OHO-09(135-180) 11/16/02 11:20 AM Page 145 ANTI-tumor ANTIBIOTICS 145 Mitoxantrone One of the anthracenediones, synthesized as analog of the anthra- cyclines. In common with the anthracyclines, mitoxantrone binds to DNA and interacts with topoisomerase II but appears less potent in generating free radicals. Mitoxantrone is also a substrate f or Pgp. Like doxorubicin, mitoxantrone is rapidly and highly tissue-bound. The main clinical use of mitoxantrone has been as an alternative to doxorubicin in advanced breast cancer, as it is substantially less cardiotoxic, less vesicant, and causes less alopecia. However, mitox- a ntrone is less effective than doxorubicin. It has some activity against other solid tumors, including non-Hodgkin \200\231s lymphoma and non- lymphocytic leukemia. Actinomycin D Structurally distinct from the anthracyclines, Actinomycin D binds strongly t o DNA by intercalation and inhibits synthesis of RNA and proteins. In pre-clinical models, reduced cellular uptake is associated with resistance to Actinomycin D; it also appears to be a substrate for the Pgp pump. Actinomycin D is especially active against childhood tumors and is used in combination therapy for: (cid:2) Wilms \200\231 tumor (cid:2) Rhabdomyosarcoma (cid :2) Neuroblastoma (cid:2) Ewing \200\231s sarcoma The dose-limiting toxicity is myelosuppre ssion, maximal around 10 days after treatment. Actinomycin D can also cause: (cid:2) Nausea and vomiting (cid:2) Mucositis (cid:2) Diarrhea (cid:2) Alopecia Mitomycin C (MMC) MMC is active against a range of solid tumors but is also used as a radio -sensitizer in chemo-irradiation. MMC is inactive in its natural form and requires reduction before binding to DNA by alkylat ion to form adducts. This bioreductive acti- vation may occur preferentially under the anae robic conditions that are known to exist within some solid tumors. Causes of resistance have not been well de \201ned, but as a naturally occuring compound it appears to be a substrate for Pgp. MMC is used in combination with other cytotoxics to treat breast cancer, non-small-cell lun g cancer, and GI cancer. It is used as a OHO-09(135-180) 11/16/02 11:20 AM Page 146 PRINCIPLES OF CHEMOTHERAPY 146 radio-sensitizer in the treatment of anal cancer. MMC is effective in reducing recurrence o f super \201cial bladder cancer where it is given intravesically, effectively eliminating systemic exposure to the drug. The most important toxicity of MMC is myelosuppression, especially thrombocytopaenia, which is delayed and can be cumulative. Accordingly, MMC is given systemically every 6 weeks in contrast to the 3-weekly schedules usually used for other anti-tumor antibiotics. Haemolyti c-uraemic syndrome, pulmonary \201brosis and, cardiac complications are all uncommon, esp ecially at low cumulative doses, but are potentially fatal. Other toxicities such as nausea and vomiting, alopecia, and stomatitis are usually mild, but extravasation can be serious. OHO-09(135-180) 11/16/02 11:20 AM Page 147 ANTI-METABOLITES 147 Anti-metabolites Anti-metabolites interfere with normal cellular metabolism of nucleic acids; they act with cell-cycle S-phase speci \201city. An early compound induced the \201rst remissions in c hildhood ALL, and they now include some of the most widely prescribed cytotoxic agents, who se indica- tions are not con \201ned to treating malignancies. Anti-folates Understanding anti-metabolite action necessitates knowledge of folate biochemi stry. The enzyme thymidylate synthase (TS) acts as rate-limit- ing step in the synthesis of thymidylate, converting dUMP into dTTP by transferring a methyl group from CH2-FH4. The su pply of reduced folate is maintained by the enzyme dihydrofolate reductase (DHFR). Methotrexate (MTX) Widely used, MTX is effective in: (cid:2) Breast cancer (cid:2) Osteogen ic sarcoma RNA 5-CHO-FH4 (Leucovorin) 10-CHO-FH4 dUMP FUTP CH2-FH4 Tegafur DHFR MTX FduMP 5-FU TS TS inhibitors DHPD Capecitabine dTMP FH2 Elimination dTTP DNA MTX Cytotoxics (MTX \200\224methotrexate; 5-FU \200\2235 \200\223fluorouracil; TS inhibitors i nclude ralitrexed) dTTP Normal metabolites Indicates enzyme inhibition Enzyme (TS \200\224thymidylate synthase; DHPD \200\224dihydropyrimidine dehydrogenase; DHFR \200\224dihydrofolate reductase) DHPD Main sites of action of anti-metabolites. OHO-09(135-180) 11/16/02 11:20 AM Page 148 PRINCIPLES OF CHEMOTHERAPY 148 (cid:2) Bladder carcinoma (cid:2) Gastric cancer (cid:2) Choriocarcinoma (cid:2) Head and n eck cancers (cid:2) Leukemia (cid:2) Lymphoma Also used for psoriasis and rheumatoid arthri tis, MTX is actively transported into cells where it is polyglutamated, binds tightly to an d inhibits DHFR. It also: Inhibits other enzymes e.g. TS Impairs DNA repair (cid:2) (cid:2) (cid:2) Causes accumulation of dihydrofolate polyglutamates (that in turn inhibit folate-dependent enzymes) Pharmacology MTX is well absorbed orally below 