Deoxyribose analogues of N6-cyclopentyladenosine (CPA): partial agonists at the adenosine A1 receptor in vivo

A novel nucleoside rescue metabolic pathway may be responsible for therapeutic effect of orally administered cordycepin

Although adenosine and its analogues have been assessed in the past as potential drug candidates due to the important role of adenosine in physiology, only little is known about their absorption following oral administration. In this work, we have studied the oral absorption and disposition pathways of cordycepin, an adenosine analogue. In vitro biopharmaceutical properties and in vivo oral absorption and disposition of cordycepin were assessed in rats. Despite the fact that numerous studies showed efficacy following oral dosing of cordycepin, we found that intact cordycepin was not absorbed following oral administration to rats. However, 3′-deoxyinosine, a metabolite of cordycepin previously considered to be inactive, was absorbed into the systemic blood circulation. Further investigation was performed to study the conversion of 3′-deoxyinosine to cordycepin 5′-triphosphate in vitro using macrophage-like RAW264.7 cells. It demonstrated that cordycepin 5′-triphosphate, the active metabolite of cordycepin, can be formed not only from cordycepin, but also from 3′-deoxyinosine. The novel nucleoside rescue metabolic pathway proposed in this study could be responsible for therapeutic effects of adenosine and other analogues of adenosine following oral administration. These findings may have importance in understanding the physiology and pathophysiology associated with adenosine, as well as drug discovery and development utilising adenosine analogues.

Discovery of Molecular Therapeutics for Glaucoma: Challenges, Successes, and Promising Directions

Glaucoma, a heterogeneous ocular disorder affecting ∼60 million people worldwide, is characterized by painless neurodegeneration of retinal ganglion cells (RGCs), resulting in irreversible vision loss. Available therapies, which decrease the common causal risk factor of elevated intraocular pressure, delay, but cannot prevent, RGC death and blindness. Notably, it is changes in the anterior segment of the eye, particularly in the drainage of aqueous humor fluid, which are believed to bring about changes in pressure. Thus, it is primarily this region whose properties are manipulated in current and emerging therapies for glaucoma. Here, we focus on the challenges associated with developing treatments, review the available experimental methods to evaluate the therapeutic potential of new drugs, describe the development and evaluation of emerging Rho-kinase inhibitors and adenosine receptor ligands that offer the potential to improve aqueous humor outflow and protect RGCs simultaneously, and present new targets and approaches on the horizon.

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.

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.

Mechanism-Based Pharmacokinetic-Pharmacodynamic Modeling: Biophase Distribution, Receptor Theory, and Dynamical Systems Analysis

Mechanism-based PK-PD models differ from conventional PK-PD models in that they contain specific expressions to characterize, in a quantitative manner, processes on the causal path between drug administration and effect. This includes target site distribution, target binding and activation, pharmacodynamic interactions, transduction, and homeostatic feedback mechanisms. As the final step, the effects on disease processes and disease progression are considered. Particularly through the incorporation of concepts from receptor theory and dynamical systems analysis, important progress has been made in the field of mechanism-based PK-PD modeling. This has yielded models with much-improved properties for extrapolation and prediction. These models constitute a theoretical basis for rational drug discovery and development.

Pharmacokinetic/pharmacodynamic modelling of the anti-hyperalgesic and anti-nociceptive effect of adenosine A1 receptor partial agonists in neuropathic pain

The objective of this investigation was to characterise the pharmacokinetic-pharmacodynamic correlation of adenosine A1 receptor partial agonists in the chronic constriction injury model of neuropathic pain. Following intravenous administration of 8-methylamino-N6-cyclopentyl-adenosine (MCPA; 10 mg/kg) and 2'deoxyribose-N6-cyclopentyl-adenosine (2'dCPA; 20 mg/kg), the time course of the effect on the mechanical paw pressure threshold was determined in conjunction with plasma concentrations. Population pharmacokinetic/pharmacodynamic analysis was applied to derive individual concentration-effect relationships. A composite model consisting of an E(max) model for the anti-hyperalgesic effect in combination with a linear model for the anti-nociceptive effect accurately described the concentration-effect relationship. For both compounds, a full anti-hyperalgesic effect was observed. The values of the EC50 for the anti-hyperalgesic effect were (mean+/-S.D.): 3170+/-1460 and 2660+/-1200 ng/ml for MCPA and 2'dCPA versus 178+/-51 ng/ml for the reference full agonist 5'deoxyribose-N6-cyclopentyl-adenosine (5'dCPA). The values of the slope for the anti-nociceptive effect were 1.9+/-0.30 and 1.2+/-0.20 g.microl/ng, respectively, versus 55+/-8 g microl/ng for 5'dCPA. Adenosine A1 receptor partial agonists behave as full agonists with regard to the anti-hyperalgesic effect in neuropathic pain, but the anti-nociceptive effect is diminished.

