The phagocytosis of rod outer segments is inhibited by selected drugs in retinal pigment epithelial cell cultures

The effects of tamoxifen, toremifene and chloroquine on the phagocytosis of rod outer segments by retinal pigment epithelium were evaluated in human r etinal pigment epithelial cell line D407 and pig retinal pigment epithelial cell culture. Retinal pigment epithelial cells were exposed to different c oncentrations of tamoxifen (1-20 muM), toremifene (1-20 muM) and chloroquin e (1-1000 muM), and challenged with FITC-labeled rod outer segments for 24 hr. The phagocytized (bound and ingested) rod outer segments were measured fluorometrically, and the effect of the drugs on the phagocytosis was deter mined. The cytotoxicity of the drugs was evaluated by measuring their effec ts on mitochondrial enzyme activities (WST-1-test). The results showed that the test compounds inhibited the phagocytosis of rod outer segments in bot h D407 and pig retinal pigment epithelial cells. The phagocytic activity wa s more sensitive to tamoxifen (EC50 7.2 muM for D407 cells and 3.6 muM for pig retinal pigment epithelial cells) and toremifene (EC50 6.2 muM and 3.1 muM respectively) than to chloroquine (EC50 77.2 muM for D407 cells). The i nhibition of rod outer segment phagocytosis in both cell cultures started a t lower dose levels of test compounds than the cytotoxicity indicated by th e WST-1-test. The experiments were carried out both in serum-free medium an d serum-containing medium. Serum seemed to be a critical factor in the medi um and caused difficulties in the interpretation of the results.