Pharmacogenetics of anticancer agents: lessons from amonafide and irinotecan.
Amonafide and irinotecan are anticancer drugs representative of the clinical relevance of N-acetyltransferase (NAT) and uridine diphosphate glucuronosyltransferase (UGT) polymorphisms in cancer chemotherapy, respectively. Amonafide, a substrate for the polymorphic NAT2, has an active metabolite, N-acetyl-amonafide. Using caffeine as a probe, slow and rapid acetylators of amonafide were identified. Fast acetylators experienced greater myelosuppression than did slow acetylators, and a reduced dose of amonafide for fast acetylators has been recommended. A pharmacodynamic model based on acetylator phenotype, pretreatment white blood cell count, and gender has been proposed for dose individualization. The strategy adopted for amonafide is a model for future investigations in pharmacogenetics, although amonafide is no longer in clinical development. SN-38, the active metabolite of irinotecan, is glucuronidated to the inactive SN-38 glucuronide by UGT1A1, the isoform catalyzing bilirubin glucuronidation. Genetic defects in UGT1A1 determine Crigler-Najjar and Gilbert's syndromes characterized by unconjugated hyperbilirubinemia. Gilbert's syndrome often remains undiagnosed and occurs in up to 19% of individuals. Gilbert's syndrome is due to a homozygous TA insertion in the TATAA promoter of UGT1A1, leading to the mutated (TA)(7) allele. Irinotecan toxicity depends on the individual glucuronidation rate of SN-38. Decreased SN-38 glucuronidating activity has been found in livers obtained from individuals carrying the (TA)(7) allele. A phenotyping procedure for UGT1A1 has not been identified and genotyping of the UGT1A1 promoter in patients receiving irinotecan may identify patients at increased risk of toxicity. A clinical trial at the University of Chicago is ongoing to demonstrate the predictive significance of UGT1A1 genotyping for irinotecan pharmacodynamics.
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Committee on Clinical Pharmacology, The University of Chicago, Illinois 60637, USA.
Iyer
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Ratain
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D000225:Adenine; D000970:Antineoplastic Agents / Q000494:pharmacology*; D002166:Camptothecin / Q000031:analogs & derivatives* / Q000494:pharmacology*; D006801:Humans; D007094:Imides / Q000494:pharmacology*; D000077146:Irinotecan; D007546:Isoquinolines / Q000494:pharmacology*; D053644:Naphthalimides; D063065:Organophosphonates; D010597:Pharmacogenetics; D011110:Polymorphism, Genetic
D000970:Antineoplastic Agents; D007094:Imides; D007546:Isoquinolines; D053644:Naphthalimides; D063065:Organophosphonates; C037020:amonafide; D000077146:Irinotecan; D000225:Adenine; D002166:Camptothecin