Population pharmacokinetic–pharmacodynamic modelling of the anti-hyperalgesic effect of 5′deoxy-N6-cylopentyladenosine in the mononeuropathic rat

The objective of this investigation was to characterise the pharmacokinetic-pharmacodynamic correlation of 5'-deoxy-N6-cyclopentyl-adenosine (5'dCPA) in the chronic constriction injury model of neuropathic pain. Following intravenous administration of 5'dCPA (0.30 or 0.75 mg kg(-1)), the time course of the drug concentration in plasma was determined in conjunction with the effect on (1) the mechanical paw pressure and (2) the Von Frey Hair monofilament withdrawal threshold. Population pharmacokinetic-pharmacodynamic analysis was applied to derive individual concentration-effect relationships. For mechanical paw pressure a composite model consisting of an Emax model for the anti-hyperalgesic effect in combination with a linear model for the anti-nociceptive effect accurately described the data. The EC50 for the anti-hyperalgesic effect was 178+/-51 ng ml(-1) and the slope of the anti-nociceptive effect 0.055+/-0.008 g ml ng(-1). For the Von Frey Hair monofilament withdrawal threshold responders and non-responders were observed. Typically, in responders, full pain relief was observed at concentrations exceeding 100 ng ml(-1). The high plasma concentrations required for the anti-hyperalgesic effect relative to the receptor affinity are consistent with restricted transport of 5'dCPA to the site of action in the spinal cord and/or the brain.

Partial agonists for A(3) adenosine receptors

Selective agonists for A(3) adenosine receptors (ARs) could potentially be therapeutic agents for a variety of disorders, including brain and heart ischemic conditions, while partial agonists may have advantages over full agonists as a result of an increased selectivity of action. A number of structural determinants for A(3)AR activation have recently been identified, including the N(6)-benzyl group, methanocarba substitution of ribose, 2-chloro and 2-fluoro substituents, various 2'- and 3'-substitutions and 4'-thio substitution of oxygen. The 2-chloro substitution of CPA and R-PIA led to A(3) antagonism (CCPA) and partial agonism (Cl-R-PIA). 2-Chloroadenosine was a full agonist, while 2-fluoroadenosine was a partial agonist. Both 2'- and 3'- substitutions have a pronounced effect on its efficacy, although the effect of 2'-substitution was more dramatic. The 4-thio substitution of oxygen may also diminish efficacy, depending on other substitutions. Both N(6)-methyl and N(6)-benzyl groups may contribute to the A(3) affinity and selectivity; however, an N(6)-benzyl group but not an N(6)-methyl group diminishes A(3)AR efficacy. N(6)-benzyl substituted adenosine derivatives have similar potency for human and rat A(3)ARs while N(6)-methyl substitution was preferable for the human A(3)AR. The combination of 2-chloro and N(6)-benzyl substitutions appeared to reduce efficacy further than either modification alone. The A(2A)AR agonist DPMA was shown to be an antagonist for the human A(3)AR. Thus, the efficacy of adenosine derivatives at the A(3)AR appears to be more sensitive to small structural changes than at other subtypes. Potent and selective partial agonists for the A(3)AR could be identified by screening known adenosine derivatives and by modifying adenosine and the adenosine derivatives.

