Photosensitization by drugs: photolysis of some chlorine-containing drugs.

Irradiation with ultraviolet light of chlorpromazine, prochlorperazine, frusemide or hydrochlorothiazide in either aqueous or methanol solution yielded free chloride ion. Potentiometric methods were used to detect one mol of chloride ion produced per mol of drug irradiated in deoxygenated solution, with a concomitant equimolar appearance of hydrogen ion. Saturation of the solutions with oxygen strongly inhibited the photolysis reaction in methanol but only partially inhibited the production of Cl- and H+ in aqueous solution. The oxidation of 2,5-dimethylfuran is photosensitized by these drugs more effectively in methanol compared with aqueous solution. The photochemical behaviour is consistent with a photodissociative process occurring predominantly in methanol while photoionization predominates in aqueous media. No chloride ion was detected after extended irradiation of chlortetracycline, demeclocycline, chlordiazepoxide and hexachlorophane. The photolability of the chlorine in the compounds tested correlated with their ability to photosensitize oxidation by the Type I (free radical) mechanism.