title: Differential activation of cyclophosphamide and ifosphamide by cytochromes P-450 2B and 3A in human liver microsomes.
abstract: The present study identifies the specific human cytochrome P-450 (CYP) enzymes involved in hydroxylation leading to activation of the anticancer drug cyclophosphamide and its isomeric analogue, ifosphamide. Substantial interindividual variation (4-9-fold) was observed in the hydroxylation of these oxazaphosphorines by a panel of 12 human liver microsomes, and a significant correlation was obtained between these 2 activities (r = 0.85, P < 0.001). Enzyme kinetic analyses revealed that human liver microsomal cyclophosphamide 4-hydroxylation and ifosphamide 4-hydroxylation are best described by a 2-component Michaelis-Menten model composed of both low Km and high Km P-450 4-hydroxylases. To ascertain whether one or more human P-450 enzymes are catalytically competent in activating these oxazaphosphorines, microsomal fractions prepared from a panel of human B-lymphoblastoid cell lines stably transformed with individual P-450 complementary DNAs were assayed in vitro for oxazaphosphorine activation. Expressed CYP2A6, -2B6, -2C8, -2C9, and -3A4 were catalytically competent in hydroxylating cyclophosphamide and ifosphamide. Whereas CYP2C8 and CYP2C9 have the characteristics of low Km oxazaphosphorine 4-hydroxylases, CYP2A6, -2B6, and -3A4 are high Km forms. In contrast, CYP1A1, -1A2, -2D6, and -2E1 did not produce detectable activities. Furthermore, growth of cultured CYP2A6- and CYP2B6-expressing B-lymphoblastoid cells, but not of CYP-negative control cells, was inhibited by cyclophosphamide and ifosphamide as a consequence of prodrug activation to cytotoxic metabolites. Experiments with P-450 form-selective chemical inhibitors and inhibitory anti-P-450 antibodies were then performed to determine the contributions of individual P-450s to the activation of these drugs in human liver microsomes. Orphenadrine (a CYP2B6 inhibitor) and anti-CYP2B IgG inhibited microsomal cyclophosphamide hydroxylation to a greater extent than ifosphamide hydroxylation, consistent with the 8-fold higher activity of complementary DNA-expressed CYP2B6 with cyclophosphamide. In contrast, troleandomycin, a selective inhibitor of CYP3A3 and -3A4, and anti-CYP3A IgG substantially inhibited microsomal ifosphamide hydroxylation but had little or no effect on microsomal cyclophosphamide hydroxylation. By contrast, the CYP2D6-selective inhibitor quinidine did not affect either microsomal activity, while anti-CYP2A antibodies had only a modest inhibitory effect. Overall, the present study establishes that liver microsomal CYP2B and CYP3A preferentially catalyze cyclophosphamide and ifosphamide 4-hydroxylation, respectively, suggesting that liver P-450-inducing agents targeted at these enzymes might be used in cancer patients to enhance drug activation and therapeutic efficacy.
authors.affiliation: Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115.
authors.forename: T K
authors.initials: TK
authors.lastname: Chang
authors.forename: G F
authors.initials: GF
authors.lastname: Weber
authors.forename: C L
authors.initials: CL
authors.lastname: Crespi
authors.forename: D J
authors.initials: DJ
authors.lastname: Waxman
mesh_terms: D000906:Antibodies / Q000276:immunology; D001711:Biotransformation; D003520:Cyclophosphamide / Q000378:metabolism*; D051544:Cytochrome P-450 CYP3A; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000276:immunology / Q000502:physiology*; D006801:Humans; D006900:Hydroxylation; D007069:Ifosfamide / Q000378:metabolism*; D066298:In Vitro Techniques; D008862:Microsomes, Liver / Q000201:enzymology* / Q000378:metabolism; D009966:Orphenadrine / Q000494:pharmacology
chemical_list: D000906:Antibodies; D065607:Cytochrome P-450 Enzyme Inhibitors; D003520:Cyclophosphamide; D003577:Cytochrome P-450 Enzyme System; D009966:Orphenadrine; C104464:CYP3A protein, human; D051544:Cytochrome P-450 CYP3A; D007069:Ifosfamide