1-(2-CHLOROETHYL)-3-CYCLOHEXYL-1-NITROSOUREA (CCNU) MODULATES RAT-LIVER MICROSOMAL CYCLOPHOSPHAMIDE AND IFOSPHAMIDE ACTIVATION BY SUPPRESSING CYTOCHROME-P450 2C11 MESSENGER-RNA LEVELS

The alkylating anticancer drug 1-(2-chloroethyl)-3-cyclohexyl-1-nitros ourea (CCNU; lomustine) is frequently administered to cancer patients as part of a combination chemotherapy regimen. Previous studies have i ndicated that CCNU treatment of adult male rats leads to prolonged dec reases in liver cytochrome P450 (CYP)-mediated enzyme activities. Beca use the alkylating agent prodrugs cyclophosphamide and ifosphamide are known to be activated by liver cyto chrome P450 enzymes, the potentia l for interaction between CCNU and these oxazaphosphorines was examine d. Treatment of adult male rats with a single dose of CCNU (30 mg/kg i p) resulted in a progressive loss of liver microsomal cyclophosphamide and ifosphamide hydroxylation activities in vitro (30-60% decrease af ter 7-27 days). The individual liver P450 forms modulated by CCNU were then identified using P450 form-specific microsomal testosterone hydr oxylase assays. CCNU treatment was found to decrease substantially CYP 2C11-dependent testosterone 2 alpha-hydroxylase activity (30-90% decre ase after 14 or 27 days), but it did not affect CYP3A2-dependent testo sterone GB-hydroxylase activity. It only modestly decreased CYP2A2-med iated testosterone 15 alpha-hydroxylase activity. The reduction in CYP 2C11 activity was not associated with a decline in liver microsomal NA DPH-cytochrome P450 reductase activity, but rather was caused by a com plete suppression of CYP2C11 mRNB levels. In contrast to other alkylat ing agents, such as cisplatin, which is known to feminize the overall expression profile of gender-specific liver enzymes, CCNU did not incr ease levels of the female-predominant liver enzymes steroid 5 alpha-re ductase and CYP2C7, nor did it deplete circulating testosterone levels . Overall, these results establish that CCNU treatment can compromise cyclophosphamide and ifosphamide activation in the rat model and that this occurs by pretranslational mechanism(s), leading to suppression o f liver CYP2C11, which contributes significantly to microsomal oxaraph osphorine activation. These results also suggest that CCNU is a select ive, long-acting modulator of liver CYP2C11.