Photochemotherapy of tumours with mesotetrahydroxyphenyl chlorin is pH dependent

Human colon carcinoma cells of the WiDr line were incubated with the tumour -localizing and photosensitizing drug mesotetrahydroxyphenyl chlorin (mTHPC ) at either pH 6.8, 7.2 or 7.8. The cellular uptake of the drug was found t o be independent of the pH value. However, under conditions when the cells contained the same amount of the drug (i.e. after incubation at different p H values) and were exposed to light, their photosensitivity increased with decreasing pH value. Furthermore, the cells were more photosensitive at 4 d egrees C than at 37 degrees C. The shoulder on the survive curves, observed for irradiation at 37 degrees C and at pH 7.2-7.8 was practically absent f or irradiation at 4 degrees C and even at 37 degrees C when the light expos ure took place at pH 6.8. The observations may be related to a reduced repa ir of photochemotherapy (PCT)-induced damage at 4 degrees C and at pH 6.8. The high PCT efficiency at pH 6.8 may contribute to the tumour selectivity of PCT with mTHPC. (C) 1999 Elsevier Science Ireland Ltd. All rights reserv ed.