Kinetics of Cyclophosphamide Metabolism in Humans, Dogs, Cats, and Mice and Relationship to Cytotoxic Activity and Pharmacokinetics

• Published in: Drug metabolism and disposition Vol. 47; no. 3; pp. 257 - 268
• Main Authors: Ramirez, Dominique A, Collins, Keagan P, Aradi, Allister E, Conger, Katherine A, Gustafson, Daniel L
• Format: Journal Article
• Language: English
• Published: United States American Society for Pharmacology and Experimental Therapeutics, Inc 01-03-2019; The American Society for Pharmacology and Experimental Therapeutics
• Subjects: Animals; Antineoplastic Agents, Alkylating - metabolism; Antineoplastic Agents, Alkylating - pharmacokinetics; Antineoplastic Agents, Alkylating - therapeutic use; Biocompatibility; Biological activity; Breast cancer; Cat Diseases - drug therapy; Cat Diseases - immunology; Cats; Cell Line, Tumor; Computer simulation; Cyclophosphamide; Cyclophosphamide - metabolism; Cyclophosphamide - pharmacokinetics; Cyclophosphamide - therapeutic use; Cytochrome; Cytochrome P-450 Enzyme Inhibitors; Cytochrome P-450 Enzyme System - metabolism; Cytochrome P-450 Enzyme System - pharmacology; Cytochromes; Cytotoxicity; Dog Diseases - drug therapy; Dog Diseases - immunology; Dogs; Female; Humans; Immune clearance; Immune system; Immunosuppressive Agents - metabolism; Immunosuppressive Agents - pharmacokinetics; Immunosuppressive Agents - therapeutic use; Inhibition; Kinetics; Liver; Male; Metabolism; Mice; Microsomes; Microsomes, Liver; Models, Biological; Neoplasms - drug therapy; Oxidation-Reduction - drug effects; Parameter estimation; Pharmacokinetics; Pharmacology; Prodrugs - metabolism; Prodrugs - pharmacokinetics; Prodrugs - therapeutic use; Reaction kinetics; Species; Toxicity; Veterinary medicine
• ISSN: 0090-9556; 1521-009X
• DOI: 10.1124/dmd.118.083766

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. We investigated the metabolism of CP in humans, dogs, cats, and mice using liver microsomes; apparent
,
, and intrinsic clearance (
/
) parameters were estimated. The interspecies and intraspecies variations in kinetics were 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 that was dependent on the species of microsomes measured by IC
: dog (31.65
M), mouse (44.95
M), cat (272.6
M), and human (1857
M). The contributions of cytochrome P450s, 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. Semiphysiologic modeling of CP metabolism using scaled metabolic parameters resulted in simulated data that closely matched published pharmacokinetic profiles, determined by noncompartmental analysis. The results highlight differential CP metabolism delineated by species and demonstrate the importance of metabolism on CP clearance.