Light dose fractionation to enhance photodynamic therapy using 5-aminolevulinic acid in the normal rat colon

5-Aminolevulinic acid (ALA) is an attractive photosensitizing agent for photodynamic therapy (PDT) as its photoactive derivative, protoporphyrin IX, is metabolized within 1-2 days, eliminating prolonged skin photosensitivity. However, at the maximum dose patients can tolerate by mouth, 60 mg/kg, only superficial effects are seen. This paper extends earlier studies on enhancing the effect by light fractionation. Experiments in the normal rat colon looked at the area of necrosis around a single light delivery fiber 3 days after PDT with a range of light-dose fractionation regimes. All animals were given 200 mg/kg ALA intravenously 2 h prior to light delivery (100 mW at 635 nm) and each interruption in illumination was for 150 s. The area of PDT necrosis (total dose 25 J) could be increased by a factor of 3 with a single interval after 5 J, compared with continuous illumination. Alternatively, with this single break, the total light dose could be reduced by 60% to achieve the same area of necrosis as with continuous illumination. This simple modification to PDT with ALA could markedly reduce current treatment times as well as increasing clinical efficacy.