25 mg/m2, but is usually adminis- tered IV, except in maintenance regimens and treatment of benign connective tissue diseases. It is el iminated in three phases, the second and third phases each taking 10 \200\22324 hours; elim ination is considerably lengthened by renal dysfunction. There is some hepatic metabolism t o the active drug 7-OH-MTX and approximately 10% of the drug is cleared by biliary excretio n. Dose adjustments are not usually neces- sary with hepatic dysfunction. Signi \201cant t hird-space effects occur in the presence of \202uid collections (e.g. ascites, pleural ef fusions) and can increase toxicity through reduced clearance. MTX excretion can also be inh ibited by: (cid:2) Probenicid (cid:2) Penicillins (and cephalosporins) (cid:2) Non-steroida l anti-in \202ammatory agents Evidence shows synergy with 5- \202uorouracil (5-FU), which is maximal if given 24 hours before the 5-FU. Resistance occurs through: Impaired uptake (cid:2) (cid:2) Gene ampli \201cation of DHFR (cid:2) Decreased polyglutamation Toxicities include : (cid:2) Mucositis (cid:2) Myelosuppression (cid:2) Nephrotoxicity (cid:2) Acute and chron ic hepatotoxicity (cid:2) Self-limiting pneumonitis (cid:2) Arachnoiditis and chronic demye lination (when given intrathecally) OHO-09(135-180) 11/16/02 11:20 AM Page 149 ANTI-METABOLITES 149 High-dose MTX has been used in sarcomas, has the advantage of some CSF penetration, and may overcome resistance. Folinic acid can be used to \200\230rescue \200\231 normal cells fro m undue toxicity, usually given 24 hours after MTX. Methotrexate analogs Aminopterin has been surpassed by MTX, though no trial comparing them has been done. Edatrexate is an analog, with better uptake characteristics and is active in breast, non-small-cell lung, and head and neck cancer. Trimetrexate is lipophilic, does no t require poly- glutamation or the folate uptake pathway, and is currently under evaluation in clinical trials. Thymidylate synthase (TS) inhibitors New agents have been developed that directly inhibit T S (in contrast to indirect inhibitors e.g. 5-FU and MTX) and interact with the folate- bind ing site of TS. Glutamated members utilize the folate-carrier system for entry into cells a nd are polyglutamated, enhancing their ef \201cacy and intracellular retention. Non-glutam ated compounds are lipophilic and may have different effects. Raltitrexed (Tomudex) is a glutamated compound that causes pro- longed inhibition of TS. It has triphasic elimination, with a rapid ini- tial fall in concentration but very prolonged \201nal phase. 50% of the drug is renally excreted unchanged. It is active in breast and colorectal cancer with toxicities including: (cid:2) Myelosuppression (cid:2) Diarrhea (ci d:2) Transaminitis It is usually administered IV once every 3 weeks. Fluropyrimidines These pro-drugs are intracellularly activated and their products inhibit p yrimidine synthesis. 5- \202urouracil (5-FU) This widely prescribed example is active in: (cid:2) Breast cancer (cid:2) Most gastrointestinal cancers (cid:2) Head and neck tumors (cid:2) Ovarian cancer It is metabolized to FdUMP that forms, in the presence of CH2-FH4, a stable complex inhibit ing TS. It also inhibits RNA synthesis and pre-ribosomal RNA processing. OHO-09(135-180) 11/16/02 11:20 AM Page 150 PRINCIPLES OF CHEMOTHERAPY 150 Pharmacology Oral absorption is erratic so 5-FU is given IV both as a bolus and a prolonged infusion. It has a short initial half-life, with signi \201cant hepatic, renal, and lung clearance. Ac tive metabolites (e.g. 5dUMP and FUTP) have variable pharmacokinetics. Toxicities of 5-FU include myelosuppression and, particularly with 5-day schedules, stomati tis and diarrhea. Prolonged infusion over- comes the initial rapid clearance, resulting in differing toxicities with minimal bone marrow effects. Instead, cutaneous toxicity known as hand \200\223foot syndrome occurs. Neurotoxicity and cardiotoxicity may also occur. Resistance to 5-FU therapy can be due to: (cid:2) Altered transportation (cid:2) Folate depletion (cid:2) Changes in TS expression (d ue to gene ampli \201cation or post- transcriptional factors) 5-FU pro-drugs Ftorafur (tegafur) Orally active, usually given in combination with uracil in a molar ratio of 1:4. Ftorafur has preferential uptake in tumors and can be given for up to 28 days at a time. It is active in many tumors includ- ing: (cid:2) Breast (cid:2) Gastric (cid:2) Colo n (cid:2) Lung Capecitabine Orally active, it is preferentially activated in tumor and liver tissue and ha s the potential to replace prolonged or continuous infusion 5-FU. Optimum schedule is thoug ht to be twice daily for 2 weeks out of every 3. Active in breast and GI tumors. Its side-e ffects include: (cid:2) Mucositis (cid:2) Diarrhea (cid:2) Hand \200\223foot syndrome 2- \202uoro-2 \200\231-deoxyuridine ( \202oxuridine) Given IV, this agent can be metaboli zed