Summary
The anti-platelet monoclonal antibody P256 is currently undergoing development for
in vivo detection of thrombus. We have examined the actions of P256 and two fragments
on human platelet function. P256, and its divalent fragment, caused aggregation at
concentrations of 10−9−3 × 10−8 M. A monovalent fragment of P256 did not cause aggregation at concentrations up to
10−7 M. P256–induced platelet aggregation was dependent upon extracellular calcium ions
as assessed by quin2 fluorescence. Indomethacin partially inhibited platelet aggregation
and completely inhibited intracellular calcium mobilisation. Apyrase caused partial
inhibition of aggregation. Aggregation induced by the divalent fragment was dependent
upon fibrinogen and was inhibited by prostacyclin. Aggregation induced by the whole
antibody was only partially dependent upon fibrinogen, but was also inhibited by prostacyclin.
P256 whole antibody was shown, by flow cytometry, to induce fibrinogen binding to
indomethacin treated platelets. Monovalent P256 was shown to be a specific antagonist
for aggregation induced by the divalent forms. In–111–labelled monovalent fragment
bound to gel-filtered platelets in a saturable and displaceable manner. Monovalent
P256 represents a safer form for in vivo applications
