Ribose modified nucleosides and nucleotides as ligands for purine receptors.

Molecular modeling of receptors for adenosine and nucleotide (P2) receptors with docked ligand, based on mutagenesis, was carried out. Adenosine 3',5'-bisphosphate derivatives act as selective P2Y1 antagonists/partial agonists. The ribose moiety was replaced with carbocyclics, smaller and larger rings, conformationally constrained rings, and acyclics, producing compounds that retained receptor affinity. Conformational constraints were built into the ribose rings of nucleoside and nucleotide ligands using the methanocarba approach, i.e. fused cyclopropane and cyclopentane rings in place of ribose, suggesting a preference for the Northern (N) conformation among ligands for P2Y1 and A1 and A3ARs.

Synthesis, biological activity, and molecular modeling of ribose-modified deoxyadenosine bisphosphate analogues as P2Y(1) receptor ligands.

The structure-activity relationships of adenosine-3', 5'-bisphosphates as P2Y(1) receptor antagonists have been explored, revealing the potency-enhancing effects of the N(6)-methyl group and the ability to substitute the ribose moiety (Nandanan et al. J. Med. Chem. 1999, 42, 1625-1638). We have introduced constrained carbocyclic rings (to explore the role of sugar puckering), non-glycosyl bonds to the adenine moiety, and a phosphate group shift. The biological activity of each analogue at P2Y(1) receptors was characterized by measuring its capacity to stimulate phospholipase C in turkey erythrocyte membranes (agonist effect) and to inhibit its stimulation elicited by 30 nM 2-methylthioadenosine-5'-diphosphate (antagonist effect). Addition of the N(6)-methyl group in several cases converted pure agonists to antagonists. A carbocyclic N(6)-methyl-2'-deoxyadenosine bisphosphate analogue was a pure P2Y(1) receptor antagonist and equipotent to the ribose analogue (MRS 2179). In the series of ring-constrained methanocarba derivatives where a fused cyclopropane moiety constrained the pseudosugar ring of the nucleoside to either a Northern (N) or Southern (S) conformation, as defined in the pseudorotational cycle, the 6-NH(2) (N)-analogue was a pure agonist of EC(50) 155 nM and 86-fold more potent than the corresponding (S)-isomer. The 2-chloro-N(6)-methyl-(N)-methanocarba analogue was an antagonist of IC(50) 51.6 nM. Thus, the ribose ring (N)-conformation appeared to be favored in recognition at P2Y(1) receptors. A cyclobutyl analogue was an antagonist with IC(50) of 805 nM, while morpholine ring-containing analogues were nearly inactive. Anhydrohexitol ring-modified bisphosphate derivatives displayed micromolar potency as agonists (6-NH(2)) or antagonists (N(6)-methyl). A molecular model of the energy-minimized structures of the potent antagonists suggested that the two phosphate groups may occupy common regions. The (N)- and (S)-methanocarba agonist analogues were docked into the putative binding site of the previously reported P2Y(1) receptor model.

Acyclic analogues of deoxyadenosine 3',5'-bisphosphates as P2Y(1) receptor antagonists.

P2Y(1) receptors are activated by ADP and occur on endothelial cells, smooth muscle, epithelial cells, lungs, pancreas, platelets, and in the central nervous system. With the aid of molecular modeling, we have designed nucleotide analogues that act as selective antagonists at this subtype. The present study has tested the hypothesis that acyclic modifications of the ribose ring, proven highly successful for nucleoside antiviral agents such as gancyclovir, are generalizable to P2Y receptor ligands. Specifically, the binding site of the P2Y(1) receptor was found to be sufficiently accommodating to allow the substitution of the ribose group with acyclic aliphatic and aromatic chains attached to the 9-position of adenine. Three groups of adenine derivatives having diverse side-chain structures, each containing two symmetrical phosphate or phosphonate groups, were prepared. Biological activity was demonstrated by the ability of the acyclic derivatives to act as agonists or antagonists in the stimulation of phospholipase C in turkey erythrocyte membranes. An acyclic N(6)-methyladenine derivative, 2-[2-(6-methylamino-purin-9-yl)-ethyl]-propane-1, 3-bisoxy(diammoniumphosphate) (10), containing an isopentyl bisphosphate moiety, was a full antagonist at the P2Y(1) receptor with an IC(50) value of 1.60 micro¿. The corresponding 2-Cl derivative (11) was even more potent with an IC(50) value of 0.84 microM. Homologation of the ethylene group at the 9-position to 3-5 methylene units or inclusion of cis- or trans-olefinic groups greatly reduced antagonist potency at the P2Y(1) receptor. Analogues containing a diethanolamine amide group and an aryl di(methylphosphonate) were both less potent than 10 as antagonists, with IC(50) values of 14 and 16 microM, respectively, and no agonist activity was observed for these analogues. Thus, the ribose moiety is clearly not essential for recognition by the turkey P2Y(1) receptor, although a cyclic structure appears to be important for receptor activation, and the acyclic approach to the design of P2 receptor antagonists is valid.

Structure-activity relationships of bisphosphate nucleotide derivatives as P2Y1 receptor antagonists and partial agonists.

The P2Y1 receptor is present in the heart, in skeletal and various smooth muscles, and in platelets, where its activation is linked to aggregation. Adenosine 3',5'- and 2',5'-bisphosphates have been identified as selective antagonists at the P2Y1 receptor (Boyer et al. Mol. Pharmacol. 1996, 50, 1323-1329) and have been modified structurally to increase receptor affinity (Camaioni et al. J. Med. Chem. 1998, 41, 183-190). We have extended the structure-activity relationships to a new series of deoxyadenosine bisphosphates with substitutions in the adenine base, ribose moiety, and phosphate groups. The activity of each analogue at P2Y1 receptors was determined by measuring its capacity to stimulate phospholipase C in turkey erythrocyte membranes (agonist effect) and to inhibit phospholipase C stimulation elicited by 10 nM 2-(methylthio)adenosine 5'-diphosphate (antagonist effect). 2'-Deoxyadenosine bisphosphate analogues containing halo, amino, and thioether groups at the 2-position of the adenine ring were more potent P2Y1 receptor antagonists than analogues containing various heteroatom substitutions at the 8-position. An N6-methyl-2-chloro analogue, 6, was a full antagonist and displayed an IC50 of 206 nM. Similarly, N6-methyl-2-alkylthio derivatives 10, 14, and 15 were nearly full antagonists of IC50 < 0.5 microM. On the ribose moiety, 2'-hydroxy, 4'-thio, carbocyclic, and six-membered anhydrohexitol ring modifications have been prepared and resulted in enhanced agonist properties. The 1,5-anhydrohexitol analogue 36 was a pure agonist with an EC50 of 3 microM, i.e., similar in potency to ATP. 5'-Phosphate groups have been modified in the form of triphosphate, methyl phosphate, and cyclic 3',5'-diphosphate derivatives. The carbocyclic analogue had enhanced agonist efficacy, and the 5'-O-phosphonylmethyl modification was tolerated, suggesting that deviations from the nucleotide structure may result in improved utility as pharmacological probes. The N6-methoxy modification eliminated receptor affinity. Pyrimidine nucleoside 3', 5'-bisphosphate derivatives were inactive as agonists or antagonists at P2Y receptor subtypes.