Non-Xanthine Antagonists for the Adenosine A1 Receptor

New fused pyrroles with rA1/A2A antagonistic activity as potential therapeutics for neurodegenerative disorders

In a pilot study, eleven pyrrolopyridine and pyrrolopyrimidine derivatives (specifically, 7-azaindole and 7-deazapurine derivatives) were synthesised by Suzuki cross-coupling reactions and evaluated via radioligand binding assays as potential adenosine receptor (AR) antagonists in order to further investigate the structure-activity relationships of these compounds. 6-Chloro-4-phenyl-1H-pyrrolo[2,3-b]pyridine, with a 7-azaindole scaffold, was identified as a selective A₁ AR antagonist with a rA₁K_i value of 0.16 µM, and interestingly, the addition of a N-atom to the aforementioned fused heterocyclic ring system, creating corresponding 7-deazapurines, led to a dual A₁/A_2A AR ligand (2-chloro-4-phenyl-7H-pyrrolo[2,3-d]pyrimidine: rA₁K_i: 0.19 ± 0.02 µM; rA_2AK_i: 0.43 ± 0.01 µM). Introducing an additional N-atom into the heterocyclic ring system was tolerable for rA₁ AR affinity and also led to rA_2A AR affinity. This pilot study concluded that new 7-azaindole and 7-deazapurine derivatives represent interesting scaffolds for design of A₁ and/or A_2A AR antagonists.

New potent and selective A1 adenosine receptor antagonists as potential tools for the treatment of gastrointestinal diseases

The synthesis of 9-alkyl substituted adenine derivatives presenting aromatic groups and cycloalkyl rings in 8- and N⁶-position, respectively, is reported. The compounds were tested with radioligand binding studies showing, in some cases, a low nanomolar A₁ adenosine receptor affinity and a very good selectivity versus the other adenosine receptor subtypes. Functional assays at human adenosine receptors and at a mouse ileum tissue preparation clearly demonstrate the antagonist profile of these molecules, with inhibitory potency at nanomolar level. A molecular modeling study, consisting in docking analysis at the recently reported A₁ adenosine receptor crystal structure, was performed for the interpretation of the obtained pharmacological results. The N⁶-cyclopentyl-9-methyl-8-phenyladenine (17), resulting the most active derivative of the series (K_i = 2.8 nM and IC₅₀ = 14 nM), was also very efficacious in counteracting the effect of the agonist CCPA on mouse ileum contractility. This new compound represents a tool for the development of new agents for the treatment of intestinal diseases as constipation and postoperative ileus.

Discovery of 2-aminoimidazole and 2-amino imidazolyl-thiazoles as non-xanthine human adenosine A3 receptor antagonists: SAR and molecular modeling studies

A small-molecule combinatorial library of 24 compounds with 2-aminoimidazole and 2-aminoimidazolyl-thiazole derivatives was synthesized using a 2-chloro trityl resin. The generated compound library was tested against all the human adenosine receptors subtypes. The 2-aminoimidazole derivatives (6a-6l) showed weak to moderate affinity towards the human adenosine receptors. Further modification to 2-aminoimidazolyl-thiazole derivatives (12a-12l) resulted in an improvement of affinity at adenosine A₁, A_2A and A₃ receptor subtypes. Compound 12b was the most potent and selective non-xanthine human adenosine A₃ receptor antagonist of this series. A receptor-based modeling study was performed to explore the possible binding mode of these novel 2-aminoimidazole and 2-aminoimidazolyl-thiazole derivatives into human adenosine A₁, A_2A and A₃ receptor subtypes.

NMR metabolomics for identification of adenosine A1 receptor binding compounds from Boesenbergia rotunda rhizomes extract

ETHNOPHARMACOLOGICAL RELEVANCE

Boesenbergia rotunda Linn. (Zingiberaceae) is traditionally used in many Asian countries as medicine for stomach pain and discomfort, viral and bacterial infection, inflammation, and as diuretic agent.

