Tricyclic imidazoline derivatives as potent and selective adenosine A1 receptor antagonists

New 2-heterocyclyl-imidazo[2,1-i]purin-5-one derivatives as potent and selective human A3 adenosine receptor antagonists

A series of 4-allyl/benzyl-7,8-dihydro-8-methyl/ethyl-2-[(substituted)isoxazol/pyrazol-3/5-yl]-1H-imidazo[2,1-i]purin-5(4H)-ones has been synthesized and evaluated in radioligand binding assays to determine their affinities at the human A(1), A(2A), and A(3) adenosine receptors. Efficacy at the hA(2B) AR and antagonism of selected ligands at the hA(3) AR were also assessed through cAMP experiments. All of the synthesized molecules exhibited high affinity at the hA(3) AR (K(i) values ranging from 1.46 to 44.8 nM), as well as remarkable selectivity versus A(1), A(2A), and A(2B) AR subtypes. Compound (R)-4-allyl-8-ethyl-7,8-dihydro-2-(3-methoxy-1-methyl-1H-pyrazol-5-yl)-1H-imidazo[2,1-i]purin-5(4H)-one (R-33) was found to be the most potent and selective ligand of the series (K(i) hA(3) = 1.46 nM, K(i) hA(2A)/K(i) hA(3) > 3425; IC(50) hA(2B)/K(i) hA(3) > 3425; K(i) hA(1)/K(i) hA(3) = 1729). Molecular modeling studies were helpful in rationalizing the available structure-activity relationships along with the selectivity profiles of the new series of ligands.

Xanthines as adenosine receptor antagonists

The natural plant alkaloids caffeine and theophylline were the first adenosine receptor (AR) antagonists described in the literature. They exhibit micromolar affinities and are non-selective. A large number of derivatives and analogues were subsequently synthesized and evaluated as AR antagonists. Very potent antagonists have thus been developed with selectivity for each of the four AR subtypes.

A1 adenosine receptor antagonists, agonists, and allosteric enhancers

Intense efforts of many pharmaceutical companies and academicians in the A(1) adenosine receptor (AR) field have led to the discovery of clinical candidates that are antagonists, agonists, and allosteric enhancers. The A(1)AR antagonists currently in clinical development are KW3902, BG9928, and SLV320. All three have high affinity for the human (h) A(1)AR subtype (hA(1) K (i) < 10 nM), > 200-fold selectivity over the hA(2A) subtype, and demonstrate renal protective effects in multiple animal models of disease and pharmacologic effects in human subjects. In the A(1)AR agonist area, clinical candidates have been discovered for the following conditions: atrial arrhythmias (tecadenoson, selodenoson and PJ-875); Type II diabetes and insulin sensitizing agents (GR79236, ARA, RPR-749, and CVT-3619); and angina (BAY 68-4986). The challenges associated with the development of any A(1)AR agonist are to obtain tissue-specific effects but avoid off-target tissue side effects and A(1)AR desensitization leading to tachyphylaxis. For the IV antiarrhythmic agents that act as ventricular rate control agents, a selective response can be accomplished by careful IV dosing paradigms. The treatment of type II diabetes using A(1)AR agonists in the clinic has met with limited success due to cardiovascular side effects and a well-defined desensitization of full agonists in human trials (GR79236, ARA, and RPR 749). However, new partial A(1)AR agonists are in development, including CVT-3619 hA(1) AR K(i) = 55nM, hA(2A:hA2B:hA(3))1,000:20, CV Therapeutics), which have the potential to provide enhanced insulin sensitivity without cardiovascular side effects and tachyphylaxis. The nonnucleosidic A(1)AR agonist BAY 68-4986 (capadenoson) represents a novel approach to angina wherein both animal studies and early human studies are promising. T-62 is an A(1)AR allosteric enhancer that is currently being evaluated in clinical trials as a potential treatment for neuropathic pain. The challenges associated with developing A(1)AR antagonists, agonists, or allosteric enhancers for therapeutic intervention are now well defined in humans. Significant progress has been made in identifying A(1)AR antagonists for the treatment of edema associated with congestive heart failure (CHF), A(1)AR agonists for the treatment of atrial arrhythmias, type II diabetes and angina, and A(1)AR allosteric enhancers for the treatment of neuropathic pain.