THE EFFECT OF IRRADIANCE ON VIRUS STERILIZATION AND PHOTODYNAMIC DAMAGE IN RED-BLOOD-CELLS SENSITIZED BY PHTHALOCYANINES

Phthalocyanines are being studied as photosensitizers for virus steril ization of red blood cells (RBC). During optimization of the reaction conditions, we observed a marked effect of the irradiance on productio n of RBC damage. Using a broad-band light source (600-700 nm) between 5 and 80 mW/cm(2), there was an inverse relationship between irradianc e and rate of photohemolysis. This effect was observed with aluminum s ulfonated phthalocyanine (AlPcS(n)) and cationic silicon (HOSiPc-OSi[C H3](2)[CH2]N-3(+)[CH3]I-3(-) phthalocyanine (Pc5) photosensitizers. Th e same effect occurred when the reduction of RBC negative surface char ges was used as an endpoint. Under the same treatment conditions, vesi cular stomatitis virus inactivation rate was unaffected by changes in the irradiance. Reduction in oxygen availability for the photochemical reaction at high irradiance could explain the effect. However, theore tical estimates suggest that oxygen depletion is minimal under our con ditions. In addition, because the rate of photohemolysis at 80 mW/cm(2 ) was not increased when irradiations were carried out under an oxygen atmosphere this seems unlikely. Likewise, formation of singlet oxygen dimoles at high irradiances does not appear to be involved because th e effect was unchanged when light exposure was in D2O. While there is no ready explanation for this irradiance effect, it could be used to i ncrease the safety margin of RBC virucidal treatment by employing expo sure at high irradiance, thus minimizing the damage to RBC.