The effects of pentoxifylline on filterability of normal red blood cells (RBCs) and their adhesiveness to cultured endothelial cells were investigated. In a balanced randomized and double blind trial, six healthy volunteers received 400 mg pentoxifylline or matching placebo 2 h before blood samples were taken. Filterability of RBCs of the subject while on pentoxifylline was significantly increased at 25 degrees C and 18 degrees C. Lowering of the filteration temperature to 18 degrees C significantly decreased filterability of RBCs. In vitro studies showed that 12 micrograms/ml pentoxifylline significantly increased RBC filterability and also partially prevented the effect of decreasing temperature on RBC filterability. 12 micrograms/ml pentoxifylline significantly decreased the adherence of normal RBCs to cultured endothelial cells. Our results suggest that in addition to increasing filterability of RBCs, pentoxifylline also decreased the adherence of RBCs to endothelial cells and this may contribute to its therapeutic effect.

The effects of dipyridamole on red cell filterability both in vitro and ex vivo were measured. In a balanced, randomised and double‐blind trial, six healthy male and female volunteers (22‐37 years) were given dipyridamole 400 mg/day or matching placebo in four divided doses for 3 days, and heparinised venous blood samples were taken 1 h after the ingestion of the last dose. Filterability of red cells was increased significantly (P less than 0.05 paired t‐test) when the subjects were on dipyridamole compared with placebo. In separate experiments, 15 min incubation with 2 or 20 micrograms/ml dipyridamole in vitro was found to have no effect on the filterability of freshly prepared red cell suspensions. After 24 h storage at 4 degrees C, the filterability of red cells was significantly decreased (P less than 0.01) and this could be partially prevented by adding dipyridamole to the stored cells (P less than 0.05). These results suggest that dipyridamole has an effect on the behaviour of the red cell membrane to increase the deformability of the cells. This may contribute to its therapeutic effect.