Structure-Guided Design of A₃ Adenosine Receptor-Selective Nucleosides: Combination of 2-Arylethynyl and Bicyclo[3.1.0]hexane Substitutions

Abstract

(N)-Methanocarba adenosine 5′-methyluronamides containing known A3 AR (adenosine receptor)-enhancing modifications, i.e. 2-(arylethynyl)adenine and N6-methyl or N6-(3-substituted-benzyl), were nanomolar full agonists of human (h) A3AR and highly selective (Ki ~0.6 nM, N6-methyl 2-(halophenylethynyl) analogues 13, 14). Combined 2-arylethynyl-N6-3-chlorobenzyl substitutions preserved A3AR affinity/selectivity in the (N)-methanocarba series (e.g. 3,4-difluoro full agonist MRS5698 31, Ki 3 nM, human and mouse A3) better than for ribosides. Polyaromatic 2-ethynyl N6-3-chlorobenzyl analogues, such as potent linearly extended 2-p-biphenylethynyl MRS5679 34 (Ki hA3 3.1 nM; A1, A2A: inactive) and fluorescent 1-pyrene adduct MRS5704 35 (Ki hA3 68.3 nM) were conformationally rigid; receptor docking identified a large, mainly hydrophobic binding region. The vicinity of receptor-bound C2 groups was probed by homology modeling based on recent X-ray structure of an agonist-bound A2AAR, with a predicted helical rearrangement requiring an agonist-specific outward displacement of TM2 resembling opsin. Thus, X-ray structure of related A2AAR is useful in guiding design of new A3AR agonists.

Rigidified A3 Adenosine Receptor Agonists: 1-Deazaadenine Modification Maintains High in Vivo Efficacy

“…The C2 group of 20 was accommodated in an exofacial interface region generated by the outward movement of TM2 in the hybrid A 3 AR model, as previously proposed for derivatives bearing rigid and extended C2 substituents. 12,13 Thus, the major conserved recognition points for A 3 AR agonists were preserved in the 1-deaza analogues, and as expected, the N1 of adenosine is not required for binding. Several different N 6 substitutions can be tolerated in this series, slightly modulating the affinity depending on their accommodation in a region a Effect shown for ipsilateral hind paw; there is no effect on the contralateral side.…”

A Single Glycine in Extracellular Loop 1 Is the Critical Determinant for Pharmacological Specificity of Dopamine D2 and D3 Receptors

“…of Gly105 in EL1 of b 1 AR and the lack of a Gly residue at the aligned position in EL1 of b 2 AR likely contribute to the difference in the flexibility of the extracellular portion of the TM2 Prokink between b 1 AR and b 2 AR, in a manner highly analogous to that observed in D3R and D2R. Similarly, modeling studies of the A 3 adenosine receptor in complex with an agonist containing rigid C2 extensions have suggested that an outward displacement of TM2 is required to accommodate such a bulky ligand (Tosh et al, 2012). Prior chimeric receptor and mutagenesis studies were consistent with a contribution of a region containing EL1 and, to a much lesser degree, EL2, in D3R over D2R selectivity for R-22 (Newman et al, 2009) and another highly D3R-selective F-analog of R-22 (Banala et al, 2011).…”

Rigid Adenine Nucleoside Derivatives as Novel Modulators of the Human Sodium Symporters for Dopamine and Norepinephrine

“…Various sterically constrained adenine nucleoside derivatives (Supplemental Material; (Supplemental Table 1) (Table 1) have been synthesized and studied for their potent binding to the A 1 AR (compound 2) (Tosh et al, 2012b) or A 3 AR (compounds 1, 3-9, 14-19, 21-28, 31, and 32) (Tosh et al, 2012a(Tosh et al, , 2015a. Many of these A 3 AR agonists reduce chronic neuropathic pain in a phenotypic screen, and the AR binding affinities of the previously reported nucleosides are provided (Supplemental Material; (Supplemental Table 2).…”

“…Adenosine analogs are under development as potential therapeutic agents for treating chronic neuropathic pain and other diseases (Tosh et al, 2012a;Borea et al, 2015;Little et al, 2015). Among these potent adenosine receptor (AR) agonists are the 9-riboside N 6 -(3-iodobenzyl)adenosine-59-N-methylcarboxamide (IB-MECA; Stoilov et al, 2014) and the carbocyclic (19S,29R,39S,49R,59S)-49-{2-chloro-6- [(3-iodophenylmethyl) amino]purin-9-yl}-1 (methylaminocarbonyl)-bicyclo[3.1.0] hexane-2,3-diol MRS1898 (Fig.…”

“…1B), which bind selectively to the A 3 AR subtype and reduce pain in the chronic constriction injury model and other models of prolonged pain (Chen et al, 2012). Recently, we enlarged the set of conformationally constrained A 3 AR agonists like (19S,29R,39S,49R,59S)-49-{2-chloro-6-[(3-chlorophenylmethyl)amino]purin-9-yl}-1 (methylaminocarbonyl)-bicyclo[3.1.0]hexane-2,3-diol that contain, in place of the natural D-ribose, a sterically rigidified bicyclic bicyclo[3.1.0]hexane (methanocarba) ring system, which maintains a receptor-preferred North (N)-methanocarba conformation (Tosh et al, 2012a). These adenosine derivatives optionally contain a rigid extension at the C2 position, consisting of an arylethynyl group, which enhances A 3 AR selectivity.…”