both into 5-FU and also directly into FdUMP, theoretically giving increased ef \201cac y. Its clinical use has largely been con \201ned to hepatic artery infusion, where trials show it to be superior to single-agent 5-FU for treating colon cancer. OHO-09(135-180) 11/16/02 11:20 AM Page 151 ANTI-METABOLITES 151 Modulation of 5-FU A number of agents have been combined with 5-FU in order to increase eit her its ef \201cacy or therapeutic index. Folinic acid 5-FU and folinic acid combinations are the mainstay of treatment of colon canc er. Folinic acid is given by infusion, before or concomitant with 5-FU. By increasing suppl y of CH2-FH4, folinic acid potentiates interaction between 5-FU and TS. Although more toxic , has higher response rate in advanced colorectal cancer with combined treatment than singl e-agent 5-FU. Levamisole the anti-helminth agent, levamisole, has some activity as an immune modulator and has been used in the treatment of colon cancer. It has been combined with 5-FU in the adjuvant setti ng and the combination is superior to no treatment, but not to intravenous 5-FU (with or wi thout folinic acid). The mechanism of action is uncertain. Interferon Interferon may enhance TS inhibition. In combination with 5-FU (with or without folinic acid), it has not been shown to be superior to single-agent 5-Fu. PALA PALA (N-phosphonacetyl-L-aspartate) inhibits de novo pyrimidine synthesis, and thus in creases 5-FU anabolism and incor- poration into RNA. DPD inhibitors Inhibit the main catabolic pathway of 5-FU and may result in increased ef \201cacy. Early clinical trials are in progress. Anti-purines Purine analogs are widely used to treat leukaemias and as immuno- suppressives (azathioprine) and anti-virals (acyclovir, gancyclovir). 6-Mercaptopurine (6-MP) and 6-Thioguanine (6-TG) both inhibit de novo purine synthesis and their nucleotide products are incorpo- rated into DNA. HGPRT produces monophosphates, which inhibit early stages of purine synthesis, and then convert into tri-phosphates which are i ncorporated into DNA, causing strand breaks. There are synergistic effects with MTX, due to PRPP build-up, facilitating phos- phorylation by HGPRT. Resistance develops due to HGPRT d e \201ciency and reduced substrate af \201nity. Variable oral bioavailability may con- tr ibute to some treatment failures in childhood ALL. Both drugs have a short half-life and are primarily metabolized \200\224 the important diff erence is that 6-MP is a substrate for xanthine oxi- dase, and dose alterations are necessa ry when co-administered with allopurinol. There is poor CSF penetration, but otherwise thes e agents are widely distributed. Main toxicity is myelosuppression, but 6-MP can also cause hepato- toxicity. Nausea, vomiti ng, and mucositis can also occur, more com- monly with 6-MP. The commonest indication is he matological malignancy: 6-MP is used for maintenance therapy of ALL, and 6-TG is used for b oth remission induction and maintenance in AML. OHO-09(135-180) 11/16/02 11:20 AM Page 152 PRINCIPLES OF CHEMOTHERAPY 152 Cytosine analogs Two main analogs are metabolized along similar pathways. Initially phospho rylated by deoxycytidine kinase, the active metabolite is triphosphate, incorporated into D NA. Agents are deaminated by cytidine deaminase, which is abundant in liver and GI tract ti ssues, and products are inactive. Cystosine arabinoside (Ara-C) Ara-C is actively transported, and its metabolite ara-CTP is incorpo- rated into DNA, inhibiting DNA polymerases and possibly phospho- lipid synthesis. Unlike gemcitabine, no further normal nucleotides are added, so that damaged DNA is suscept ible to DNA repair. Ara-C is active in NHL and AML, but not in solid tumors. There is renal excretion of deanim ated compound and because of rapid clear- ance better activity is observed when Ara-C is gi ven by continuous infu- sion. CSF levels are about half-plasma concentration, Side-effects are: (cid:2) Nausea (cid:2) Vomiting (cid:2) Alopecia (cid:2) Myelosuppression It can also cause \200\230ara-C syndrome \200\231 with fevers, myalgias, rash, kerato- conjunctivitis, and arthralgias. Rarely, lung and pancreatic damage occurs. There are signi \201cant inte ractions with many cytotoxics, including: (cid:2) Hydroxyurea (cid:2) Cisplatin (cid:2) Cyc lophosphamide (cid:2) BCNU 2,2-di \202uorodeoxycytidine (gemcitabine) This \202uorinated analog has better membrane permeation and af \201nity for deoxycytidine kinase than Ara-C. Intracellular retention i s prolonged, partly due to a unique self-potentiation in which the bi- and tri-phosphates f acilitate the phosphorylation of the parent com- pound, as well as inhibiting its catabolis m. Active metabolite dF-CTP is incorporated into DNA, followed only by one more normal nucleot ide, resulting in protection of the DNA from repair enzymes ( \200\230masked termination \200\231). It is probably the saturable