Basic study on therapeutic effect distribution formation along depth using high-intensity pulsed photodynamic therapy

Photodynamic therapy (PDT) against murine macrophage like cells with the second-generation hydrophilic photosensitizer ME2906 (mono-L-aspartyl chlorin e₆: NPe6) was performed in vitro to study therapeutic effect distribution formation along depth with high-intensity pulsed irradiation. The photocytotoxicity of macrophage like cell with ME2906 under various fluence rates was measured. We found photocytotoxicity suppression from 64 % to 16% in the cell lethality ranging the fluence rate of a pulsed laser from 20 to 400mW/cm² (corresponding pulse peak power: from 0.07 to 1.4 MW/cm²). The cell lethality of about 80 % was obtained with continues wave (cw) laser irradiation under the fluence of 10 J/cm². Photobleaching and oxygen consumption of the photosensitizer solution, were measured to know photoreaction of the photosensitizer solution under the high fluence rate pulsed irradiation. Type-II photochemical reaction suppression was indicated with the high fluence rate pulsed irradiation. The transient absorption of the photosensitizer solution during pulse irradiation was measured by the pump-and-probe technique with pulse peak power density up to 1.2 MW/cm² to investigate absorption saturation. In the case of the pump beam peak power of 1.2MW/cm², the transmittance of the probe beam increased approximately 7% from that of without the pump beam, so that huge absorption saturation did not occur in this case. We think the main cause of the photocytotoxicity suppression in this study may not to be attributed to the absorption saturation. This photocytotoxicity suppression induced by the high-intensity irradiation may be available to control treatment depth of PDT to preserve healthy internal wall of a hollow organ.