Synthesis and evaluation of new N6-substituted adenosine-5′-N-methylcarboxamides as A3 adenosine receptor agonists Original Research Article

A number of N6-substituted adenosine-5′-N-methylcarboxamides were synthesised and their pharmacology, in terms of their receptor affinity, selectivity and cardioprotective effects, were explored. The first series of compounds, 4a–4f and 5a–5f, showed modest receptor affinity for the A3AR with Ki values in the low to mid μM range. However, the incorporation of a 4-(2-aminoethyl)-2,6-di-tert-butylphenol group in the N6-position (in compounds 4g and 5g) significantly improved the affinity with Ki values of 30 and 9 nM, respectively. Improvements in affinity, as well as selectivity were seen when a functionalised linker was introduced. The N6-phenyl series, compounds 7a–7d, demonstrated low to mid nanomolar receptor affinities (Ki = 2.3–45.0 nM), with 7b displaying 109-fold selectivity for the A3AR (vs A1). The N6-benzyl series 9a–9c also proved to be potent and selective A3AR agonists and the longer chain length linker 13 was tolerated at the A3AR without abrogation of affinity or selectivity. Cardioprotection was demonstrated by a simulated ischaemia cell culture assay, whereby 7b, 7c, 9a, 9b and 9c all showed cardioprotective effects at 100 nM comparable or better than the benchmark A3AR agonist IB-MECA, but which were indistinguishable by statistical analysis. For example, compound 9c reduced cell death by 68.0 ± 3.6%.