The 12 isomeric phenols of benzo(a)pyrene were tested for their ability to inhibit the mutagenic activity of (±)-7β,8α-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene [B(a)P 7,8-diol-9,10-epoxide-2], an ultimate mutagenic and carcinogenic metabolite of benzo(a)pyrene. 3-Hydroxybenzo(a)pyrene [3-HO-B(a)P], a major metabolite of benzo(a)pyrene, was the most potent antagonist tested. Approximately 3 nmol of 3-HO-B(a)P, 14 nmol of 10-HO-B(a)P, and 5–8 nmol of 1-, 2-, 4-, 5-, 6-, 7-, 8-, 9-, 11-, and 12-HO-B(a)P inhibited the mutagenic activity of 0.05 nmol of B(a)P 7,8-diol-9,10-epoxide-2 by 50% in Salmonella typhimurium strain TA 100. The importance of the phenolic group for antimutagenic activity was indicated by the lack of antimutagenic activity of benzo(a)pyrene itself. 3-HO-B(a)P also inhibited the mutagenic activity resulting from the metabolic activation of benzo(a)pyrene and (±)-trans-7,8-dihydroxy-7,8-dihydrobenzo(a)pyrene by rat liver microsomes. This inhibition may have resulted from an effect of 3-HO-B(a)P on the metabolic activation of these carcinogens and/or from a direct effect on the action of B(a)P 7,8-diol-9,10-epoxide-2. In a mammalian cell culture system utilizing Chinese hamster V79 cells, 3-HO-B(a)P (8 µm) inhibited the mutagenicity of B(a)P 7,8-diol-9,10-epoxide-2 (0.2 µm) by 50%. Although 3-HO-B(a)P was a potent inhibitor of the mutagenic activity of bay-region diol epoxides of benzo(a)pyrene, dibenzo(a,h)pyrene, and dibenzo(a,i)pyrene in S. typhimurium strain TA 100, higher concentrations of 3-HO-B(a)P were needed to inhibit the mutagenicity of the chemically less reactive benzo(a)pyrene 4,5-oxide and the bay-region diol epoxides of benz(a)anthracene, chrysene, and benzo(c)phenanthrene.
Both 3-HO-B(a)P and 10-HO-B(a)P accelerated the disappearance of B(a)P 7,8-diol-9,10-epoxide-2 from 1:9 dioxane-water solutions at pH 7 and 25°C. 3-HO-B(a)P, the most effective antimutagen of the B(a)P phenols tested, was much more reactive with the diol epoxide than 10-HO-B(a)P, the least effective antimutagen. The rate constant for the reaction of 3-HO-B(a)P with the diol epoxide exhibited a nonlinear (>first-order) dependence on the concentration of the phenol. Evidence was obtained for covalent adduct formation between the diol epoxide and each of the two phenols. A 3-HO-B(a)P adduct was isolated and characterized spectroscopically as the adduct derived from cis addition of the 3-hydroxyl group of 3-HO-B(a)P to the C-10 position of B(a)P 7,8-diol-9,10-epoxide-2.
Although a 2500-nmol dose of 3-HO-B(a)P had weak tumorinitiating activity on mouse skin, the topical application of this amount of the phenol 5 min before a tumor-initiating dose of 200 nmol of B(a)P 7,8-diol-9,10-epoxide-2 caused a more than 70% decrease in the number of diol epoxide-induced skin tumors that were observed after 16-20 weeks of promotion with 12-O-tetradecanoylphorbol-13-acetate. This dosing regimen of 3-HO-B(a)P had little or no effect on the tumor-initiating activity of 50 nmol of B(a)P, but the application of 2500 nmol of 3-HO-B(a)P 5 min before and 60 min after a 50-nmol initiating dose of B(a)P caused a modest inhibition in the mean number of B(a)P-induced skin tumors.
- Received July 8, 1985.
- Revision received October 16, 1985.
- Accepted October 17, 1985.
- ©1986 American Association for Cancer Research.