Reticuloses and epidermal tumors in hairless mice after topical skin applications of cantharidin and asiaticoside.

Cantharidin, a vesicant agent, and asiaticoside, which promotes the healing of skin ulcers, were tested for carcinogenicity by topical applications to the skin of hairless mice. A short-term tetrazolium test indicated that both compounds were weak carcinogens. The compounds were then separately painted twice weekly on the dorsal skin up to about 20 months; some of the mice had previously been initiated with a small dose of 20-methylcholanthrene (MCA). A control group which received only the solvent benzene after MCA initiation was also studied. Cantharidin acted as a weak but complete carcinogen on the skin, causing carcinomas in 6.3% of the animals. These carcinomas did not appear before about 16 months of observation. Before this, the painted, MCA-initiated animals had a significantly lower number of papillomas of the skin than the corresponding control group with only benzene treatment. It therefore seemed that Cantharidin in the early stages of painting was tumor inhibitory, or "anticarcinogenic,"