Digoxin and Platelet Activation in Patients With Atrial Fibrillation: In Vivo and In Vitro Study

BACKGROUND: Digoxin use was shown to be associated with an increased risk of cardiovascular events in atrial fibrillation (AF). We hypothesized that digoxin may affect cardiovascular risk by increasing platelet activation. METHODS AND RESULTS: Post hoc analysis of a prospective study of anticoagulated patients with AF. Patients were divided into 2 groups balanced for age, sex, and cardiovascular risk factors: digoxin users (n=132) and nonusers (n=388). Urinary excretion of 11‐dehydro‐thromboxane B(2) (TxB(2)), a marker of platelet activation, and serum digoxin concentration (SDC) were measured. In vitro experiments were performed on platelets from healthy subjects and AF patients, which were incubated with scalar doses of digoxin (0.6–2.4 ng/mL) with or without prestimulation with a sub‐threshold of collagen. Median 11‐dehydro‐TxB(2) was 105.0 (interquartile range, 60.0–190.0) ng/mg creatinine, and median SDC was 0.65 (interquartile range, 0.40–1.00) ng/mL. Urinary 11‐dehydro‐TxB(2) and SDC were correlated (r (s)=0.350, P<0.001). Patients in the upper tertile of SDC showed higher 11‐dehydro‐TxB(2) compared with non–digoxin users (P=0.019). In vitro study showed an increased basal platelet activation in patients with AF compared with healthy subjects. Digoxin (2.4 ng/mL) induced calcium mobilization, PAC‐1 (procaspase‐activating compound 1) and platelet aggregation in AF patients but not in healthy subjects. After pretreatment with a sub‐threshold of collagen, digoxin dose‐dependent induced calcium mobilization, arachidonic acid release, TxB(2) biosynthesis, PAC‐1 and soluble platelet selectin expression, and platelet aggregation, which were inhibited by antibody against digoxin. CONCLUSIONS: We found a significant in vivo correlation between SDC and platelet activation. Supratherapeutic SDC increased in vitro platelet aggregation via calcium‐related phospholipase A(2) phosphorylation. Our findings may have clinical implications for AF patients treated with digoxin.