Modulation of the cytochrome P450–mediated metabolism of ifosfamide by ketoconazole and rifampin

The autoinducible metabolic transformation of the anticancer agent ifosfamide involves activation through 4-hydroxyifosfamide to the ultimate cytotoxic ifosforamide mustard and deactivation to 2- and 3-dechloroethylifosfamide with concomitant release of the neurotoxic chloroacetaldehyde. Activation is mediated by cytochrome P450 (CYP) 3A4 and deactivation by CYP3A4 and CYP2B6. The aim of this study was to investigate modulation of the CYP-mediated metabolism of ifosfamide with ketoconazole, a potent inhibitor of CYP3A4, and rifampin (INN, rifampicin), an inducer of CYP3A4/CYP2B6. In a double-randomized, 2-way crossover study a total of 16 patients received ifosfamide 3 g/m(2) per 24 hours intravenously, either alone or in combination with 200 mg ketoconazole twice daily (1 day before treatment and 3 days of concomitant administration) or 300 mg rifampin twice daily (3 days before treatment and 3 days of concomitant administration). Plasma pharmacokinetics and urinary excretion of ifosfamide, 2- and 3-dechloroethylifosfamide, and 4-hydroxyifosfamide were assessed in both courses. Data analysis was performed with a population pharmacokinetic model with a description of autoinduction of ifosfamide. Rifampin increased the clearance of ifosfamide at the start of therapy at 102%. The fraction of ifosfamide metabolized to the dechloroethylated metabolites was increased, whereas exposure to the metabolites was decreased as a result of increased elimination. The fraction metabolized and the exposure to 4-hydroxyifosfamide were not significantly influenced. Ketoconazole did not affect the fraction metabolized or the exposure to the dechloroethylated metabolites, whereas both parameters were reduced with 4-hydroxyifosfamide. Coadministration of ifosfamide with ketoconazole or rifampin did not produce changes in the pharmacokinetics of the parent or metabolites that may result in an increased benefit of ifosfamide therapy