocarditis syndrome," may begin in the days following drug infusion and is characterized by severe disturbances in impulse conduction and frank congestive heart failure, often associated with pericardial effusion.
Chronic, cumulative dose-related toxicity (usually total doses of ≥550 mg/m2) progress to congestive heart failure. The mortality rate in patients with congestive failure approaches 50%. Total doses of doxorubicin as low as 250 mg/m2 can cause pathological changes in the myocardium, as demonstrated by subendocardial biopsies. Nonspecific alterations, including a decrease in the number of myocardial fibrils, mitochondrial changes, and cellular degeneration, are visible by electron microscopy. The most promising noninvasive techniques used to detect the early development of drug-induced congestive heart failure are radionuclide cineangiography, which assesses ejection fraction, and echocardiography, which reveals abnormalities in contractility and ventricular dimensions. Sequential echocardiograms have detected structural abnormalities in 25% of children who received up to 300 mg/m2 of doxorubicin, although <10% have clinical manifestations of cardiac disease in long-term follow-up. Although no completely practical and reliable predictive tests are available, the frequency of clinically apparent cardiomyopathy is 1-10% at total doses <450 mg/m2. The risk increases markedly, with estimates as high as 20% at total doses of 550 mg/m2. This total dosage should be exceeded only under exceptional circumstances or with the concomitant use of dexrazoxane, a cardioprotective iron-chelating agent that appears not to compromise the anticancer activity of the drug (Swain et al., 1997). Cardiac irradiation, administration of high doses of cyclophosphamide or another anthracycline, or concomitant trastuzumab (Slamon et al., 2001) increases the risk of cardiotoxicity. Late-onset cardiac toxicity, with congestive heart failure years after treatment, may occur in both pediatric and adult populations. In children treated with anthracyclines, there is a 3- to 10-fold elevated risk of arrhythmias, congestive heart failure, and sudden death in adult life. A total dose limit of 300 mg/m2 is advised for pediatric cases. Concomitant administration of dexrazoxane may reduce troponin T elevations and avert later cardiotoxicity (Lipshultz et al., 2004).
Epirubicin (ellence, others)
This anthracycline is indicated as a component of adjunctive therapy for treatment of breast cancer. It is administered in doses of 100-120 mg/m2 intravenously every 3-4 weeks. Total doses >900 mg/m2 sharply increase the risk of cardiotoxicity. Its toxicity profile is the same as that of doxorubicin.
Valrubicin is a semi-synthetic analog of doxirubicin, used exclusively for intravesicular treatment of bladder cancer. Eight hundred mg are instilled into the bladder once a week for 6 weeks. Less than 10% of instilled drug is absorbed systemically. Side effects relate to bladder irritation (Kuznetsov et al., 2001).
Mitoxantrone has been approved for use in AML, prostate cancer, and late-stage, secondary progressive multiple sclerosis. Mitoxantrone has limited ability to produce quinone-type free radicals and causes less cardiac toxicity than does doxorubicin. It produces acute myelosuppression, cardiac toxicity, and mucositis as its major toxicities; the drug causes less nausea, vomiting, and alopecia than does doxorubicin.
Mitoxantrone (novantrone, others) is administered by intravenous infusion. To induce remission in acute nonlymphocytic leukemia in adults, the drug is given in a daily dose of 12 mg/m2 for 3 days with cytarabine. It also is used in advanced hormone-resistant prostate cancer in a dose of 12-14 mg/m2 every 21 days.
Podophyllotoxin, extracted from the mandrake plant (mayapple; Podophyllum peltatum), was used as a folk remedy by the American Indians and early colonists for its emetic, cathartic, and anthelmintic effects. Two synthetic derivatives have significant therapeutic activity in pediatric leukemia, small cell carcinomas of the lung, testicular tumors, Hodgkin's disease, and large cell lymphomas.
These derivatives, shown below, are etoposide (VP-16-213) and teniposide (VM-26). Although podophyllotoxin binds to tubulin, etoposide and teniposide have no effect on microtubular structure or function at usual concentrations.
Mechanism of Action. Etoposide and teniposide are similar in their actions and in the spectrum of human tumors affected. Like the anthracyclines, they form a ternary complex with topoisomerase II and DNA and prevent resealing of the break that normally follows topoisomerase binding to DNA. The enzyme remains bound to the free end of the broken DNA strand, leading to an accumulation of DNA breaks and cell death. Cells in the S and G2 phases of the cell cycle are most sensitive to etoposide and teniposide. Resistant cells demonstrate 1) amplification of the mdr-1 gene that encodes the P-glycoprotein drug efflux transporter, 2) mutation or decreased expression of topoisomerase II, or 3) mutations of the p53 tumor suppressor gene, a required component of the apoptotic pathway (Lowe et al., 1993).
Absorption, Fate, and Excretion. Oral administration of etoposide results in variable absorption that averages ~50%. After intravenous injection, peak plasma concentrations of 30 μg/mL are achieved; there is a biphasic pattern of clearance with a terminal t1/2 of ~6-8 hours in patients with normal renal function. Approximately 40% of an administered dose is excreted intact in the urine. In patients with compromised renal function, dosage should be reduced in proportion to the reduction in CrCl (Arbuck et al., 1986). In patients with advanced liver disease, increased toxicity may result from a low serum albumin (decreased drug binding) and elevated bilirubin (which displaces etoposide from albumin). However, guidelines for dose reduction in this circumstance have not been defined. Drug concentrations in the CSF average 1-10% of those in plasma.
Therapeutic Uses. The intravenous dose of etoposide (vepesid, others) for testicular cancer in combination therapy is 50-100 mg/m2 for 5 days, or 100 mg/m2 on alternate days for three doses. For small cell carcinoma of the lung, the dosage in combination therapy is 35 mg/m2/day intravenously for 4 days or 50 mg/m2/day intravenously for 5 days. The oral dose for small cell lung cancer is twice the IV dose. Cycles of therapy usually are repeated every 3-4 weeks. When given intravenously, the drug should be administered slowly over a 30- to 60-minute period to avoid hypotension and bronchospasm, which likely result from the additives used to dissolve etoposide, a relatively insoluble compound.
