The Role of Constitutive Androstane Receptor in the Bioactivation of Oxazaphosphorines

Abstract

Prodrugs are pharmaceutical substances that are administered in an inactive form and are subsequently converted to the active therapeutic moiety through bioactivation. Among them, oxazaphosphorines represent a major class of anti-cancer prodrugs, with cyclophosophamide (CPA) and ifosfamide (IFO) being the most widely used ones. Because of the increased polypharmacy in oxazaphosphorine-based chemotherapy, drug-drug interactions have been raising concerns. Hepatic CYP2B6 and CYP3A4 differentially contribute to the activation and inactivation of CPA and IFO and many clinically used drugs and environmental compounds can influence the expression of these enzymes. Constitutive Androstane Receptor (CAR) and Pregnane X Receptor (PXR)are important regulators of CYP2B6 and CYP3A4 expression. Activation of PXR induces both isozymes, while selective activation of CAR leads to preferential induction of CYP2B6 over CYP3A4 in human liver. Since CPA is predominantly bioactivated by CYP2B6 while deactivated through CYP3A4, we hypothesized that a combination of CPA and CAR activators may enhance the therapeutic effect of CPA by preferential induction of CYP2B6 over 3A4. The hypothesis was tested in an innovative hepatocyte-hematopoietic cell co-culture system that was demonstrated to be a useful in vitro model for studying the biotransformation and therapeutic effects of CPA and potentially other prodrugs in an environment that closely mimics in vivo conditions.Based on this system, CITCO was proved to preferentially induce CYP2B6 over CYP3A4 and subsequently enhance CPA anti-cancer activity. Similar trends were observed using rodent primary hepatocytes and corresponding CAR activators. Because CITCO is not clinically available, a panel of FDA approved drugs together with 800 Chinese herbal medical products were screened for preferential induction profile of CYP2B6 over CYP3A4. Matrine (Figure S1), the major component of two Chinese medical products was found to selectively induce CYP2B6 over CYP3A4 and enhanced CPA therapeutic effect in vitro in the co-culture system. The autoinduction profile and the underlying mechanism of CPA and IFO were also addressed. Both compounds induced CYP2B6 and CYP3A4 at mRNA and protein levels in human primary hepatocyte. While both drugs were proved to be PXR activators which indiscriminately induce CYP2B6 and CYP3A4, CPA but not IFO was demonstrated to be an indirect CAR activator.