Highly selective adenosine A2 receptor agonists in a series of N-alkylated 2-aminoadenosines

Dual A1/A3 Adenosine Receptor Antagonists: Binding Kinetics and Structure-Activity Relationship Studies Using Mutagenesis and Alchemical Binding Free Energy Calculations

Drugs targeting adenosine receptors (AR) can provide treatment for diseases. We report the identification of 7-(phenylamino)-pyrazolo[3,4-c]pyridines L2-L10, A15, and A17 as low-micromolar to low-nanomolar A₁R/A₃R dual antagonists, with 3-phenyl-5-cyano-7-(trimethoxyphenylamino)-pyrazolo[3,4-c]pyridine (A17) displaying the highest affinity at both receptors with a long residence time of binding, as determined using a NanoBRET-based assay. Two binding orientations of A17 produce stable complexes inside the orthosteric binding area of A₁R in molecular dynamics (MD) simulations, and we selected the most plausible orientation based on the agreement with alanine mutagenesis supported by affinity experiments. Interestingly, for drug design purposes, the mutation of L250^6.51 to alanine increased the binding affinity of A17 at A₁R. We explored the structure-activity relationships against A₁R using alchemical binding free energy calculations with the thermodynamic integration coupled with the MD simulation (TI/MD) method, applied on the whole G-protein-coupled receptor-membrane system, which showed a good agreement (r = 0.73) between calculated and experimental relative binding free energies.

Potential use of inhibitors of bacteria spore germination in the prophylactic treatment of anthrax andClostridium difficile-associated disease

Spore germination is the first step in establishing Bacillus and Clostridium infections. Germination is triggered by the binding of small molecules by the resting spore. Subsequently, the activated spore secretes dipicolinic acid and calcium, the spore core is rehydrated and spore structures are degraded. Inhibition of any of the germination-related events will prevent development to the vegetative stage. Inhibition of spore germination has been studied intensively in the prevention of food spoilage. In this perspective, we propose that similar approaches could be used in the prophylactic control of Bacillus anthracis and Clostridium difficile infections. Inhibition of B. anthracis spore germination could protect military and first-line emergency personnel at high risk for anthrax exposure. Inhibition of C. difficile could prevent human C. difficile-associated disease during antibiotic treatment of immunocompromised patients.

Adenosine receptors and the heart: role in regulation of coronary blood flow and cardiac electrophysiology

Adenosine is an autacoid that plays a critical role in regulating cardiac function, including heart rate, contractility, and coronary flow. In this chapter, current knowledge of the functions and mechanisms of action of coronary flow regulation and electrophysiology will be discussed. Currently, there are four known adenosine receptor (AR) subtypes, namely A(1), A(2A), A(2B), and A(3). All four subtypes are known to regulate coronary flow. In general, A(2A)AR is the predominant receptor subtype responsible for coronary blood flow regulation, which dilates coronary arteries in both an endothelial-dependent and -independent manner. The roles of other ARs and their mechanisms of action will also be discussed. The increasing popularity of gene-modified models with targeted deletion or overexpression of a single AR subtype has helped to elucidate the roles of each receptor subtype. Combining pharmacologic tools with targeted gene deletion of individual AR subtypes has proven invaluable for discriminating the vascular effects unique to the activation of each AR subtype. Adenosine exerts its cardiac electrophysiologic effects mainly through the activation of A(1)AR. This receptor mediates direct as well as indirect effects of adenosine (i.e., anti-beta-adrenergic effects). In supraventricular tissues (atrial myocytes, sinuatrial node and atriovetricular node), adenosine exerts both direct and indirect effects, while it exerts only indirect effects in the ventricle. Adenosine exerts a negative chronotropic effect by suppressing the automaticity of cardiac pacemakers, and a negative dromotropic effect through inhibition of AV-nodal conduction. These effects of adenosine constitute the rationale for its use as a diagnostic and therapeutic agent. In recent years, efforts have been made to develop A(1)R-selective agonists as drug candidates that do not induce vasodilation, which is considered an undesirable effect in the clinical setting.

Recent developments in adenosine A2A receptor ligands

The development of potent and selective agonists and antagonists of adenosine receptors (ARs) has been a target of medicinal chemistry research for several decades, and recently the US Food and Drug Administration has approved Lexiscan, an adenosine derivative substituted at the 2 position, for use as a pharmacologic stress agent in radionuclide myocardial perfusion imaging. Currently, some other adenosine A(2A) receptor (A(2A)AR) agonists and antagonists are undergoing preclinical testing and clinical trials. While agonists are potent antiinflammatory agents also showing hypotensive effects, antagonists are being developed for the treatment of Parkinson's disease.However, since there are still major problems in this field, including side effects, low brain penetration (for the targeting of CNS diseases), short half-life, or lack of in vivo effects, the design and development of new AR ligands is a hot research topic.This review presents an update on the medicinal chemistry of A(2A)AR agonists and antagonists, and stresses the strong need for more selective ligands at the human A(2A)AR subtype, in particular in the case of agonists.

