Comparative tumor initiating activities of cyclopentano and methyl derivatives of 5-methylchrysene and chrysene.

Previous studies showed that 5,6-dimethylchrysene (5,6-diMeC) and 5,7-diMec were significantly less tumorigenic than 5-methylchrysene (5-MeC). These results were unexpected based on the known mechanism of metabolic activation of 5-MeC and indicated the presence of critical steric requirements for tumorigenicity at the 6 and 7 positions of 5-MeC. In this study, the structure activity relationships were further extended by comparing the tumor-initiating activities on mouse skin of 5-MeC, 6,7-cyclopentano-5-MeC, 5,6-diMeC, 6,7-diMeC, 5,7-diMeC, chrysene and 6,7-cyclopentanochrysene. 5-MeC was the most tumorigenic compound, with activity significantly higher than all other compounds tested. Among the other compounds. Only 5,6-diMeC was significantly tumorigenic. The results demonstrate that substitution of methyl or methylene groups at the 6 or 7 positions of 5-MeC leads to a significant reduction of tumor initiating activity.