Further characterization of PAF receptor and novel antagonists.

To investigate the possible existence of subtypes of PAF receptors and antagonists, the relative activities of four typical competitive PAF antagonists, kadsurenone, BN 52021, WEB 2086 and L-659,989, were compared. Some differences in their inhibition of the specific binding of [3H]-PAF to human platelet and PMN membranes and PAF-induced platelet aggregation were observed. Synergism of the inhibition of platelet aggregation was indicated by combinations of suboptimal levels of two structurally unrelated antagonists, which suggested that the binding sites of these antagonists may not be identical. For 2,5-diaryl tetrahydrofurans, an unsymmetrical molecular configuration represented by the s,s-enantiomer of L-659,989 and its cyclopentane analogues is clearly preferred for optimal potency in vitro. The PAF-binding proteins from human platelet and bovine lung membranes have been solubilized and partially purified. As a research probe, L-662,025, which is an azido analogue of L-659,989, has been characterized as a photoactivable and irreversible PAF antagonist.