A2A Adenosine Receptor Affinity Research Articles

A New Series of 1,3-Dimethylxanthine Based Adenosine A2A Receptor Antagonists as a Non-Dopaminergic Treatment of Parkinson's Disease.

Adenosine receptors (AR) have emerged as competent and innovative nondopaminergic targets for the development of potential drug candidates and thus constitute an effective and safer treatment approach for Parkinson's disease (PD). Xanthine derivatives are considered as potential candidates for the treatment Parkinson's disease due to their potent A2A AR antagonistic properties. The objectives of the work are to study the impact of substituting N7-position of 8-m/pchloropropoxyphenylxanthine structure on in vitro binding affinity of compounds with various AR subtypes, in vivo antiparkinsonian activity and binding modes of newly synthesized xanthines with A2A AR in molecular docking studies. Several new 7-substituted 8-m/p-chloropropoxyphenylxanthine analogues have been prepared. Adenosine receptor binding assays were performed to study the binding interactions with various subtypes and perphenazine induced rat catatonia model was used for antiparkinsonian activity. Molecular docking studies were performed using Schrödinger molecular modeling interface. 8-para-substituted xanthine 9b bearing an N7-propyl substituent displayed the highest affinity towards A2A AR (Ki = 0.75 μM) with moderate selectivity versus other AR subtypes. 7-Propargyl analogue 9d produced significantly long-lasting antiparkinsonian effects and also produced potent and selective binding affinity towards A2A AR. In silico docking studies further highlighted the crucial structural components required to develop xanthine derived potential A2A AR ligands as antiparkinsonian agents. A new series of 7-substituted 8-m/p-chloropropoxyphenylxanthines having good affinity for A2A AR and potent antiparkinsonian activity has been developed.

C2-substituted quinazolinone derivatives exhibit A1 and/or A2A adenosine receptor affinities in the low micromolar range

Antagonists of the adenosine receptors (A1 and A2A subtypes) are widely researched as potential drug candidates for their role in Parkinson’s disease-related cognitive deficits (A1 subtype), motor dysfunction (A2A subtype) and to exhibit neuroprotective properties (A2A subtype). Previously the benzo-α-pyrone based derivative, 3-phenyl-1H-2-benzopyran-1-one, was found to display both A1 and A2A adenosine receptor affinity in the low micromolar range. Prompted by this, the α-pyrone core was structurally modified to explore related benzoxazinone and quinazolinone homologues previously unknown as adenosine receptor antagonists. Overall, the C2-substituted quinazolinone analogues displayed superior A1 and A2A adenosine receptor affinity over their C2-substituted benzoxazinone homologues. The benzoxazinones were devoid of A2A adenosine receptor binding, with only two compounds displaying A1 adenosine receptor affinity. In turn, the quinazolinones displayed varying degrees of affinity (low micromolar range) towards the A1 and A2A adenosine receptor subtypes. The highest A1 adenosine receptor affinity and selectivity were favoured by methyl para-substitution of phenyl ring B (A1Ki = 2.50 μM). On the other hand, 3,4-dimethoxy substitution of phenyl ring B afforded the best A2A adenosine receptor binding (A2AKi = 2.81 μM) among the quinazolinones investigated. In conclusion, the quinazolinones are ideal lead compounds for further structural optimization to gain improved adenosine receptor affinity, which may find therapeutic relevance in Parkinson’s disease-associated cognitive deficits and motor dysfunctions as well as exerting neuroprotective properties.

New 8-amino-1,2,4-triazolo[4,3-a]pyrazin-3-one derivatives. Evaluation of different moieties on the 6-aryl ring to obtain potent and selective human A2A adenosine receptor antagonists

In this work, further structural investigations on the 8-amino-2-phenyl-6-aryl-1,2,4-triazolo[4,3-a]pyrazin-3-one series were carried out to achieve potent and selective human A2A adenosine receptor (AR) antagonists. Different ether and amide moieties were attached at the para-position of the 6-phenyl ring, thus leading to compounds 1–9 and 10–18, respectively. Most of these moieties contained terminal basic rings (pyrrolidine, morpholine, piperidine and substituted piperazines) which were thought to confer good physicochemical and drug-like properties.Compounds 11–16, bearing the amide linker, possessed high affinity and selectivity for the hA2A AR (Ki = 3.6–11.8 nM). Also derivatives 1–9, featuring an ether linker, preferentially targeted the hA2A AR but with lower affinity, compared to those of the relative amide compounds. Docking studies, carried out at the hA2A AR binding site, highlighted some crucial ligand-receptor interactions, particularly those provided by the appended substituent whose nature deeply affected hA2A AR affinity.

