Acyclic and cyclopropyl analogues of adenosine bisphosphate antagonists ofthe P2Y(1) receptor: Structure-activity relationships and receptor docking

The activation of P2Y(1) receptors in platelets contributes to platelet agg regation, and selective antagonists are sought as potential antithrombotic agents. We reported (Kim et al. J. bled. Chem. 2000, 43, 746-755) that acyc lic analogues of adenine nucleotides, containing two phosphate groups on a symmetrically branched aliphatic chain, attached at the 9-position of adeni ne, are moderately potent P2Y(1) receptor antagonists. In this study we hav e varied the chain structure, to include asymmetric substitution, olefinic, and cyclopropyl groups. These antagonists inhibited the stimulation of pho spholipase C in turkey erythrocyte membranes induced by 30 nM 2-MeS-ADP in the micromolar range. In the series of symmetrically branched aliphatic gro ups substituted with two phosphate groups, the optimal antagonist potency o ccurred with the 2-methylpropyl group. A 2-chloro-N-6-methyladenine derivat ive, 2-[2-(2-chloro-6-methylaminopurin-9-yl)methyl]propane-1,3-bisoxy(diamm oniumphosphate) (7), was a full antagonist at the P2Y(1) receptor with an I C50 value of 0.48 muM. Esterification of one of the phosphate groups or sub stitution with O-acetyl greatly reduced the antagonist potency at the P2Y(1 ) receptor. Removal of a methylene group of 7 or inclusion of an olefinic o r cyclopropyl group also reduced potency. A pair of enantiomeric glycerol d erivatives demonstrated a 5-fold stereoselectivity for the S-isomer. Stereo isomerically defined analogues of 7 containing a cyclopropyl group in place of the branched carbon were less potent than 7 as antagonists, with IC50 v alues of 2-3 muM. No agonist activity was observed for these analogues. A n ew rhodopsin-based molecular model of the P2Y(1) receptor indicated that th e optimal docked orientation of the two monophosphate moieties relative to the adenine N-6 (compared to a rigid, bicyclic analogue) was consistent wit h the dependence of antagonist potency on chain length. The 3 ' -phosphate was predicted to occupy a restricted space, deeper in the binding cleft tha n the 5 ' -phosphate location. In summary, modification of the flexible spa cer chain linking bisphosphate groups to the adenine moiety provided many m oderately potent antagonists.