Critical parameters in the cytotoxicity of photodynamic therapy using a pulsed laser

Photodynamic therapy (PDT) using a pulsed laser is becoming popular, but its cytotoxic effect is still not clear. We therefore studied the cytotoxicity of PDT using a pulsed laser by changing its irradiation parameters and compared the degrees of cytotoxicity with those of PDT using continuous-wave (CW) light sources. Mice renal cell carcinoma cells were incubated with PAD-S31, a water-soluble photosensitiser of which the excitation peak is 670 nm, and were then irradiated with either a tungsten lamp, a CW diode laser, or a nanosecond pulsed ND:YAG laser-based optical parametric oscillator system. When the PAD-S31 concentration and total light dose were constant (12 μg/ml and 40 J/cm², respectively), the CW laser caused fiuence fluence rate-dependent decrease in cellular proliferation until the fluence rate reached 90 mW/cm², at which point inhibition of cellular proliferation was more than 80%. The cytotoxicity then became almost saturated at fluence rates of > 90 mW/cm². On the other hand, inhibition of cellular proliferation in samples irradiated with the pulsed laser reached 80% even at the fluence rate of 15 mW/cm², and, interestingly, the cytotoxicity paradoxically decreased with increase in the fluence rate. Moreover, the cytotoxicity in the PDT using the pulsed laser depended on the repetition rate. The inhibition of cellular proliferation by PDT using 30-Hz irradiation was greater than that by PDT using 5-Hz irradiation when the same fluence rates were used. These results suggest that the efficacy of PDT using a pulsed laser depends considerably on fluence rate and repetition rate.