Simple model for the effectiveness of PDT with pulsed-laser sources

Irradiation with pulsed lasers can change mechanisms and efficacy of photodynamic therapy (PDT) depending on the laser pulse parameters. Since most photosensitizers have a relatively high triplet quantum yield and triplet lifetimes of tens of microseconds, even moderate power densities below 100 kW/cm2 can lead to a saturation of the singlet oxygen production, thereby reducing the PDT effect. A simple quantitative model is developed to estimate this effect. According to this, for laser pulses not longer than the triplet lifetime, the PDT efficacy depends on the product of single pulse energy, irradiation wavelength, and the extinction coefficient of the photosensitizer. Peak irradiance and pulse width have minor influence on the efficacy of pulsed irradiation, which decreases as the triplet quantum yield reaches one. The model is supported by in vitro experiments with Photosan 3 and in vivo and in vitro experiments with Photosan 3, Photofrin, aluminum sulphonated phthalocyanine (AlSPc) and benzoporphyrin derivative monoacid ring A (BPD-MA) reported in the literature.