N6/5’-DISUBSTITUTED ADENOSINE AND 2-CHLORO-ADENOSINE DERIVATIVES AS POTENT AND SELECTIVE A1 ADENOSINE RECEPTOR AGONISTS: SYNTHESIS, BINDING ASSAYS AND ANTINOCICEPTIVE ACTIVITY IN MICE

The A1 adenosine receptor (A1AR) is the best-characterized subtype of the four known adenosine receptors (ARs). Selective A1AR agonists mediate antiarrhythmic, antinociceptive and neuro- and cardioprotective effects. Moreover, A1AR agonists reduce lipolysis in adipose tissue. The A1AR is abundantly expressed in spinal cord and other neuronal tissue and its activation produced pain-relieving effects in a number of preclinical animal models [Gao Z-G and Jacobson KA, Expert Opin Emerg Drugs 2011, 16, 597]. Even though several selective A1AR agonists are being developed as analgesic, e.g. SDZ WAG 994, GR79236, GW-493838, it seems that all of them have been withdrawn possibly due to cardiovascular side effects. As A1AR is abundant in a number of tissues including CNS, heart and adipose tissues, A1AR full agonists can lead to severe cardiovascular side effects such as hypotension and bradycardia, limiting their clinical usefulness. Thus recent efforts have focused on developing ligands that will partially stimulate the A1AR [Cappellacci L et al., Bioorg. Med. Chem. 2008, 16, 336] or which are selective for certain signalling pathways (i.e. biased agonists). In our continuously efforts in searching potent and selective A1AR agonists, we have identified 5’-chloro-5’-deoxy-N6-(±)-(endo-norborn-2-yl)-adenosine (5’Cl5'd-(±)-ENBA) as a potent and highly selective full agonist of human A1AR (hA1AR (Ki) = 0.51 nM, A3/A1 = 2530) [Franchetti P et al., J. Med. Chem. 2009, 52, 2393]. Moreover, 5’Cl5’d-(±)-ENBA showed acute nocifensive effect in the formalin test and anti-neuropathic properties in a model of neuropathic pain, the spared nerve injury (SNI) of the sciatic nerve in mice. Interestingly, 5’Cl5’d-(±)-ENBA did not modify motor coordination and blood pressure or heart rate in unanaesthetized sham and SNI mice [Luongo L et al., Molecules 2012, 17, 13712]. The interesting biological properties of 5’Cl5’d-(±)-ENBA demonstrated that the substitution of 5’-hydroxyl group in adenosine analogues with a chlorine is well tolerated by A1AR and 5'-modified adenosine derivatives could be more druggable than 5'-unmodified ones, since normal ribonucleosides may be phosphorylated by adenosine kinases and then by nucleotide kinases to 5'-mono-, 5'-di-, or 5'-triphosphates and subsequently interact with P2Y receptors and/or other biological targets. As a proof of the concept, N6/5’-disubstituted adenosine and 2-chloro-adenosine derivatives were synthesized and evaluated for affinity and selectivity at all human adenosine receptor subtypes. Moreover, antinociceptive activity in a formalin test in mice was assayed. The results of these studies will be discussed.