The arylpropionic acid derivatives (APADs) ketoprofen and tiaprofenic acid can provoke photoallergic dermatitis. Possible cross-reactivity between APADs is of importance in patients using nonsteroidal anti-inflammatory drugs. Because of the similarities in chemical structures, we investigated patients with photoallergy to ketoprofen or tiaprofenic acid, in order to study cross-reactivity between APADs and a possible pattern of cross-reactivity between benzophenone-containing molecules, so as to determine the molecular basis of photoallergy to ketoprofen or tiaprofenic acid. 10 patients with photoallergy to topical ketoprofen, 2 with photoallergy to oral tiaprofenic acid, and 15 control subjects with no history of contact dermatitis from APADs, nor from benzophenone-containing molecules, were photopatch tested in triplicate with ketoprofen, tiaprofenic acid, other APADs (alminoprofen, fenoprofen, flurbiprofen, ibuprofen and naproxen), benzophenone-containing molecules (fenofibrate, oxybenzone, sulisobenzone), and unsubstituted benzophenone. 1 set was irradiated with UVA light, 1 with solar-simulated irradiation and 1 dark control. Tests were read at 2, 3 and 4 days. Patients reacted to both ketoprofen and tiaprofenic acid (12/12), fenofibrate (8/12), oxybenzone (3/12) and unsubstituted benzophenone (11/12), but not to other APADs, nor to sulisobenzone. Tests remained negative in control patients. Photoallergy is due to the benzophenone moiety of ketoprofen, or to the very similar thiophene-phenylketone of tiaprofenic acid, but not to their arylpropionic function. This induces cross-reactivity to fenofibrate and oxybenzone but not to APADs without a benzophenone moiety, which may therefore probably be used in such patients. Unsubstituted benzophenone should be added to standard phototesting series.

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