Probing Substituents in the 1- and 3-Position: Tetrahydropyrazino-Annelated Water-Soluble Xanthine Derivatives as Multi-Target Drugs With Potent Adenosine Receptor Antagonistic Activity.

Tetrahydropyrazino-annelated theophylline (1,3-dimethylxanthine) derivatives have previously been shown to display increased water-solubility as compared to the parent xanthines due to their basic character. In the present study, we modified this promising scaffold by replacing the 1,3-dimethyl residues by a variety of alkyl groups including combinations of different substituents in both positions. Substituted benzyl or phenethyl residues were attached to the N8 of the resulting 1,3-dialkyl-tetrahydropyrazino[2,1-f ]purinediones with the aim to obtain multi-target drugs that block human A1 and A2A adenosine receptors (ARs) and monoaminoxidase B (MAO-B). 1,3-Diethyl-substituted derivatives showed high affinity for A1 ARs, e.g., 15d (PSB-18339, 8-m-bromobenzyl-substituted) displayed a Ki value of 13.6 nM combined with high selectivity. 1-Ethyl-3-propargyl-substituted derivatives exhibited increased A2A AR affinity. The 8-phenethyl derivative 20h was selective for the A2A AR (Ki 149 nM), while the corresponding 8-benzyl-substituted compound 20e (PSB-1869) blocked A1 and A2A ARs with equal potency (Ki A1, 180 nM; A2A, 282 nM). The 1-ethyl-3-methyl-substituted derivative 16a (PSB-18405) bearing a m,p-dichlorobenzyl residue at N8 blocked all three targets, A1 ARs (Ki 396 nM), A2A ARs (Ki 1,620 nM), and MAO-B (IC50 106 nM) with high selectivity vs. the other subtypes (A2B and A3 ARs, MAO-A), and can thus be considered as a multi-target drug. Our findings were rationalized by molecular docking studies based on previously published X-ray structures of the protein targets. The new drugs have potential for the treatment of neurodegenerative diseases, in particular Parkinson's disease.

Benzopyrone represents a privilege scaffold to identify novel adenosine A1/A2A receptor antagonists

Adenosine receptor antagonists are under investigation as potential drug candidates for the treatment of certain cancers, neurological disorders, depression and potentially improve tumour immunotherapy. The benzo-γ-pyrone scaffold is well-known in medicinal chemistry with diverse pharmacological activities attributed to them, however, their therapeutic potential as adenosine receptor antagonists have not been investigated in detail. To expand on the structure–activity relationships, the present study explored the adenosine A1 and A2A receptor binding affinities of a selected series of benzo-γ-pyrone analogues. In vitro evaluation led to the identification of 5-hydroxy-2-(3-hydroxyphenyl)-4H-1-benzopyran-4-one with the best adenosine A2A receptor affinity among the test compounds and was found to be non-selective (A1Ki = 0.956 µM; A2AKi = 1.44 µM). Hydroxy substitution on ring A and/or B play a key role in modulating the binding affinity at adenosine A1 and A2A receptors. Adenosine A1 receptor affinity was increased to the nanomolar range with hydroxy substitution on C6 (ring A), while meta-hydroxy substitution on ring B governed adenosine A2A receptor affinity. The double bond between C2 and C3 of ring C as well as C2 phenyl substitution was shown to be imperative for both adenosine A1 and A2A receptor affinity. Selected benzo-γ-pyrone derivatives behaved as adenosine A1 receptor antagonists in the performed GTP shift assays. It may be concluded that benzo-γ-pyrone based derivatives are suitable leads for designing and identifying adenosine receptor antagonists as treatment of various disorders.