A disturbing complication of etoposide therapy has emerged in long-term follow-up of patients with childhood acute lymphoblastic leukemia, who develop an unusual form of acute nonlymphocytic leukemia with a translocation in chromosome 11q23. At this locus is a gene (the MLL gene) that regulates the proliferation of pluripotent stem cells. The leukemic cells have the cytological appearance of acute monocytic or monomyelocytic leukemia but may express lymphoid surface markers. Another distinguishing feature of etoposide-related leukemia is the short time interval between the end of treatment and the onset of leukemia (1-3 years), compared to the 4- to 5-year interval for secondary leukemias related to alkylating agents, and the absence of a myelodysplastic period preceding leukemia (Pui et al., 1995). Patients receiving weekly or twice-weekly doses of etoposide, with cumulative doses >2000 mg/m2, seem to be at higher risk of leukemia.
Etoposide primarily is used for treatment of testicular tumors, in combination with bleomycin and cisplatin, and in combination with cisplatin and ifosfamide for small cell carcinoma of the lung. It also is active against non-Hodgkin's lymphomas, acute nonlymphocytic leukemia, and Kaposi sarcoma associated with acquired immunodeficiency syndrome (AIDS). Etoposide has a favorable toxicity profile for dose escalation in that its primary acute toxicity is myelosuppression. In combination with ifosfamide and carboplatin, it frequently is used for high-dose chemotherapy in total doses of 1500-2000 mg/m2 (Josting et al., 2000).
Clinical Toxicities. The dose-limiting toxicity of etoposide is leukopenia, with a nadir at 10-14 days and recovery by 3 weeks. Thrombocytopenia occurs less often and usually is not severe. Nausea, vomiting, stomatitis, and diarrhea complicate treatment in ~15% of patients. Alopecia is a common but reversible adverse effect. Hepatic toxicity is particularly evident after high-dose treatment. For both etoposide and teniposide, toxicity increases in patients with decreased serum albumin, an effect related to decreased protein binding of the drug.
Teniposide (vumon) is administered intravenously. It has a multiphasic pattern of clearance from plasma; after distribution, a t1/2 of 4 hours and another t1/2 of 10-40 hours are observed. Approximately 45% of the drug is excreted in the urine, but in contrast to etoposide, as much as 80% is recovered as metabolites. Anticonvulsants such as phenytoin increase the hepatic metabolism of teniposide and reduce systemic exposure (Baker et al., 1992). Dosage need not be reduced for patients with impaired renal function. Less than 1% of the drug crosses the blood-brain barrier. Teniposide is available for treatment of refractory ALL in children and is synergistic with cytarabine. It is administered by intravenous infusion in dosages that range from 50 mg/m2/day for 5 days to 165 mg/m2/day twice weekly. The drug has limited utility and primarily is given for acute leukemia in children and monocytic leukemia in infants, as well as glioblastoma, neuroblastoma, and brain metastases from small cell carcinomas of the lung. Myelosuppression, nausea, and vomiting are its primary toxic effects.
DRUGS OF DIVERSE MECHANISMOF ACTION
The bleomycins, a unique group of DNA-cleaving antibiotics, were discovered by Umezawa and colleagues as fermentation products of Streptomyces verticillus. The drug currently employed clinically is a mixture of the two copper-chelating peptides, bleomycins A2 and B2. The various bleomycins differ only in their terminal amino acid (Figure 61–14).
Bleomycins have attracted interest because of their significant antitumor activity against both Hodgkin's lymphoma and testicular tumors. They are minimally myelo- and immunosuppressive but cause unusual cutaneous side effects and pulmonary fibrosis. Because their toxicities do not overlap with those of other cytotoxic drugs, and because of their unique mechanism of action, bleomycin maintains an important role in treating Hodgkin's disease and testicular cancer.
Chemistry. The bleomycins are water-soluble, basic glycopeptides (Figure 61–14). The core of the bleomycin molecule assumes a metal-binding cage consisting of a pyrimidine chromophore linked to propionamide, a β-aminoalanine amide side chain, and the sugars, l-gulose and 3-O-carbamoyl-d-mannose. Bound in this complex are either Fe2+ or Cu2+. Attached to the metal ion binding core are a tripeptide chain and a terminal, DNA-binding bithiazole carboxylic acid.
Mechanism of Action. Bleomycin's cytotoxicity results from its ability to cause oxidative damage to the deoxyribose of thymidylate and other nucleotides, leading to single and double-stranded breaks in DNA. Studies in vitro indicate that bleomycin causes accumulation of cells in the G2 phase of the cell cycle, and many of these cells display chromosomal aberrations, including chromatid breaks, gaps, and fragments, as well as translocations (Twentyman, 1983).
Bleomycin cleaves DNA by generating free radicals. In the presence of O2 and a reducing agent, such as dithiothreitol, the metal–drug complex becomes activated and functions as a ferrous oxidase, transferring electrons from Fe2+ to molecular oxygen to produce oxygen radicals (Burger, 1998). Metallobleomycin complexes can be activated by reaction with the flavin enzyme, NADPH-cytochrome P450 reductase. Bleomycin binds to DNA, and the activated complex generates free radicals that are responsible for abstraction of a proton at the 3′ position of the deoxyribose backbone of the DNA chain, opening the deoxyribose ring and generating a strand break in DNA. The process for repair of this break is poorly understood, but an excess of breaks generates apoptosis.
Bleomycin is degraded by a specific hydrolase found in various normal tissues, including liver. Hydrolase activity is low in skin and lung, perhaps contributing to the serious toxicity at those sites. Some bleomycin-resistant cells contain high levels of hydrolase activity (Sebti et al., 1991). In other cell lines, resistance has been attributed to decreased uptake, repair of strand breaks, or drug inactivation by thiols or thiol-rich proteins. A polymorphism of the hydrolase gene, SNP A1450G, has been identified in 10% of patients with testicular cancer, and the G/G genotype is associated with a 20% decreased survival in patients treated with bleomycin combination therapy, suggesting that this single nucleotide polymorphism is associated with increased hydrolase activity (de Haas et al., 2008).
Absorption, Fate, and Excretion. Bleomycin is administered intravenously, intramuscularly, or subcutaneously or instilled into the bladder for local treatment of bladder cancer. After intravenous infusion, relatively high drug concentrations are detected in the skin and lungs of experimental animals, and these organs become major sites of toxicity. Having a high molecular mass, bleomycin crosses the blood-brain barrier poorly.
