Effect of ultraviolet irradiation of photopatch test substances in vitro.

The mechanisms behind allergic photocontact reactions are not clear. For certain photoactive substances the generation of photoproducts may be one step in the sequence of events inducing photoallergy. 26 photoactive substances were studied by thin layer chromatography (TLC) before and after long-wave or medium-wave ultraviolet irradiation for 1, 3 and 5 h (10.8, 32.4, 54 J/cm2 and 1.1, 3.2, 5.4 J/cm2). The formation of photoproducts was demonstrated for 13 substances. For 8 of these, photoproducts were formed regardless of type of ultraviolet exposure while for 5 compounds photoproducts were demonstrated only after long-wave ultraviolet irradiation. All photoactive compounds forming photoproducts did so after UVA irradiation which is in accordance with the finding of action spectra for photoallergic reactions in human skin within this region. No photoproducts were demonstrated for 13 of the tested substances.