ENHANCED CYCLOPHOSPHAMIDE AND IFOSFAMIDE ACTIVATION IN PRIMARY HUMAN HEPATOCYTE CULTURES - RESPONSE TO CYTOCHROME-P-450 INDUCERS AND AUTOINDUCTION BY OXAZAPHOSPHORINES

The anticancer oxazaphosphorine prodrugs cyclophosphamide and ifosfami de are activated in human liver by a 4-hydroxylation reaction catalyze d by multiple cytochrome P450 (CYP) enzymes, In the present study, we used a cultured human hepatocyte model to identify possible inducers o f the CYP-catalyzed activation of these two anticancer prodrugs, Treat ment of primary cultures of human hepatocytes with phenobarbital, dexa methasone, or rifampin elevated hepatocyte microsomal oxazaphosphorine 4-hydroxylation by up to 200-400% of control for both drug substrates , These inductions were associated with corresponding increases in imm unoreactive CYP2B6, CYP2C8, CYP2C9, and CYP3A4, all previously shown t o catalyze oxazaphosphorine activation, Rifampin (1 mu M, 96-h exposur e) was a particularly potent inducer of ifosfamide and cyclophosphamid e 4-hydroxylation, as well as of CYP3A protein levels and CYP3A-depend ent testosterone 6 beta-hydroxylation, CYP3A4, CYPZC8, and CYP2C9 prot ein levels were also increased by exposure of the hepatocytes to cyclo phosphamide or ifosfamide (50 mu M), which thereby enhanced their own rates of 4-hydroxylation in the cultured hepatocytes, In one human hep atocyte culture that contained the polymorphically expressed CYP3A5 in addition to the more widely expressed CYP3A4, only CYP3A4 was induced by cyclophosphamide, ifosfamide, and rifampin, These studies: (a) dem onstrate an underlying metabolic basis for the clinically important ox azaphosphorine autoinduction pharmacokinetics seen with these drugs in cancer patients; and (b) identify rifampin and other CYP inducers as potentially useful for increasing the rates of cyclophosphamide 4-hydr oxylation and ifosfamide 4-hydroxylation in human liver in a manner th at could favorably impact the clinical pharmacokinetics of these antic ancer prodrugs.