5'-Deoxy congeners of 9-(3-amido-3-deoxy-beta-D-xylofuranosyl)-N(6)-cyclopentyladenine: new adenosine A(1) receptor antagonists and inverse agonists

Polymer-assisted structural modification on nucleosides and nucleotides

Nucleosides and their analogues play important roles in biological research and clinical therapeutics. Polymer-assisted structural modifications of nucleosides and nucleotides enable parallel and rapid construction of nucleoside library. For some nucleosides, higher chemical selectivity and regioselectivity can be achieved using solid-phase synthesis when compared to classic solution-phase synthesis.

Structure-activity relationships of inverse agonists for G-protein-coupled receptors

It has been recently established that G-protein-coupled receptors (GPCRs) can be constitutively active, i.e., they can be active in the absence of an agonist. This activity can be inhibited by so-called inverse agonists. For a number of GPCRs, such inverse agonists have been developed and studied, now enabling for the first time a study into their structure-activity relationships.

A "locked-on," constitutively active mutant of the adenosine A1 receptor

We studied the wild-type human adenosine A1 receptor and three mutant receptors, in which the glycine at position 14 had been changed into an alanine, a leucine, or a threonine residue. All receptors were characterized in radioligand binding experiments, the wild-type and the Gly14Thr mutant receptor in greater detail. Both receptors were allosterically modulated by sodium ions and PD81,723 (2-amino-4,5-dimethyl-3-thienyl-[3(trifluoromethyl)-phenyl]methanone), although in a different way. All mutant receptors appeared to be spontaneously or "constitutively" active in a [35S]GTPgammaS binding assay, the first demonstration of the existence of such CAM (constitutively active mutant) receptors for the adenosine A1 receptor. The Gly14Thr mutant receptor was also constitutively active in another functional assay, i.e., the inhibition of forskolin-induced cAMP production in intact cells. Importantly, this mutant displayed a peculiar "locked-on" phenotype, i.e., neither agonist nor inverse agonist was capable of modulating the basal activity in both the GTPgammaS and the cAMP assay, unlike the wild-type and the two other mutant receptors.

N-Acyl-N-alkyl-sulfonamide anchors derived from Kenner’s safety-catch linker: powerful tools in bioorganic and medicinal chemistry

In 1971 Kenner et al. introduced the safety-catch principle into solid phase peptide synthesis. Thus two contradicting needs were addressed. On the one hand, sufficient stability of the linker substrate bond to impede hydrolysis or similar side reactions, on the other hand mild chemical conditions allowing for unscathed liberation of the precious products. Over the years this linker type emerged in several different chemical disciplines and nowadays it presents a useful and broadly applicable tool. Recent advancements and applications based on Kenner's safety-catch linker are reviewed.

Novel amino acid derived natural products from the ascidian Atriolum robustum: identification and pharmacological characterization of a unique adenosine derivative

Investigation of the methanolic extract of the Australian ascidian Atriolum robustum led to the isolation and characterization of five new amino acid derived structures (1-5). The structures were elucidated employing spectroscopic techniques (NMR, MS, UV, and IR). The absolute stereochemistry of 1 and 2 was established by chemical degradation, derivatization, and chiral GC-MS analysis. Structures 4 and 5 are complex nucleosides containing rare methylthioadenosine and methylsulfinyladenosine moieties, respectively. In radioligand binding studies the 5'-deoxy-5'-methylthioadenosine-2',3'-diester 4 exhibited affinity for A(1) and A(3) adenosine receptors with K(i) values below 10 microM. Its affinity was somewhat lower for A(2A) (K(i) = 17 microM) and much lower for A(2B) adenosine receptors. Analytical experiments using capillary electrophoresis showed that compound 4 was stable under the conditions of radioligand binding studies. Incubation with carboxylesterase resulted in slow hydrolysis of the adenosine derivative to 5'-deoxy-5'-methylthioadenosine (MTA), which was about 10-fold more potent at adenosine receptors than compound 4. Thus, the 2',3'-diester derivative 4 may act as a lipophilic prodrug of MTA in addition to its own adenosine receptor activity. GTP shift experiments indicated that the adenosine derivative was a partial agonist at A(1) adenosine receptors of rat brain cortical membranes. Compound 4 inhibited cAMP accumulation in Chinese hamster ovary (CHO) cell membranes recombinantly expressing the human A(3) adenosine receptor, thus indicating that the adenosine derivative also acted as a partial agonist at A(3)ARs. Homology models of the A(1) and the A(3) adenosine receptors in their putative active and inactive conformations were built and used for docking of the sterically demanding compound 4. It was found that this ligand fit well into the binding pockets of both receptor subtypes because of its highly flexible structure, although in somewhat different binding modes.

N6-cyclopentyl-2-(3-phenylaminocarbonyltriazene-1-yl)adenosine (TCPA), a very selective agonist with high affinity for the human adenosine A1 receptor

Four subtypes of adenosine receptors are currently known, that is, A(1), A(2A), A(2B), and A(3) receptors. Interestingly, quite substantial species differences exist especially between human and rat A(3) receptors. As a result, ligands such as CCPA, which are very selective for the rat A(1) receptor versus the human A(3) receptor, are substantially less selective when the human A(1) and A(3) receptors are compared. New 2-substituted and 2,N(6)-disubstituted adenosines were synthesized, and their affinities for the human adenosine A(1), A(2A), A(2B), and A(3) receptors were determined. Although large substituents on the C2-position are generally thought to yield adenosine A(2A) receptor selective ligands, the reported series of 2-triazeno-substituted adenosines had a very high affinity for the A(1) receptor. For example, 2-(3-phenylaminocarbonyltriazene-1-yl)adenosine had an affinity of 6.1 +/- 1.3 nM for the human adenosine A(1) receptor. Introduction of a diphenethyl substituent at the N(6)-position of this compound resulted in a high-affinity agonist, 3.1 +/- 0.9 nM, for the human adenosine A(1) receptor with 316- and 45-fold selectivity versus the human A(2A) and human A(3) receptors, respectively. The most selective, high-affinity human adenosine A(1) receptor agonist was the disubstituted compound N(6)-cyclopentyl-2-(3-phenylaminocarbonyltriazene-1-yl)adenosine (TCPA). TCPA had an affinity of 2.8 +/- 0.8 nM for the human adenosine A(1) receptor and was 75-fold and 214-fold selective versus the human A(2A) and human A(3) receptors, respectively. In addition, TCPA was a full agonist and inhibited the forskolin-induced cAMP production of CHO cells stably transfected with the human adenosine A(1) receptor with an IC(50) of 1.5 +/- 0.5 nM.