In vitro inhibition of platelet function and coagulation by pentamidine isethionate

Pentamidine isethionate is a trypanocidal drug used for the treatment of Pneumocystis carinii pneumonitis. Hematological complications have occasionally been reported and include anemia, leukopenia, and thrombocytopenia. We report here several qualitative abnormalities of in vitro platelet function and coagulation that have not been described previously. Platelets were exposed in vitro to concentrations of pentamidine isethionate ranging from 0.5 to 100 mug/ml of platelet-rich plasma. Clot retraction, platelet adhesiveness to glass beads, and platelet aggregation (adenosine 5'-diphosphate [ADP], thrombin, epinephrine, collagen, and ristocetin) were inhibited in a dose-dependent fashion. The addition of pentamidine isethionate after aggregation had been initiated with ADP reversed both primary and, to a lesser degree, secondary aggregation. Platelet factor 3 availability and serotonin uptake and release (using collagen as the releasing agent) were not inhibited. Serotonin release with 10(-4) M ADP was slightly inhibited. Pentamidine isethionate prolonged the thrombin time of plasma at concentrations of 5 mug/ml and greater. The prothrombin time was prolonged at concentrations greater than 10 mug/ml of plasma. The inhibition of aggregation was reversed by washing and resuspension in plasma or by the addition of calcium or magnesium ions.