Absorption and action spectra studies on bithionol and halogenated salicylanilide photosensitivity.

Abstract

SUMMARY. The action spectra were determined with a high intensity light source on 11 patients (including 4 persistent light reactors) known to be photosensitive to bithionol and halogenated salicylanilides. Maximum photosensitivity observed on patch tests containing the sensitizer was in the range of 360 nm. but no reaction occurred above 420 nm. The persistent light reactors had a considerably lowered sunburn MED below 330 nm. compared with control groups. The absorption spectra of the sensitizers was determined in dimethyl formamide and in 95%, ethanol at various pH. The halogenated salicylanilides gave a maximum absorption at 282 nm. in an acid pH, but in a basic pH above 8, when phenolic ionization occurs, a second absorption peak at 360 nm. is found. Maximum fluorescence was achieved by excitation in the region of 360 nm. If the concentration of the sensitizer is equal (1%) then the order of decreasing photosensitizing ability of the tested substances was tetrachlorosalicylanilide (TCSA), bithionol. 4′,5-dibromosalicylanilide (DBS) and 3,4′,5-tribromosalicylanillde (TBS). Guinea-pigs could be sensitized with TCSA within one week using combined long U.V. (320–420 nm.) and sunburn irradiation, but no sensitivity occurred with irradiation for 16 weeks using only long U.V. Patch tests on sensitized patients to photodecomposition products of TBS and DBS were negative. The possible mechanisms in the production of the persistent light reactors are discussed.