The Measurement of Singlet Oxygen Production in an Experimental Tongue Cancer Model as an Indicator for Photodynamic Therapy

Abstract

Purpose: Photodynamic therapy (PDT) induced singlet oxygen (1O2) can cause rapid necrosis of solid tumors. Using a new near-infrared photomultiplier tube system, the 1O2 production was monitored in realtime for HSC-3 tongue cancer cells and in vivo tissue, and the relationship between the 1O2 production and anti-tumor effects was investigated. Experimental Design: Using 5-aminolevulinic acid (5-ALA), 1O2 generation was induced with a variety of laser parameters (fluence and irradiation) in vivo. During this time, tumor death was evaluated. Based on the 1O2 production, the optimal irradiation conditions and the long-term outcome were investigated on a subcutaneous tongue cancer model over 90 days. Methods and Results: The 1O2 production showed a strong correlation with the laser output in a dose-dependent manner. At high laser output, the 1O2 levels were high and caused gradual but strong antitumor effects, whereas at low laser outputs, the 1O2 levels were low and the anti-tumor effects were rapid but superficial. We then performed PDT in an experimental tongue cancer model at an irradiance lower than 150 mW/cm, which was identified in the first part of the study as the optimal irradiance value based on the 1O2 production. Within the 90 days post PDT, PDT-induced hemorrhagic necrosis was observed in 8 mice out of 10 within 5 days. In the other 2 mice, no sign of necrosis was seen until 25 days after PDT. The tumor volume of the PDT group continued to decline except for 2 mice which showed some slight growth over 70 days after PDT. In the control group, tumor size increased dramatically over 70 days. Conclusions: The present studies established 1O2 levels as an indicator for optimal irradiation during PDT, and showed a relationship between the 1O2 production and the photodynamic effects. Although 5ALA mediated PDT showed a strong anti-tumor effect on larger subcutaneous tongue tumors, it might be more effective for small tumors of oral cavity.