The effects of GR79236 on plasma fatty acid concentrations, heart rate and blood pressure in the conscious rat

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.

Antilipolytic Activity of a Novel Partial A1Adenosine Receptor Agonist Devoid of Cardiovascular Effects: Comparison with Nicotinic Acid

Elevated lipolysis and circulating free fatty acid (FFA) levels have been linked to the pathogenesis of insulin resistance. A1 adenosine receptor agonists are potent inhibitors of lipolysis. Several A1 agonists have been tested as potential antilipolytic agents; however, their effect on the cardiovascular system remains a potential problem for development of these agents as drugs. In the present study, we report that CVT-3619 [(2-{6-[((1R,2R)-2-hydroxycyclopentyl) amino] purin9-yl} (4S,5 S,2R,3R)5-[(2fluorophenylthio) methyl] oxolane-3,4-diol)], a novel partial A1 receptor agonist, significantly reduces circulating FFA levels without any effect on heart rate and blood pressure in awake rats. Rats were implanted with indwelling arterial and venous cannulas to obtain serial blood samples, record arterial pressure, and administer drug. CVT-3619 decreased FFA levels in a dose-dependent manner at doses from 1 up to 10 mg/kg. The FFA-lowering effect was blocked by the A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine. Triglyceride (TG) levels were also significantly reduced by CVT-3619 treatment in the absence and presence of Triton. Tachyphylaxis of the antilipolytic effect of CVT-3619 (1 mg/kg i.v. bolus) was not observed with three consecutive treatments. An acute reduction of FFA by CVT-3619 was not followed by a rebound increase of FFA as seen with nicotinic acid. The potency of insulin to decrease lipolysis was increased 4-fold (p < 0.01) in the presence of CVT-3619 (0.5 mg/kg). In summary, CVT-3619 is an orally bioavailable A1 agonist that lowers circulating FFA and TG levels by inhibiting lipolysis. CVT-3619 has antilipolytic effects at doses that do not elicit cardiovascular effects.

Lack of effect of the adenosine A1 receptor agonist, GR79236, on capsaicin-induced CGRP release in anaesthetized pigs

Migraine is a common neurological disorder that is associated with an increase in plasma calcitonin gene-related peptide (CGRP) levels. CGRP, a potent vasodilator released from the activated trigeminal sensory nerves, dilates intracranial blood vessels and transmits vascular nociception. Hence, inhibition of trigeminal CGRP release may prevent neurotransmission and, thereby, ameliorate migraine headache. Therefore, the present study in anaesthetized pigs investigates the effects of a selective adenosine A(1) receptor agonist, GR79236 (3, 10 and 30 microg/kg, i.v.) on capsaicin-induced carotid haemodynamic changes and on plasma CGRP release. Intracarotid (i.c.) infusion of capsaicin (10 microg/kg/min, i.c.) increased the total carotid blood flow and conductance as well as carotid pulsations, but decreased the difference between arterial and jugular venous oxygen saturations. These responses to capsaicin were dose-dependently attenuated by GR79236. However, the increases in the plasma CGRP concentrations by capsaicin remained essentially unmodified after GR79236 treatment. The above results suggest that GR79236 may have an antimigraine potential due to its postjunctional effects (carotid vasoconstriction) rather than to prejunctional inhibition of trigeminal CGRP release.