After intravenous administration of a bolus dose of 15 mg/m2, peak concentrations of 1-5 mg/mL are achieved in plasma. The half-time for elimination is ~3 hours. About two-thirds of the drug are excreted intact in the urine. Concentrations in plasma are greatly elevated if usual doses are given to patients with renal impairment and if such patients are at high risk of developing pulmonary toxicity. Doses of bleomycin should be reduced in the presence of a CrCl <60 mL/min (Dalgleish et al., 1984).
Therapeutic Uses. The recommended dose of bleomycin (blenoxane, others) is 10-20 units/m2 given weekly or twice weekly by the intravenous, intramuscular, or subcutaneous route. A test dose of ≤2 units before the first two doses is recommended for lymphoma patients. A variety of regimens are employed clinically, with bleomycin doses expressed in units. In treating testicular cancer, a standard total dose of 30 mg is given weekly for 3 consecutive weeks, and for three to four cycles of treatment. Total courses exceeding 250 mg should be given with caution, and usually only in high-risk testicular cancer treatment, because of a marked increase in the risk of pulmonary toxicity above this total dose. Bleomycin also may be instilled into the pleural cavity in doses of 5-60 mg (depending on the technique) to ablate the pleural space in patients with malignant effusions.
Bleomycin is highly effective against germ cell tumors of the testis and ovary. In testicular cancer, it is curative when used with cisplatin and vinblastine or cisplatin and etoposide. It is a component of the standard curative ABVD regimen (doxorubicin [adriamycin], bleomycin, vinblastine, and dacarbazine) for Hodgkin's lymphoma.
Clinical Toxicities. Because bleomycin causes little myelosuppression, it has significant advantages in combination with other cytotoxic drugs. However, it does cause a constellation of cutaneous toxicities, including hyperpigmentation, hyperkeratosis, erythema, and even ulceration. Rarely, patients with severe skin toxicity may experience Raynaud's phenomenon. Skin changes may begin with tenderness and swelling of the distal digits and progress to erythematous, ulcerating lesions over the elbows, knuckles, and other pressure areas. Healing of these lesions often leaves a residual hyperpigmentation, and lesions may recur when patients are treated with other antineoplastic drugs. Rarely, bleomycin causes a flagellate dermatitis consisting of bands of pruritic erythema on the arms, back, scalp, and hands. This rash responds readily to topical corticosteroids.
The most serious adverse reaction to bleomycin is pulmonary toxicity, which begins with a dry cough, fine rales, and diffuse basilar infiltrates on X-ray and may progress to life-threatening pulmonary fibrosis. Radiological changes of bleomycin-induced lung disease may be indistinguishable from interstitial infection or tumor, and show strong PET-positivity, but may progress from patchy infiltrates to dense fibrosis, cavitation and pneumothorax, atelectasis, or lobar collapse. Approximately 5-10% of patients receiving bleomycin develop clinically apparent pulmonary toxicity, and ~1% die of this complication (O'Sullivan et al., 2003). Most who recover experience a significant improvement in pulmonary function, but fibrosis may be irreversible. Pulmonary function tests are not of predictive value for detecting early onset of this complication. The CO diffusion capacity declines in patients receiving doses >250 mg. The risk of pulmonary toxicity is related to total dose, with a significant increase in risk in total doses >250 mg and in patients >40 years of age, in those with a CrCl of <80 mL/min, and in those with underlying pulmonary disease; single doses of ≥30 mg/m2 also are associated with an increased risk of pulmonary toxicity. Administration of high inspired O2 concentrations during anesthesia or respiratory therapy may aggravate or precipitate pulmonary toxicity in patients previously treated with the drug. There is no known specific therapy for bleomycin lung injury except for symptomatic management and standard pulmonary care. Steroids are of variable benefit, with greatest effectiveness in the earliest inflammatory stages of the lesion.
The etiology of bleomycin pulmonary toxicity has been investigated in rodent models (Moeller et al., 2008). These studies implicate various factors secreted by macrophages, including cytokines (such as transforming growth factor β [TGFβ] and tumor necrosis factor α [TNFα]), and chemokines (such as CCL2 and CXCLI2), as causative factors in leading to fibrosis in response to epithelial damage. Other contributing factors may be disordered coagulation cascades and imbalances in eicosanoids, leading to overproduction of profibrotic leukotrienes and underproduction of antifibrotic prostaglandins. Fibroblasts are recruited to the site of injury by release of lysophosphatide acid from inflammatory cells and contribute to the development of fibrosis (Tager et al., 2008). Various agents (e.g., thalidomide, anti-Her 2 antibodies, PPAR-γ agonists, N-acetylcysteine, anticoagulants, pirfenidone, and bosentan) have attenuated bleomycin toxicity in the animal model, given either before or after the toxic agent. The last four agents are being evaluated in clinical trials for the treatment of idiopathic pulmonary fibrosis (Walter et al., 2006), for which bleomycin lung disease in rodents is the primary disease model.
Other toxic reactions to bleomycin include hyperthermia, headache, nausea and vomiting, and a peculiar acute fulminant reaction observed in patients with lymphomas. This reaction is characterized by profound hyperthermia, hypotension, and sustained cardiorespiratory collapse; it does not appear to be a classical anaphylactic reaction and possibly may be related to release of an endogenous pyrogen. This reaction has occurred in ~1% of patients with lymphomas or testicular cancer.
This antibiotic was isolated from Streptococcus caespitosus by Wakaki and associates in 1958. It has limited clinical utility, having been replaced by less toxic and more effective drugs in most settings, with the exception of anal cancers, for which it is curative.
Mitomycin contains an azauridine group and a quinone group in its structure, as well as a mitosane ring, and each of these participates in the alkylation reactions with DNA.
Mechanism of Action. After intracellular enzymatic or spontaneous chemical reduction of the quinone and loss of the methoxy group, mitomycin becomes a bifunctional or trifunctional alkylating agent. Reduction occurs preferentially in hypoxic cells in some experimental systems. The drug inhibits DNA synthesis and cross-links DNA at the N6 position of adenine and at the O6 and N7 positions of guanine. Attempts to repair DNA lead to strand breaks. Mitomycin is a potent radiosensitizer, teratogen, and carcinogen in rodents. Resistance has been ascribed to deficient activation, intracellular inactivation of the reduced quinone, and P-glycoprotein-mediated drug efflux (Dorr, 1988).
