PSB 10 hydrochloride

Antinociceptive effects of novel A2B adenosine receptor antagonists.[Pubmed:14563788]

J Pharmacol Exp Ther. 2004 Jan;308(1):358-66.

Caffeine, an adenosine A1, A2A, and A2B receptor antagonist, is frequently used as an adjuvant analgesic in combination with nonsteroidal anti-inflammatory drugs or opioids. In this study, we have examined the effects of novel specific adenosine receptor antagonists in an acute animal model of nociception. Several A2B-selective compounds showed antinociceptive effects in the hot-plate test. In contrast, A1- and A2A-selective compounds did not alter pain thresholds, and an A3 adenosine receptor antagonist produced thermal hyperalgesia. Evaluation of psychostimulant effects of these compounds in the open field showed only small effects of some antagonists at high doses. Coadministration of low, subeffective doses of A2B-selective antagonists with a low dose of morphine enhanced the efficacy of morphine. Our results indicate that analgesic effects of caffeine are mediated, at least in part, by A2B adenosine receptors.

Medicinal chemistry of adenosine A3 receptor ligands.[Pubmed:12570761]

Curr Top Med Chem. 2003;3(4):445-62.

A(3) Adenosine receptors (ARs) exhibit large species differences. Potent, selective agonists for rat (e.g. Cl-IB-MECA, 5) and human A(3) ARs (e.g. PENECA, 17, and analogs) have been developed during the past years. Potent, selective antagonists for human A(3) ARs include the imidazopurinones PSB-10 (28) and PSB-11 (29), the pyrazolotriazolopyrimidines MRE-3005F20 (38) and analogs, and the dihydropyridines (e.g. MRS-1334, 50). For rat A(3) ARs only moderately potent antagonists have been identified, such as the pyridine derivative MRS-1523 (51) and the flavonoid MRS-1067 (52), both of which exhibit only a low degree of selectivity versus the other AR subtypes. Selective antagonist radioligands for the human A(3) receptor, [(3)H]MRE-3008F20 and [(3)H]PSB-11, have been prepared, while A(3)-selective agonist radioligands are still lacking. Recent developments also include allosteric modulators, irreversibly binding antagonists, fluorescence-labelled agonists, partial agonists and inverse agonists for A(3)ARs. Site-directed mutagenesis and molecular modeling studies have been performed in order to obtain information about the ligand binding site and the process of receptor activation. A(3)Adenosine receptors have recently attracted considerable interest as novel drug targets. A(3) Agonists may have potential as cardioprotective and cerebroprotective agents, for the treatment of asthma, as antiinflammatory and immunosuppressive agents, and in cancer therapy as cytostatics and chemoprotective compounds. A(3) AR antagonists might be therapeutically useful for the acute treatment of stroke, for glaucoma, and also as antiasthmatic and antiallergic drugs, since A(3)receptors cannot only mediate antiinflammatory, but also proinflammatory responses. The future development of further pharmacological tools, including potent, selective antagonists for rat A(3) receptors and selective agonist radioligands for rat and human receptors will facilitate the evaluation of the (patho)physiological roles of A(3) receptors and the pharmacological potential of their ligands.

2-Phenylimidazo[2,1-i]purin-5-ones: structure-activity relationships and characterization of potent and selective inverse agonists at Human A3 adenosine receptors.[Pubmed:12517430]

Bioorg Med Chem. 2003 Feb 6;11(3):347-56.

Structure-activity relationships of 2-phenyl-imidazo[2,1-i]purin-5-ones as ligands for human A(3) adenosine receptors (ARs) were investigated. An ethyl group in the 8-position of the imidazoline ring of 4-methyl-2-phenyl-imidazopurinone leading to chiral compounds was found to increase affinity for human A(3) ARs by several thousand-fold. Propyl substitution instead of methyl at N4 decreased A(3) affinity but increased A(1) affinity leading to potent A(1)-selective AR antagonists. The most potent A(1) antagonist of the present series was (S)-8-ethyl-2-phenyl-4-propyl-4,5,7,8-tetrahydro-1H-imidazo[2,1-i]purin-5-one (S-3) exhibiting a K(i) value of 7.4 nM at rat A(1) ARs and greater than 100-fold selectivity versus rat A(2A) and human A(3) ARs. At human A(1) ARs 2-phenylimidazo[2,1-i]purin-5-ones were generally less potent and therefore less A(1)-selective (S-3: K(i)=98 nM). 2-, 3-, or 4-Mono-chlorination of the 2-phenyl ring reduced A(3) affinity but led to an increase in affinity for A(1) ARs, whereas di- (3,4-dichloro) or polychlorination (2,3,5-trichloro) increased A(3) affinity. The most potent and selective A(3) antagonist of the present series was the trichlorophenyl derivative (R)-8-ethyl-4-methyl-2-(2,3,5-trichlorophenyl)-4,5,7,8-tetrahydro-1H-imidazo[2,1- i]purin-5-one (R-8) exhibiting a subnanomolar K(i) value at human A(3) ARs and greater than 800-fold selectivity versus the other AR subtypes. Methylation of 4-alkyl-2-phenyl-substituted imidazo[2,1-i]purin-5-ones led exclusively to the N9-methyl derivatives, which exhibited largely reduced AR affinities as compared to the unmethylated compounds. [35S]GTP gamma S binding studies of the most potent 2-phenyl-imidazo[2,1-i]purin-5-ones at membranes of Chinese hamster ovary cells expressing the human A(3) AR revealed that the compounds were inverse agonists at A(3) receptors under standard test conditions. Due to their high A(3) affinity, selectivity, and relatively high water-solubility, 2-phenyl-imidazo[2,1-i]purin-5-ones may become useful research tools.