Population pharmacokinetic modeling of blood-brain barrier transport of synthetic adenosine A1 receptor agonists

A population pharmacokinetic model is proposed for estimation of the brain distribution clearance of synthetic A1 receptor agonists in vivo. Rats with permanent venous and arterial cannulas in combination with a microdialysis probe in the striatum received intravenous infusions of 8-methylamino-N6-cyclopentyladenosine (MCPA) and 2'-deoxyribose-N6-cyclopentyladenosine (2'-dCPA) (10 mg kg(-1)). The clearance for transport from blood to the brain was estimated by simultaneous analysis of the blood and extracellular fluid concentrations using a compartmental pharmacokinetic model. The proposed pharmacokinetic model consists of three compartments describing the time course of the concentration in blood in combination with three compartments for the brain extracellular fluid concentrations. The blood clearance was 7.4 +/- 0.5 for MCPA and 7.2 +/- 1.4 ml min(-1) for 2'-dCPA. The in vivo microdialysis recoveries determined by the dynamic-no-net-flux method were independent of time with values of 0.21 +/- 0.02 and 0.22 +/- 0.01 for MCPA and 2'-dCPA, respectively. The values of the intercompartmental clearance for the distribution from blood to brain were 1.9 +/- 0.4 versus 1.6 +/- 0.3 mul min(-1) for MCPA and 2'-dCPA, respectively. It is concluded that on basis of the novel six-compartment model precise estimates of the rate of brain distribution are obtained that are independent of eventual differences in systemic exposure. The low brain distribution rates of MCPA and 2'-dCPA were consistent with in vitro tests. Furthermore, a slow elimination from the brain compartment was observed, indicating that the duration of central nervous system effects may be much longer than expected on the basis of the terminal half-life in blood.

Characterization of adenosine-A1 receptor-mediated antilipolysis in rats by tissue microdialysis, 1H-spectroscopy, and glucose clamp studies

Increased supply of fatty acids to muscle and liver is causally involved in the insulin resistance syndrome. Using a tissue microdialysis technique in Wistar and Zucker fatty (ZF) rats, we determined tissue glycerol levels as a marker of lipolysis in gastrocnemius muscle (gMT), subcutaneous adipose (SAT), and visceral adipose tissue (VAT) as well as the reduction of plasma free fatty acids, glycerol, and triglycerides caused by the antilipolysis-specific adenosine-A1 receptor agonist (ARA). In Wistar and ZF rats, ARA significantly lowered dialysate glycerol levels in SAT, VAT, and gMT. Whereas in SAT and VAT the decrease in dialysate glycerol indicated adipocytic antilipolysis, this decrease in gMT was not caused by a direct effect of ARA on intramyocellular lipolysis, as demonstrated by the lack of inhibition of the protein kinase A activity ratio in gMT. In addition, no differences of the fed-starved-refed dynamics of intramyocellular triglyceride levels compared with untreated controls were measured by in vivo (1)H-spectroscopy, excluding any adenylate cyclase-independent antilipolysis in muscle. Treatment with ARA resulted in pronounced reductions of plasma free fatty acids, glycerol, and triglycerides. Furthermore, in ZF rats, ARA treatment caused an immediate improvement of peripheral insulin sensitivity measured by the euglycemic-hyperinsulinemic glucose clamp technique.

Characterization of the pharmacokinetics, brain distribution, and therapeutic efficacy of the adenosine A1 receptor partial agonist 2'-deoxy-N6-cyclopentyladenosine in sarin-poisoned rats

The objective of the present study was to determine (1) the influence of sarin poisoning (144 microg/kg s.c.) on the pharmacokinetics and brain distribution of the adenosine A1 receptor partial agonist 2'-deoxy-N6-cyclopentyladenosine (2'dCPA), and (2) the effect of 2'dCPA (20 mg/kg i.v.) on the central acetylcholine (ACh) release and protection against sarin toxicity. A five-compartment model successfully described the pharmacokinetic profile of 2'dCPA in blood and brain microdialysate. A covariate analysis revealed that the volume of distribution of 2'dCPA in blood was different in sarin-poisoned rats, 177 +/- 7 versus 148 +/- 8 ml in control rats. However, the transport of 2'dCPA from blood to the brain was unaffected as reflected by the values of the intercompartmental transport clearances, 0.21 +/- 0.02 and 0.21 +/- 0.04 microl/min in control and sarin-poisoned rats, respectively. Also the area-under-curve (AUC) ratios of brain microdialysate and blood were identical with values of 0.02 +/- 0.001 and 0.02 +/- 0.002, respectively, demonstrating the restricted transport of 2'dCPA into the brain in both treatment groups. Treatment of sarin-poisoned rats by 2'dCPA did not adequately prevent the accumulation of ACh in the central nervous system. 2'dCPA delayed the emergence of concomitant symptoms compared to untreated rats, but eventually only 29% of the animals survived 24 h. In conclusion, the pharmacokinetic profile of 2'dCPA in blood was slightly changed by sarin, but not the distribution of 2'dCPA into the brain. The therapeutic efficacy of 2'dCPA against sarin was limited, presumably due to insufficient quantities of 2'dCPA reaching the brain.

