Propenyl carboxamide derivatives as antagonists of platelet-activating factor

A series of N-[4-(3-pyridinyl)butyl] 3-substituted propenyl carboxamide derivatives bearing an unsaturated bicyclic moiety in the 3-position was prepared and evaluated for PAF (platelet activating factor) antagonist activity. These compounds represent conformationally constrained direct analogues of the corresponding potent 5-aryl-pentadienecarboxamides (5). Most of the new compounds were active in a PAF-binding assay employing whole, washed dog platelets as the receptor source and inhibited PAF-induced bronchoconstriction in guinea pigs after intravenous administration. However, oral activity in the PAF-induced bronchoconstriction model was highly sensitive to the nature and substitution of the bicyclic ring system. The most interesting compounds included [R-(E)]-(1-butyl-6-methoxy-2-naphthyl)-N-[1-methyl-4-(3- pyridinyl)butyl]-2-propenamide (4b), [R-(E)]-(3-butyl-6-methoxy-2- benzo[b]thiophene-yl)-N-[1-methyl-4-(3-pyridinyl)butyl]-2-propenamide (4k), and [R-(E)]-(3-butyl-6-methoxy-1-methyl-2-indoly)-N-[1-ethyl-4- (3-pyridinyl)butyl]-2-propenamide (4l) which inhibited PAF-induced broncho-constriction in guinea pigs with IC50s of 3.0-5.4 mg/kg, when the animals were challenged 2 h after drug treatment. They were also highly effective 6 h after a 50 mg/kg oral dose. This study supports the notion that the key remote aromatic ring present in the 5-arylpentadienecarboxamides (5) is preferentially coplanar with the diene system for good PAF antagonist activity.