Highlights on the development of A(2A) adenosine receptor agonists and antagonists

Although significant progress has been made in the past few decades demonstrating that adenosine modulates a variety of physiological and pathophysiological processes through the interaction with four subtypes of a family of cell-surface G-protein-coupled receptors, clinical evaluation of some adenosine receptor ligands has been discontinued. Major problems include side effects due to the wide distribution of adenosine receptors, low brain penetration (which is important for the targeting of CNS diseases), short half-life of compounds, or a lack of effects, in some cases perhaps due to receptor desensitization or to low receptor density in the targeted tissue. Currently, three A(2A) adenosine receptor agonists have begun phase III studies. Two of them are therapeutically evaluated as pharmacologic stress agents and the third proved to be effective in the treatment of acute spinal cord injury (SCI), while avoiding the adverse effects of steroid agents. On the other hand, the great interest in the field of A(2A) adenosine receptor antagonists is related to their application in neurodegenerative disorders, in particular, Parkinson's disease, and some of them are currently in various stages of evaluation. This review presents an update of medicinal chemistry and molecular recognition of A(2A) adenosine receptor agonists and antagonists, and stresses the strong need for more selective ligands at the A(2A) human subtype.

Identification of an in vivo inhibitor of Bacillus anthracis spore germination

Germination of Bacillus anthracis spores into the vegetative form is an essential step in anthrax pathogenicity. This process can be triggered in vitro by the common germinants inosine and alanine. Kinetic analysis of B. anthracis spore germination revealed synergy and a sequential mechanism between inosine and alanine binding to their cognate receptors. Because inosine is a critical germinant in vitro, we screened inosine analogs for the ability to block in vitro germination of B. anthracis spores. Seven analogs efficiently blocked this process in vitro. This led to the identification of 6-thioguanosine, which also efficiently blocked spore germination in macrophages and prevented killing of these cells mediated by B. anthracis spores. 6-Thioguanosine shows potential as an anti-anthrax therapeutic agent.

2-(N-Acyl) and 2-N-acyl-N6-substituted analogues of adenosine and their affinity at the human adenosine receptors

A series of 2-(N-acyl) and 2-(N-acyl)-N(6)-alkyladenosine analogues have been synthesized from the intermediate 2-amino-6-chloroadenosine derivatives (2b and 7) and evaluated for their affinity at the human A(1), A(2A), and A(3) receptors. We found that 2-(N-acyl) derivatives of adenosine showed relatively low affinity at A(2A) and A(3) receptors, while the N(6)-cyclopentyl substituent in 4h and 4i induced high potency [A(1) (K(i))=20.7 and 31.8 nM respectively] at the A(1) receptor and resulted therefore in increased selectivity for this subtype. The general synthetic methods and their binding studies are presented herein.

2,8-Disubstituted adenosine derivatives as partial agonists for the adenosine A2A receptor

Novel 2,8-disubstituted adenosine derivatives were synthesized in good overall yields starting from 2-iodoadenosine. Binding affinities were determined for rat adenosine A(1) and A(2A) receptors and human A(3) receptors. Some compounds displayed good adenosine A(2A) receptor affinities, with most of the 2-(1-hexynyl)- and 2-[(E)-1-hexenyl]-substituted derivatives having K(i) values in the nanomolar range. Although the introduction of an 8-alkylamino substituents decreased the affinity for the adenosine A(2A) receptor somewhat, the selectivity for this receptor compared to A(3) was improved significantly. The 8-methylamino (12) and 8-propylamino (14) derivatives of 2-(1-hexynyl)adenosine (3), showed reasonable A(2A) receptor affinities with K(i) values of 115 and 82nM, respectively, and were 49- and 26-fold selective for the adenosine A(2A) receptor compared to the A(3) receptor. The compounds were also evaluated for their ability to stimulate the cAMP production in CHO cells expressing the human adenosine A(2A) receptor. 2-(1-Hexynyl)adenosine (3) and 2-[(E)-1-hexenyl]adenosine (4) both showed submaximal levels of produced cAMP, compared to the reference full agonist CGS 21680, and thus behaved as partial agonists. Most 8-alkylamino-substituted derivatives of 3, displayed similar cAMP production as 3, and behaved as partial agonists as well. Introduction of alkylamino groups at the 8-position of 4, showed a slight reduction of the efficacy compared to 4, and these compounds were partial agonists also.

