WAVELENGTH-DEPENDENT PHOTORESPONSE OF BIOLOGICAL AND AQUEOUS MODEL SYSTEMS USING THE PHOTODYNAMIC PLANT PIGMENT HYPERICIN

Photodynamic eradication of tumour cells in vivo depends on the presen ce of a photosensitizer, light delivery to the cells, and an oxygen su pply. Hypericin, a polycyclic quinone with absorption maxima in the ul traviolet and visible ranges, was prepared for clinical use as a photo sensitizer. Due to antitumoral and antineoplastic activities as well a s the generation of singlet oxygen after photoexcitation, hypericin wa s applied in clinical oncology and photodynamic therapy. Hypericin was administered subcutaneously (20 mu g hypericin in 200 mu l Nacl/pyrid ine solution) into the ante brachium (forearm) of two volunteers. Afte r the diffusion and equilibration of 120 min phototesting was carried out using outdoor light exposure, halogen lamp, laser 514 nm (argon), laser 632 nm (argon dye) and laser 670 nm (diode laser), from 60 to 12 0 J cm(-2). Positive phototests to outdoor light exposure, halogen lam p and laser 514 nm were characterized by rubescence, oozing, vesiculat ion and darting pain. Phototests with laser 632 nm and 670 nm showed n o effects after irradiation. When hypericin was administered topically on skin, erythema and flaring could not be induced by any irradiation . These results suggest that hypericin is a potent photosensitizer onl y within the UV and green light ranges. This characteristic photorespo nse could also be obtained in guinea pig papillary muscle (GPPM) bioas say, which may be established as a model for photosensitizer testing. Irradiation of hypericin-incubated GPPM with 514 nm (20 J cm(-2)) led to a decrease of the contractile force of about 31%. However, excitati on with 632 nm and 670 nm did not cause inotropic effects on GPPM. In addition, hypericin and Photosan 3 were shown to be capable of sensiti zing the photo-oxidation of sodium linoleate. This assay should be est ablished for testing interactions between photosensitizers and light s ources in vitro.