Partial adenosine A(1) receptor agonists inhibit sarin-induced epileptiform activity in the hippocampal slice

Organophosphate poisoning can result in seizures and subsequent neuropathology. One possible therapeutic approach would be to employ adenosine A(1) receptor agonists, which have already been shown to have protective effects against organophosphate poisoning. Using an in vitro model of organophosphate-induced seizures, we have investigated the ability of several adenosine A(1) receptor agonists to inhibit epileptiform activity induced by the organophosphate sarin, in the CA1 stratum pyramidale of the guinea pig hippocampal slice. Application of the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) or the partial adenosine A(1) receptor agonists 2-deoxy-N(6)-cyclopentyladenosine (2-deoxy-CPA) and 8-butylamino-N(6)-cyclopentyladenosine (8-butylamino-CPA) abolished epileptiform activity in a concentration-related manner. The rank order of potency was CPA (IC(50) 4-5 nM) >2-deoxy-CPA (IC(50) 113-119 nM)=8-butylamino-CPA (IC(50) 90-115 nM). These data suggest that partial adenosine A(1) receptor agonists, which have fewer cardiovascular effects, should be further evaluated in vivo as potential treatments for organophosphate poisoning.

International Union of Pharmacology. XXV. Nomenclature and classification of adenosine receptors

Four adenosine receptors have been cloned and characterized from several mammalian species. The receptors are named adenosine A(1), A(2A), A(2B), and A(3). The A(2A) and A(2B) receptors preferably interact with members of the G(s) family of G proteins and the A(1) and A(3) receptors with G(i/o) proteins. However, other G protein interactions have also been described. Adenosine is the preferred endogenous agonist at all these receptors, but inosine can also activate the A(3) receptor. The levels of adenosine seen under basal conditions are sufficient to cause some activation of all the receptors, at least where they are abundantly expressed. Adenosine levels during, e.g., ischemia can activate all receptors even when expressed in low abundance. Accordingly, experiments with receptor antagonists and mice with targeted disruption of adenosine A(1), A(2A), and A(3) expression reveal roles for these receptors under physiological and particularly pathophysiological conditions. There are pharmacological tools that can be used to classify A(1), A(2A), and A(3) receptors but few drugs that interact selectively with A(2B) receptors. Testable models of the interaction of these drugs with their receptors have been generated by site-directed mutagenesis and homology-based modelling. Both agonists and antagonists are being developed as potential drugs.

Allosteric modulation of G protein-coupled receptors

Allosteric modulation of G protein-coupled receptors is a relatively novel and unexplored pharmacological concept that may lead to more selective and more 'natural' drugs for these receptors. In particular, allosteric enhancers may serve as tools to intensify selectively a weakened hormone or neurotransmitter signal caused by a localized deficit, such as in Alzheimer's or Parkinson's disease. In this paper, attention is paid to the adenosine A1 receptor, for which novel allosteric enhancers were synthesized and characterized that proved superior to the prototypic allosteric enhancer PD 81,723.

