Cytochrome P450 Gene-directed Enzyme Prodrug Therapy (GDEPT) for Cancer

Abstract

Several commonly used cancer chemotherapeutic prodrugs, including cyclophosphamide and ifosfamide, are metabolized in the liver by a cytochrome P450 (CYP)-catalyzed prodrug activation reaction that is required for therapeutic activity. Preclinical studies have shown that the chemosensitivity of tumors to these prodrugs can be dramatically increased by P450 gene transfer, which confers the capability to activate the prodrug directly within the target tissue. This P450 gene-directed enzyme prodrug therapy (P450 GDEPT) greatly enhances the therapeutic effect of P450-activated anti-cancer prodrugs without increasing host toxicity associated with systemic distribution of active drug metabolites formed by the liver. The efficacy of P450 GDEPT can be enhanced by further increasing the partition ratio for tumor:liver prodrug activation in favor of increased intratumoral metabolism. This can be achieved by co-expression of P450 with the flavoenzyme NADPH-P450 reductase, which increases P450 metabolic activity, by localized prodrug delivery, or by the selective pharmacologic inhibition of liver prodrug activation. P450 GDEPT prodrug substrates are diverse in their structure, mechanism of action, and optimal prodrug-activating P450 gene; they include both established and investigational anticancer prodrugs, as well as bioreductive drugs that can be activated by P450/P450 reductase in a hypoxic tumor environment. Several strategies may be employed to achieve the tumor-selective gene delivery that is required for the success of P450 GDEPT; these include the use of tumor-targeted cellular vectors and tumor-selective oncolytic viruses. Overall, P450-based GDEPT presents several important, practical advantages over other GDEPT strategies that should facilitate the incorporation of P450 GDEPT into existing cancer treatment regimens. A recent report of clinical efficacy in a P450-based phase I/II gene therapy trial for pancreatic cancer patients supports this conclusion.

Enhancement of intratumoral cyclophosphamide pharmacokinetics and antitumor activity in a P450 2B11-based cancer gene therapy model

“…6,7 These enzymes catalyze the metabolism of more than 35 anticancer drugs, 8,9 of which 12 drugs, including the widely used alkylating agent prodrug cyclophosphamide (CPA) and its isomer ifosfamide (IFA), are activated via P450 metabolism. 10 CPA is used in both adjuvant and highdose chemotherapy settings and is effective against a broad range of tumors, including breast cancer and lymphomas. 11 Two hepatic cytochromes P450, P450 2B6 and P450 3A4, catalyze a substantial fraction of CPA activation in human liver with electron input from NADPH P450 reductase.…”

Enhanced antitumor activity of P450 prodrug-based gene therapy using the low Km cyclophosphamide 4-hydroxylase P450 2B11

“…Tumor-selective delivery of a prodrug activation gene can be achieved in a variety of ways, including the use of viral vectors that incorporate tumor-specific promoters (4) or selectively replicate in tumor cells that present specific genetic defects, such as p53 deficiency (5), or are characterized by pathophysiologic conditions, such as hypoxia (6). One GDEPT strategy uses genes that encode mammalian cytochrome P450 enzymes (7,8), which metabolize anticancer prodrugs, including the oxazaphosphorines cyclophosphamide and ifosfamide, to active, cytotoxic metabolites (9,10). Prodrug-activating P450 enzymes are expressed at high levels in liver but are generally deficient or present at very low levels in human tumors (11 -14).…”

“…Prodrug-activating P450 enzymes are expressed at high levels in liver but are generally deficient or present at very low levels in human tumors (11 -14). Consequently, substantial increases in intratumoral production of active drug metabolites and enhanced anticancer activity may be realized by delivery of prodrug activation P450 genes to tumor cells (9,15,16).…”

Cytochrome P450 Profile of Colorectal Cancer: Identification of Markers of Prognosis

“…The bioreductive prodrug AQ4N, a topoisomerase inhibitor, is activated to the cytotoxic amine AQ4 by cytochrome P450 -mediated bioreduction selectively under the hypoxic conditions found in tumor tissue. CYP3A4, CYP1A1, and CYP1B1 all contribute to bioreduction of AQ4N (17,18 (20) and AVI-4557, an antisense construct specifically targeted against CYP3A4, has recently completed a phase 1 study (21). Gene-directed prodrug therapy is also being used to deliver exogenous P450s (21,22).…”