Photocytotoxicity of protohypericin after photoconversion to hypericin

In the present study, protohypericin was synthesised in order to compare its intrinsic photocytotoxicity with that of hypericin. The experimental work was performed in specific filtered light conditions that prevented both an unintended photoconversion of protohypericin and photosensitization of the cells. Assessing the photocytotoxicity as a function of irradiation time, it was found that the photocytotoxicity of both compounds converged after a long irradiation time (i.e., 15 min), while the difference between the photocytotoxicities was maximal after a short irradiation time (i.e., 1 min). Since this could not be accounted for by a redistribution of protohypericin during irradiation, and the different irradiation times corresponded to different degrees of photoconversion of protohypericin into hypericin, the results clearly suggest that protohypericin exhibits intrinsically a dramatically lower photoactivity as compared to hypericin.