Adenosine modulation: a novel approach to analgesia and inflammation

Adenosine (ADO) is an endogenous purine nucleoside that functions as an extracellular signalling molecule. It is released locally at sites of cellular trauma, and acts on specific cell-surface purinergic receptors (termed P1 receptors) near its site of release to exert its effects. Four subtypes of the P1 family of G-protein-coupled receptors have been identified and cloned: A1, A2A, A2B and A3. A considerable body of evidence, including experimental animal data and preliminary clinical reports, indicates that ADO is involved in modulating endogenous antinociceptive processes in the brain and spinal cord. ADO analogues provide analgesic activity after systemic or spinal administration in a broad spectrum of animal pain models. In addition, iv. ADO infusion has shown benefit in human pain states. The spinal cord is a key site for ADO-mediated modulation of nociception. ADO is well known to act as an inhibitory neuromodulator in the central and peripheral nervous system, and it may act to control N-methyl-D-aspartate (NMDA)- and substance P-mediated events in nociception and central sensitisation at the spinal level. ADO is also released at sites of inflammation and it exerts anti-inflammatory effects via multiple mechanisms involving several cell types. These include effects on neutrophil function, endothelial cell permeability, in vivo and in vitro release of tumour necrosis factor (TNF-alpha and collagenase expression in synoviocytes. Accordingly, ADO analogues are effective in several animal models of inflammation, including the rat adjuvant arthritis model. Several therapeutic approaches to pain and inflammation, based on mimicking or modulating the effects of endogenous ADO, are currently under preclinical and clinical investigation. These include the use of ADO itself, the use of direct-acting ADO receptor agonists and the use of agents designed to modulate the levels and, therefore, the actions of ADO in the extracellular space (ADO kinase (AK) inhibitors). Data emerging in the next several years should indicate whether these strategies represent a therapeutically useful new approach to analgesia and inflammation.

The therapeutic potential of agents acting via purine receptors

A host of physiological processes associated with the cardiovascular (CV) system, central nervous system (CNS), and a variety of other organ systems and tissues are regulated by agents, primarily adenosine (ado) and adenosine triphosphate (ATP), that act via cell-surface purine receptors. These receptors have therefore been the focus of a variety of programmes directed at the discovery and development of new therapeutic agents, most notably for the treatment of disorders of the CV system. Currently, only a handful of agents, including ado, theophylline, dipyridamole, and ticlopidine, are approved for clinical use. A variety of new agents intended for use in CV disease, disorders of the CNS, such as Parkinson's disease, treatment of pain, inflammatory disorders, and diverse other pathophysiological conditions are in clinical development. Historically, ado receptors have been the primary target. Recent research efforts have begun to examine alternative strategies including agents that modulate endogenous levels of extracellular ado and agents that act via P(2) receptors.

Effect of the adenosine A1 receptor agonist GR79236 on trigeminal nociception with blink reflex recordings in healthy human subjects

GR79236 is a highly potent and selective adenosine A1 receptor agonist that has analgesic and anti-inflammatory actions in humans and animals. In animal models it inhibits trigeminal nerve firing and calcitonin gene-related peptide release which play a pivotal role in migraine pathophysiology. Thus GR79236 may have therapeutic potential in migraine. Although there are no validated human models of migraine, the trigeminal nociceptive pathways may be studied with a novel electrode to elicit nociception-specific blink reflex responses. Twelve healthy female volunteers were randomized in a double-blind, placebo-controlled, cross-over trial to investigate the effect of GR79236 on trigeminal nociceptive pathways, as measured by the blink reflex. A secondary objective was to compare the use of two types of electrode, the standard (SE) and nociception-specific electrodes (NE), to investigate human trigeminal pharmacology. Blink reflexes were elicited with SE and NE before and 30 min after GR79236 (10 microg/kg i.v.) or placebo. The median area under the curve of repeated sweeps of the R2 component of the blink reflex was analysed using analysis of covariance with baseline as covariate. Using NE, GR79236 produced a non-significant reduction of the ipsilateral R2 compared with placebo (P = 0.097) and a significant reduction contralaterally (P = 0.008). No significant changes were observed using SE. There were no significant adverse events. The results suggest that NE is more sensitive than SE to detect pharmacological effects in the trigeminal nociceptive system. Furthermore, the adenosine A1 receptor agonist GR79236 inhibits trigeminal nociception in humans. These results support a possible therapeutic role for GR79236 in primary headache disorders.

