Tumor-initiating ability of the twelve monomethylbenz[a]-anthracenes

The 12 possible monomethylbenz[a]anthracenes (MBAs) were examined for their tumor-initiating activity in the classical two-stage initiation-promotion experiment. Based on the average number of tumors/mouse, 7-MBA was the most tumorigenic compound of the series causing 4.9 papillomas/mouse at an initiating dose of 400 nmol/mouse. At this same dose level 8-MBA and 12-MBA were equally potent causing 1.0 papillomas/mouse. 6- and 9-MBAs caused ∼0.6 papillomas/mouse at this dose level. These data are in general agreement with previous experiments using other model systems. Of interest is the low tumorigenicity of the 1-, 2-, 3- and 4-MBAs which has been cited in support of the bay region theory of polycyclic aromatic hydrocarbon carcinogenesis. The tumor-initiating ability of anthracene and 9,10-dimethylanthracene was also examined. Neither of these compounds possessed tumor-initiating activity.