The fate of tetrachlorosalicylanilide in photosensitized guinea pigs.

The distribution and fate of 3, 3', 4', 5-tetrachlorosalicylanilide (TCSA) in photosensitized guinea-pig skin following topical application at varying times was investigated by fluorescence microscopy in unfixed, frozen sections. When the site to which TCSA had been applied was irradiated with long-wave UV light, TCSA was detected in the entire epidermis only 3 days after application and in the horny layer up to 10 days. Unless TCSA-treated sites were exposed to light, TCSA was still recognized in the entire epidermis 10 days after application and in the horny layer even at 3 weeks. The irradiated photosensitized guinea pigs in the present study eliminated TCSA more rapidly than normal control animals in our previous work. The mechanism of persistent light reaction was discussed and we concluded that TCSA can remain in the skin for a long time, because it is a prohapten, not a hapten.