5'-O-alkyl ethers of N,2-substituted adenosine derivatives: partial agonists for the adenosine A1 and A3 receptors.

New N,5'-di- and N,2,5'-trisubstituted adenosine derivatives were synthesized in good overall yields. Appropriate 5-O-alkyl-substituted ribose moieties were coupled to 6-chloropurine or 2,6-dichloropurine via Vorbrüggen's glycosylation method. Subsequent amination and deprotection of the intermediates yielded compounds 18-35. Binding affinities were determined for rat adenosine A1 and A2A receptors and the human A3 receptor. The ability of compounds 18-35 to inhibit forskolin-induced (10 microM) cyclic AMP (cAMP) production and their ability to stimulate guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding, via either the adenosine A1 receptor or the adenosine A3 receptor, were assessed. N-Cyclopentyl-substituted adenosine derivatives displayed affinities in the low nanomolar range for the adenosine A1 receptor, whereas N-(3-iodobenzyl)-substituted derivatives had high affinity for the adenosine A3 receptor. Compound 22 had the highest affinity for the adenosine A1 receptor (K(i) value of 16 nM), and compounds 20 and 26 had the highest affinities for the adenosine A3 receptor (K(i) values of 4 and 3 nM, respectively). A chlorine substituent at the 2-position either did not affect or slightly increased the adenosine A1 receptor affinity, whereas the A3 receptor affinity was affected differently, depending on the N-substituent. Furthermore, the introduction of chlorine slightly increased the A3/A1 selectivity ratio. At the 5'-position, an O-methyl substituent induced the highest adenosine A1 receptor affinity, whereas an O-ethyl substituent did so for the A3 receptor. All compounds showed partial agonistic effects in both the cAMP and [35S]GTPgammaS assays, although more marked in the latter assay. In general, the 2-chloro derivatives seemed to have lower intrinsic activities compared to the 2-H-substituted compounds on both the adenosine A1 and the adenosine A3 receptors. The compounds with an N-(3-iodobenzyl) substituent displayed the lowest intrinsic activities. Finally, all compounds also showed partially antagonistic behavior in the [35S]GTPgammaS assay.

N-cycloalkyl derivatives of adenosine and 1-deazaadenosine as agonists and partial agonists of the A(1) adenosine receptor.

A number of cycloalkyl substituents (from C-3 to C-8) have been introduced on the 6-amino group of adenosine, 1-deazaadenosine, and 2'-deoxyadenosine, bearing or not a chlorine atom at the 2-position, to evaluate the influence on the A(1) and A(2A) affinity of steric hindrance and lipophilicity. Furthermore, the guanosine 5'-triphosphate (GTP) shift and the maximal induction of guanosine 5'-(gamma-thio)triphosphate ([(35)S]GTPgammaS) binding to G proteins in rat brain membranes were used to determine the intrinsic activity of these nucleosides at the A(1) adenosine receptor. All compounds of the ribose-bearing series proved to be full agonists, the 1-deaza derivatives showing affinities for the A(1) receptor about 10-fold lower than the corresponding adenosines. On the other hand, all the 2'-deoxyribose derivatives bind to the A(1) receptor with affinities in the high nanomolar range, with the 2-chloro substituted compounds showing slightly higher affinities than the 2-unsubstituted counterparts. In terms of the potencies, the most potent compounds proved to be those bearing four- and five-membered rings. Both GTP shifts and [(35)S]-GTPgammaS experiments showed that most of the 2'-deoxyadenosine derivatives are partial agonists. The 2'-deoxyadenosine derivatives which were identified as partial agonists consistently detected fewer A(1) receptors in the high-affinity state than full agonists. However, it is worthwhile noting that there was not a simple linear relationship between receptor occupancy and activation. These results indicate that a critical density of A(1) adenosine receptors in the high-affinity state is required for G protein activation.

Pyrazole derivatives as partial agonists for the nicotinic acid receptor.

Nicotinic acid as a hypolipidemic agent appears unique due to its potential to increase HDL cholesterol levels to a greater extent than other drugs. However, it has some side effects, among which severe skin flushing is the most frequent and often limits patients' compliance. In a search for novel agonists for the recently identified and cloned G protein-coupled nicotinic acid receptor, we synthesized a series of substituted pyrazole-3-carboxylic acids that proved to have substantial affinity for this receptor. The affinities were measured by inhibition of [(3)H]nicotinic acid binding to rat spleen membranes. Potencies and intrinsic activities relative to nicotinic acid were determined by their effects on [(35)S]GTPgammaS binding to rat adipocyte and spleen membranes. Interestingly, most compounds were partial agonists. In particular, 2-diazabicyclo[3,3,0(4,8)]octa-3,8-diene-3-carboxylic acid (4c) and 5-propylpyrazole-3-carboxylic acid (4f) proved active with K(i) values of approximately 0.15 microM and EC(50) values of approximately 6 microM, while their intrinsic activity was only approximately 50% when compared to nicotinic acid. Even slightly more active was 5-butylpyrazole-3-carboxylic acid (4g) with a K(i) value of 0.072 microM, an EC(50) value of 4.12 microM, and a relative intrinsic activity of 75%. Of the aralkyl derivatives, 4q (5-(3-chlorobenzyl)pyrazole-3-carboxylic acid) was the most active with a relatively low intrinsic activity of 39%. Partial agonism of the pyrazole derivatives was confirmed by inhibition of G protein activation in response to nicotinic acid by these compounds. The pyrazoles both inhibited the maximum effect elicited by 100 microM nicotinic acid and concentration dependently shifted nicotinic acid concentration-response curves to the right, pointing to a competitive mechanism of action.

Characterization of a G protein-coupled receptor for nicotinic acid.

Nicotinic acid is a lipid-lowering agent widely used to treat hypertriglyceridemia and to elevate low high density lipoprotein levels. However, the underlying mechanisms are poorly understood. In this study, G protein activation by nicotinic acid and derivatives was assessed as stimulation of guanosine 5'-(gamma-[(35)S]-thio)triphosphate ([(35)S]GTPgammaS) binding, and [(3)H]nicotinic acid was used for specific labeling of binding sites. Nicotinic acid (EC(50) approximately 1 microM) stimulated [(35)S]GTPgammaS binding in membranes from rat adipocytes and spleen, but not from other tissues. G protein activation in adipocyte membranes in the presence of maximally activating concentrations of the selective A(1) adenosine receptor agonist 2-chloro-N(6)-cyclopentyladenosine and nicotinic acid was almost additive, indicating that G proteins of mostly distinct pools were activated by these agonists. G protein activation by nicotinic acid and related substances in spleen and adipocytes revealed identical pharmacological profiles. [(3)H]Nicotinic acid specifically detected guanine nucleotide-sensitive binding sites of identical pharmacology in adipocyte and spleen membranes. The site of action of nicotinic acid is distinct from other G protein-coupled receptors. These data indicate that nicotinic acid most probably acts on a specific G protein-coupled receptor.