Ondansetron and tropisetron are highly specific and selective members of the 5-HT3 receptor antagonists and are used for the prevention of chemotherapy-induced, radiation-induced and postoperative nausea and vomiting. While tropisetron is extensively metabolized by CYP2D6 to inactive metabolites, ondansetron is metabolized by multiple cytochrome P450 enzymes including CYP3A4, CYP1A2 and CYP2D6, though there is substantial data to support a major role of CYP2D6 in ondansetron metabolism. For both drugs, there is evidence linking the CYP2D6 genotype with phenotypic variability in drug efficacy. CYP2D6 ultrarapid metabolizers may have increased metabolism of the drugs, resulting in decreased drug efficacy. There are suitable alternatives to ondansetron and tropisetron that are not affected by CYP2D6 metabolism. Therapeutic guidelines for ondansetron and tropisetron based on CYP2D6 genotype have been published in Clinical Pharmacology and Therapeutics by the Clinical Pharmacogenetics Implementation Consortium (CPIC) and are available on the PharmGKB website. [from
PharmGKB]