The ability was tested of appropriate substituents of benzo[a]pyrene (BP) at C-6 to decrease or suppress the carcinogenic activity for these BP derivatives relative to the parent compound. 8-week-old female Swiss mice in 9 groups of 30 were treated on the back with 0.2 μmol of compound in acetone 4 times weekly for 20 weeks. The following compounds were administered: BP, 6-methylbenzo[a]pyrene (BP-6-CH3), 6-hydroxymethylbenzo[a]pyrene (BP-6-CH2OH), benzo[a]pyrene-6-carboxaldehyde (BP-6-CHO), benzo[a]pyrene-6-carboxylic acid, 6-methoxybenzo[a]pyrene, 6-acetoxybenzo[a]pyrene, 6-bromobenzo[a]pyrene, and 6-iodobenzo[a]pyrene. Two additional groups received BP or BP-6-CH3 twice weekly for 20 weeks at a total dose 25% of that above. In addition, the metabolism of selected 6-substituted BP derivatives was studied, using mouse skin homogenates in vitro and mouse skin in vivo. Only four compounds were carcinogenic; the order of potency was BP>BP-6-CH3>BP-6-CH2OH and BP-6-CHO. The difference in carcinogenicity between BP-6-CH2OH and BP-6-CHO could not be assessed by this experiment. In a further tumorigenesis experiment the carcinogenicity of BP-6-CH2OH was compared to that of BP-6 CHO, BP-6-CH3 and 6-hydroxymethylbenzo[a]pyrene sulfate ester (BP-6-CH2OSO3Na) on mouse skin. 9-week-old female Swiss mice in groups of 28 were treated at three dose levels with 0.8, 0.2 and 0.05 μmol of compound in dioxane-dimethyl sulfoxide (75:25) twice weekly for 40 weeks. After 40 experimental weeks BP-6-CH2OSO3Na proved to be a more potent carcinogen than BP-6-CH2OH, which, in turn was more active than BP-6-CHO. The greater carcinogenicity of BP-6-CH3 relative to BP-6-CH2OH and BP-6-CHO is confirmed, suggesting that BP-6-CH2OH is not a proximate carcinogenic metabolite for BP-6-CH3. Since BP-6-CHO is a weaker carcinogen than BP-6-CH2OH and is efficiently reduced metabolically to BP-6-CH2OH, the latter compound may be a common proximal carcinogenic metabolite. The stronger potency of BP-6-CH2OSO3Na, compared to its alcohol, suggests that an ester of BP-6-CH2OH might be the ultimate alkylating compound reacting with cellular nucleophiles.
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