Adenosine A1 receptor agonists inhibit trigeminovascular nociceptive transmission

There is a considerable literature to suggest that adenosine A1 receptor agonists may have anti-nociceptive effects, and we sought to explore the role of adenosine A1 receptors in a model of trigeminovascular nociceptive transmission. Cats were anaesthetized (alpha-chloralose 60 mg/kg, intraperitoneally), and prepared for physiological monitoring. The superior sagittal sinus (SSS) was stimulated electrically, and linked units were recorded in the trigeminocervical complex. Post-stimulus histograms were constructed to analyse the responses and the effect of drug administration. Blood was sampled from the external jugular vein to determine levels of calcitonin gene-related peptide (CGRP) release before and after drug administration. Intravenous administration of the highly selective adenosine A1 receptor agonist, GR79236 (3-100 microg/kg) had a dose-dependent inhibitory effect on SSS-evoked trigeminal activity. The maximal effect (80 +/- 6% reduction in probability of firing) was seen at 100 microg/kg. The neuronal inhibitory effect of GR79236 could be inhibited by the selective adenosine A1 receptor antagonist DPCPX (300 microg/kg; P < 0.05). SSS stimulation increased cranial CGRP levels from 33 +/- 2 pmol/l (n = 6) to 64 +/- 3 pmol/l, an effect substantially reduced by pre-treatment with GR79236 (30 microg/kg; P < 0.01). The selective low efficacy adenosine A1 receptor agonist, GR190178 (30-1000 microg/kg i.v.), also inhibited SSS-evoked neuronal activity in a dose-dependent fashion. In this model of trigeminovascular nociception, adenosine A1 receptor activation leads to neuronal inhibition without concomitant vasoconstriction, suggesting a novel avenue for the treatment of migraine and cluster headache.

The time course of cardioprotection induced by GR79236, a selective adenosine A1-receptor agonist, in myocardial ischaemia-reperfusion injury in the pig

The cardioprotective effects of the selective adenosine A1-receptor agonist, GR79236 (N-[(1S, trans)-2-hydroxycyclopentyl]adenosine), were examined in a porcine model of myocardial ischaemia-reperfusion injury. When pigs were subjected to a 50-min coronary artery occlusion followed by 3-h reperfusion, GR79236 (10 nmol/kg, i.v.) significantly reduced infarct size whether given 10 min before the onset of ischaemia or reperfusion. This effect was independent of the bradycardia induced by GR79236, as it was also observed in animals in which heart rate was maintained by electrical pacing. However, GR79236 administered 10 min after reperfusion did not reduce infarct size. GR79236 had no effect on the incidence or outcome of ventricular dysrhythmias in this pig model of infarction. Similarly, ischaemic preconditioning (IPC, 2 x 10-min ischaemia and 10-min reperfusion) significantly reduced infarct size. The selective adenosine A1-receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 3.3 micromol/kg, i.v.), abolished the haemodynamic and cardioprotective effects of GR79236 and the cardioprotective effects of IPC in anaesthetised pigs. In conclusion, GR79236 exerted a marked cardioprotective effect in a porcine model of myocardial ischaemia-reperfusion injury, provided that it was administered before reperfusion. This suggests that GR79236 may have clinical utility in the treatment of various aspects of ischaemic heart disease.

Selectivity of action of 8-alkylamino analogues of N6-cyclopentyladenosine in vivo: haemodynamic versus anti-lipolytic responses in rats

