Analogues of platelet activating factor. 4. Some modifications of the phosphocholine moiety.

Racemic analogues of platelet activating factor (PAF) in which the methylene bridge separating the phosphate and trimethylammonium moieties is altered in length (7a-f) have been prepared. Increasing the length of this bridge results in a progressive decrease in the hypotensive and platelet aggregation responses. Analogues in which the phosphocholine group is substituted with a methyl group (7h and 7i) or a phenyl group (5j) or in which the methylene bridge is replaced with a meta-substituted benzyl group (5k) have been prepared. With respect to both the blood pressure and platelet aggregation responses, 7i and 5k showed little if any changes in potency compared to racemic C16-PAF (1a). While 7h is more potent than 1a with respect to both the hypotensive and platelet aggregation properties, 5j is less potent. Replacement of the phosphate moiety of C18-PAF (1b) with a phosphonate group (7g) leads to decreased activity in both assays. Analogue 11, in which the phosphocholine group has been replaced with a 4-(trimethylammonio)butoxy group, exhibited no detectable hypotensive or platelet aggregating activity. None of the analogues exhibited a separation of the blood pressure and platelet aggregation activities.