Absorption, Fate, and Excretion. Mitomycin is administered intravenously. It disappears rapidly from the blood after injection, with a t1/2 of 25-90 minutes. Peak concentrations in plasma are 0.4 μg/mL after doses of 20 mg/m2 (Dorr, 1988). The drug distributes widely throughout the body but is not detected in the CNS. Inactivation occurs by hepatic metabolism or chemical conjugation with sulfhydryls. Less than 10% of the active drug is excreted in the urine or the bile.
Therapeutic Uses. Mitomycin (mitomycin-C; mutamycin, others) is administered by intravenous infusion; extravasation may result in severe local injury. The usual dose (6-20 mg/m2) is given as a single bolus every 6-8 weeks. Dosage is modified based on hematological recovery. Mitomycin also may be used by direct instillation into the bladder to treat superficial carcinomas (Boccardo et al., 1994).
Mitomycin is used in combination with 5-FU and cisplatin, for anal cancer. Mitomycin is used off label (in the form of an extemporaneously compounded eye drop) as an adjunct to surgery to inhibit wound healing and reduce scarring; it appears to have benefit in the management of malignant and nonmalignant ophthalmic pathologies (see the review by Abraham, 2006).
Clinical Toxicities. The major toxic effect is myelosuppression, characterized by marked leukopenia and thrombocytopenia; after higher doses, the nadirs may be delayed and cumulative, with recovery only after 6-8 weeks of pancytopenia. Nausea, vomiting, diarrhea, stomatitis, rash, fever, and malaise also are observed. A hemolytic uremic syndrome represents the most dangerous toxic manifestation of mitomycin and is believed to result from drug-induced endothelial damage. Patients who have received >50 mg/m2 total dose may acutely develop hemolysis, neurological abnormalities, interstitial pneumonia, and glomerular damage resulting in renal failure. The incidence of renal failure increases to 28% in patients who receive total doses of ≥70 mg/m2. There is no effective treatment for the disorder. It must be recognized early, and mitomycin must be discontinued immediately. Mitomycin causes interstitial pulmonary fibrosis, and total doses >30 mg/m2 have infrequently led to congestive heart failure. It also may potentiate the cardiotoxicity of doxorubicin when used in combination with this drug.
Mitotane (o,p′-DDD), a compound chemically similar to the insecticides DDT and DDD, is used in the treatment of neoplasms derived from the adrenal cortex. In studies of the toxicology of related insecticides in dogs, it was noted that the adrenal cortex was severely damaged, an effect caused by the presence of the o,p′ isomer of DDD.
Cytotoxic Action. The mechanism of action of mitotane has not been elucidated, but its relatively selective destruction of adrenocortical cells, normal or neoplastic, is well established. Thus, administration of the drug causes a rapid reduction in the levels of adrenocorticosteroids and their metabolites in blood and urine, a response that is useful in both guiding dosage and following the course of hyperadrenocorticism (Cushing's syndrome) resulting from an adrenal tumor or adrenal hyperplasia. It does not damage other organs.
Absorption, Fate, and Excretion. Approximately 40% of mitotane is absorbed after oral administration. After daily doses of 5-15 g, concentrations of 10-90 μg/mL of unchanged drug and 30-50 μg/mL of a metabolite are present in the blood. After discontinuation of therapy, plasma concentrations of mitotane are still measurable for 6-9 weeks. Although the drug is found in all tissues, fat is the primary site of storage. A water-soluble metabolite of mitotane found in the urine constitutes 25% of an oral or parenteral dose. About 60% of an oral dose is excreted unchanged in the stool.
Therapeutic Uses. Mitotane (lysodren) is administered in initial daily oral doses of 2-6 g, usually in three or four divided portions, and usually increased to 9-10 g/day if tolerated. The maximal tolerated dose may vary from 2-16 g/day. Treatment should continue for at least 3 months; if beneficial effects are observed, therapy should be maintained indefinitely. Spironolactone should not be administered concomitantly, because it interferes with the adrenal suppression produced by mitotane (Wortsman and Soler, 1977).
Treatment with mitotane is indicated for the palliation of inoperable adrenocortical carcinoma, producing symptomatic benefit in 30-50% of such patients.
Clinical Toxicity. Although the administration of mitotane produces anorexia and nausea in most patients, somnolence and lethargy in ~34%, and dermatitis in 15-20%, these effects do not contraindicate the use of the drug at lower doses. Because this drug damages the adrenal cortex, administration of replacement doses of adrenocorticosteroids is necessary (Hogan et al., 1978).
Trabectedin (yondelis) is the only drug used clinically that is derived from a sea animal. It was isolated from the marine tunicate, Ecteinascidin turbinate, as part of the National Cancer Institute's natural products discovery program. Trabectedin is designated as an orphan drug in the U.S. for ovarian cancer, sarcoma, and pancreatic cancer and is approved in the E.U. for second-line treatment of soft-tissue sarcomas and ovarian cancer.
Mechanism of Action. This large structure binds to the minor groove of DNA, allowing the alkylation of the N2 position of guanine and bending the helix toward the major groove (Tavecchio et al., 2008). A portion of the molecule protrudes from the minor groove and may play a role in attracting repair or transcription complexes. The bulky DNA adduct is recognized by the transcription-coupled nucleotide excision repair complex, and these proteins initiate attempts to repair the damaged strand, converting the adduct to a double-stranded break. Tumor cell sensitivity to trabectedin exhibits interesting molecular features. Trabectedin has particular cytotoxic effects on cells that lack components of the Fanconi anemia complex or those that lack the ability to repair double-strand DNA breaks through homologous recombination (Soares et al., 2007). Unlike cisplatin and other DNA adduct– forming drugs, its activity requires the presence of intact components of NER, including XPG, which may be important for initiation of single breaks and attempts at adduct removal (Stevens et al., 2008).
Absorption, Fate, and Excretion. Trabectedin is administered as a 24-hour infusion of 1.3 mg/m2 every 3 weeks. Its approval in Europe was based on a trial in soft-tissue sarcoma in which a superior time to progression was found with the longer infusion, as compared to a more convenient 3-hour infusion. It is administered with dexamethasone, 4 mg BID, starting 24 hours before drug infusion to diminish hepatic toxicity. The drug is slowly cleared by CYP3A4, with a plasma t1/2 of ~24-40 hours.
Therapeutic Uses. Trabectedin is approved outside the U.S. for second-line treatment of soft-tissue sarcomas and for ovarian cancer in combination with a doxorubicin formulation (doxil). It produces a very high (>50%) disease control rate in myxoid liposarcomas, a tumor characterized by a particular genomic translocation, although the reasons for this sensitivity are unclear (Grosso et al., 2009).
