Inhibition by 2,6-dithiopurine and thiopurinol of binding of a benzo(a)pyrene diol epoxide to DNA in mouse epidermis and of the initiation phase of two-stage tumorigenesis.

The chemotherapeutic agent 6-mercaptopurine was previously shown to inhibit the binding of 7r,8t-dihydroxy-9,10t-oxy-7,8,9,10-tetrahydro-benzo(a) pyrene (BPDE-I) to DNA in Chinese hamster ovary cells. Two compounds related to 6-mercaptopurine, 2,6-dithiopurine (DTP) and thiopurinol (TP), have been tested for inhibition of the binding of BPDE-I to epidermal DNA in mouse skin. Doses of test compound (0.2-20 mumol) or solvent control were applied to the shaved backs of female SENCAR mice. Fifteen min later, 200 nmol [3H]BPDE-I were applied to the same area and 3 h later the mice were sacrificed and epidermal DNA was purified and adduct formation was quantitated radiometrically. At the highest doses studied, DTP and TP inhibited DNA binding by 90 and greater than 80%, respectively. The dose necessary to inhibit DNA binding by 50% was about 0.8 mumol for DTP and about 2 mumol for TP. To test whether this protective effect was long-lasting, the time between application of purinethiol and [3H]BPDE-I was systematically increased. Although the level of protection was decreased by increasing the time between applications, both compounds inhibited binding 50-60% even after 24-48 h. A radioactive compound tentatively identified as a TP-BPDE-I adduct could be recovered from epidermal homogenates following topical application of TP and BPDE-I. We used a standard two-stage initiation-promotion protocol to test the effects of these compounds on mouse skin carcinogenesis. Mice were treated with 0, 1, or 10 mumol of either TP or DTP, and 15 min later were treated with an initiating dose of BPDE-I (200 nmol). Twice weekly promotion with 12-O-tetradecanoylphorbol-13-acetate was begun 2 weeks later and continued for 23 weeks. A dose-dependent inhibition of tumor incidence and multiplicity was noted with both compounds. Treatment of skin with 10 mumol of DTP prior to initiation lowered the number of papillomas per mouse by greater than 90% compared to solvent controls; a 10-fold lower dose resulted in about 50% inhibition. The 10-mumol dose of TP resulted in about 50% inhibition. Mice were examined for 50 weeks for the presence of squamous cell carcinomas. Compared to the positive control group, 10 mumol DTP inhibited carcinoma incidence and lowered the total number of carcinomas by 90-95%. Treatment with 10 mumol TP had no significant effect on carcinoma incidence, and only slightly lowered the total number of carcinomas.