Truncated Nucleosides as A₃ Adenosine Receptor Ligands: Combined 2-Arylethynyl and Bicyclohexane Substitutions

Abstract

C2-Arylethynyladenosine-5′-N-methyluronamides containing a bicyclo[3.1.0]hexane ((N)-methanocarba) ring are selective A3 adenosine receptor (AR) agonists. Similar 4′-truncated C2-arylethynyl-(N)-methanocarba nucleosides containing alkyl or alkylaryl groups at the N6 position were low-efficacy agonists or antagonists of the human A3AR with high selectivity. Higher hA3AR affinity was associated with N6-methyl and ethyl (Ki 3–6 nM), than with N6-arylalkyl groups. However, combined C2-phenylethynyl and N6-2-phenylethyl substitutions in selective antagonist 15 provided a Ki of 20 nM. Differences between 4′-truncated and nontruncated analogues of extended C2-p-biphenylethynyl substitution suggested a ligand reorientation in AR binding, dominated by bulky N6 groups in analogues lacking a stabilizing 5′-uronamide moiety. Thus, 4′-truncation of C2-arylethynyl-(N)-methanocarba adenosine derivatives is compatible with general preservation of A3AR selectivity, especially with small N6 groups, but reduced efficacy in A3AR-induced inhibition of adenylate cyclase.

Structure-Activity Analysis of Biased Agonism at the Human Adenosine A3 Receptor

“…A similar maximal effect to the reference agonist IB-MECA was observed for each agonist assessed with the exception of MRS5916, MRS7030, and MRS7034 that mediated a partial response, increasing cell survival to approximately 75% ( Table 2). The 49-truncated (N)-methanocarba derivative MRS5776 has previously been suggested to act as a low-efficacy partial agonist (Tosh et al, 2012b). Consistent with these findings, at signaling pathways assessed in the current study, MRS5776 stimulated either no detectable response (Akt 1/2/3 phosphorylation and calcium mobilization) or behaved as a weak partial agonist (inhibition of cAMP accumulation, ERK1/2 phosphorylation, and cell survival) ( Tables 1 and Table 2).…”

“…However, docking of rigid (N)-methanocarba 59-N-methyluronamide nucleoside derivatives with elongated C2 substituents at the A 3 AR model required a significant outward movement of TM2 to maintain conserved polar contacts surrounding the ribose and adenine moieties. As such, these compounds were better accommodated using a hybrid A 2A AR-b 2 adrenergic receptor template or an A 2A AR-opsin template, which have an approximate 4 Å and 7 Å outward movement of the extracellular portion of TM2 at the Ca atom of Ser73, respectively (Tosh et al, 2012b). These findings highlight the significant conformational changes that are likely to occur upon binding of biased A 3 AR agonists.…”

“…Within the A 2A AR crystal structure, nonconserved disulphide bonds constrain the extracellular portion of TM2 toward the TM bundle (Tosh et al, 2012b). However, docking of rigid (N)-methanocarba 59-N-methyluronamide nucleoside derivatives with elongated C2 substituents at the A 3 AR model required a significant outward movement of TM2 to maintain conserved polar contacts surrounding the ribose and adenine moieties.…”

“…The North (N)-methanocarba ring system maintains conformation of the ribose ring that is favored at the A 3 AR. Furthermore, the subtype selectivity of A 3 AR agonists can be enhanced through 59-N-methyluronamide substitution (Tosh et al, 2012b). Collectively, these studies provide a valuable framework, particularly with respect to the N 6 and C2 position, for the rational design of potent A 3 AR agonists with high efficacy and subtype selectivity.…”

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

“…The truncated derivatives were included because 49-truncation tends to convert selective A 3 AR agonists into selective antagonists (Tosh et al, 2012c). The triazole linker was explored as a substitute for the ethynyl group that would maintain key interactions with the A 3 AR agonists (Tosh et al, 2015b).…”