Distribution of 14C-labelled 2-[p-(1-oxo-2-isoindolinyl)phenyl] butyric acid, (indobufen or K 3920) in blood, platelets and platelet components

The behaviour of 14C-labelled indobufen, an inhibitor of platelet aggregation, was examined and in 4 patients.The half-life of radioactivity in plasma was about 7 h, activity being mostly in cell-free plasma. Activity in platelets was never higher than the plasma level. When platelet fractions were examined, there was only a trace of activity in the sediment obtained by ultracentrifugation of disrupted platelets, and most of the platelet activity was found in cytosol. The half-life of activity was longest in cytosol and shortest in solid structures of platelets. From these results, the conclusion was drawn that indobufen does not bind irreversibly to platelets and that the activities found in platelets are a consequence of passive migration of the substance through the platelet membrane. These results also explain the shorter action of indobufen after a single dose in comparison with ASA which, in contrast, irreversibly binds to membrane proteins.