Kinetics of Cyclophosphamide Metabolism in Human, Dog, Cat, and Mouse and Relationship to Cytotoxic Activity and Pharmacokinetics.

Cyclophosphamide (CP), a prodrug that is enzymatically converted to the cytotoxic 4-hydroxycyclophosphamide (4OHCP) by hepatic enzymes, is commonly used in both human and veterinary medicine to treat cancers and modulate the immune system. The metabolism of CP in humans, dogs, cats, and mice was investigated with liver microsomes and apparent KM , Vmax , and intrinsic clearance (Vmax /KM ,) parameters were estimated. The inter- and intra-species variation in kinetics was vast. Dog microsomes were, on average, 55-fold more efficient than human microsomes, 2.8-fold more efficient than cat microsomes, and 1.2-fold more efficient than mouse microsomes at catalyzing CP bioactivation. These differences translated to cell-based systems. Breast cancer cells exposed to 4OHCP via CP bioactivation by microsomes resulted in a stratification of cytotoxicity dependent on the species of microsomes measured by IC50 : dog (31.65 μM), mouse (44.95 μM), cat (272.6 μM), and human (1857 μM). The contribution of cytochrome P450s (CYP), specifically CYP2B, CYP2C, and CYP3A to CP bioactivation were examined: CYP3A inhibition resulted in no change in 4OHCP formation; CYP2B inhibition slightly reduced 4OHCP in humans, cats, and mice; and CYP2C inhibition drastically reduced 4OHCP formation in each species. Semi-physiologic modeling of CP metabolism using scaled metabolic parameters resulted in simulated data that closely matched published pharmacokinetic (PK) profiles, determined by non-compartmental analysis. The results highlight differential CP metabolism delineated by species and demonstrate the importance of metabolism on CP clearance.