1. A1 adenosine receptor agonists with reduced intrinsic activity may be therapeutically useful as result of an increased selectivity of action. In this study the tissue selectivity of three 8-alkylamino substituted analogues of N6-cyclopentyladenosine (CPA) was investigated for haemodynamic and anti-lipolytic effects using an integrated pharmacokinetic-pharmacodynamic approach. 2. Chronically instrumented male Wistar rats received intravenous infusions of 4.0 mg kg(-1) 8-methylaminoCPA (8MCPA), 12.0 mg kg(-1) 8-ethylaminoCPA (8ECPA), 20.0 mg kg(-1) 8-butylaminoCPA (8BCPA) or vehicle during 15 min. During experimentation, serial arterial blood samples were drawn for the determination of agonist concentrations and plasma non-esterified fatty acid (NEFA) levels. Blood pressure and heart rate were monitored continuously. In addition to the CPA analogues, each rat received a rapid bolus infusion of CPA to determine the maximal effects of the full agonist. 3. The concentration-time profiles of the CPA analogues could be described by a bi-exponential function. Values for clearance, volume of distribution at steady state and elimination half-life were 44+/-5, 48+/-6 and 39+/-2 ml min(-1) kg(-1), 0.97+/-0.09, 0.84+/-0.10 and 1.05+/-0.07 1 kg(-1) and 25+/-2, 28+/-2 and 40+/-2 min for 8MCPA, 8ECPA and 8BCPA, respectively (mean+/-s.e.mean, n=6-8). 4. Different models were used to derive the concentration-effect relationships for heart rate and NEFA, yielding estimates of potency (EC50) and intrinsic activity (Emax) for both effects of the compounds in vivo. On heart rate the compounds acted as partial agonists, with Emax values of -173+/-14, -131+/-11 and -71+/-6 beats min(-1) for 8MCPA, 8ECPA and 8BCPA, respectively. These Emax values were significantly lower than the maximal effect of CPA (-208+/-8 beats min(-1)). With regard to the anti-lipolytic effect all three compounds were full agonists and lowered NEFA levels to the same extent as CPA (69%). The estimated Emax values were 63+/-5, 63+/-4 and 68+/-2%, respectively. 5. Furthermore, the compounds were more potent in causing anti-lipolytic than cardiovascular effects. The EC50 values for the NEFA and heart rate lowering effects were 37+/-15, 68+/-22 and 659+/-108 ng ml(-1) and 164+/-22, 341+/-76 and 975+/-190 ng ml(-1) for 8MCPA, 8ECPA and 8BCPA, respectively (mean+/-s.e.mean, n=6-8). 6. This study demonstrates that partial agonists for the A1 adenosine receptor have increased selectivity of action in vivo. The 8-alkylamino analogues of CPA may be useful anti-lipolytics with less pronounced haemodynamic side effects.

Physiological indirect effect modeling of the antilipolytic effects of adenosine A1-receptor agonists

The relationship between blood concentrations of the adenosine A1-receptor agonist N6-(p-sulfophenyl) adenosine (SPA) and its effect on both plasma nonesterified fatty acid (NEFA) and glycerol release was described on the basis of an integrated pharmacokinetic-pharmacodynamic model. An indirect response model rather than a hypothetical "link" model was used to account for the delayed response. For that purpose an empirical solution to the differential equation describing the physiological indirect response model is presented. The model-estimated rate constant for the output of the glycerol response was compared to the elimination rate constant after exogenous administration of glycerol. In a crossover designed study, chronically cannulated male Wistar rats were subjected to either SPA administration (120 microgram/kg for 15 min) for measurement of the effects on glycerol, or glycerol administration for determination of glycerol pharmacokinetics. Glycerol pharmacokinetics was determined in the presence of a stable level of SPA (171 +/- 6 ng/ml) to suppress endogenous glycerol levels completely. The indirect response model adequately described the relationship between SPA concentrations and plasma glycerol levels. The PD parameter estimates for EC50, EMAX, and Hill factor were 23 +/- 2 ng/ml, 74 +/- 3% (change from baseline), and 3.3 +/- 0.5, respectively. These values were not different from those obtained when analyzing the data on basis of the differential equation directly. Furthermore, the EC50 values for the reduction in glycerol or NEFA levels were identical (23 +/- 2 and 21 +/- 3 ng/ml, respectively) indicating that both PD endpoints reflect the same physiological process. The concentration-time profile after administration of glycerol could be described best on the basis of a biexponential function. The value for kout in the PK/PD model (0.19 +/- 0.03 min-1) corresponded very well to the terminal elimination rate constant determined after i.v. administration of glycerol (0.25 +/- 0.03 min-1). In conclusion, the antilipolytic effects of adenosine A1-receptor agonists can be described by the indirect suppression model. The rate constant describing the delay between concentration and glycerol effect was shown to be a true reflection of the removal of glycerol.

