Photosensitivity to piroxicam is induced by sensitization to thimerosal and thiosalicylate

Piroxicam (PXM), a nonsteroidal anti-inflammatory drug, has been well known to often induce photosensitive eruptions within a few days after its administration. It has been reported that this photosensitivity correlates well with a positive patch-test reaction to thimerosal (TMS) and also to thiosalicylate (TOS), which is an active hapten of TMS. But it has not yet been concluded whether this correlation is caused by a cross-reaction among the drugs or not. In our experiments, animals contact-sensitized with TMS or TOS developed positive photopatch-test reactions to PXM, and those photocontact-sensitized with PXM had positive patch-test reactions to TMS and TOS. Photosensitive reactions were also induced by UVA irradiation (photo test) performed 90 min after perioral administration of PXM in the animals contact-sensitized with TMS or TOS. Analysis of the UVA-treated PXM by nuclear magnetic resonance spectrography and thin-layer chromatography revealed that the high dose of UVA induced photodecomposition of PXM, and generated several other chemicals different from PXM. But the PXM treated with the high dose of UVA could not induce positive patch-test reactions in many of the animals contact-sensitized with TMS or TOS. The cross-reacting hapten generated from PXM by UVA treatment may not be stable in the absence of carrier proteins. These results taken together indicate that the PXM photosensitivity in man is induced by contact-sensitization with TMS, as shown in our animal model, and then is photoallergic in nature. But the identity of the cross-reacting substance remains unknown.