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Evidence That Benzo(a)anthracene 3,4-Diol-1,2-Epoxide Is an Ultimate Carcinogen on Mouse Skin

Department of Biochemistry and Drug Metabolism, Hoffmann-LaRoche Inc., Nutley, N. J. 07110 [W. L., A. W. W., R. L. C., A. H. C.], and Section on Oxidation Mechanisms, Laboratory of Chemistry, National Institute of Arthritis, Metabolism and Digestive Diseases, NIH, Bethesda, Maryland 20014 [D. R. T., R. E. L., D. M. J.]

Benzo(a)anthracene (BA) and several benzo-ring derivatives of BA were tested for their tumor-initiating activity on mouse skin. A single topical application of 0.1 to 2.0 µmol of hydrocarbon was followed 7 days later by twiceweekly applications of the tumor promoter 12-O-tetradecanoylphorbol-13-acetate for 20 weeks. Comparisons of the percentage of mice with papillomas and the number of papillomas observed per mouse indicated that both diastereomeric 3,4-dihydroxy-1,2-epoxy-1,2,3,4-tetrahydrobenzo(a)anthracenes, in which the epoxide oxygen is either cis (diol-epoxide 1) or trans (diol-epoxide 2) to the benzylic 4-hydroxyl group, were 10- to 40-fold more tumorigenic (papillomas/mouse) than was the parent hydrocarbon, benzo(a)anthracene. Diol-epoxide 2 was 2- to 3-fold more tumorigenic than was diol-epoxide 1 after 20 weeks of promotion. The immediate metabolic precursor of the 3,4-dihydroxy-1,2-epoxy-1,2,3,4-tetrahydrobenzo(a)anthracenes, trans-3,4-dihydroxy-3,4-dihydrobenzo(a)anthracene, had intermediate tumorigenic activity compared to diol-epoxide 1 and diol-epoxide 2 and was approximately 20-fold more tumorigenic (papillomas/mouse) than was BA. Resolution of racemic trans-3,4-dihydroxy-3,4-dihydrobenzo(a)anthracene into the (-)- and (+)-enantiomers and assignment of their absolute stereochemistry revealed that the (-)-(3R,4R) isomer was at least 5-fold more active as a tumor initiator than was the (+)-(3S,4S) isomer. 1,2-Dihydrobenzo(a)anthracene and 3,4-dihydrobenzo(a)anthracene were also tested for tumorigenic activity. 3,4-Dihydrobenzo(a)anthracene was the most potent tumorigenic compound used in this study. 1,2-Dihydrobenzo(a)anthracene was a very weak tumor initiator, with activity comparable to BA. These results support the concept that a diol-epoxide in which the epoxide on a saturated, angular benzo ring forms part of a "bay region" of the hydrocarbon is a prime candidate for an ultimate carcinogenic metabolite of the hydrocarbon. In the case of BA, this metabolite is either or both of the diastereomeric 3,4-dihydroxy-1,2-epoxy-1,2,3,4-tetrahydrobenzo(a)anthracenes.

¹ To whom requests for reprints should be addressed.

Received 12/27/77. Accepted 3/10/78.

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