Formaldehyde and skin tumorigenesis in SENCAR mice

Previous experiments involving topical applications of formaldehyde on hairless mouse skin were repeated with SENCAR mice, which are bred for maximum sensitivity to chemical tumorigenesis. Most experimental groups consisted of 32 mice. Topical skin applications of either 100 μl acetone or about 200 μl 4% formaldehyde in water twice weekly, resulted in two tumor-bearing animals, each with one small, benign papilloma. A group of 96 mice, treated once with 51/2 μg DMBA in acetone, developed a total of 107 tumors in 40 tumor-bearing animals. Thus, DMBA is a strong, complete tumorigen also in SENCAR mice. Animals given 51.2 μg DMBA first and then treated twice weekly with 1% formaldehyde developed a total of 30 tumors in 8 tumor-bearing animals, whereas mice given 51.2 μg DMBA first, followed by twice weekly treatment with 4% formaldehyde, developed 51 tumors in 15 animals. When two widely accepted, statistical methods were used, there was no significant difference between the groups treated once with DMBA alone and that treated once with DMBA followed by 4% formaldehyde. The results in SENCAR mice confirm that formaldehyde has no skin tumorigenic or carcinogenic potency of its own. It seems doubtful whether it may act as a very weak enhancer of DMBA-induced tumorigenesis, but it has no significant influence on DMBA-induced carcinogenesis.