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

The adenosine (A1) receptor agonist, GR79236 (N-[(1S,trans)-2-hydroxycyclopentyl]adenosine), inhibits catecholamine-induced lipolysis in vitro, but the short-term metabolic and haemodynamic effects have not been previously reported in the fructose fed model of insulin resistance, dyslipidaemia and hypertension. This study reports the effects of GR79236 (1 mg/kg/day for 8 days) on nonesterified free fatty acid and triglyceride metabolism, oral and i.v. glucose tolerance, blood pressure and heart rate, and insulin sensitivity, in normal rats and rats fed a fructose-enriched diet. In normal rats, GR79236 significantly reduced fasting glucose (25%), free fatty acid (50%) and triglyceride (55%) concentrations, and improved glucose tolerance (AUC[glu] 21.2 +/- 1.3 vs. 16.5 +/- 1.1 mmol h/l, p < 0.05). Fructose feeding induced a state of insulin resistance and dyslipidaemia, as shown by an increase in steady-state plasma glucose levels (7.1 vs. 6.1 mmol/l), impaired i.v. glucose tolerance and a 3-fold rise in fasting triglyceride levels; fructose-fed rats also developed a significant increase in blood pressure. GR79236 ameliorated the effects of fructose feeding on fatty acid and triglyceride levels, and blood pressure, and improved i.v. glucose tolerance in fructose-fed rats. The hypotriglyceridaemic effect was due to a reduction in triglyceride secretion rate (17.3 +/- 1.7 vs. 30.2 +/- 1.1). Thus, in normal rats and in a dietary-induced rodent model of insulin resistance, dyslipidaemia and hypertension, GR79236 has lipid-lowering and glucose-lowering activity, as well as haemodynamic effects, which are potentially useful for treating both the metabolic and haemodynamic features of insulin resistance and NIDDM in humans.

Pharmacokinetic-pharmacodynamic modelling of the anti-lipolytic and anti-ketotic effects of the adenosine A1-receptor agonist N6-(p-sulphophenyl)adenosine in rats

1. The purpose of this study was to develop and validate an integrated pharmacokinetic-pharmacodynamic model for the anti-lipolytic effects of the adenosine A1-receptor agonist N6-(p-sulphophenyl)adenosine (SPA). Tissue selectivity of SPA was investigated by quantification of haemodynamic and anti-lipolytic effects in individual animals. 2. After intravenous infusion of SPA to conscious normotensive Wistar rats, arterial blood samples were drawn for determination of blood SPA concentrations, plasma non-esterified fatty acid (NEFA) and beta-hydroxybutyrate levels. Blood pressure and heart rate were monitored continuously. 3. The relationship between the SPA concentrations and the NEFA lowering effect was described by the indirect suppression model. Administration of SPA at different rates and doses (60 microg kg[-1] in 5 min and 15 min, and 120 microg kg[-1] in 60 min) led to uniform pharmacodynamic parameter estimates. The averaged parameters (mean+/-s.e., n=19) were Emax: -80+/-2% (% change from baseline), EC50: 22+/-2 ng ml(-1), and Hill factor: 2.2+/-0.2. 4. In another group, given 400 microg kg(-1) SPA in 15 min, pharmacodynamic parameters for both heart rate and anti-lipolytic effect were derived within the same animal. The reduction in heart rate was directly related to blood concentration on the basis of the sigmoidal Emax model. SPA inhibited lipolysis at concentrations lower than those required for an effect on heart rate. The EC50 values (mean+/-s.e., n=6) were 131+/-31 ng ml(-1) and 20+/-3 ng ml(-1) for heart rate and NEFA lowering effect, respectively. 5. In conclusion, the relationship between blood SPA concentrations and anti-lipolytic effect was adequately described by the indirect suppression model. For SPA a 6 fold difference in potency was observed between the effects on heart rate and NEFAs, indicating some degree of tissue selectivity in vivo.