Fixed drug eruption to piroxicam. Positive patch tests with cross-sensitivity to tenoxicam

In our experience, fixed drug eruption (FDE) is not a rare example of cutaneous adverse drug reactions, but the identification of the drug responsible often remains a challenge. We report on a patient in whom piroxicam was identified as the cause of recurring lesions of FDE. Epicutaneous testing with the offending drug was positive on residual lesions, but not on uninvolved skin. Histopathological and immunohistochemical findings on the positive tests were identical to those in an acute lesion of FDE. Patch testing on lesional skin revealed cross-sensitivity between piroxicam and tenoxicam, but no reactivity to thiosalicylic acid. This suggests that the molecular moiety involved in FDE to piroxicam is shared by tenoxicam, in contrast to piroxicam photosensitivity where there is no cross-reaction between these two drugs and the offending moiety is antigenically and structurally similar to thiosalicylic acid.