As an approach for elucidation of structure activity relationships that underlie the exceptionally large difference in carcinogenic activity between
benz[a]anthracene and 7,12-dimethylbenz[a]
anthracene (7,12-DMBA), 11 methyl- and/or
fluorine-substituted
benz[a]anthracenes were evaluated for
tumor-initiating activity on mouse skin. Outbred CD-1 and outbred Sencar mice received a single topical application of the
hydrocarbons followed by twice weekly applications of the
tumor promoter 12-O-tetradecanoylphorbol 13-acetate for 16-26 weeks. 7,12-DMBA was almost two orders of magnitude more active as a
tumor-initiator than 7- and 12-methylbenz[a]
anthracene. Methyl substitution at the 7- and 7,12-positions of
benz[a]anthracene was significantly more effective in the enhancement of tumorigenic activity than
fluorine substitution at these positions. Although 7-fluorobenz[a]
anthracene, 12-fluorobenz[a]
anthracene, and 7,12-difluorobenz[a]
anthracene had only 0.15, 0.26, and less than 0.005 times the
tumor-initiating activity of their respective methyl-substituted derivatives, they were severalfold more active than
benz[a]anthracene. 7-Fluorobenz[a]
anthracene was slightly less active than 12-fluorobenz[a]
anthracene, whereas 7-methylbenz[a]
anthracene was about twofold more than 12-methylbenz[a]
anthracene. For 7,12-di-substituted
benz[a]anthracenes, 7-methyl-12-fluorobenz[a]
anthracene was more than twice as tumorigenic as 7-fluoro-12-methylbenz[a]
anthracene, but each was individually more active than 7-methylbenz[a]
anthracene and 12-methylbenz[a]
anthracene, respectively. Both fluorinated compounds were much less active than 7,12-DMBA. Substitution of
fluorine or methyl at the 5-position of 7-methylbenz[a]
anthracene and substitution of
fluorine at the 5-position of 12-methylbenz[a]
anthracene dramatically reduced their tumorigenic activity.