Critical Importance of the Triplet Lifetime of Photosensitizer in Photodynamic Therapy of Tumor

The relation between the lifetimes of the triplet states of various porphyrins and their photosensitizing effects on the photodynamic therapy (PDT) of tumor has been examined. Diethylene-triamine pentaacetic acid ester of 4-[1-(2-hydroxy-ethyloxy)ethyl]-2-vinyl deuteroporphyrin-IX gallium (III) complex (Ga-DP), zinc (II) complex (Zn-DP), and manganese (III) complex (Mn-DP) and Photofrin II (PII) are used as the photosensitizer. The triplet lifetimes have been measured for the samples adsorbed on filter paper (FP) and found to be 57 ms (Ga-DP), 26 ms (Zn-DP), less than or equal to 10 microseconds (Mn-DP) and 9 ms (PII). The phosphorescence of Ga-DP in tumor-bearing golden hamsters are measured both in tumor tissue and in liver. They show bi-exponential decay with the lifetimes of about 5 and 20 ms. From the values, the generation rate, kct[3O2], of singlet molecular oxygen in living animal tissue may be estimated to be an order of 10(2) s-1. The PDT effects have been quantitatively investigated for in vitro experiments; upon irradiation the growth inhibitions of mouse p388 leukemia cells are obtained as a function of concentration of Ga-DP, Zn-DP, Mn-DP and PII. The experimental results indicate that the PDT effects depend essentially on the triplet lifetimes of the photosensitizers.