A2B adenosine receptor agonists: synthesis and biological evaluation of 2-phenylhydroxypropynyl adenosine and NECA derivatives.

In the search for agonists for the elusive A2B adenosine receptor subtypes, 2-phenylhydroxypropynyl-5'-N-methylcarboxamido adenosine (PHPMECA, 14), 2-phenylhydroxypropynyl-5'-N-propylcarboxamido adenosine (PHPPECA, 15), and N6-ethyl-2-phenylhydroxypropynyl-5'-N-ethylcarboxamidoadenosine (19) were synthesized on the basis that introduction of alkynyl chains in 2-position of adenosine derivatives resulted in reasonably good A2B potency compared to NECA [see N6-ethyl-2-phenylhydroxypropynyl adenosine (5) EC50 = 1,700 nM and 2-phenylhydroxypropynyl-5'-N-ethylcarboxamido adenosine (PHPNECA, 8) EC50 = 1,100 nM, respectively]. Radioligand binding studies and adenylyl cyclase assays, performed with recently cloned human A1, A2A, A2B, and A3 adenosine receptors, showed that these modifications produced a decrease in potency at A2B receptor, as well as a general reduction in affinity at the other receptor subtypes. On the other hand, the contemporary presence of an ethyl substituent in N6-position and of a 4'-ethylcarboxamido group in the same compounds led to (R,S)-N6-ethyl-2-phenylhydroxypropynyl-5'-N-ethylcarboxamidoadenosine and (S)-N6-ethyl-2-phenylhydroxypropynyl-5'-N-ethylcarboxamidoadenosine, which did not show the expected increase in potency at A2B subtype. Hence, (S)-2-phenylhydroxypropynyl-5'-N-ethylcarboxamidoadenosine [(S)-PHPNECA] with EC50 A2B = 220 nM remains the most potent agonist at A2B receptor reported so far.