Structure–affinity relationships of the affinity of 2-pyrazolyl adenosine analogues for the adenosine A2A receptor

The structure-affinity relationships of two novel 2-substituted adenosine series containing a substituted pyrazole attached at the N-1 or C-4 position for the adenosine (ADO) A2A receptor are described. Compounds in the 2-(N-1-pyrazolyl) adenosine series IV provided the highest affinity for the ADO A2A receptor as compared to the 2-(C-4-pyrazolyl) series V. The main structural differences between the two series point to the N-1 nitrogen of series IV imparting more favorable binding interactions with the receptor than those of series V.

Hydrocarbon activation. Synthesis of beta-cycloalkyl (di)nitriles through photosensitized conjugate radical addition

Photoinduced hydrogen abstraction from aliphatic cyclic hydrocarbons (C(5) to C(7), C(12), as well as adamantane) by triplet aromatic ketones in the presence of alpha,beta-unsaturated (di)nitriles offers a straightforward entry to the corresponding alkylated (di)nitriles via the alkyl radicals. Yields are moderate to good depending on the olefins structure (substitution in beta slows down the addition to mononitriles, but with alpha,alpha-dinitriles electronic activation allows efficient alkylation also of beta,beta-disubstituted substrates). A tandem alkylation-cyclization process has been obtained with (1-methylpent-4-enylidene)malononitrile.

High-pressure synthesis of enantiomerically pure C-6 substituted pyrazol

The synthesis of enantiomerically pure C-6 substituted pyrazolo[3,4-d]pyrimidines has been performed by aromatic nucleophilic substitution of 4-amino-6-chloro-1-phenylpyrazolo[3,4-rd]pyrimidine under conditions of high pressure at ambient temperature. Conventional synthetic conditions (reflux at atmospheric pressure) were unsuccessful. The S enantiomer 11 displayed higher affinity and selectivity for the adenosine A1 receptor than the R enantiomer 12.

The discovery and synthesis of highly potent, A2a receptor agonists

A series of N6,2-disubstituted adenosine analogues have been synthesized and their functional activity measured against A2a and A1 receptors. Examples of compounds with both a lipophilic N6-substituent and amino-functionalized 2-position were highly active at the A2a receptor on the human neutrophil.

The synthesis and biological activity of a highly selective adenosine A2a receptor agonist

Three novel nucleosides 1, 2, and 3 were prepared that contained side chains at the 2-position of adenosine. Compound 1 was shown to be the most selective A2a receptor agonist reported to date having an A1/A2 ratio of 2400. In addition, compound 1 was shown to reduce blood pressure in rats and dogs with only minimal effects on heart rate.

Adenosine receptors: new opportunities for future drugs

This review summarises current knowledge on adenosine receptors, an important G protein-coupled receptor. The four known adenosine receptor subtypes A1, A2A, A2B, and A3 are discussed with special reference to the opportunities for drug development.

Molecular modeling of adenosine receptors. The ligand binding site on the rat adenosine A2A receptor

The amino acid sequence of the rat adenosine A2A receptor and the atomic coordinates of bacteriorhodopsin were combined to generate a three-dimensional model for the adenosine A2A receptor. This model consists of seven amphipathic alpha-helices, forming a pore that is rather hydrophilic compared to the hydrophobic outside of the protein. Subsequently, a highly potent and selective ligand for this receptor, 2-(cyclohexylmethylidinehydrazino)adenosine (SHA 174), was docked into this cavity. A binding site is proposed that takes into account the conformational characteristics of the ligand. Moreover, it involves two histidine residues that were shown to be important for ligand coordination from chemical modification studies. Subsequently, the deduced binding site was used to model other potent ligands, including 8-(3-chlorostyryl)caffeine, a new A2-selective antagonist, that could all be accommodated consistent with earlier biochemical and pharmacological findings. Finally, some thoughts on how adenosine receptor activation might proceed are put forward, based on structural analogies with the enzyme family of serine proteases.

Adenosine receptors and their modulators

The identification and characterization of adenosine receptors and the development of potent, receptor subtype-selective agonists and antagonists has been an active area of research for the past 20 years. Major recent advances in the field have been the cloning of several adenosine receptor subtypes of different species, including the discovery of a new subtype, designated A3, the discovery and development of new agonists and antagonists, particularly those with selectivity for the A2a adenosine receptor, the characterization of signal transduction pathways, and the development of agents which act indirectly on the adenosine receptor system. The present article focusses on aspects of pharmaceutical/medicinal chemistry related to adenosine receptors.