for- mation of dF-CTP that contributes to the clini cal schedule dependency of gemcitabine, usually given IV, weekly for 3 weeks out of 4. Toxicities include: (cid:2) Flu-like symptoms (cid:2) Transaminitis (cid:2) Peripheral oede ma OHO-09(135-180) 11/16/02 11:20 AM Page 153 ANTI-METABOLITES 153 (cid:2) Myelosuppression (cid:2) Possible nephrotoxicity There is some evidence for synergy with cisplatin, the extent of which appears to be schedule-dependent. It is active in panc reatic cancer (where there is improved symptom control in comparison with single-agent 5-FU ) as well as in lung, breast, and bladder cancer. Adenosine analogs Three adenosine analogs have come into clinical practice, active in low-g rade NHL, Waldenstr m \200\231s macroglobulinaemia, and CLL. All have similar effects and interact with enzyme adenosine deaminase (ADA), a de \201ciency of which causes severe com bined immuno- de \201ciency. Toxicity includes myelosuppression with particular effects on lymphocytes, including depression of CD3 and CD4 levels, and reduced NK activity. Infectiv e complications are more likely to be opportunistic than with most cytotoxics. Neurotoxicit y can occur, usually, but not exclusively, associated with higher doses. Fludarabine Resistant to ADA, it is particularly useful in treating CLL. It is actively tra nsported into the cells and its mode of action is a consequence of phosphorylation followin g which it is incorporated into DNA and, probably RNA, and may even cause topoisomerase II inhibition. Can cause haemolytic anemia. 2 \200\231-deoxycoformycin (pentostatin) Has a very high af \201nity for ADA, and the resu ltant complex is stable for over 24 hours, resulting in enzyme inhibition. Its major indica tion is treatment of hairy-cell leukemia. Actively transported into cells, it is phosphoryl ated and incorporated into DNA and also produces inhibitory dATP. It inhibits both DNA synt hesis and DNA repair. 2-Chlorodeoxyadensine Resistant to ADA, phosphorylated and incorporated into DNA; and is us ed for hairy-cell leukemia. Hydroxyurea This oral agent inhibits ribonucleotide reductase, which reduces availability o f all deoxynucleotides. It crosses the blood \200\223brain barrier, and is used in myelopro liferative disorders. Toxicities are: (cid:2) Myelosuppression (cid:2) Gastrointestinal tox icities (cid:2) Sometimes hyperpigmentation OHO-09(135-180) 11/16/02 11:20 AM Page 154 PRINCIPLES OF CHEMOTHERAPY 154 Cisplatin and derivatives Cisplatin is one of the most active anti-cancer drugs in clinical use since the early 1970s , with a very wide spectrum of anti-tumor activ- ity. In view of its considerable toxicity pro \201le, many attempts have been made to develop analogs with less toxicity, increased ef \201cacy, or both. Carboplatin A large number of analogs have been subject to clinical trials but only carbopl atin has emerged as a viable clinical candidate. All plat- inum compounds possess \200\230 leaving groups \200\231 that are substituents on the platinum atom that are lost when DNA-p latinum cross-links are formed. Physical properties for analogs suggests a correlation betw een reactivity of leaving groups and nephrotoxicity. There is still a degree of controversy regarding the clinical equiva- lence of cisplatin an d carboplatin; there are limited situations such as germ cell tumors where cisplatin still appears to be the agent of choice. However, carboplatin in most other circumstances has sup - planted the use of cisplatin. Side-effects of carboplatin Signi \201cant (cid:2) Thrombocytopenia, worse at day 14 (cid: 2) Leucopenia, worse at day 14 Less signi \201cant toxicities (cid:2) Renal (cid:2) Neurological (cid:2) Otological (cid: 2) Nausea and vomiting \200\224occasionally (cid:2) Alopecia \200\224absent/mild (cid:2) Vi sual disturbances \200\224rarely (cid:2) Allergy \200\224in 2% Dosage of carboplatin Initially, a dosage of carboplatin based on body-surface area resulte d in a variable degree of thrombocytopenia with a number of patients requiring platelet tra nsfusion. Pharmacokinetically-based dosing is now the adopted standard. With simple pharmacokinetics, about 65% of an administered dose of carboplatin is excreted in the urine within 24 hours and renal clear- OHO-09(135-180) 11/16/02 11:20 AM Page 155 CISPLATIN AND DERIVATIVES 155 ance is virtually the same as the glomerular \201ltration rate (GFR). The remaining drug remains covalently bound to tissues for long periods (months to years) and is biologically inert. Antiproliferative toxicities of carboplatin are related to drug con- centration and time, w hich is given by the area under the plasma concentration time curve (AUC). The simple pharm acokinetics of carboplatin allow a dosing formula to be derived from which the dose require d to achieve a speci \201c AUC can be calculated for an individual patient. The most widel y used formula is: Dose = H(GFR + 25) Where: