THE INFLUENCE OF DIHYDRODIOL CONFORMATION ON THE METABOLIC-ACTIVATIONOF CYCLOPENTA[A]PHENANTHRENES

The present study was undertaken in order to rationalise the apparent biological inactivity of 16-dihydro-6-methylcyclopenta[a]phenanthren-1 7-one (4) when other methyl isomers of 15,16-dihydrocyclopenta[a]phena nthren-17-one, e.g, the 11-methyl derivative (2), display appreciable tumorigenicity. In vitro metabolism of the 6-methyl-ketone-17-one (4) demonstrated that its principal metabolite was the 3,4-dihydro-3,4-dio l(3,4-dihydroxy-6-methyl-3,4, 15,16-tetrahydrocydopenta[a]phenanthren- 17-one) (5) which, in the case of the active 11-methyl derivative, is the proximate genotoxin. Thus the inactivity of this 6-methyl-17-keton e cannot be ascribed to lack of formation of the 3,4-dihydro-3,4-diol, the precursor of the 3,4-diol-1,2-epoxides (the ultimate mutagens in this series). However, the 6-methyl-3,4-dihydro-3,4-diol exists in a p seudo-diaxial rather than a pseudo-diequatorial conformation character istic of the 3,4-dihydro-3,4-diols of the other members of the series. It is therefore suggested that a diequatorial conformation in the dih ydrodiol is essential to the metabolic activation of the cyclopenta[a] phenanthren-17-ones.