Adenosine Receptor Agonists. X-Ray Crystal Structure of Neca 1-(6-Amino-9H-Purin-9-Yl)-1-Deoxy-N-Ethyl-β-D-Ribofuranuronamide

N6-Substituted 5'-N-Methylcarbamoyl-4'-selenoadenosines as Potent and Selective A3 Adenosine Receptor Agonists with Unusual Sugar Puckering and Nucleobase Orientation

Potent and selective A₃ adenosine receptor (AR) agonists were identified by the replacement of 4'-oxo- or 4'-thionucleosides with bioisosteric selenium. Unlike previous agonists, 4'-seleno analogues preferred a glycosidic syn conformation and South sugar puckering, as shown in the X-ray crystal structure of 5'-N-methylcarbamoyl derivative 3p. Among the compounds tested, N⁶-3-iodobenzyl analogue 3d was found to be the most potent A₃AR full agonist (K_i = 0.57 nM), which was ≥800- and 1900-fold selective for A₁AR and A_2AAR, respectively. In the N⁶-cycloalkyl series, 2-Cl analogues generally exhibited better hA₃AR affinity than 2-H analogues, whereas 2-H > 2-Cl in the N⁶-3-halobenzyl series. N⁷ isomers 3t and 3u were much weaker in binding than corresponding N⁹ isomers, but compound 3t lacked A₃AR activation, appearing to be a weak antagonist. 2-Cl-N⁶-3-iodobenzyl analogue 3p inhibited chemoattractant-induced migration of microglia/monocytes without inducing cell death at ≤50 μM. This suggests the potential for the development of 4'-selenonucleoside A₃AR agonists as novel antistroke agents.

Different efficacy of adenosine and NECA derivatives at the human A3 adenosine receptor: insight into the receptor activation switch

A3 Adenosine receptors are promising drug targets for a number of diseases and intense efforts are dedicated to develop selective agonists and antagonists of these receptors. A series of adenosine derivatives with 2-(ar)-alkynyl chains, with high affinity and different degrees of selectivity for human A3 adenosine receptors was tested for the ability to inhibit forskolin-stimulated adenylyl cyclase. All these derivatives are partial agonists at A3 adenosine receptors; their efficacy is not significantly modified by the introduction of small alkyl substituents in the N(6)-position. In contrast, the adenosine-5'-N-ethyluronamide (NECA) analogs of 2-(ar)-alkynyladenosine derivatives are full A3 agonists. Molecular modeling analyses were performed considering both the conformational behavior of the ligands and the impact of 2- and 5'-substituents on ligand-target interaction. The results suggest an explanation for the different agonistic behavior of adenosine and NECA derivatives, respectively. A sub-pocket of the binding site was analyzed as a crucial interaction domain for receptor activation.

Design, synthesis, and evaluation of novel A2A adenosine receptor agonists

We have been interested in the design, synthesis, and evaluation of novel adenosine A2A agonists. Through the use of comparative molecular field analysis (CoMFA) we have generated a training model that includes 78 structurally diverse A2A agonists and correlated their affinity for isolated rat brain receptors with differences in their structural and electrostatic properties. We validated this model by predicting the activity of a test set that included 24 additional A2A agonists. Our CoMFA model, which incorporates the physiochemical property of dipole and selects against A1 receptor activity, generated a correlated final model (r2 = 0.891) that provides for enhanced A2A selectivity and predictability. Synthesis, pharmacological evaluation, and modeling of four novel ligands further validate the utility and predictive power (r2 = 0.626) of the CoMFA model.

Adenosine receptor agonists: synthesis and biological evaluation of the diastereoisomers of 2-(3-hydroxy-3-phenyl-1-propyn-1-yl)NECA

Among the recently reported 2-(ar)alkynyl derivatives of 5'-N-ethylcarboxamidoadenosine (NECA), the (R,S)-2-(3-hydroxy-3-phenyl-1-propyn-1-yl)NECA [(R,S)-PHPNECA or SCH 59761] was found to be a very potent agonist at A1 and A2A receptor subtypes, with a Ki of 2.5 nM and 0.9 nM, respectively. Furthermore, this compound showed an inhibitory activity on platelet aggregation 16-fold higher than NECA, being the most potent anti-aggregatory nucleoside reported so far. Since this compound bears a chiral carbon in the side chain, the diastereoisomer separation was undertaken both by chiral HPLC and by a stereospecific synthetic method. Binding assays have shown that the (S)-diastereomer is about fivefold more potent and selective than the (R)-diastereomer as agonist of the A2A receptor subtype [(S)-PHPNECA, KiA2A = 0.5 nM; (R)-PHPNECA, KiA2A = 2.6 nM]. Functional studies indicated that (S)-PHPNECA possesses marked vasodilating activity and produces a relevant decrease in heart rate. Moreover, the (S)-diastereomer proved to be about ten times more potent than the (R)-diastereomer in inducing cardiovascular effects, in in vivo hemodynamic studies. However, the greatest difference between these two enantiomers resulted in the platelet aggregation test: in fact, the (R)-diastereomer displayed an inhibitory activity similar to that of NECA, whereas the (S)-diastereomer was 37-fold more active than NECA as an inhibitor of rabbit platelet aggregation, induced by ADP. These data suggest that (S)-PHPNECA could be a useful tool to investigate the mode of binding of agonists to the platelet adenosine receptor subtype.

Nucleoside inhibitors of rhodopsin kinase

The specificity of the ATP-binding site of rhodopsin kinase was studied with adenosine analogues that are competitive inhibitors. Systematic changes in the ribose ring (position 5') and the purine ring (positions 2, 6, 7, 8, and 9) and determination of the inhibitory properties of these analogues lead to the following conclusions: (1) The N6 nitrogen in the purine ring is essential for binding at the active site, which may explain the marked preference for ATP rather than GTP as substrate. (2) The configuration of the sugar moiety is critical for the binding. (3) Positions 2, 3, and 8 of the purine ring, as well as the polyphosphate chain, play a minor role in substrate recognition by rhodopsin kinase. (4) ATP gamma S is a good substrate for rhodopsin kinase (thus rhodopsin phosphorothioate, a phosphatase-resistant product, can be formed in order to study the role of phosphorylation in rod outer segments). Pyrrolopyrimidine derivatives are very potent inhibitors of rhodopsin kinase. The Ki of one of these, sangivamycin, is 180 nM. Sangivamycin in solution assumes the anti conformation, as determined by nuclear Overhauser measurement. These measurements show that the most potent inhibitors of rhodopsin kinase, sangivamycin and toyocamycin, occur in solution preferentially in the anti conformation. Many nucleotides and nucleosides tested that are not inhibitors are syn, and many that are inhibitors form a mixture of syn and anti. The hypothesis that inhibitors may have a conformation intermediate between syn and anti was strengthened by testing a cyclic nucleoside locked in an anti conformation.(ABSTRACT TRUNCATED AT 250 WORDS)