AIM OF THE STUDY

The study aimed to identify adenosine A1 receptor binding compounds from Boesenbergia rotunda rhizome extract by using comprehensive extraction coupled to the NMR metabolomics method.

MATERIALS AND METHODS

Dried and powdered Boesenbergia rotunda rhizomes were extracted with the comprehensive extraction method to obtain several fractions with different polarity. Each fraction was divided into two: for NMR analysis and for adenosine A1 receptor binding test. Orthogonal projection to the least square analysis (OPLS) was used to study the correlation between metabolites profile and adenosine A1 receptor binding activity of the plant extracts. Based on Y-related coefficient and variable of important (VIP) value, signals in active area of OPLS loading plot were studied and the respective compounds were then elucidated

RESULTS AND DISCUSSIONS

Based on OPLS Y-related coefficient plot and variable of importance value plot, several characteristic signals were found to positively correlate to the binding activity. By using 1D and 2D NMR spectra of one of the most active fraction, pinocembrine and hydroxy-panduratin were identified as the possible active compounds. Two signals from ring C of pinocembrine flavanone skeleton with negative coefficient correlations possibly overlapped with those of non-active methoxylated flavanones which were also presence in the extract. NMR based metabolomics applied in this study was able to quickly identify bioactive compounds from plant extract without necessity to purify them. Further confirmation by isolating pinocembrine and hydroxy-panduratin and testing their adenosine A1 receptor binding activity to chemically validate the method are required.

CONCLUSION

Two flavonoid derivatives, pinocembrine and hydroxy-panduratin, have been elucidated as possible active compounds bind to adenosine A1 receptor. Flavonoid was reported to be one of natural antagonist ligand for adenosine A1 receptor while antagonistic activity to the receptor is known to associate with diuretic activity. Thus, the result of this research supports the traditional use of Boesenbergia rotunda rhizome extract as diuretic agent.

Synthesis and pharmacology of pyrido[2,3-d]pyrimidinediones bearing polar substituents as adenosine receptor antagonists

Amino-substituted pyrido[2,3-d]pyrimidinediones have previously been found to bind to adenosine A1 and A2A receptors in micromolar concentrations. The present study was aimed at studying the structure-activity relationships of this class of compounds in more detail. Most of the investigated compounds were provided with polar substituents, such as ethoxycarbonyl groups and basic amino functions, in order to improve their water-solubility. The compounds were synthesized starting from 6-amino-1,3-dimethyluracil via different reaction sequences involving (cyano)acetylation, Vilsmeier formylation, or reaction with diethyl ethoxymethylenemalonate (EMME). The most potent and selective compound of the present series was 6-carbethoxy-1,2,3,4-tetrahydro-1,3-dimethyl-5-(2-naphthylmethyl)aminopyrido[2,3-d]pyrimidine-2,4-dione (11c) with a Ki value of 5 nM at rat and 25 nM at human A1 receptors. The compound was more than 60-fold selective versus A3 and more than 300-fold selective versus A2A receptors. It showed an over 300-fold improvement with respect to the lead compound. In GTPgammaS binding studies at membranes of Chinese hamster ovary cells recombinantly expressing the human adenosine A1 receptor, 11c behaved as an antagonist with inverse agonistic activity. A regioisomer of 11c, 6-carbethoxy-1,2,3,4-tetrahydro-1,3-dimethyl-7-(2- naphthylmethyl)aminopyrido[2,3-d]pyrimidine-2,4-dione (7a) in which the 2-naphthylmethylamino substituent at position 5 of 11c was moved to the 7-position, was a relatively potent (Ki=226 nM) and selective (>20-fold) A3 ligand. In the series of compounds lacking an electron-withdrawing ethoxycarbonyl or cyano substituent in the 6-position, compounds with high affinity for adenosine A2A receptors were identified, such as 1,2,3,4-tetrahydro-1,3-dimethyl-5-(1-naphthyl)aminopyrido[2,3-d]pyrimidine-2,4-dione 16b (Ki human A2A=81.3 nM, Ki human A1=153 nM, and Ki human A3>10,000 nM).