PT - JOURNAL ARTICLE AU - Chong-Sheng Chen AU - Youssef Jounaidi AU - David J. Waxman TI - ENANTIOSELECTIVE METABOLISM AND CYTOTOXICITY OF <em>R-</em>IFOSFAMIDE AND <em>S-</em>IFOSFAMIDE BY TUMOR CELL-EXPRESSED CYTOCHROMES P450 AID - 10.1124/dmd.105.004788 DP - 2005 Sep 01 TA - Drug Metabolism and Disposition PG - 1261--1267 VI - 33 IP - 9 4099 - http://dmd.aspetjournals.org/content/33/9/1261.short 4100 - http://dmd.aspetjournals.org/content/33/9/1261.full SO - Drug Metab Dispos2005 Sep 01; 33 AB - The anticancer prodrug ifosfamide (IFA) contains a chiral phosphorous atom and is administered in the clinic as a racemic mixture of R-IFA and S-IFA. Hepatic cytochrome P450 (P450) enzymes exhibit enantioselective preferences in the metabolism of R-IFA and S-IFA; however, the impact of this selectivity on P450-dependent anticancer activity is not known. Presently, the metabolism and cytotoxicity of R-IFA and S-IFA were determined in 9L gliosarcoma and Chinese hamster ovary tumor cells expressing an IFA-activating P450 enzyme and by in vitro steady-state kinetic analysis using cDNA-expressed P450 enzymes. Tumor cells expressing P450 enzyme CYP3A4 were the most sensitive to R-IFA cytotoxicity, whereas tumor cells expressing CYP2B1 or CYP2B6 were most sensitive to cyclophosphamide (CPA), an isomer of IFA. Correspondingly, CYP3A4-expressing cells and cDNA-expressed CYP3A4 metabolized R-IFA to yield the active, 4-hydroxylated metabolite at a 2- to 3-fold higher rate than they metabolized S-IFA or CPA. CYP2B cells and cDNA-expressed CYP2B enzymes metabolized CPA almost exclusively by 4-hydroxylation, whereas R-IFA and S-IFA were substantially converted to inactive, N-dechloroethylated metabolites. Further investigation revealed that CYP3A1, a rat enzyme, exhibited superior kinetic properties compared with the human enzyme CYP3A4, with R-IFA and S-IFA both metabolized with high catalytic efficiency by 4-hydroxylation and with a Km value of 200 μM, ∼5-fold lower than CYP3A4. Based on these kinetic parameters and metabolic profiles, R-IFA is expected to exert greater anticancer activity than S-IFA or CPA against tumors that express CYP3A enzymes, whereas tumors expressing CYP2B enzymes may be more sensitive to CPA treatment. The American Society for Pharmacology and Experimental Therapeutics