INHIBITION OF PAF-INDUCED HUMAN PLATELET-AGGREGATION BY ANTITHROMBOTIC NIPECOTAMIDES

Nipecotamides (piperidine-3-carboxamides) are potent inhibitors of pla telet aggregation induced by a variety of agonists in vitro and in viv o, The inhibitory effects of six structural types of nipecotamides on human platelet aggregation induced by platelet-activating factor (PAF) in vitro, are studied, Evaluation of 15 racemates and stereoisomers o f two nipecotamides showed that bis-nipecotoyl alkanes were more activ e than their mono congeners, Mono- and bis-nipecotoyl decanes were mor e potent than the corresponding hexanes, Lipophilicity was found to pl ay a significant role in the antiplatelet activity of these compounds, The stereoselectivity in the PAF-antagonist potential of nipecotamide s was less pronounced than that resulting from their action on ADP- or collagen-induced aggregation, Oxidation of the two benzylic carbon at oms of [3-(N,N-diethylcarbamoyl)piperidino]-p-xylene-2HBr (A-1) to for m 1,4-bis[3-N,N-diethylcarbamoyl) piperidino]benzenedicarboxamide (A-4 0K), which has a second set of carbonyl oxygens but lacks basic N atom s, resulted in a remarkable loss of ADP-antagonist potency while retai ning PAF-antagonist activity, It is suggested that in addition to thei r membrane effects, nipecotamides act at other sites, including the PA F receptor, Double reciprocal plots of PAF binding to gel-filtered pla telets (GFP) in the presence and absence of a typical nipecotamide (A- 1C) were indicative of competitive inhibition (K-i = 19.28 mu M). Scat chard analysis of H-3-PAF binding to GFP suggested the presence of hig h, intermediate (I) and low affinity binding sites, of which the I sit e gave a K-D/app of 0.332 nM with an estimated 564 sites/platelet. Key interactions of nipecotamides with the PAF receptor appear to be the following (i) electrostatic interactions of the two amide oxygens with a primary set of electropositive areas spaced at 5-7 Angstrom, (ii) i n the case of appropriate compounds, electrostatic interactions of the two amide oxygens spaced at 10-12 Angstrom with corresponding seconda ry receptor sites carrying positive electrostatic potential, (iii) a h ydrophobic moiety fitting into a hydrophobic pocket in the receptor, a nd (iv) the cationic piperidine N+ (at pH 7.4) interacting with a coun terion, probably aspartic acid.