Fractionated illumination in oesophageal ALA-PDT: effect on ferrochelataseactivity

Background and objective: Administration of 5-aminolevulinic acid (ALA) ind uces accumulation of the photosensitive compound protoporphyrin IX (PpIX) i n certain tissues. PpIX can be used as photosensitizer in photodynamic ther apy (PDT). More selective or higher PpIX: accumulation in the area to be tr eated could optimize the results of ALA-PDT. Porphobilinogen deaminase (PBG D) is rate-limiting in PpIX formation whereas ferrochelatase converts PpIX into haem by chelation of ferrous iron into PpIX. This results in a moment of close interaction (ferrochelatase binding to PpIX) during which ferroche latase could selectively be destroyed resulting in an increased PpIX concen tration. The aim of the present study was to investigate whether illuminati on before PDT can selectively destroy ferrochelatase, and whether this resu lts in higher PpIX accumulation and thereby increases the PDT effect. Furth ermore, the effect of a second ALA dose was tested. Study design/materials and methods: Oesophageal tissue of 60 rats were allocated to 2 groups of 30 animals each. In one group, enzyme and PpIX measurements were performed af ter ALA administration (200 mg/kg orally, n=20), or a second dose of 200 mg /kg ALA at 4 h (n=10), half of each group with and without illumination at 1 h with 12.5 J/cm diffuser length. In the second group, PDT was performed. Ten animals were illuminated at 3 h after ALA administration with 20 (n=5) or 32.5 J/cm (n=5), 10 animals were illuminated at 1 h (12.5 J/cm) and rec eived intra-oesophageal PDT treatment (20 J/cm) at 3 h (n=5) or 4 h (n=5) a fter ALA, Additionally, 10 animals received a second dose of 200 mg/kg ALA at 4 h and were illuminated (20 J/cm) at 7 h after the first dose of ALA wi th (n=5) or without (n=5) illumination at 4 h (12.5 J/cm). Results: Illumin ation with 12.5 J/cm at 1 h after ALA administration caused inhibition of t he activity of ferrochelatase at 3 and 4 h after ALA (P=0.02 and P<0.001, r espectively), but not at 7 h (P=0.3). In animals sacrificed at 4 h the rati o PBGD:ferrochelatase was higher in animals illuminated at 1 h compared to non-illuminated animals (P<0.001). PpIX: concentration was highest (42.7+/- 3.2 pmol/mg protein) at 3 h after ALA administration and did not increase b y illumination at 1 h. Administration of a second dose of ALA did not resul t in higher PpIX accumulation. After PDT, no difference in epithelial or mu scular damage was found between the various groups. Conclusion: Illuminatio n at 1 h after ALA administration can cause selective destruction of ferroc helatase, resulting in a higher ratio of PBGD:ferrochelatase. This does not result in accumulation of more porphyrins, even when a second dose of ALA is given. Therefore, under the conditions used in this study fractionated i llumination does not enhance ALA-PDT-induced epithelial ablation of the rat oesophagus. (C) 2000 Elsevier Science S.A. All rights reserved.