Covalent binding of a selective agonist irreversibly activates guinea pig coronary artery A2 adenosine receptors

Experiments employing guinea pig heart Langendorff preparations compared the coronary vasoactivity of a functionalized congener of adenosine, 2-[(2-aminoethyl-aminocarbonylethyl)phenylethylamino]-5'-N-e thyl- carboxamidoadenosine, APEC, with the vasoactivity of the product of the reaction of APEC with 1,4-phenylene-diisothiocyanate, 4-isothiocyanatophenylaminothiocarbonyl-APEC (DITC-APEC). Previous experiments showed that whereas APEC binds reversibly to the A2A adenosine receptor of brain striatum, DITC-APEC binds irreversibly. APEC caused concentration-dependent coronary vasodilation that persisted unchanged when agonist administration continued for up to 165 min, but promptly faded when the agent was withdrawn. The unselective adenosine receptor antagonist 8-(4-sulfophenyl)theophyline (8-SPT) antagonized the vasoactivity of APEC. By contrast, DITC-APEC (0.125-1.0 nM) caused progressive, concentration-independent vasodilation that persisted unchanged for as long as 120 min after the agent was stopped and that was insensitive to antagonism by subsequently applied 8-SPT. However, perfusion of the heart with buffer containing 0.1 mM 8-SPT strongly antagonized the coronary vasodilatory action of DITC-APEC given subsequently. Such observations indicate that the covalent binding of DITC-APEC causes irreversible activation of the guinea pig coronary artery A2A adenosine receptor. Neither APEC nor DITC-APEC appeared to desensitize the coronary adenosine receptor during two or more hours of exposure to either agonist.

Correlation of frameshift mutagenicity with DNA intercalation by CGS 20928A using an in vitro DNA unwinding assay

A compound's mutagenicity in different Salmonella tester strains can suggest its mechanism of reaction with DNA. Clear confirmation of such a mechanism, however, requires a direct test of the compound's reaction with DNA, often relying on specific in vitro studies. We report the use of a rapid in vitro test designed to measure DNA unwinding, a characteristic of DNA intercalators and many frameshift mutagens. CGS 20928A, an adenosine antagonist, produced a significant (> 2-fold) increase in revertants only for Salmonella tester strain TA1537, and only without metabolic activation. These data indicated that the compound was a direct acting frameshift mutagen and possibly intercalated into DNA. Our DNA unwinding assay indicated that at concentrations of > 0.1 mM CGS 20928A behaved like known intercalating compounds in that it unwound DNA. These concentrations of compound are comparable to those found mutagenic to TA1537. By comparison, the frameshift mutagen and known intercalating compound 9-aminoacridine unwound DNA in this assay in a concentration dependent fashion between 6-12 microM. ICR-191, another acridine frameshift mutagen, also unwound DNA. A compound structurally related to CGS 20928A, which was not mutagenic in Salmonella tester strains, did not produce any DNA unwinding even at 10 mM. Because the assay uses microgram quantities of material, it should be ideal for screening small amounts of congeneric series suspected of frameshift mutagenicity.

Vasodilatory effects of adenosine A2 receptor agonists CGS 21680 and CGS 22492 in human vasculature

The vasodilatory effects of the adenosine analogs, 5'-N-ethylcarboxamidoadenosine (NECA), 2-[p-(2-carboxyethyl)phenethyl amino]-5'-N-ethylcarboxamidoadenosine (CGS 21680) and 2-[(2-cyclohexylethyl)amino]adenosine (CGS 22492) in human coronary, internal mammary artery and saphenous vein were examined in vitro. All produced concentration-dependent relaxations in arterial as well as venous rings contracted with 35 mM KCl. The concentration-response curves for NECA and CGS 21680 were parallel in the coronary. The adenosine A2 receptor antagonist, 9-chloro-2-(2-furyl)[1,2,4]triazolo[1,5-c]quinazolin-5-amine (CGS 15943A) significantly attenuated the relaxing response to the adenosine analogs in coronary artery. Although NECA and CGS 22492 were equally as effective at the highest concentration administered (both achieving approximately 70% relaxation at 10(-4) M) NECA (EC50 = 1.25 +/- 0.11 microM) induced greater vasodilation at lower concentrations than CGS 22492 (EC50 = 11.27 +/- 1.53 microM). CGS 21680 was the least potent of the agents tested achieving only 44% relaxation at 10(-4) M (EC50 = 4.71 +/- 0.46 microM). Coronary artery appeared to be more responsive than internal mammary artery or saphenous vein which displayed only marginal relaxation to these agents.