Differential property of antigenic characterization between piroxicam and ampiroxicam in contact hypersensitivity

Piroxicam (PXM; a non-steroidal anti-inflammatory drug) has been reported t o induce photosensitivity. In our previous report, however, ultraviolet-A ( UVA)-irradiated or non-irradiated PXM did not induce any reactions in the i n vivo model of contact hypersensitivity, while positive patch testing was shown by ampiroxicam (APX; a prodrug of PXM). The purpose of the present st udy was to clarify the influence of protein on the antigenicity of PXM usin g this model. Animals sensitized by UVA-irradiated 1% APX showed positive p atch testing (open application) in UVA-irradiated 1% APX, while they were n egative in challenge by UVA-irradiated PXM with or without 5% human serum a lbumin (HSA). Although animals sensitized by 1% thiosalicylate (TOS), which is thought to be an active hapten of PXM, were cross-reacted with UVA-irra diated 1% APX, they failed to react with UVA-irradiated 1% PXM with or with out HSA. On the other hand, intra-dermal testing (intra-dermal application) in UVA-irradiated 0.1% PXM with 5% HSA was positive in animals sensitized by UVA-irradiated 1% APX, while 5% HSA alone, 0.1% PXM with 5% HSA and UVA- irradiated 0.1% PXM did not induce any reactions under this condition. Furt hermore, concentration of PXM in the presence of HSA was reduced by UVA-irr adiation in a time dependent manner, while the degradation of PXM was not o bserved in the absence of HSA. Finally, PXM almost disappeared at 120 min a fter the initiation of UVA-irradiation. The degradation of PXM irradiated b y UVA was dependent on the concentration of HSA at the range of 0 to 4%. He nce, these results suggest that the presence of protein is necessary for th e induction of the antigenic activity of PXM and the antigenic characteriza tion of PXM is different from that of APX in contact hypersensitivity.