A cross-reaction between piroxicam-photosensitivity and thiosalicylate hypersensitivity in lymphocyte proliferation test

Development of novel in vitro photosafety assays focused on the Keap1-Nrf2-ARE pathway

Although photoallergens require UV energy for antigen formation, the subsequent immune response is considered to be the same as in ordinary skin sensitization. Therefore, in vitro tests for skin sensitization should also be applicable for photoallergy testing. In this study, we examined whether activation of the Keap1 (Kelch-like ECH-associated protein 1)-Nrf2 (nuclear factor-erythroid 2-related factor 2)-ARE (antioxidant response element) pathway could be used to assess the photoallergenic potential of chemicals, using the reporter cell line AREc32 or KeratinoSens(TM) . First, we identified an appropriate UVA irradiation dose [5 J cm(-2) irradiation in phosphate-buffered saline (PBS)] by investigating the effect of UV irradiation on ARE-dependent gene induction using untreated or 6-methylcoumarin (6-MC)-treated cells. Irradiation of well-known photoallergens under this condition increased ARE-dependent gene expression by more than 50% compared with both vehicle and non-irradiated controls. When the cut-off value for detecting photoallergens was set at 50% induction, the accuracy of predicting photoallergenic/phototoxic chemicals was 70% in AREc32 cells and 67% in KeratinoSens(TM) cells, and the specificity was 100% in each case. We designate these assays as a photo-ARE assay and photo-KeratinoSens(TM) , respectively. Our results suggest that activation of the Keap1-Nrf2-ARE pathway is an effective biomarker for evaluating both photoallergenic and phototoxic potentials. Either of the above tests might be a useful component of a battery of in vitro tests/in silico methods for predicting the photoallergenicity and phototoxicity of chemicals. Copyright © 2015 John Wiley & Sons, Ltd.

Antigenic characterization in ampiroxicam-induced photosensitivity using an in vivo model of contact hypersensitivity

Ampiroxicam (APX), a prodrug of piroxicam (PXM), has been reported to induce photosensitivity. Antigenic characterization of these photosensitivities, however, is still insufficient. The purpose of the present study was to elucidate further mechanism of photosenstivity induced by APX and PXM using an in vivo model of contact hypersensitivity in guinea pigs. Animals sensitized with ultraviolet-A (UVA)-irradiated 1% APX showed positive reaction in the patch testing to UVA-irradiated 1% APX and 1% thiosalicylate (TOS), while they were negative in challenge with UVA-irradiated 1% PXM, non-irradiated APX and PXM, whereas none of UVA-irradiated or non-irradiated APX and PXM showed positive patch test reaction in animals sensitized with UVA-irradiated 1% PXM or control vehicles. Animals sensitized with 1% TOS were successfully challenged by 1% TOS and cross-reacted with UVA-irradiated 1% APX; however, they failed to react with UVA-irradiated PXM, non-irradiated APX and PXM. Indeed, the in vitro study revealed that the concentration of APX was easily reduced by the increase of UVA irradiation dose, as compared with that of PXM. Interestingly, absorption spectrum of UVA-irradiated APX was similar to that of TOS, which is thought to be an active hapten of PXM. In the present study, we succeeded in the development of a novel animal model reflecting the clinical observations. Furthermore, these results suggested that contact hypersensitivity induced by UVA-irradiated APX is developed by photoproducts of APX itself, but not by the biotransformation of APX to PXM.

Lymphocyte stimulation test with drug-photomodified cells in patients with quinolone photosensitivity

Quinolone antibacterial agents, known to elicit photosensitive dermatitis as an adverse effect, have both phototoxicity and photoallergenicity. The latter potency is mainly derived from their photohaptenic moiety; quinolones covalently bind to protein and cells upon exposure to ultraviolet A (UVA) light. Our previous study has shown the in vivo and in vitro antigenicity of quinolone-photomodified cells in mice. Here, we examined the presence of sensitized lymphocytes that react with quinolone-photomodified autologous cells in patients with photosensitivity to quinolones. A flow cytometric analysis using a monoclonal antibody specific to quinolone photoadducts demonstrated that peripheral blood mononuclear cells (PBMC) were successfully photomodified with quinolones upon exposure to UVA. PBMC from quinolone-photosensitive patients were cocultured with autologous PBMC photomodified with the causative drug. Modest but significant proliferative responses of responder lymphocytes were found in patients photosensitive to lomefloxacin, fleroxacin, and enoxacin, indicating photoallergic mechanism in these patients. On the other hand, sparfloxacin-photosensitive patients exhibited negative lymphocyte stimulation test, suggesting that its photosensitivity is mainly phototoxic. When UVA-preirradiated quinolones were used as stimulators, only fleroxacin exceptionally stimulated patients' PBMC, indicating its prohaptenic as well as photohaptenic properties. These findings suggest the presence of circulating sensitized T cells in patients with photosensitivity to certain quinolones.

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.