Design, synthesis and evaluation of thienopyridines as ligands of adenosine receptors

Abstract

Despite the availability of various classes of antiepileptic drugs (AEDs), about one third of epileptic patients do not experience satisfactory seizure control with present treatments. This has been an important drive in the search for alternative epilepsy treatment. The endogenous nucleoside adenosine is a known anticonvulsant through activation of adenosine A1 receptors. The development of adenosine derivatives such as N6-cyclohexyladenosine (CHA) as anticonvulsants had limitations which include pronounced peripheral side effects, mainly cardiovascular effects. Over the years, non-nucleoside agonists have been investigated as an alternative set of compounds which are highly potent and selective to specific adenosine receptor (AR) subtypes. The aim of this study was to investigate the use of amino-3,5-dicyanopyridine and thieno[2,3-b]pyridine derivatives as potential A1 AR agonists. A total of 23 test compounds were synthesised (6a–s and 7a–d) and 7 of these were novel (6d and 6k–p), while 4 compounds (7a–d) have been synthesised before but have never been tested for AR affinity. The effect of intramolecular cyclisation that occurs during synthesis of thieno[2,3-b]pyridines from the intermediate compounds, amino-3,5-dicyanopyridines, in relation to AR binding was evaluated. Overall, amino-3,5-dicyanopyridine displayed superior activity towards rA1 ARs compared to thieno[2,3]pyridines. The general poor activity of thieno[2,3-b]pyridines suggest that the intramolecular cyclisation results in molecular stiffening or rigidity which negatively affects binding to the receptors, perhaps, due to steric hindrance. For instance, compound 6f (open ring) had a six-fold better inhibition constant (Ki) value of 48 nM for the A1 subtype compared to its closed ring counterpart compound 7d (rA1Ki = 305 nM). Generally, most amino-3,5-dicyanopyridines exhibited greater affinity toward the rA1 AR (Ki <10 nM) than the rA2A AR. Compound 6c had the overall best rA1 affinity (rA1Ki = 0.076 nM). 7 novel compounds synthesised (6d, 6k, 6l, 6m, 6n, 6o, 6p) proved to be highly selective with low nanomolar rA1 affinity (Ki values between 0.179 to 21.0 nM). Compounds 6n, 6q and 7c acted as potent, highly selective agonists at A1 ARs; however, compounds 6c, 6d and 6o (notably all containing a 3-OCH3 group at position R2) behaved as rA1 antagonists. Due to their high selectivity for A1 receptors, amino-3,5-dicyanopyridines could be investigated further for use as A1 ligands in pharmaco-resistant epilepsy with the right structural optimisations and formulations.