Toxicity. Without dexamethasone pretreatment, trabectedin causes significant hepatic enzyme elevations and fatigue in at least one-third of patients. With the steroid, the increases in transaminase are less pronounced and rapidly reverse (Grosso et al., 2009). Other toxicities include mild myelosuppression and, rarely, rhabdomyolysis.
In 1953, Kidd reported that guinea pig serum had anti-leukemic effects and identified L-asparaginase (L-ASP) as the source of this activity (Kidd, 1953). Fifteen years later, the purified enzyme from Escherichia coli proved to have dramatic antitumor activity against malignant lymphoid cells, based on the dependence of those tumors on exogenous sources of l-asparagine (Broome, 1981). The enzyme has become a standard agent for treating ALL.
Mechanism of Action. Most normal tissues are able to synthesize l-asparagine in amounts sufficient for protein synthesis, but lymphocytic leukemias lack adequate amounts of asparagine synthetase, and derive the required amino acid from plasma. l-ASP, by catalyzing the hydrolysis of circulating asparagine to aspartic acid and ammonia, deprives these malignant cells of asparagine, leading to cell death. l-ASP is used in combination with other agents, including methotrexate, doxorubicin, vincristine, and prednisone for the treatment of ALL and for high-grade lymphomas.
Resistance arises through induction of asparagine synthetase in tumor cells. For unknown reasons, hyperdiploid ALL cells or those with translocations involving the TEL oncogene are particularly sensitive to l-ASP (Pui et al., 2004), while cells containing the bcr-abl translocation, more common in adult ALL, are more resistant.
Absorption, Fate, Excretion, and Therapeutic Use. l-Asparaginase (elspar), a 144-kDa tetramer, is given intramuscularly or intravenously, but usually by the former route. After intravenous administration, E. coli–derived l-ASP has a clearance rate from plasma of 0.035 mL/min/kg, a volume of distribution that approximates the volume of plasma in humans, and a t1/2 of 24 hours (Asselin et al., 1993). It is given in doses of 6000-10,000 IU every third day for 3-4 weeks, although doses up to 25,000 IU once per week may be more effective in childhood ALL (Moghrabi et al., 2007). Enzyme levels must be maintained at >0.2 IU/mL in plasma to deplete asparagine in the bloodstream. Pegaspargase (peg-l-asparaginase; oncaspar) a preparation in which the enzyme is conjugated to 5000-Da units of monomethoxy polyethylene glycol, has much slower clearance from plasma (t1/2 of 6-7 days), and it is administered in doses of 2500 IU/m2 intramuscularly no more frequently than every 14 days, producing rapid and complete depletion of plasma and tumor cell asparagine for 21 days in most patients (Appel et al., 2008). Pegaspargase has much reduced immunogenicity (<20% of patients develop antibodies) (Hawkins et al., 2004) and has been approved for first-line ALL therapy.
Intermittent dosage regimens and longer durations of treatment increase the risk of inducing hypersensitivity. In hypersensitive patients, neutralizing antibodies inactivate l-ASP. Not all patients with neutralizing antibodies experience clinical hypersensitivity, although enzyme may be inactivated and therapy may be ineffective. In previously untreated ALL, pegaspargase produces more rapid clearance of lymphoblasts from bone marrow than does the E. coli preparation and circumvents the rapid antibody-mediated clearance seen with E. coli enzyme in relapsed patients (Avramis et al., 2002). Asparaginase preparations only partially deplete CSF asparagine.
Clinical Toxicity. l-ASP toxicities result from its antigenicity as a foreign protein and its inhibition of protein synthesis. Hypersensitivity reactions, including urticaria and full-blown anaphylaxis, occur in 5-20% of patients and may be fatal. These reactions usually are heralded by the earlier appearance of circulating neutralizing antibody and accelerated enzyme clearance from plasma. In these patients, pegaspargase is a safe and effective alternative. So-called "silent" enzyme inactivation by antibodies occurs in a higher percentage of patients than overt hypersensitivity and may be associated with a negative clinical outcome, especially in high-risk ALL patients (Mann et al., 2007).
Other toxicities result from inhibition of protein synthesis in normal tissues (e.g., hyperglycemia due to insulin deficiency, clotting abnormalities due to deficient clotting factors, hypertriglyceridemia due to effects on lipoprotein production, hypoalbuminemia). Pancreatitis may result from extreme triglyceridemia and has been treated by plasma exchange. The clotting problems may take the form of spontaneous thrombosis—more frequent in thrombophilic patients with underlying deficiencies in factor S, factor C, antithrombin III mutation, or factor V Leiden—or, less frequently, hemorrhagic episodes (Caruso et al., 2006). Thrombosis of cortical sinus vessels frequently goes unrecognized. Brain magnetic resonance imaging studies should be considered in patients treated with l-ASP who present with seizures, headache, or altered mental status. Intracranial hemorrhage in the first week of l-ASP treatment is an infrequent but devastating complication. l-ASP suppresses immune function as well.
l-ASP terminates the antitumor activity of methotrexate when given shortly after the antimetabolite. By lowering serum albumin concentrations, it may decrease protein binding and accelerate plasma clearance of other drugs.
The syntheses of hydroxyurea (HU) was first reported in 1869, but its potential anticancer activity was not recognized until 90 years later, when the drug was found to inhibit the growth of both leukemia and solid tumors. This drug has unique and surprisingly diverse biological effects as an antileukemic drug, radiation sensitizer, and an inducer of fetal hemoglobin in patients with sickle cell disease. The drug is orally administered, and its toxicity in most patients is modest and limited to myelosuppression.
Cytotoxic Action. HU inhibits the enzyme ribonucleoside diphosphate reductase, which catalyzes the reductive conversion of ribonucleotides to deoxyribonucleotides, a rate-limiting step in the biosynthesis of DNA. HU binds the iron molecules that are essential for activation of a tyrosyl radical in the catalytic subunit (hRRM2) of RNR. The drug is specific for the S phase of the cell cycle, during which RNR concentrations are maximal. It causes cells to arrest at or near the G1–S interface through both p53-dependent and independent mechanisms.
