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.

2-Phenylhydroxypropynyladenosine derivatives as high potent agonists at A2B adenosine receptor subtype.

Adenosine derivatives bearing in 2-position the (R,S)-phenylhydroxypropynyl chain were evaluated for their potency at human A2B adenosine receptor, stably transfected on CHO cells, on the basis that (R,S)-2-phenylhydroxy-propynyl-5'-N-ethylcarboxyamidoadenosine [(R,S)-PHPNECA] was found to be a good agonist at the A2B receptor subtype. Biological studies demonstrated that the presence of small alkyl groups in N6-position of these molecules are well tolerated, whereas large groups abolished A2B potency. On the other hand, the presence of an ethyl group in the 4'-carboxamido function seems to be optimal, the (S)-PHPNECA resulting the most potent agonist at A2B receptor reported so far.

Human cytidine deaminase: understanding the catalytic mechanism.

In the absence of an experimentally elucidated three-dimensional structure of the human CDA, we built an homology model of this enzyme starting from the crystal structure of its E. coli homologous. Furthermore, we docked in the active site alternatively the substrate, the intermediate or the product. By means of molecular dynamics simulations, we determined the topology of the active site, identifying the amino acids involved in the catalytic mechanism, and outlining the central role played by E67.