Potential contribution of cytochrome P450 2B6 to hepatic 4-hydroxycyclophosphamide formation: in vitro and in vivo investigations.

Publication Type:

Journal Article

Source:

Drug metabolism and disposition: the biological fate of chemicals, Volume 40, Issue 1, p.54-63 (2012)

Keywords:

2012, Adolescent, Aryl Hydrocarbon Hydroxylases, Center-Authored Paper, Child, Child, Preschool, Clinical Research Division, Cyclophosphamide, Genetic Variation, Humans, Infant, Microsomes, Liver, Nov 11, November 2011, Oxidoreductases, N-Demethylating, Public Health Sciences Division

Abstract:

Results from retrospective studies on the relationship between cytochrome P450 2B6 (CYP2B6) genotype and cyclophosphamide (CY) efficacy and toxicity in adult cancer patients have been conflicting. We evaluated this relationship in children, who have faster CY clearance and receive different CY-based regimens than adults. These factors may influence the cytochrome P450s (CYPs) metabolizing CY to 4-hydroxycyclophosphamide (4HCY), the principal precursor to CY's cytotoxic metabolite. Therefore, we sought to characterize the in vitro and in vivo roles of hepatic CYP2B6 and its main allelic variants in 4HCY formation. CYP2B6 is the major isozyme responsible for 4HCY formation in recombinant CYP Supersomes(®). In human liver microsomes (HLM) 4HCY formation correlated with known phenotypic markers of CYP2B6 activity, specifically formation of (S)-2-ethyl-1,5-dimethyl-3,3-diphenyl pyrrolidine and hydroxybupropion. However, in HLM, CYP3A4/5 also contributes to 4HCY formation at the CY concentrations similar to plasma concentrations achieved in children (0.1 mM). 4HCY formation was not associated with CYP2B6 genotype at low (0.1 mM) or high (1 mM) CY concentrations potentially because CYP3A4/5 and other isozymes also form 4HCY. To remove this confounder, 4HCY formation was evaluated in recombinant CYP2B6 enzymes, which demonstrated that 4HCY formation was lower for CYP2B6.4 and CYP2B6.5 compared to CYP2B6.1. In vivo, CYP2B6 genotype was not directly related to CY clearance or ratio of 4HCY/CY areas under the curve (AUCs) in 51 children receiving CY-based regimens. Concomitant chemotherapy agents did not influence 4HCY formation in vitro. We conclude that CYP2B6 genotype is not consistently related to 4HCY formation in vitro or in vivo.