3-Amino-1-methyl-5H-pyrido[4,3-b]-indole initiates two-stage carcinogenesis in mouse skin but is not a complete carcinogen.

In two separate experiments, 2.0 mg of 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2) was applied to the back of female CD-1 mice twice weekly for 5 weeks followed by application of 2.5 micrograms of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) twice weekly for 47 weeks. Skin squamous cell papillomas and carcinomas developed in 6 of 20 mice (30%) in Experiment 1 and in 3 of 19 mice (16%) in Experiment 2. In contrast, application of 2.0 mg of Trp-P-2 twice weekly for 52 weeks did not produce any skin tumors. These data suggest that Trp-P-2 acts primarily as an initiator, not as a complete carcinogen, for mouse skin.