Effects of Antimalarial Tafenoquine on Blood Platelet Activity and Survival.

BACKGROUND/AIMS: The 8-aminoquinoline tafenoquine has been shown to be effective against Plasmodia, Leishmania and Trypanosoma. The substance is at least in part effective by triggering apoptosis of the parasites. Moreover, tafenoquine has been shown to trigger eryptosis, the suicidal erythrocyte death characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine translocation to the erythrocyte surface. The effect of tafenoquine on eryptosis is in part due to stimulation of Ca2+ entry and oxidative stress. Ca2+ entry is a critical event in the activation of blood platelets by thrombin and collagen related peptide (CRP). The present study explored, whether tafenoquine influences Ca2+ entry, activation and apoptosis of blood platelets.

METHODS: Platelets isolated from wild-type mice were exposed for 30 minutes to tafenoquine (2.5 µg/ml) without or with an additional treatment with thrombin (0.01 U/ml) or CRP (2 µg/ml or 5 µg/ml). Flow cytometry was employed to estimate cytosolic Ca2+-activity ([Ca2+]i ) from Fluo-3 fluorescence, platelet degranulation from P-selectin abundance, integrin activation from αIIb β3 integrin abundance, phosphatidylserine abundance from annexin-V-binding, relative platelet volume from forward scatter, reactive oxygen species (ROS) from DCF fluorescence, caspase 3 activity with an active caspase-3 Staining kit, and aggregation utilizing staining with CD9-APC and CD9-PE.

RESULTS: Both, thrombin (0.01 U/ml) and CRP (2 µg/ml or 5 µg/ml), significantly increased [Ca2+]i , P-selectin abundance, active αIIb β3 integrin, and annexin-V-binding, and both significantly decreased platelet volume, activated caspase 3 and stimulated aggregation. Administration of tafenoquine (2.5 µg/ml, 30 min) significantly decreased [Ca2+]i both, in the absence and presence of thrombin and CRP. Tafenoquine significantly blunted the effect of thrombin and CRP on [Ca2+]i , P-selectin abundance, and active αIIb β3 integrin, but significantly increased ROS and annexin-V-binding, significantly augmented the effect of thrombin on caspase 3 activity and platelet volume and significantly enhanced platelet aggregation.

CONCLUSIONS: Tafenoquine counteracts thrombin and CRP induced increase of cytosolic Ca2+ activity and platelet activation, but enhances platelet apoptosis and platelet aggregation.