Modifying excitation light dose of novel photosensitizer PVP-Hypericin for photodynamic diagnosis and therapy

Conventional photodynamic diagnosis (PDD) and therapy (PDT) makes use of photosensitizers that are excited by continuous light irradiation of specific wavelengths. In the case of PDT, the overdose of continuous excitation may lead to an expansion of necrosis in cancer cells or morbidity in healthy surroundings. The present study involves 5-h fluorescence imaging of living human lung epithelial carcinoma cells (A549) in the presence of a novel photosensitizer, PVP-Hypericin (PVP: polyvinylpyrrolidone) to optimize the excitation light doses for PDD and PDT. A number of time-lapse imaging experiments were performed using a low-power blue LED operating in either continuous or pulsed mode. The irradiances I* were 1.59, 6.34 and 14.27 mW/cm2, the pulse lengths L being 0.127, 1.29, 13, 54.5, 131 and 60,000 ms. Then, the relation between irradiance, various exposure times, photobleaching and phototoxicity of PVP-Hyperycin was investigated. Results showed a nonlinear relationship between the amounts of excitation dose, cell viability and toxicity. For all experimental I*, minimal phototoxicity and photobleaching was detected when cells were exposed to brief pulses of light (L ⩽ 13 ms). On the other hand, pulsed excitation with I* = 14.27 mW/cm2 and L = 131 ms induced high percentages of apoptosis comparable to the long exposures of L = 60,000 ms and the continuous excitation. Thus, replacement of continuous excitation by a pulsed method seems applicable for PDT.