PHOTOBINDING OF DRUGS TO CELLS AS AN INDICATOR OF POTENTIAL PHOTOALLERGY

The photobinding of drugs to cells has been investigated as a possible indicator of the photoallergic potential. Tiaprofenic acid (TA), carprofen (CP), benoxaprofen (BP) and ibuprofen (IP) were selected as test substances and human fibroblasts as model biological system. Radioactive labelling of the drugs was achieved through alkaline exchange of the alpha-carboxyl hydrogens by tritium, using (H2O)-H-3 as solvent. When the labelled compounds were co-irradiated with fibroblasts, TA gave rise to the highest amount of radioactivity covalently bound to cells. This also occurred, albeit to a lower extent, with CP and BP; by contrast, no effect was observed for IP. The observed rank order is in agreement with the available in vivo data. The time course of the process was determined for TA, distinguishing between covalent and non-covalent binding. The results, together with gas chromatographic analyses of the irradiation mixtures and binding studies with TA and its major photoproduct decarboxytiaprofenic acid (decarboxy-TA) in the dark, indicated that most of the observed photobinding might be due to the photoproduct rather than to the parent drug. It is conceivable that hydrogen abstraction by the excited ketone could take place with proteins as reaction partners. This process would lead to the generation of a radical pair, whose coupling would result in the formation of a covalent bond. The contribution of this mechanistic pathway to the in vivo photobinding has to be considered when a lipophilic photoproduct is formed which, as for decarboxy-TA, still contains an active chromophore.