Ben-Hur, E., Department of Radiobiology, Nuclear Research Center-Negev, P.O. Box 9001, Beer-Sheva, Israel
Kol, R., Department of Food Science, A.R.O., The Volcani Center, P.O. Box 6, Bet-Dagan, Israel
Riklis, E., Department of Radiobiology, Nuclear Research Center-Negev, P.O. Box 9001, Beer-Sheva, Israel
Marko, R., Department of Radiobiology, Nuclear Research Center-Negev, P.O. Box 9001, Beer-Sheva, Israel
Rosenthal, I., Department of Radiobiology, Nuclear Research Center-Negev, P.O. Box 9001, Beer-Sheva, Israel

Phthalocyanines are photosensitizers evaluated for use in photodynamic therapy of cancer. As such, the dependence of the bioresponse on the light fluence rate may be of clinical importance. The effect of the fluence rate of white light from 0·165 to 3·3 kJ m-2 min-1 was studied in Chinese hamster cells and human lymphocytes, using as endpoints colony-forming ability and inhibition of [3H]thymidine incorporation following mitogenic stimulation and dye-photoactivation, respectively. Using Chinese hamster cells exposed to photoexcited chloroaluminium phthalocyanine tetrasulphonate in PBS solution, cytotoxicity was diminished as the fluence rate was reduced. In human lymphocytes changing the fluence rate between 0·33 and 3·3 kJ m-2 min-1 affected the response in a way similar to that of Chinese hamster cells. Human lymphocytes, when exposed to incremental increasing light fluences, 4 h after a conditioning dose, were able to recover from phthalocyanine-induced photo-damage, as evidenced by the reappearance of a shoulder on the dose-effect curve. This recovery process during a protracted light exposure, could explain the reduced sensitivity to phthalocyanine photosensitization, compared to exposure at high fluence rates. © 1987 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.