Increased free locomotor activity in rats after intraperitoneal injection of 1-Methylxanthine versus caffeine: a comparative study

The A2A antagonist receptors are potential molecules for pharmacological Parkinson disease therapy. The 1-Methylxanthine is the principal product in caffeine biotransformation and has affinity for A2A adenosine receptors, this effect is related with the xanthinic core in molecular structure, with high affinity for these receptors.

Evaluation of 2-benzylidene-1-tetralone derivatives as antagonists of A1 and A2A adenosine receptors.

Antagonists of the adenosine receptors (A1 and A2A ) are thought to be beneficial in neurological disorders, such as Alzheimer's and Parkinson's disease. The aim of this study was to explore 2-benzylidene-1-tetralone derivatives as antagonists of A1 and/or A2A adenosine receptors. In general, the test compounds were found to be selective for the A1 adenosine receptor, with only three test compounds possessing affinity for both the A1 and A2A adenosine receptor. The 2-benzylidene-1-tetralones bearing a hydroxyl substituent at either position C5, C6 or C7 of ring A displayed favourable adenosine A1 receptor binding, while C5 hydroxy substitution led to favourable A2A adenosine receptor affinity. Interestingly, para-hydroxy substitution on ring B in combination with ring A bearing a hydroxy at position C6 or C7 provided the 2-benzylidene-1-tetralones with both A1 and A2A adenosine receptor affinity. Compounds 4 and 8 displayed the highest A1 and A2A adenosine receptor affinity with values below 7μm. Both these compounds behaved as A1 adenosine receptor antagonists in the performed GTP shift assays. In conclusion, the 2-benzylidene-1-tetralone derivatives can be considered as lead compounds to design a new class of dual acting adenosine A1 /A2A receptor antagonists that may have potential in treating both dementia and locomotor deficits in Parkinson's disease.

Die adenosien A<sub>1</sub>- en A<sub>2A</sub>-reseptoraffiniteit van ’n reeks 3,4-dihidropirimidoon-analoë

Parkinson’s disease is a complex neurodegenerative condition with current treatment only focussed on symptomatic therapy that does not slow or stop the progression of the disease. Since the discovery that adenosine A1 and A2A receptors are potential drug targets for the therapy of Parkinson’s disease, various research groups have attempted to identify adenosine antagonists. So the possibility exists that the administration of an adenosine A2A receptor antagonist may prevent further neurodegeneration. Furthermore, the antagonism of adenosine A1 receptors has the potential of treating Parkinson’s disease-associated cognitive deficits. Therefore, dual antagonism of adenosine A1 and A2A receptors would be of great benefit since this would potentially treat both the motor as well as the cognitive impairment associated with Parkinson’s disease. Based on the observation that a series of 1,4-dihydropyridine derivatives possess adenosine A1 and A2A receptor affinity, the current study investigated the potential of the structurally related 3,4-dihydropyrimidone analogues as adenosine A1 and A2A receptor antagonists. Overall, the 3,4-dihyropyrimidone analogues were found to possess weak affinity for the adenosine A2A receptor, but more promising adenosine A1 receptor affinity was found, ranging in the low micromolar range. Among the investigated compounds, the p-bromophenyl substituted dihydropyrimidone (6b) possesses the best adenosine A1 receptor affinity with a Ki value of 7.39 µM. In conclusion, this 3,4-dihydropyrimidone derivative can be used as a lead for the design of novel adenosine A1 receptor antagonists, although further structural modifications are required to enhance the adenosine A2A receptor affinity before a clinically viable candidate will be available as potential treatment of Parkinson’s disease.

