Genetic polymorphisms of CYP2B6 affect the pharmacokinetics/pharmacodynamics of cyclophosphamide in Japanese cancer patients

Objective To evaluate the effects of genetic polymorphisms of drug metabolizing enzymes on the pharmacolkinetics of cyclophosphamide and its active metabolite, 4-hydroxycyclophosphamide, and on the pharmacodynamics. Experimental Design One hundred and three Japanese patients with malignant lymphoma or breast cancer treated with cyclophosphamide (500-750 mg/m(2)) participated in this study. The plasma concentrations of cyclophosphamide and 4-hydroxycyclophosphamide were determined by high-performance liquid chromatography, and pharmacolkinetic parameters were calculated. The genotypes of CYP2B6, CYP2C19, CYP3A4, CYP3A5, ALDMA1, GST genes were determined by allele-specific polymerase chain reaction or polymerase chain reaction-restriction -fragment length polymorphism. Results A large interindividual difference (54-fold) was observed in the area under the curve ratio of 4-hydroxycyclophosphamide/cyclophosphamide calculated as the metabolic index. We first proved that leukocytopenia and neutropenia were significantly (P<0.01) related to the area under the curve of 4-hydroxycyclophosphamide. We found that the homozygotes of CYP2B6*6 (Q172H and K262R) showed significantly (P<0.05) higher clearance and shorter half-life of cyclophosphamide than heterozygotes and homozygotes of CYP2B6*1. The small sample size, however, limited the impact. On the other hand, it was clearly demonstrated that the patients possessing the single nucleotide polymorphisms of the CYP2B6 gene, g.-2320T>C, g.-750T>C (5'-flanking region), g.15582C>T (intron 3), or g.18492T>C (intron 5), had significantly lower area under the curve ratios of 4-hydroxycyclophosphamide/cyclophosphamide, indicating a decreased cyclophosphamide 4-hydroxylation. Of particular importance was the finding that leukocytopenia was significantly related to the single nucleotide polymorphisms g. - 2320T > C, g. - 750T > C, and g.18492T>C in CYP2B6 gene, which are highly linked. No relationship was observed between the pharmacokinetics of cyclophosphamide or 4-hydroxycyclophosphamide and genetic polymorphisms of the other enzymes. Conclusions We clarified that the single nucleotide polymorphisms in the promoter region or introns in the CYP2B6 affect the potency of cyclophosphamide activation to 4-hydroxycyclophosphamide. This information would be valuable for predicting adverse reactions and the clinical efficacy of cyclophosphamide.