Characterization of adenosine A1 receptor in cultured myoblast C2C12 cells of mice

In an attempt to investigate the presence of adenosine A1 receptor in cell line, we used N6-cyclopentyladenosine (CPA), an agonist of adenosine A1 receptor, to incubate with C2C12 cells in vitro. CPA increased the uptake of radioactive glucose into C2C12 cells in a concentration-dependent manner and this action was abolished by the antagonists, both 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) (1,3-dipropy1-8-cyclopentylxanthine) and 8-(p-sulfophenyl)theophylline (8-SPT), at concentrations sufficient to block adenosine A1 receptor. Northern blot analysis showed the expression of adenosine A1 receptor mRNA by C2C12 cells. Western blotting also indicated a positive correlation (r = 0.99) of antibody recognized adenosine A1 receptor with membrane protein. The presence of adenosine A1 receptor in C2C12 cells can thus be considered. In the presence of U73312 (1-[6[[(17 beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H- pyrrole-2,5-dione), the specific inhibitor of phospholipase C, glucose uptake stimulated by CPA into C2C12 cells was reduced concentration-dependently while it was not modified by U73343 (1-[6[[(17 beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-2,5- pyrrolidinedione), the negative control of U73312. Moreover, chelerythrine and GF 109203X (3-[1-[3-(dimethylamino)propyl]-1H-indol-3-yl]-4-(1H-indol-3- yl)-1H-pyrrole-2,5-dione) also diminished the CPA-stimulated glucose uptake at concentrations sufficient to inhibit protein kinase C. The obtained data suggest that activation of adenosine A1 receptor in C2C12 cells may increase the glucose uptake via phospholipase C-protein kinase C pathway.

Thermodynamic in vitro studies as a method to investigate the pharmacodynamic behavior of adenosine A1 receptor ligands

PURPOSE

A thermodynamic analysis of the binding to rat cortex adenosine A1 receptor of N6-substituted (full agonists) and N6-substituted-deoxyribose (partial agonists) adenosine derivatives was performed. The intrinsic activity of the compounds was evaluated by measurements of the inhibition of forskolin stimulated 3', 5'-cyclic adenosine monophosphate (c-AMP) levels in isolated epididymal rat adipocytes.

METHODS

The thermodynamic parameters deltaG(o) (standard free energy), deltaH(o) (standard enthalpy), and deltaS(o) (standard entropy) of the binding equilibrium were determined by means of affinity measurements carried out at different temperatures (0, 10, 20, 25, 30 degrees C). Levels of c-AMP were evaluated performing competitive protein binding assays.

RESULTS

The binding of the ligands increases with temperature enhancement and, as a consequence, is totally entropy driven. Standard entropy values correlate significantly with intrinsic activity ones.

CONCLUSIONS

It is proposed the data obtained by these in vitro experiments can be used to investigate the in vivo pharmacodynamic of A, full and partial agonists.

Multivariate quantitative structure-pharmacokinetic relationships (QSPKR) analysis of adenosine A1 receptor agonists in rat

The aim of this study was to investigate the feasibility of a quantitative structure-pharmacokinetic relationships (QSPKR) method based on contemporary three-dimensional (3D) molecular characterization and multivariate statistical analysis. For this purpose, the programs SYBYL/CoMFA, GRID, and Pallas, in combination with the multivariate statistical technique principal component analysis were employed to generate a total of 16 descriptor variables for a series of 12 structurally related adenosine A1 receptor agonists. Subsequently, the multivariate regression method, partial least squares, was used to predict clearance (CL), volume of distribution (VdSS) and protein binding (fraction unbound, fU). The QSPKR models obtained could account for most of the variation in CL, VdSS, and fU (R2 = 0.82, 0.61 and 0.78, respectively). Cross-validation confirmed the predictive ability of the models (Q2 = 0.59, 0.41 and 0.62 for CL, VdSS, and fU, respectively). In conclusion, we have developed a multivariate 3D QSPKR model that could adequately predict overall pharmacokinetic behavior of adenosine A1 receptor agonists in rat. This methodology can also be used for other classes of compounds and may facilitate the further integration of QSPKR in drug discovery and preclinical development.