Because cells are highly sensitive to irradiation at the G1–S boundary, HU and irradiation cause synergistic antitumor effects. Through depletion of physiological deoxynucleotides, HU potentiates the antiproliferative effects of DNA-damaging agents such as cisplatin, alkylating agents, or topoisomerase II inhibitors and facilitates the incorporation of antimetabolites such as Ara-C, gemcitabine, and fludarabine into DNA. It also promotes degradation of the p21 cell-cycle checkpoint and thereby enhances the effects of HDAC (histone deacetylase) inhibitors in vitro (Kramer et al., 2008). The role of nitric oxide release in its differentiating activity and in its antitumor effects is uncertain but intriguing (Cokic et al., 2003).
HU has become the primary drug for improving the control of sickle cell (HbS) disease in adults and is also used for inducing fetal hemoglobin (HbF) in thalassemia HbC and HbC/S patients (Brawley et al., 2008). It reduces vaso-occlusive events, painful cries, hospitalizations, and the need for blood transfusions in patients with sickle cell disease. It does so via several potential mechanisms. Increased synthesis of HbF promotes solubility of hemoglobin and prevents sickling. The mechanism of HbF production is uncertain. It may simply result from suppression of erythroid precursor proliferation with compensatory stimulation of a distinct set of fetal Hb-producing cells. Sar1a, a specific promoter that upregulates in response to HU, induces HbF synthesis (Kumkhaek, 2008). Polymorphisms in this promoter may explain differential responses to HU. An alternative mechanism for HbF production has been offered because of the ability of HU to generate nitric oxide both in vitro and in vivo, causing nitrosylation of small-molecular-weight GTPases, a process that stimulate γ-globin production in erythroid precursors. Another property of HU that may be relevant is its ability to reduce L-selectin expression and thereby to inhibit adhesion of red cells and neutrophils to vascular endothelium. Also, by suppressing the production of neutrophils, it decreases their contribution to vascular occlusion.
Tumor cells become resistant to HU through increased synthesis of the hRRM2 subunit of ribonucleoside diphosphate reductase, thus restoring enzyme activity.
Absorption, Fate, and Excretion. The oral bioavailability of HU is excellent (80-100%), and comparable plasma concentrations are seen after oral or intravenous dosing. Peak plasma concentrations are reached 1-1.5 hours after oral doses of 15-80 mg/kg. HU disappears from plasma with a t1/2 of 3.5-4.5 hours. The drug readily crosses the blood-brain barrier, and it appears in significant quantities in human breast milk. From 40-80% of the drug is recovered in the urine within 12 hours after either intravenous or oral administration. Although precise guidelines are not available, it is advisable to modify initial doses for patients with abnormal renal function until individual tolerance can be assessed. Animal studies suggest that metabolism of HU does occur, but the extent and significance of its metabolism in humans have not been established.
Therapeutic Uses. In cancer treatment, two dosage schedules for HU (hydrea, droxia, others), alone or in combination with other drugs, are most commonly used in a variety of solid tumors: 1) intermittent therapy with 80 mg/kg administered orally as a single dose every third day or 2) continuous therapy with 20-30 mg/kg administered as a single daily dose. In patients with essential thrombocythemia and in sickle cell disease, HU is given in a daily dose of 15 mg/kg, adjusting that dose upward or downward according to blood counts. The neutrophil count responds within 1-2 weeks to discontinuation of the drug. In treating subjects with sickle cell and related diseases, a neutrophil count of at least 2500 cells/mL should be maintained (Platt, 2008). Treatment typically is continued for 6 weeks in malignant diseases to determine its effectiveness; if satisfactory results are obtained, therapy can be continued indefinitely, although leukocyte counts at weekly intervals are advisable.
The principal use of HU has been as a myelosuppressive agent in various myeloproliferative syndromes, particularly CML, polycythemia vera, myeloid metaplasia, and essential thrombocytosis, for controlling high platelet or white cell counts. Many of the myeloproliferative syndromes harbor activating mutations of JAK2, a gene that is downregulated by HU. In essential thrombocythemia, it is the drug of choice for patients with a platelet count >1.5 million cells/mm3 or with a history of arterial or venous thrombosis. In this disease, it dramatically lowers the risk of thrombosis by lowering the platelet, neutrophil, and red cell counts and by reducing expression of L-selectin and increasing nitric oxide production by neutrophils.
In CML, HU has been largely replaced by imatinib. Although it has produced anecdotal, temporary remissions in patients with solid tumors (e.g., head and neck cancers, cervical cancers), HU rarely is used in such patients as a single agent. HU is a potent radiosensitizer as a consequence of its inhibition of RNR (Flanagan et al., 2007) and has been incorporated into several treatment regimens with concurrent irradiation (i.e., cervical carcinoma, primary brain tumors, head and neck cancer, non–small-cell lung cancer).
Clinical Toxicity. Leukopenia, anemia, and occasionally thrombocytopenia—are the major toxic effects; recovery of the bone marrow is prompt if the drug is discontinued for a few days. Other adverse reactions include a desquamative interstitial pneumonitis, GI disturbances, and mild dermatological reactions; more rarely, stomatitis, alopecia, and neurological manifestations have been encountered. Increased skin and fingernail pigmentation may occur, as well as painful leg ulcers, especially in elderly patients or in those with renal dysfunction. HU does not increase the risk of secondary leukemia in patients with myeloproliferative disorders or sickle cell disease. It is a potent teratogen in all animal species tested and should not be used in women with childbearing potential (Platt, 2008).
One of the hallmarks of malignant transformation is a block in differentiation. It is not clear whether the block is complete or partial, in that tumor cells with the features of stem cells can be found in most tumors, while the greater bulk of tumor cells do not carry the markers or the biological potential of continuous proliferation. Nonetheless, there is growing evidence that many human tumors are generated by mutations that block specific steps in differentiation, an example being the t(15;17) translocation in APL (acute promyelocytic leukemia). This translocation joins the retinoic acid receptor-α (RAR-α, a dimerizing protein critical for differentiation) and the PML gene, which encodes a transcription factor important in inhibiting proliferation and promoting myeloid differentiation. Four other translocation partners for APL have been identified in less common varieties of APL. Under physiological conditions, RAR-α binds retinoic acid and regulates the expression of a number of specific genes that control myeloid differentiation. The oncogenic PML–RAR-α gene produces a protein that binds retinoids with much decreased affinity, lacks PML regulatory function, and fails to upregulate transcription factors (C/EBP and PU.1) that promote myeloid differentiation (Collins, 2008). The fusion protein forms homo- and heterodimers that regulate expression of genes that increase leukemic stem cell renewal, suppress checkpoint and apoptotic signals, and suppress expression of DNA repair functions, thereby enhancing mutability of APL cells. Epigenetic regulation of gene expression by histone acetylation and methylation also is disrupted by the fusion protein.
