Photosensitivity by Demethylchlortetracycline and Sulphanilamide

SUMMARY. Photosensitization of the skin by demethylchlortetracycline and sulphanilamide has been demonstrated in mice. Demethylchlortetracycline has an action spectrum extending from 350 to 420 nm with a peak at 400 nm; it is most effective in mice when given by the intradermal route, less effective intraperitoneally and without action orally. Sulphanilamide has a photosensitizing action spectrum from 375 to 400 nm, i.e. mostly in the visible spectrum, with less certain activity at 280–300 and 550–600 nm; it is effective intradermally and inactive orally. The majority of the photosensitizing effects of sulphanilamide must be due either to the alteration of its constitution in the skin, or to a metabolic change in the skin itself since sulphanilamide does not absorb visible light. Possible mechanisms are discussed and a speculative hypothesis is proposed that a local disturbance of porphyrin metabolism may be implicated.