ENHANCEMENT OF PHOTODYNAMIC THERAPY WITH 5-AMINOLEVULINIC ACID-INDUCED PORPHYRIN PHOTOSENSITIZATION IN NORMAL RAT COLON BY THRESHOLD AND LIGHT FRACTIONATION STUDIES

5-Aminolaevulinic acid (ALA)-induced porphyrin photosensitisation is a n attractive option for photodynamic therapy (PDT) since skin photosen sitivity is limited to 1-2 days. However, early clinical results on co lon tumours using the maximum tolerated oral dose of 60 mg kg(-1) show ed only superficial necrosis, presumably owing to insufficient intratu moral porphyrin levels, although inadequate light dosimetry may also b e a factor. We undertook experiments using ALA, 25-400 mg kg(-1) intra venously, to establish the threshold doses required for a PDT effect. Laser light al 630 nm (100 mW, 10-200 J) was delivered to a single sit e in the colon of photosensitised normal Wistar rats at laparotomy. Th e animals were killed 3 days later and the area of PDT-induced necrosi s measured. No lesion was seen with 25 mg kg(-1). The lesion size incr eased with larger ALA doses and with the light dose but little benefit was seen from increasing the ALA dose above 200 mg kg(-1) or the ligh t dose above 100 J. Thus there is a fairly narrow window for optimum d oses of drug and light. Further experiments showed that the PDT effect can be markedly enhanced by fractionating the light dose. A series of animals was sensitised with 200 mg kg(-1) ALA and then treated with 2 5 J. With continuous irradiation, the lesion area was 13 mm(2), but wi th a single interruption of 150 s the area rose to 94 mm(2) with the s ame total energy. Results were basically similar for different interva ls between fractions (10-900 s) and different numbers of fractions (2- 25). This suggests that a single short interruption in the light irrad iation may dramatically reduce the net light dose required to achieve extensive necrosis.