Dose is the total dose in mg to be given to the patient. H is the desired AUC in mg/ml.mm. Typical AUCs are between 4 and 7, depending on frequency of administration, previous treatment, and the drugs being used in combination. GFR is glomerular \201ltration rate of patient (ml/min), unadjusted for surface area (sho uld ideally be measured by an isotope method such as 51CrEDTA clearance, but a carefully pe rformed 24-hour urinary creatinine clearance is also acceptable). AUC-based dosing results in a predictable toxicity and the adminis- tration of a larger ave rage dose. AUC has largely superseded surface area-based dosing. When high-dose carboplatin is used with stem-cell rescue regimens there appears to be a relationship between AUC and non-hematological toxicities. Activity of carboplatin Carboplatin can be regarded as a less toxic substitute for cisplati n and is used for similar indications. Patients resistant to cisplatin will also be resista nt to carboplatin and vise versa. However, the increased thrombocytopenia seen with carbopl atin may be a disadvantage in some combinations, while reduced non-hematological toxicities may be an advantage in others. Further, a low level of non-hematological toxicity makes ca rboplatin suitable for inclusion in high-dose regimens with bone marrow or stem cell rescue . Indications for carboplatin (cid:2) Ovarian cancer \200\224equal ef \201cacy for carboplatin and CAP (cyclophosphamide, doxorubicin, and cisplatin) \200\224less toxicity for carboplatin compared to CAP \200\224addition of paclitaxel less ens risk of thrombocytopenia (cid:2) Germ cell tumors \200\224testicular teratoma: lower relapse rate for cisplatin compared to carboplatin: OHO-09(135-180) 11/16/02 11:20 AM Page 156 PRINCIPLES OF CHEMOTHERAPY 156 \200\224pediatric germ cell tumors: equal ef \201cacy for cisplatin and carboplatin \200\224seminoma: carboplatin, highly ef \201cacious (cid:2) Pediatric cancers \200\224cisplatin has role in many cancers \200\224carboplatin less toxic (cid:2) Small-cell lung cancer \200\224carboplatin, high response rate (cid:2) Non-small-cell lung cancer, bladder cancer, cervical cancer \200\224carboplatin, slightly better response than cisplatin \200\224carboplatin combined with paclitaxel, high response rate in non-small-cell lung cancer (cid:2) Head and neck cancer \200\224carboplatin used as alternative to cisplatin (cid:2) Breast cancer \200\224not used as \201rst-line treatment \200\224used in high-dose regimes (cid:2) Colon cancer \200\224little activity (cid:2) Prostate cancer \200\224little data (cid:2) Upper GI tumors \200\224little data (cid:2) Brain tumors \200\224response in medulloblastoma Pharmacokinetic interactions with carboplatin Unlike cisplatin, carboplatin does not affect hepatic cytochrome P450 enzyme and pharmacokinetic interactions with other drugs seem to b e rare. Summary Carboplatin has major advantages in terms of ease of administration and non-hematol ogical toxicities than cisplatin, although the higher incidence of thrombocytopenia may be a problem in some circum- stances. In the main, it can be regarded as an alternative to cis platin, but current data suggest cisplatin should still be used for treating testicular ter atoma. Unlike cisplatin, carboplatin can be used in high- dose regimens. Carboplatin should generally be dosed on a pharma- cokinetic basis. In the future, a major role for carboplat in is likely to be in combination with paciltaxel for a variety of tumors. Cisplatin mechanism of action Cisplatin binds directly to DNA, inhibiting synthesis by alte ring the DNA template via formation of intra-strand and inter-strand cross- OHO-09(135-180) 11/16/02 11:20 AM Page 157 CISPLATIN AND DERIVATIVES 157 links. These cross-links are generated by an aquated complex that acts as a bifunctional al kylating agent. Cytotoxic effects of cisplatin are cell-cycle independent, and synergy betw een cisplatin and anti- metabolites has been demonstrated both in vitro and in clinical tri als. The mechanism behind this synergy has not been fully explained; the most commonly held hypothesis is that this is due to a malfunction in DNA repair processing. Side-effects of cisplatin (cid:2) Dose-dependant nephrotoxicity \200\224prehydrate \200\224may need diuretics (cid:2) Nausea, vomiting \200\224all require anti-emetics (cid:2) Peripheral neuropathy \200\224sensory loss \200\224segmental demyelination (cid:2) Central nervous toxicity \200\224less common (cid:2) Leucopenia \200\224recovers in 21 days (cid:2) Thrombocytopenia \200\224recovers in 21 days (cid:2) Anemia \200\224common Dosage of cisplatin Cisplatin is used in a variety of dosage schedules. The standard dose l imit for high-dose therapy is 100 mg/m2 as a single daily dose; higher doses have been expl ored in clinical trials, particularly in con- junction with neuroprotective agents. Alterna te schedules such as \201ve daily