Metabolism of Cyclophosphamide by the Human Cytochrome P450 2B6 Polymorphic Variants

Abstract

The alkylating agent, cyclophosphamide, is a common therapeutic agent used in the treatment of many cancers. Importantly, the efficacy and toxicity of cyclophosphamide varies greatly in patient populations. Cytochrome 2B6 is the primary P450 isoform involved in the activation and metabolism of cyclophosphamide and a potential explanation for the observed therapeutic discrepancies is believed to be the highly polymorphic nature of CYP2B6. Common CYP2B6 polymorphisms are known to cause alterations in the metabolism of other known substrates, such as bupropion and efavirenz. Using a reconstituted purified system, the kinetics of cyclophosphamide metabolism have been determined for wild type CYP2B6 as well as several common polymorphisms including CYP2B6.4–7 and CYP2B6.9. Interestingly, CYP2B6.4 shows only mild variations from CYP2B6.1 in the Km, Vmax, and kcat whereas CYP2B6.7 and CYP2B6.9 have significantly lower kcat values. Other mutants to be tested are CYP2B6.5 and CYP2B6.6. In addition the kinetics of cyclophosphamide metabolism by CYP2B6 WT and the other polymorphic variants will be investigated in the lipid bilayer system, the Nanodisc, to reveal the impact that the lipid bilayer has on P450 function. [This work is supported in part by CA16954]