Processing of adenosine receptor agonists in rat and human whole blood

A stability study of adenosine receptor agonists in rat and human whole blood was performed. The compounds were incubated at 37 degrees in fresh blood, and aliquots of the incubation mixture were hemolyzed at regular time intervals and analyzed with HPLC. N6-cyclopentyladenosine (CPA) and N6-cyclobutyladenosine (CBA) were degraded, whereas N6-cyclohexyladenosine, N6-cycloheptyladenosine and N6-sulfophenyladenosine were not. 2-Chloroadenosine had a half-life very similar to that of CPA. However, the 2'-, 3'-, and 5'-deoxyribose derivatives of CPA remained intact. The nucleoside transport inhibitor nitrobenzylthioinosine attenuated CBA and CPA metabolism in rat blood as did the inhibitor of adenosine deaminase erythro-9-(2-hydroxy-3-nonyl)adenine, albeit at relatively high concentrations. Complete blockade of CBA and CPA degradation was achieved by a preincubation of rat and human blood with the adenosine kinase (AK) inhibitor 5'-amino-5'-deoxyadenosine. We conclude that the two adenosine analogues are metabolized by AK both in rat and in human whole blood.

Thermodynamics of full agonist, partial agonist, and antagonist binding to wild-type and mutant adenosine A1 receptors

A thermodynamic analysis of the binding of a full agonist (N6-cyclopentyladenosine), a partial agonist (8-butylamino-N6-cyclopentyladenosine) and an antagonist (8-cyclopentyltheophylline) to human wild-type and mutant (mutation of a threonine (Thr) to an alanine (Ala) residue at position 277) adenosine A1 receptors expressed on Chinese hamster ovary (CHO) cells, and to rat brain adenosine A1 receptors was undertaken. The thermodynamic parameters deltaGo (standard free energy), deltaHo (standard enthalpy) and deltaSo (standard entropy) of the binding equilibrium to rat brain receptors were determined by means of affinity measurements carried out at four different temperatures (0, 10, 20 and 25 degrees) and van't Hoff plots. Two temperatures (0 and 25 degrees) were considered for human receptors. Affinity constants were obtained from inhibition assays on membrane preparations of rat brain and CHO cells by use of the antagonist [3H]1,3-dipropyl-8-cyclopentylxanthine ([3H]DPCPX) as selective adenosine A1 receptor radioligand. As for rat brain receptors, full agonist binding was totally entropy driven, whereas antagonist binding was essentially enthalpy driven. Partial agonist binding appeared both enthalpy and entropy driven. As for human receptors, full agonist affinity was highly dependent on the presence of Thr277. Moreover, affinity to both wild-type and mutant receptors was enhanced by temperature increase, suggesting a totally entropy-driven binding. Antagonist binding did not depend on the presence of Thr277. Antagonist affinity decreased with an increase in temperature, suggesting a mainly enthalpy-driven binding. Partial agonist binding was significantly dependent on the presence of Thr277 at 25 degrees, whereas such a dependence was not evident at 0 degrees. It is concluded that Thr277 contributes only to the binding of adenosine derivatives and that its role changes drastically with the receptor conformation and with the type of agonist (full or partial) interacting with the adenosine A1 receptors.

Activation of various subtypes of G-protein alpha subunits by partial agonists of the adenosine A1 receptor

The activation of different G protein subtypes by the rat adenosine A1 receptor initiated by stimulation with the full agonist 2-chloro-N6-cyclopentyladenosine (CCPA) and by six structurally distinct partial agonists of this receptor was investigated. Endogenous G protein alpha subunits in rat cortical membranes were inactivated by N-ethylmaleimide (NEM). Activation of rat recombinant myristoylated alpha(o), alpha(i1), alpha(i2) and alpha(i3) by partial agonists in comparison to the full agonist was assessed by guanosine-5'-(gamma-[35S]thio)triphosphate ([35S]GTPgammaS) binding after reconstitution of G protein alpha subunits with the adenosine A1 receptor in N-ethylmaleimide-treated membranes. 2-Chloro-N6-cyclopentyladenosine and 3' -deoxy-N6-cyclopentyladenosine (3'-d-CPA), the partial agonist with the highest intrinsic activity, were significantly more potent in activation of alpha(i) subtypes than alpha(o). In contrast, 5'-methylthioadenosine (MeSA), 2'-deoxy-2-chloroadenosine (cladribine), 2'-deoxy-N6-cyclopentyladenosine (2'-d-CPA), 2-phenylaminoadenosine (CV 1808) and C8-aminopropyl-N6-cyclopentyladenosine (C8-aminopropyl-CPA) did not exhibit higher potency for Go or any Gi subtype. All partial agonists, although carrying structurally different modifications, showed higher relative intrinsic activities in activation of Gi than of Go, indicating that Gi-coupled pathways may be activated selectively via the A1 receptor by partial agonists, but not Go-mediated responses.

