Short-term metabolic and haemodynamic effects of GR79236 in normal and fructose-fed rats

Therapeutic potentials of agonist and antagonist of adenosine receptors in type 2 diabetes

Type 2 diabetes has been a global health challenge over the decades and is among the leading causes of death. Several treatment approaches have been developed, but more effective and new therapies are still needed. The role of adenosine in glucose and lipid homeostasis has offered a different therapeutic approach. Adenosine mediates its physiological role through the activation of adenosine receptors. These adenosine receptors have been implicated in glucose and lipid homeostasis. The ability of agonists and antagonists of adenosine receptors to activate or inhibit the adenosine signalling cascade and thereby affecting the balance of glucose and lipid homeostasis has challenged the studies of agonists and antagonists of adenosine receptors, both preclinical and clinical, as potential anti-diabetic drugs. This review provides a background on different anti-diabetic therapeutic approaches, outlining the role of adenosine receptors in glucose and lipid homeostasis, and mechanisms underlying the action of agonists/antagonists of adenosine receptors as a therapeutic potential towards type 2 diabetes.

Orally active adenosine A(1) receptor agonists with antinociceptive effects in mice

Adenosine A(1) receptor (A(1)AR) agonists have antinociceptive effects in multiple preclinical models of acute and chronic pain. Although numerous A(1)AR agonists have been developed, clinical applications of these agents have been hampered by their cardiovascular side effects. Herein we report a series of novel A(1)AR agonists, some of which are structurally related to adenosine 5'-monophosphate (5'-AMP), a naturally occurring nucleotide that itself activates A(1)AR. These novel compounds potently activate A(1)AR in several orthogonal in vitro assays and are subtype selective for A(1)AR over A(2A)AR, A(2B)AR, and A(3)AR. Among them, UNC32A (3a) is orally active and has dose-dependent antinociceptive effects in wild-type mice. The antinociceptive effects of 3a were completely abolished in A(1)AR knockout mice, revealing a strict dependence on A(1)AR for activity. The apparent lack of cardiovascular side effects when administered orally and high affinity (K(i) of 36 nM for the human A(1)AR) make this compound potentially suitable as a therapeutic.

A1 adenosine receptor: role in diabetes and obesity

Adenosine mediates its diverse effects via four subtypes (A(1), A(2A), A(2B) and A(3)) of G-protein-coupled receptors. The A(1) adenosine receptor (A(1)AR) subtype is the most extensively studied and is well characterized in various organ systems. The A(1)ARs are highly expressed in adipose tissue, and endogenous adenosine has been shown to tonically activate adipose tissue A(1)ARs. Activation of the A(1)ARs in adipocytes reduces adenylate cyclase and cAMP content and causes inhibition of lipolysis. The role of A(1)ARs in lipolysis has been well characterized by using several selective A(1)AR agonists as well as A(1)AR knockout mice. However, the contribution of A(1)ARs to the regulation of lipolysis in pathological conditions like insulin resistance, diabetes and dyslipidemia, where free fatty acids (FFA) play an important role, has not been well characterized. Pharmacological agents that reduce the release of FFA from adipose tissue and thus the availability of circulating FFA have the potential to be useful for insulin resistance and hyperlipidemia. Toward this goal, several selective and efficacious agonists of the A(1)ARs are now available, and some have entered early-phase clinical trials; however, none have received regulatory approval yet. Here we review the existing knowledge on the role of A(1)ARs in insulin resistance, diabetes and obesity, and the progress made in the development of A(1)AR agonists as antilipolytic agents, including the challenges associated with this approach.

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.

Carnosine, the anti-ageing, anti-oxidant dipeptide, may react with protein carbonyl groups

Carnosine (beta-alanyl-L-histidine) is a physiological dipeptide which can delay ageing and rejuvenate senescent cultured human fibroblasts. Carnosine's anti-oxidant, free radical- and metal ion-scavenging activities cannot adequately explain these effects. Previous studies showed that carnosine reacts with small carbonyl compounds (aldehydes and ketones) and protects macromolecules against their cross-linking actions. Ageing is associated with accumulation of carbonyl groups on proteins. We consider here whether carnosine reacts with protein carbonyl groups. Our evidence indicates that carnosine can react non-enzymically with protein carbonyl groups, a process termed 'carnosinylation'. We propose that similar reactions could occur in cultured fibroblasts and in vivo. A preliminary experiment suggesting that carnosine is effective in vivo is presented; it suppressed diabetes-associated increase in blood pressure in fructose-fed rats, an observation consistent with carnosine's anti-glycating actions. We speculate that: (i) carnosine's apparent anti-ageing actions result, partly, from its ability to react with carbonyl groups on glycated/oxidised proteins and other molecules; (ii) this reaction, termed 'carnosinylation,' inhibits cross-linking of glycoxidised proteins to normal macromolecules; and (iii) carnosinylation could affect the fate of glycoxidised polypeptides.