title: Enantioselective metabolism and cytotoxicity of R-ifosfamide and S-ifosfamide by tumor cell-expressed cytochromes P450.
abstract: 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 K(m) value of 200 microM, approximately 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.
authors.affiliation: Division of Cell and Molocular Biology, Department of Biology, Boston University, Boston, MA 02215, USA.
authors.forename: Chong-Sheng
authors.initials: CS
authors.lastname: Chen
authors.forename: Youssef
authors.initials: Y
authors.lastname: Jounaidi
authors.forename: David J
authors.initials: DJ
authors.lastname: Waxman
mesh_terms: D000079:Acetaldehyde / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D018906:Antineoplastic Agents, Alkylating / Q000494:pharmacology*; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D006224:Cricetinae; D003412:Cricetulus; D003520:Cyclophosphamide / Q000494:pharmacology*; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D018076:DNA, Complementary / Q000378:metabolism; D015870:Gene Expression; D006801:Humans; D007069:Ifosfamide / Q000031:analogs & derivatives / Q000494:pharmacology*; D009251:NADPH-Ferrihemoprotein Reductase / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism; D011355:Prodrugs / Q000494:pharmacology*; D051381:Rats; D013237:Stereoisomerism
chemical_list: D018906:Antineoplastic Agents, Alkylating; D018076:DNA, Complementary; D011355:Prodrugs; D003520:Cyclophosphamide; D003577:Cytochrome P-450 Enzyme System; C004656:chloroacetaldehyde; D009251:NADPH-Ferrihemoprotein Reductase; D000079:Acetaldehyde; D007069:Ifosfamide