Potent inhibitory effects of suicide inhibitors of P450 isozyines on 7, 12-dimethylbenzℵaℵanthracene and benzoℵaℵpyrene initiated skin tumors

Abstract

A single dose of 1-ethynylpyrene (EP), 1-vinylpyrene (VP) or 2-ethynylnaphthalene (EN) was applied to the skin of SENCAR mice 5 mice before an initiating dose of 7,l2dimethylbenz[a]anthracene (DMBA) or benzo [a] pyrene (B[a]P and the development of skin tumors then promoted with biweekly topical applications of 12-O-tetradecanoylphor-bol-13-acetate (TPA). The application of EP strongly inhibited the formation of skin tumors initiated by either DMBA or B[a]P in a dose-dependent manner. Application of 44 pmol of EP inhibited tumor initiation by 10 nmol of DMBA ˜25%; application of 440 nmol of EP inhibited tumor Initiation by 200 nmol of B[a]P ˜51%. A high single dose of EP (4.4–44 μmol) nearly eliminated skin tumor initiation by either 10 nmol of DMBA or 200 nmol of B[a]P ˜56%. Application of EN yielded contrasting results. EN inhibited the formation of skin tumors initiated by 10 nmol of DMBA, but the observed dose-dependence was minimal; tumors were decreased about 40% by 3.3 μmol of EN and only about 65/ by 132 mol of EN. A high single dose of EN (132 μmol) increased both the mean number of tumors per mouse and the percentage of mice that developed tumors after initiation by 200 nmol of B[a]P Topical application of 4.4 μmol of EP, 22 μmol of VP or 33 μmol of EN to the skin of SENCAR mice 5 min before a single initiation dose of 2.5 μmol of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) had a minimal inhibitory effect (14–28%) on the development of skin tumors produced by subsequent biweekly promotion with TPA. A single dose of 44 μmol of EP or 132 μmol of EN followed by biweekly applications of TPA did not produce skin tumors; however, a dose of 44 μmol of VP followed by promotion with TPA produced a low but significant number of skin tumors. Except for the inhibition of B[a]P-initated skin tumorigenesis by VP, the observed inhibitions of polycycic aromatic hydrocarbon (PAH) tumorigenesis by EP, VP and EN corresponded with the inhibitions of PAH—DNA adduct formation in mouse skin by these agents previously reported. VP was more effective at preventing B[a]P-initiated tumorigenesis than anticipated based upon its inhibition of B[a]P—DNA adduct formation in mouse epidermis. These data are discussed in terms of the potential of the pyrene moiety of EP and VP to direct binding preferentially to the active sites of P450 isozymes involved in PAIl activation and of the ethynyl group of EP to function additionally as a mechanism-based inactivator (suicide inhibitor) of certain of these P450 isozymes.