injections of 20 mg/m2 are favored in the treatment of teratoma. The initial clearance of cisplatin is rapid, followed by a much slower decline due to bindi ng to plasma proteins. Clearance is prolonged in patients with renal insuf \201ciency. Unl ike carboplatin, there is no clear evidence of a pharmacodynamic/pharmacokinetic relationsh ip with cisplatin; therefore dosage is usually based on empirical body-surface calculations . Clinical indications for cisplatin Cisplatin was a major step forward in the treatment of t esticular can- cer. In patients with metastatic disease, cisplatin-based combination therap y results in a complete clinical response in over 80% of patients, with the majority of the se achieving long-term cure. Cisplatin is also a major component of treatment of: OHO-09(135-180) 11/16/02 11:20 AM Page 158 PRINCIPLES OF CHEMOTHERAPY 158 (cid:2) Ovarian cancer (cid:2) Bladder cancer (cid:2) Penile cancer (cid:2) Cervical carcin oma (cid:2) Other squamous carcinomas, particularly those in the head and neck and non-small-cell bronchogenic carcinoma Combinations of cisplatin with other cytotoxic agents are common and are used in a variety of human solid cancers and pediatric tumors. Oxaliplatin Oxaliplatin is a platinum analog that differs from carboplatin and cisplatin, i n both chemical behavior and possibly its mechanism of action. Availability is limited. In vitro oxaliplatin has a broad spectrum of activity with marked differences from the spectru m seen with cis- platin or carboplatin e.g. oxaliplatin appears to be active in those cell lines that are resistant to cisplatin, and a high level of activity is seen in colorectal c ell lines. This has led to multiple Phase I, II, and III studies of the agent, evaluating i ts activity in colorectal cancer. Dosage of oxaliplatin Two commonly used regimens exist: (cid:2) First: 85 mg/m2 every two w eeks as a 2 \200\2236 hour infusion. (cid:2) Second: 130 mg/m2 over a similar length of tim e repeated every three weeks. However, a multitude of studies exist using a variety of different dosing regimens, includi ng chronomodulated infusion together with 5- \202uorouracil. When used as a single agent i n colorectal cancer, an overall response rate of around l0% is achieved in patients who are resistant to 5- \202uorouracil and around 20% in those not previously exposed to 5- \202 uorouracil. Of clinical importance, the highest response rates are seen when oxaliplatin is added to regimens with 5-FU and folinic acid. There is synergy between these agents. A stu dy in \201rst-line colorectal cancer has shown a marked increase in response rates in tho se patients given oxaliplatin; however there is no clear sur- vival advantage. OHO-09(135-180) 11/16/02 11:20 AM Page 159 TOPOISOMERASE INHIBITORS 159 Topoisomerase inhibitors Topoisomerase enzymes are a family of nuclear proteins with essential functions in regulati ng the topology of the DNA helix. Topoisomerase proteins appear to constantly monitor DNA s tructure, looking for points of increased tension. The protein then alters DNA ter- tiary s tructure, by creating transient strand breakage in DNA backbone. Eukaryotics has two forms of topoisomerase enzyme: (cid:2) Topoisomerase I (topoI) binds to double-stranded DNA and c leaves and religates one strand of duplex DNA. Relaxation of supercoiled DNA is then used d uring processes of replication, transcription, and recombination. (cid:2) Topoisomerase II (topoII) creates transient double-stranded breakage of DNA, allowi ng subsequent passage of a second intact DNA duplex through the break. Biochemical analysis has identi \201ed camptothecin (CPT) and its ana- logues as inhibitor s of topoI, while epipodophyllotoxins, etoposide and teniposide, are inhibitors of topoII. Topoisomerase I inhibitors CPT has been identi \201ed as the active constituent of an extr act isolated from the Chinese Tree Camptotheca acuminata. Mechanism of action studies demon strated that CPT stabilized co-valent adducts between genomic DNA and topoI. Early clinical studies with CPT observed anti-tumor activity in a variety of common solid tumors. However , a high rate of severe and unpredictable toxicities, including haemorhagic cystitis and ga strointestinal effects, were seen. This led to discontinuation of CPT \200\231s develop- me nt. The novelty of topoI as a cellular target for chemotherapy has result- ed in the developmen t of several analogs, such as irinotecan (CPT- 11) and topotecan. Both CPT-11 and topotecan have greater aqueous solubility than CPT. Analogs of CPT with an amino group at the 9-posi tion have enhanced activity, but are less soluble than topotecan or CPT-11. CPT and all CPT derivatives have a basic 5-ring structure. Reversible, pH-dependent non-enzymatic hydrolys is of the lactone ring of all CPT derivatives results in an open-ring carboxylate moiety. T he two are in equilibrium in aqueous buffers, but