Association of cyclophosphamide pharmacokinetics to polymorphic cytochrome P4502C19

Cyclophosphamide (CP), a widely used cytostatic, is metabolized by polymorphic drug metabolizing enzymes particularly cytochrome P450 (CYP) enzymes. Its side effects and clinical efficacy exhibit a broad interindividual variability, which might be due to differences in pharmacokinetics. CP-kinetics were determined in 60 patients using a global and a population pharmacokinetic model considering functionally relevant polymorphisms of CYP2B6, CYP2C9, CYP2C19, CYP3A5, and GSTA1. Moreover, metabolic ratios were calculated for selected CP metabolites, analyzed by P-31-NMR-spectroscopy. Analysis of variance revealed that the CYP2C19*2 genotype influenced significantly pharmacokinetics of CP at doses <= 1000 mg/m(2), whereas there was no evidence of an association of other genotypes to CP elimination or clearance. Mean (+/- SD) CP elimination constants k(e) ( (-1)) were 0.109 +/- 0.025 in 44 CYP2C19*1/*1 subjects, 0.088 +/- 0.018 in 13 CYP2C19* 1/* 2, and 0.076 +/- 0.014 in three inactive CYP2C19* 2/* 2 carriers ( P = 0.009). At CP doses higher than 1000 mg/m(2), a significantly increase of elimination was observed ( P = 0.001), possibly due to CYP induction. Further studies should link these findings with the clinical outcome.