A number of chemical entities (vitamin D and its analogs, retinoids, benzamides and other inhibitors of histone deacetylase, various cytotoxics and biological agents, and inhibitors of DNA methylation) can induce differentiation in tumor cell lines in vitro. Fittingly, the first and best example of differentiating therapy was discovered in the treatment of APL (Wang and Chen, 2008).
Tretinoin. The biology and pharmacology of retinoids and related compounds are discussed in detail in Chapter 65. The most important of these for cancer treatment is tretinoin (all-trans retinoic acid; ATRA), which induces a high rate of complete remission in APL as a single agent and, in combination with anthracyclines, cures most patients with this disease.
Under physiological conditions, the RAR-α receptor dimerizes with the retinoid X receptor to form a complex that binds ATRA tightly. ATRA binding displaces a repressor from the complex and promotes differentiation of cells of multiple lineages. In APL cells, physiological concentrations of retinoid are inadequate to displace the repressor. Pharmacological concentrations, however, are effective in activating the differentiation program and in promoting degradation of the PML–RAR-α fusion gene (Collins, 2008). ATRA also binds and activates RAR-γ and thereby promotes stem-cell renewal, perhaps through its effects on the microenvironment (Drumea et al., 2008), and this action may help restore normal bone marrow renewal. Resistance to ATRA arises by further mutation of the fusion gene, abolishing ATRA binding; by induction of the CYP26A1 in liver or leukemic cells; or by loss of expression of the PML–RAR-α fusion gene (Roussel and Lanotte, 2001). Sensitivity can be restored by transfection of a functional PML–RAR-α gene.
Clinical Pharmacology. The usual dosing regimen of orally administered ATRA (vesanoid, others) is 45 mg/m2/day until 30 days after remission is achieved (maximum course of therapy is 90 days). ATRA as a single agent reverses the hemorrhagic diathesis associated with APL and induces a high rate of temporary remission. However, clinical trials have clearly established the benefit of giving ATRA in combination with an anthracycline for remission induction, achieving ≥80% relapse-free long-term survival.
ATRA concentrations reach 400 ng/mL in plasma. ATRA is cleared by a CYP3A4-mediated elimination with a t1/2 of <1 hour. Treatment with inducers of CYP3A4 leads to more rapid drug disappearance and, in some patients, resistance to ATRA (Gallagher, 2002). Inhibitors, such as antifungal imidazoles, block its degradation and may lead to hypercalcemia and renal failure (Cordoba et al., 2008), which responds to diuresis, bisphosphonates, and ATRA discontinuation. Corticosteroids and chemotherapy sharply decrease the occurrence of "retinoic acid syndrome," which is characterized by fever, dyspnea, weight gain, pulmonary infiltrates, and pleural or pericardial effusions. When used as a single agent for remission induction, especially in patients with >5000 leukemic cells/mm3 in the peripheral blood, ATRA induces an outpouring of cytokines and mature-appearing neutrophils of leukemic origin. These cells express high concentrations of integrins and other adhesion molecules on their surface and clog small vessels in the pulmonary circulation, leading to significant morbidity in 15-20% of patients. The syndrome of respiratory distress, pleural and pericardial effusions, and mental status changes may have a fatal outcome. Pretreatment dexamethasone should be given to patients with leukemic cell counts of >5,000/mL to counteract "retinoic acid syndrome".
Toxicity. Retinoids as a class, including ATRA, cause dry skin, cheilitis, reversible hepatic enzyme abnormalities, bone tenderness, pseudotumor cerebri, hypercalcemia, and hyperlipidemia, and as mentioned in the previous paragraph, the retinoic acid syndrome.
Although recognized as a heavy metal toxin for centuries, arsenicals attracted interest as a medicinal agent nearly a century ago for syphilis and parasitic disease, and eventually CML. Arsenic trioxide has become a highly effective treatment for relapsed APL, producing complete responses in >85% of such patients (Wang and Chen, 2008). It now is a standard treatment for patients who relapse after ATRA and chemotherapy, cures a significant fraction of these patients, and has entered trials as primary therapy in combination with ATRA and chemotherapy. The chemistry and toxicity of arsenic is considered in detail in Chapter 67.
The basis for its antitumor activity remains uncertain. APL cells have high levels of reactive oxygen species (ROS) and are quite sensitive to further ROS induction. ATO inhibits thioredoxin reductase and thereby generates ROS. It inactivates glutathione and other sulfhydryls that scavenge ROS and thereby aggravates ROS damage. Cells exposed to ATO also upregulate p53, Jun kinase, and caspases associated with the intrinsic pathway of apoptosis and downregulate anti-apoptotic proteins such as bcl-2. Of particular relevance to APL, it promotes the phosphorylation, sumoylation, and degradation of the APL fusion protein (Lallemand-Breitenbach et al., 2008), as well as the degradation of NF-κB, a transcription factor that stimulates angiogenesis and dampens apoptotic responses in cells with DNA damage. ATO's cytotoxic effects are antagonized by cell survival signals emanating from activation of components of the PI3 kinase cell survival pathway, including Akt kinase, S6 kinase, and mammalian target of rapamycin (mTOR). Inhibition of mTOR by rapamycin enhances its cytotoxic activity in culture systems.
It induces differentiation of leukemic cell lines in vitro, and in both experimental and human leukemias in vivo, but the mechanisms of differentiation and their relationship to the above pharmacological activities are not known.
Clinical Pharmacology. ATO (trisenox) is well absorbed orally, but in cancer treatment is administered as a 2-hour intravenous infusion in dosages of 0.15 mg/kg/day for up to 60 days, until remission is documented. Consolidation therapy begins after a 3-week break. It enters cells via one of several glucose transporters. The primary mechanism of elimination is through enzymatic methylation. Methylated metabolites have uncertain biological effects. Peak steady-state concentrations of arsenic in plasma reached 5-7 μM in one study in adults, while 20-fold lower levels were reported in children using more specific atomic absorption methods (Fox, 2008). Multiple methylated metabolites form rapidly and are excreted in urine. Less than 20% of administered drug is excreted unchanged in the urine. No dose reductions are indicated for hepatic or renal dysfunction.