the carboxylate form is a less potent inh ibitor of topoI and a much less potent anti-tumor agent. To date, two CPT analogs have received regulatory approval for use in patients with solid tumors: OHO-09(135-180) 11/16/02 11:20 AM Page 160 PRINCIPLES OF CHEMOTHERAPY 160 (cid:2) Topotecan (hycamtin) in Europe and the US for treatment of advanced ovarian and non-small-cell lung cancers. (cid:2) CPT-11 (camptosar) in patients with advanced colorectal cancer. Both agents are cur rently administered as a 30 \200\22390 minute infusion daily for \201ve days every three weeks (topotecan) or weekly for four weeks, every six weeks (CPT-l1). Alternate intravenous , intra- peritoneal, and oral schedules are under evaluation and demonstrate anti-tumor act ivity. Side-effects (cid:2) Neutropenia \200\224common (cid:2) Diarrhea \200\224common (early or late) (cid:2) Thrombocytopenia (cid:2) Anemia (cid:2) Alopecia (cid:2) Nausea, vomiting Clinical pharmacology Clinical pharmacology of CPT-11 and topotecan is now relatively well developed. Both can be absorbed orally, with topotecan bioavailability of 30 \200\22350%; b oth are widely distributed throughout the body, with cerebrospinal \202uid topotecan conc entrations 30-50% of simultanous plasma concentrations. Topotecan undergoes negligible metabolism. CPT-11 is in itself relatively inactive and must be converted by carboxylesterases to SN-38 that has potent topoI inhibitory activity. SN-3 8 undergoes glucuronidation to inactive metabolite by uridine-diphosphosphate glucuronosyla transferase lAl and genetic polymorphism in the forma- tion of SN-38 glucuronide has been p roposed. Topotecan is primarily eliminated by the kidneys, with evidence for renal tubular secretion . A linear relationship between creatinine clear- ance and clearance of both total topoteca n and lactone form has been demonstrated. Approximately 20% of the total dose of CPT-11 is excreted unchanged in urine, whereas less than 1% is excreted as SN-38. Glucuronidation and biliary excretion appear to be principal mechanisms of elimination for SN-38. Several mechanisms of resistance to topoI inhibitors have been described in in vitro systems. These include: (cid:2) Factors that affect the stabilization of the topoI \200\223DNA complex (cid:2) Alterations in drug accumulation (cid:2) Decrease d activation of CPT-11 (cid:2) Decreased cellular content/activity of topoI (cid:2) Mutatio ns of the topo I enzyme OHO-09(135-180) 11/16/02 11:20 AM Page 161 TOPOISOMERASE INHIBITORS 161 Little is know about mechanisms of resistance to CPT or its analogs in human tumors. Topoisomerase II inhibitors In the 1960s, podophylin derivatives \200\224etoposide and teni poside \200\224 were found to have a unique mechanism of action, subsequently identi \201e d as inhibition of topoII. Etoposide and teniposide exert their action on topoII by: Inhibiting the ability of the enzyme to relegate the cleaved DNA complex (cid:2) (cid:2) Generating high levels of DNA with potentially toxic double- stranded breaks (cid:2) Promoting mutation (cid:2) Permanent double-stranded breaks Illegitimate recombination (cid:2) (cid:2) Apoptosis Chemically, teniposide only differs from etoposide in the substitution of a thenylidene ring in place of a methyl group. However, teniposide is approximately 10% mo re potent then etoposide, in terms of both in vitro cytotoxicity and DNA strand breakage, b ut the difference is less apparent in vivo where teniposide is only 1.5 \200\2233 times mor e potent. Etoposide and teniposide are poorly water-soluble and are formu- lated with a num ber of excipients including polysorbate (etoposide) or cremophor EL (teniposide). Etoposide can be administered by either oral or intravenous routes, teniposide only by intravenous i njection. Recently, etoposide phosphate was introduced, a more solu- ble yet bioequivalent form of etoposide designed to decrease infusion- related toxicity. Teniposide and etoposide are widely used in treatment of adult and pediatric malignancies. Etoposide has been more broadly used in front-line therapy for: (cid:2) Small-cell lung can cer (cid:2) Germ-cell tumors (cid:2) Kaposi \200\231s sarcoma (cid:2) As part of preoperati ve regimens for bone marrow transplantation Both agents are used in front-line therapy for childhood cancer, including: (cid:2) Acute lymphoblastic leukemia (cid:2) Neuroblastoma (ci d:2) Rhabdomyosarcoma The pattern of toxicity is very similar between both agents and inclu des: OHO-09(135-180) 11/16/02 11:20 AM Page 162 PRINCIPLES OF CHEMOTHERAPY 162 (cid:2) Neutropenia (cid:2) Alopecia (cid:2) Mucositis Infusion-related blood pressure changes (cid:2) (cid:2) Hypersensitivity reactions Clinical pharmacology Oral etoposide capsules have bioavailability of 60% with extensive va riation. Etoposide absorption appears to be non-linear with decreased bioavailability at do ses above 200 mg. Etoposide phosphate