Relationship of drug metabolizing enzyme genotype to plasma levels as well as myelotoxicity of cyclophosphamide in breast cancer patients

Purpose The cytotoxic drug cyclophosphamide (CP) is bioactivated into 4-hydroxy-cyclophosphamide (4-OH-CP) through cytochrome P450 enzymes and cleared through aldehyde dehydrogenase and glutathione S-transferase. This prospective study analyzes the influence of drug metabolizing enzyme genotype on (1) plasma 4-OH-CP:CP ratio and (2) myelotoxicity in breast cancer patients on 500 mg/m ² cyclophosphamide. Methods Sixty-eight female breast cancer patients on FAC (fluorouracil, adriamycin, cyclophosphamide) were included. Genotyping of cytochrome P450 enzymes CYP2B6, CYP2C9, CYP2C19, CYP3A5, aldehyde dehydrogenase (ALDH3A1), and glutathione S-transferase (GSTA1) was done either through RFLP or pyrosequencing. Plasma CP and 4-OH-CP were measured immediately and 1 and 2 h after the end of infusion through LCMS. The leukocyte count was determined on day 10 and 20 after chemotherapy. Results At CP dose of 500 mg/m2, the 4-OH-CP:CP ratio was negatively affected by CYP2C19*2 genotype (p=0.039) showing a gene-dose effect. Moreover ALDH3A1*2 genotype increased 4-OH-CP:CP ratio (p=0.037). These effects did not remain significant in a univariate analysis of variance including all genotypes. GSTA1*B carriers were at increased risk of severe leucopenia (OR 6.94; 95% CI 1.75-27.6, p=0.006). Conclusion The myelotoxicity in patients receiving FAC is related to the activity of the phase-II enzyme GSTA1 but is independent of the formation of 4-OH-CP.