Piroxicam has at least two epitopes for contact photoallergy

We have demonstrated previously in guinea pigs that the induction of photocontact sensitivity to piroxicam (PXM) also induces a state of cross-reactive contact hypersensitivity to two compounds having structurally related elements, thimerosal (TMS) and thiosalicylate (TOS). The present study was conducted to determine whether oral administration of TOS would desensitize guinea pigs previously photosensitized with PXM. At the same time, the spectrum of reactivities against these compounds and against tenoxicam (TXM) which resembles only piroxicam was assessed by appropriate sensitizing and eliciting protocols. As expected, animals photosensitized to PXM developed reactivities against all four compounds, PXM and TXM (photosensitivity) and TMS and TOS (contact sensitivity). By contrast, photosensitization with TXM induced cross-reactivity only against PXM. Moreover, the induction of contact sensitivity against TMS or TOS induced photosensitive cross-reactivity to PXM, but not to TXM. Finally, the oral administration of TOS produced a transient desensitization only for TMS and TOS. These results suggest that photosensitization with PXM induces two distinct reactivities. The first reactivity cross-reacts with TMS and TOS and is suppressible with orally administered TOS. The second cross-reacts only with TXM and is not suppressible with oral TOS. We conclude that PXM acquires at least two distinct immunogenic epitopes when exposed to UVA irradiation.