has a similar degree of bioavailability and suggestio ns of non-linear absorption. Both etoposide and teniposide are heavily protein-bound; use in patients with low albumin c oncentrations will result in greater than expected systemic toxicity due to the larger free (unbound) drug con- centrations. Although both etoposide and teniposide are distributed in to the CSF, they achieve concentrations of 0.1 \200\2234% of that measured simultaneously i n plasma. Both etoposide and teniposide are extensively metabolized. Production of a catechol metabol ite is mediated through P450 3A4 and may have intrinsic cytotoxic activity. Etoposide glucu ronide has been found to account for up to 30% of the administered dose. Etoposide is more rapidly eliminated than teniposide with: (cid:2) Faster systemic clearance (cid:2) Greater renal clearance (cid:2) Shorter eliminati on half-life Linear relationships between etoposide systemic clearance and creatinine clear ance have been described for both adult and pediatric patients. Both etoposide and teniposi de have demonstrated pharma- codynamic relationships, where measures of systemic exposure ( AUC, Css, trough concentrations, etc.) are correlated with hematological toxicity. OHO-09(135-180) 11/16/02 11:20 AM Page 163 ANTI-MICROTUBULE AGENTS 163 Anti-microtubule agents Tubulin-interactive agents, commonly known as \200\230spindle poisons \200\231 have a long history of use in cancer treatment. They act by binding to speci \201c sites on tubulin, a protein that polymerizes to form cellular microtubules. Microtubules are important struct ural units involved in a number of cellular activities, including formation of the mitotic spindle. Agents that bind to tubulin can be categorized according to their main tubulin binding site: (cid:2) Vinca alkaloid binding site (cid:2) Colchicine binding s ite (cid:2) Rhizoxin/maytansine binding site (cid:2) Tubulin sulfhydryl groups (cid:2) A se parate class of as yet uncharacterized binding sites The table focusses on important anti-m icrotubule agents in pre-clinical and clinical development, grouped into families to summar ize: (cid:2) Mechanism of action (cid:2) Major indications (cid:2) Administration (cid:2) P harmacokinetic data for clinical practice (cid:2) Selected important information for the cl inic Tubulin is an important target for anti-cancer drug development; several anti-tubulin agents have signi \201cant anti-cancer activity in the clinic. Taxanes were the most encou raging development in anti-cancer chemotherapy of the 1990s; paclitaxel, when incorporated in a \201rst- line chemotherapy regimen for advanced ovarian cancer leads to signi \201c ant prolongation of survival, while docetaxel can make a small but signi \201cant impact o n survival of metastatic breast cancer patients, even when given in a second- or third-line setting to heavily pre-treated patients. Recent progress observed with taxanes has led to renewed interest in anti-microtubule analo gs or drugs interacting with different sites on tubulin. In particular, agents with an impr oved pharmacological pro \201le and/or activity in vinca/taxane-resistant cell lines are o f interest. Several new anti-tubulin agents are in pre-clinical development. Most of the basic research into drug resistance has involved using pairs of sensitive and r esistant tumor cells derived from the same parental cell line, usually by serial passage in increasing concentra- tions of the drug under investigation. This is an arti \201cial sit uation O H 1 6 4 O 0 9 Table 9.1 Anti-microtubule agents ( 1 3 Class of spindle poison Useful indications (mechanism of action) Drug administration (IV doses in mg/m2) Main toxicities Pharmacokinetics and Comments of clinical 5 metabolism interest 1 8 Vincristine (VCR) (destabilization of polymerised tubulin ((cid:2)-tubulin)) 0.5 \200\2231.4 q 1 \200\2234 w Metabolized in the liver VCR induces multi-drug Neuropathy Leukaemias, lymphomas, pediatric (total individual dose: tumors, small-cell lung cancer, mu ltiple myeloma 0 ) resistance (MDR) by P-glycoprotein (PgP). Mutations in (cid:3)and (cid:2) tubulin proteins enhance stability against depolymerization. 2 mg) 1 1 / 1 6 / 0 2 Vinblastine (VBL) (same as VCR) Neutropenia, neuropathy Lymphomas, germ cell 6 \200\22310 q 2 \200\2234 w tumors, Kaposi \200\231s sarcoma, breast cancer Metabolized in the liver Neuropathy occurs less frequently than with VCR 1 1 : 2 Vindesine (VDS) same as VCR) prostate, 2 \200\2234 q 1 \200\2233 w Neutropenia, neuropathy Non-small-cell lung cancer (NSCLC), Metabolized in the liver Randomized trials (breast, 0 NSCLC, sarcomas, A M breast cancer, lymphomas and melanoma) with VDS showed no advantage over treatments without VDS. P a g e 1 Vinorelbine (NVB) (same as VCR) NSCLC, breast cancer 25 \200\22330 / w combinations: Neutropenia, constipation, neuropathy Metabolized in the liver Selective binding to the Tau 6