Toxicity. Pharmacological doses of ATO are well tolerated. Patients may experience reversible side effects, including hyperglycemia, hepatic enzyme elevations, fatigue, dysesthesias, and light-headedness. Ten percent or fewer of patients will experience a leukocyte maturation syndrome similar to that seen with ATRA, including pulmonary distress, effusions, and mental status changes. Oxygen, corticosteroids, and temporary discontinuation of ATO lead to full reversal of this syndrome (Soignet et al., 1998). Another important and potentially dangerous side effect is lengthening of the QT interval on the electrocardiogram in 40% of patients, but rarely do patients develop torsades de pointes, a dangerous form of ventricular tachycardia. Simultaneous treatment with other QT-prolonging drugs, such as macrolide antibiotics, quinidine, or methadone, should be avoided. QT prolongation by ATO results from inhibition of the rapid K+ efflux channels in myocardial tissue by As2O3. This change leads to slow repolarization of myocardium, and ventricular arrhythmias. Monitoring of serum electrolytes and repletion of serum K+ in patients with hypokalemia are precautionary measures in patients receiving ATO therapy. In patients exhibiting a significantly prolonged QT (>470 milliseconds), treatment should be suspended, K+ supplemented, and therapy resumed only if the QT returns to normal. Torsades de pointes requires treatment with intravenous magnesium sulfate, K+ repletion, and defibrillation if the arrhythmia persists (Gupta et al., 2007) (see Chapter 29).
Histone Deacetylase Inhibitors
Vorinostat. A new field of cancer research, called epigenetics, concerns the control of cell proliferation and differentiation by processes beyond pure genetic alterations. These processes include cellular modification of expression of genes by microRNAs, histones, and proteins, and post-translational modification of proteins. Vorinostat (zolinza), also known as suberoylanilide hydroxamic acid (SAHA), is unique as an epigenetic modifier that directly affects histone function (Figure 61–15). To understand its action, it is important to review the complex structure of DNA, which wraps itself around histone proteins to form the nucleosome. This higher-order packaging controls gene expression. Acetylation of lysine residues on histones increases the spatial distance between DNA strands and the protein core, allowing access for transcription factor complexes, and thereby enhancing transcriptional activity. Acetyl groups are added by histone acetyltransferases and removed by histone deacetylases (HDACs). HDAC inhibitors such as vorinostat increase histone acetylation and thus enhance gene transcription. Many nonhistone proteins also are subject to lysine acetylation and thus are affected by treatment with HDAC inhibitors. The role of their acetylation status in the antitumor action of HDAC inhibitors is unclear.
Chemical structures of vorinostat (A), and its metabolites, vorinostat O-glucuronide (B) and 4-anilino-4-oxobutanoic acid (C).
Mechanism of Action. Vorinostat is a hydroxamic acid modeled after hybrid polar compounds, such as hexamethylene bisacetamide (HMBA); as a class, these compounds cause differentiation of malignant cells in vitro, as do other classes of compounds with HDAC-inhibitory activity, including cyclic tetrapeptides, benzamides, and short-chain aliphatic acids. These compounds bind to a critical Zn++ ion in the active site of HDAC enzymes. Vorinostat inhibits the enzymatic activity of HDACs at micromolar concentrations. An important distinction between vorinostat and other HDAC inhibitors is that vorinostat and the hydroxymates are pan-HDAC inhibitors, whereas other compounds have selectivity for HDAC isoenzyme subsets. The biological and clinical implications of this specificity are not clear, and the specific mechanism by which HDAC inhibitors exert their antitumor activity is uncertain. They induce cell-cycle arrest, differentiation, and apoptosis of cancer cells; nonmalignant cells are relatively resistant to these effects. They increase transcription of cell-cycle regulators, affect levels of nuclear transcription factors, and induce pro-apoptotic genes. HDAC inhibition directly blocks function of the chaperone HSP90 and stabilizes the tumor suppressor p53 (Bolden et al., 2006).
Absorption, Fate, and Excretion. Vorinostat is administered as a once-daily oral dose of 400 mg. It is inactivated by glucuronidation of the hydroxyl amine group, followed by hydrolysis of the terminal carboxamide bond and further oxidation of the aliphatic side chain (Figure 61–15). The metabolites are pharmacologically inactive. The terminal t1/2 of vorinostat in plasma is ~2 hours. Interestingly, histones remain hyperacetylated up to 10 hours after an oral dose of vorinostat, suggesting that its effects persist beyond drug metabolism and elimination.
Therapeutic Uses. Vorinostat is approved for use in refractory cutaneous T-cell lymphoma (CTCL). In patients with refractory CTCL, vorinostat produced an overall response rate of 30%, with a median time to progression of 5 months (Duvic et al., 2007). Vorinostat and other HDAC inhibitors, including romidepsin (depsipeptide; FK228) and MGCD 0103, have shown activity in CTCL, other B and T-cell lymphomas, and myeloid leukemia.
Toxicity. The most common side effects of vorinostat are fatigue, nausea, diarrhea, and thrombocytopenia. Deep venous thrombosis and pulmonary embolism were infrequent but serious adverse events in CTCL patients receiving vorinostat. Most HDAC inhibitors in development cause QTc prolongation, although no serious cardiac toxicity has been reported with vorinostat. A small number of patients receiving infusional depsipeptide romidepsin (see below) and the hydroxamate dacinostat (NVP-LAQ 824) have developed ventricular arrhythmias while on treatment, but the causal relationship to the drugs has not been clearly established, and the cardiac risk may be lower with orally administered and/or less potent HDAC inhibitors (Piekarz et al., 2006). Caution is advised when using HDAC inhibitors in patients with underlying cardiac abnormalities, and careful monitoring of the QTc interval and correction of electrolyte (K+, Mg++) abnormalities is necessary.
Romidepsin. Romidepsin, a bicyclic polypeptide derived from a soil bacterium, inhibits HDAC at low nanomolar concentrations and is approved for treatment of CTCL and for peripheral T-cell lymphomas. In a Phase II trial it produced complete responses in 4 patients with CTCL, and partial responses in 20, from a total of 71 patients treated. Its primary toxities include GI complaints (nausea vomiting) and transient myelosuppression. Its administration leads to T-wave flattening, but without clear cardiac toxicity (Piekarz et al., 2009).