Enhancement of ALA-PDT damage by IR-induced hyperthermia on a colon carcinoma model

The interaction of photodynamic therapy (PDT) with 5-aminolevulinic acid (ALA) and hyperthermia is not well understood. In the present study, significant enhancement of tumor damage was observed after simultaneous application of ALA-PDT and IR-induced hyperthermia using a broad-band incoherent light source. One hour after systemic administration of ALA at a dose of 200 mg/kg, subcutaneously transplanted C26 colon carcinoma tumors were irradiated with two bands of the VersaLight® system, red (R, 580-720 nm) and red plus IR (R+IR, 580-720 nm and 1250-1600 nm). Photoirradiation using the R+IR band at different fluence rates and exposures caused mild heating of the tumor to 39-43°C at a 3 mm depth. Electron microscopy after ALA+R, ALA+R+IR and R+IR treatments showed early mitochondrial swelling that was more pronounced in the ALA+R+IR group. Tumor necrosis assessment, using histology and vital staining, revealed an enhancement of tumor necrosis depth in the ALA+R+IR group compared to ALA+R and R+IR. The results showed that subhyperthermic heating to 39-39.5°C in the ALA+R+IR group decreased the threshold light dose required for 100% tumor necrosis from 210 J/cm² (observed in the ALA+R group) to 140 J/cm². A tumor growth delay test, based on tumor volume measurement, also revealed significant enhancement of antitumor effect after application of ALA+R+IR compared to ALA+R.