The constitutive androstane receptor is a novel therapeutic target facilitating cyclophosphamide-based treatment of hematopoietic malignancies

Cyclophosphamide (CPA) is one of the most widely used chemotherapeutic prodrugs that undergoes hepatic bioactivation mediated predominantly by cytochrome P450 (CYP) 2B6. Given that the CYP2B6 gene is primarily regulated by the constitutive androstane receptor (CAR, NR1I3), we hypothesize that selective activation of CAR can enhance systemic exposure of the pharmacologically active 4-hydroxycyclophosamide (4-OH-CPA), with improved efficacy of CPA-based chemotherapy. In this study, we have developed a unique human primary hepatocyte (HPH)-leukemia cell coculture model; the chemotherapeutic effects of CPA on leukemia cells can be directly investigated in vitro in a cellular environment where hepatic metabolism was well maintained. Our results demonstrated that activation of CAR preferentially induces the expression of CYP2B6 over CYP3A4 in HPHs, although endogenous expression of these enzymes in leukemia cells remains negligible. Importantly, coadministration of CPA with a human CAR activator led to significantly enhanced cytotoxicity in leukemia cells by inducing the apoptosis pathways, without concomitant increase in the off-target hepatotoxicity. Associated with the enhanced antitumor activity, a time and concentration-dependent increase in 4-OH-CPA formation was observed in the coculture system. Together, our findings offer proof of concept that CAR as a novel molecular target can facilitate CPA-based chemotherapy by selectively promoting its bioactivation. (Blood. 2013;121(2):329-338)