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.

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.

Biological activities of N6,C8-disubstituted adenosine derivatives as partial agonists at rat brain adenosine A1 receptors

C8-substituted derivatives of the adenosine A1 receptor-selective agonist N6-cyclopentyladenosine (CPA) were evaluated as potential partial adenosine A1 receptor agonists in rat brain. Potencies and efficacies of 8-alkylamino-CPA derivatives were determined in G protein activation assays by their ability to stimulate binding of [35S]guanosine-5'-(gamma-thio)triphosphate ([35S]GTPgammaS) to rat forebrain membranes, by their ability to inhibit forskolin-stimulated adenylate cyclase, and by inhibition of evoked field excitatory postsynaptic potentials (field EPSPs) in hippocampal slices. EC50 values around 1 microM were determined for all C8-substituted CPA derivatives. Increase in chain length of the substituent gradually reduced agonist efficacy in [35S]GTPgammaS binding studies. Only C8-methylamino-, C8-ethylamino- and C8-propylamino-CPA inhibited forskolin-stimulated adenylate cyclase. In contrast, 8-methylamino- and 8-butylamino-CPA were the compounds of highest intrinsic activity in inhibition of field EPSPs in the hippocampus, followed by 8-ethylamino-CPA. 8-Cyclopentylamino-CPA was without effect in this tissue, and the propylamino derivative, when applied cumulatively, caused an inhibition which was smaller the higher the concentration used and the longer the application, which is suggestive of drug-induced desensitization. These data indicate that 8-aminoalkyl-substituted CPA derivatives act as partial agonists on the brain and may serve as valuable tools to dissect adenosine A1 receptor mediated signal trafficking in various organs.

Pharmacokinetic-haemodynamic relationships of 2-chloroadenosine at adenosine A1 and A2a receptors in vivo

1. The purpose of the present study was to develop an experimental strategy for the quantification of the cardiovascular effects of non-selective adenosine receptor ligands at the adenosine A1 and A2a receptor in vivo. 2-Chloroadenosine (CADO) was used as a model compound. 2. Three groups of normotensive conscious rats received an short intravenous infusion of 1.4 mg kg-1 CADO during constant infusions of the A1-selective antagonist, 8-cyclopentyltheophylline (CPT; 20 micrograms min-1 kg-1), the A2a-selective antagonist, 8-(3-chlorostyryl) caffeine (CSC; 32 micrograms min-1 kg-1) or the vehicle. The heart rate (HR) and mean arterial blood pressure (MAP) were recorded continuously during the experiment and serial arterial blood samples were taken for analysis of drug concentrations. The ratio MAP/HR was also calculated, which may reflect changes in total peripheral resistance on the assumption that no changes in stroke volume occur. 3. During the infusion of CPT, CADO produced a reduction in both blood pressure and MAP/HR by activation of the A2a receptor. The concentration-effect relationships were described according to the sigmoidal Emax model, yielding potencies based on free drug concentrations (EC50,u) of 61 and 68 ng ml-1 (202 and 225 nM) for the reduction of blood pressure and MAP/HR, respectively. During the infusion of CSC, an EC50,u value of 41 ng ml-1 (136 nM) was observed for the A1 receptor-mediated reduction in heart rate. The in vivo potencies correlated with reported receptor affinities (Ki(A1) = 300 nM and Ki(A2a) = 80 nM). The maximal reductions in MAP/HR and heart rate were comparable to those of full agonists, with the Emax values of -12 +/- 1 x 10(-2) mmHg b.p.m.-1 and -205 b.p.m. respectively. 4. It is concluded that this integrated pharmacokinetic-pharmacodynamic approach can be used to obtain quantitative information on the potency and intrinsic activity of new non-selective adenosine receptor agonists at different receptor subtypes in vivo.