Photocytotoxicity of hypericin in normoxic and hypoxic conditions

The normoxic and hypoxic photocytotoxicity of hypericin has been examined o n A431 cells as assessed by the Neutral Red method, using cell-culture flas ks made of polystyrene and glass, different hypericin concentrations and li ght fluences. Using polystyrene flasks, lower hypoxic photoactivities of hy pericin than those in normoxic conditions are seen under low fluence. In th ese conditions the hypoxic photocytotoxic effect can be (partially) rescued by increasing the fluence. However, a completely different outcome is obse rved when using glass flasks, since most of the hypoxic photocytotoxicity i s lost under these conditions. The differences can be explained in terms of efficiency of deoxygenation of the medium present in polystyrene or glass flasks. Polystyrene holds large amounts of oxygen that effuses very slowly. Glass, on the other hand, does not cause this inconvenience. Therefore the type of material of the container used to investigate the oxygen dependenc y of the photobiological activity of photosensitizers dramatically influenc es the outcome of the hypoxic experiments. Our results unequivocally prove that the cytotoxic effect induced by photoactivated hypericin is completely oxygen dependent. Hence hypericin does not differ from other phototherapeu tics used in photodynamic therapy of cancer, since haematoporphyrin derivat ive and the second-generation photosensitizers used all seem to depend on t he presence of oxygen for their antitumour activity. (C) 2000 Elsevier Scie nce S.A. All rights reserved.