CYTOCHROME-P450-DEPENDENT METABOLISM AND MUTAGENICITY OF 6-DIHYDRO-11-METHYLCYCLOPENTA[A]PHENANTHREN-17-ONE AND THEIR IMPLICATIONS IN ITS CARCINOGENICITY

Methylation of the non-carcinogen 15,16-dihydrocyclopenta[a]phenanthre n-17-one (CPP-17-one) at the bay region to form 11-CH3-CPP-17-one conf ers carcinogenic potential. In the present study we have investigated the in vitro metabolism and mutagenicity of the methylated compound by hepatic microsomal preparations from rats pretreated with various pro totype inducers of cytochrome P450 proteins in order to provide a rati onale for this marked difference in carcinogenic activity. The most ef fective metabolism of 11-CH3-CPP-17-one occurred in the presence of Ar oclor 1254-induced microsomes, the principal metabolites being oxidati ve products of the A- and D-rings and of the methyl substituent. When benzo[a]pyrene-induced microsomes served as the metabolising system, t he major A-ring metabolite was the 3,4-diol. A similar metabolic patte rn was seen with microsomes from rats treated with 11-CH3-CPP-one itse lf, but the overall effect of metabolism was lower than that observed with benzo[a]pyrene-treated microsomes but higher than that of control animals. In contrast, microsomes from rats treated with clofibrate, d examethasone, isoniazid and phenobarbitone failed to enhance the metab olism of 11-CH3-CPP-17-one when compared with control microsomes and t he metabolites reflected primarily oxidation of the D-ring. When 11-CH 3-CPP-17-one was employed as a promutagen in the Ames test, a mutageni c response was evident only in the presence of microsomes from benzo[a ]pyrene-induced rats, but induction with phenobarbitone, isoniazid, de xamethasone, clofibrate and the compound itself, failed to elicit a po sitive mutagenic response. When 3,4-dihydroxy-11-CH3-CPP-17-one served as the promutagen, a mutagenic response was observed in the presence of benzo[a]pyrene-induced and, to a lesser extent, 11-CH3-CPP-17-one-i nduced microsomes. Treatment of rats with 11-CH3-CPP-17-one caused a m arked increase in the O-deethylation of ethoxyresorufin and, to a much lesser extent in epoxide hydrolase activity. It is concluded that (i) 11-CH3CPP-17-one is an inducer of the CYP1 family; (ii) under the pre sent experimental conditions only the CYP1 family can oxidise the A-ri ng to form the 3,4-dihydroxy-11-CH3-CPP-17-one, the precursor of the u ltimate carcinogen and (iii) only the CYP1 family oxidises the diol to generate the ultimate carcinogen. Finally, the carcinogenic potential of the 11-methylated CPP-17-one, when compared with the inactive unsu bstituted compound, may be attributed, at least partly, to (1) higher CYP1-catalysed metabolism of the 11-methylated compound to the 3,4-dih ydrodiol and (2) more potent induction of the CYP1 family by the It-su bstituted derivative, when compared to the parent, unsubstituted compo und.