Discovery of 1,3-diethyl-7-methyl-8-(phenoxymethyl)-xanthine derivatives as novel adenosine A1 and A2A receptor antagonists

Based on a previous report that a series of 8-(phenoxymethyl)-xanthines may be promising leads for the design of A1 adenosine receptor antagonists, selected novel and known 1,3-diethyl-7-methyl-8-(phenoxymethyl)-xanthine and 1,3,7-trimethyl-8-(phenoxymethyl)-xanthine analogs were synthesized and evaluated for their A1 and A2A adenosine receptor affinity. Generally, the study compounds exhibited affinity for both the A1 and A2A adenosine receptors. Replacement of the 1,3-dimethyl-substition with a 1,3-diethyl-substition pattern increased A1 and A2A binding affinity. Overall it was found that para-substitution on the phenoxymethyl side-chain of the 1,3-diethyl-xanthines decreased A1 affinity except for the 4-Br analog (4f) exhibiting the best A1 affinity in the submicromolar range. On the other hand A2A affinity for the 1,3-diethyl-xanthines were increased with para-substitution and the 4-OCH3 (4b) analog showed the best A2A affinity with a Ki value of 237nM. The 1,3-diethyl-substituted analogs (4a, and 4f) behaved as A1 adenosine receptor antagonists in GTP shift assays performed with rat whole brain membranes expressing A1 adenosine receptors. This study concludes that para-substituted 1,3-diethyl-7-methyl-8-(phenoxymethyl)-xanthine analogs represent novel A1 and A2A adenosine receptor antagonists that are appropriate for the design of therapies for neurodegenerative disorders such as Parkinson’s and Alzheimer’s disease.

Selected C8 two-chain linkers enhance the adenosine A1/A2A receptor affinity and selectivity of caffeine

Recent research exploring C8 substitution on the caffeine core identified 8-(2-phenylethyl)-1,3,7-trimethylxanthine as a non-selective adenosine receptor antagonist. To elaborate further, we included various C8 two-chain-length linkers to enhance adenosine receptor affinity. The results indicated that the unsubstituted benzyloxy linker (1e A1Ki = 1.52 μM) displayed the highest affinity for the A1 adenosine receptor and the para-chloro-substituted phenoxymethyl (1d A2AKi = 1.33 μM) linker the best A2A adenosine receptor affinity. The position of the oxygen revealed that the phenoxymethyl linker favoured A1 adenosine receptor selectivity over the benzyloxy linker and, by introducing a para-chloro substituent, A2A adenosine receptor selectivity was obtained. Selected compounds (1c, 1e) behaved as A1 adenosine receptor antagonists in GTP shift assays and therefore represent selective and non-selective A1 and A2A adenosine receptor antagonists that may have potential for treating neurological disorders.

Evaluation of antidepressant-like and anxiolytic-like activity of purinedione-derivatives with affinity for adenosine A2A receptors in mice.

It has recently been suggested that the adenosine A2A receptor plays a role in several animal models of depression. Additionally, A2A antagonists have reversed behavioral deficits and exhibited a profile similar to classical antidepressants. In the present study, imidazo- and pyrimido[2,1-f]purinedione derivatives (KD 66, KD 167, KD 206) with affinity to A2A receptors but poor A1 affinity were evaluated for their antidepressant- and anxiolytic-like activity. The activity of these derivatives was tested using a tail suspension and forced swim test, two widely-used behavioral paradigms for the evaluation of antidepressant-like activity. In turn, the anxiolytic activity was evaluated using the four-plate test. The results showed the antidepressant-like activity of pyrimido- and imidazopurinedione derivatives (i.e. KD 66, KD 167 and KD 206) in acute and chronic behavioral tests in mice. KD 66 revealed an anxiolytic-like effect, while KD 167 increased anxiety behaviors. KD 206 had no effect on anxiety. Furthermore, none of the tested compounds increased locomotor activity. Available data support the proposition that the examined compounds with adenosine A2A receptor affinity may be an interesting target for the development of antidepressant and/or anxiolytic agents.

The 1-metylxantine affinity for A2A adenosine receptors is similar to caffeine, theobromine, theophylline and adenosine. The importance of xanthine core.

The 1-metylxantine affinity for A2A adenosine receptors is similar to caffeine, theobromine, theophylline and adenosine. The importance of xanthine core.

Insight into the binding mode and the structural features of the pyrimidine derivatives as human A2A adenosine receptor antagonists

The interaction of 278 monocyclic and bicyclic pyrimidine derivatives with human A2A adenosine receptor (AR) was investigated by employing molecular dynamics, thermodynamic analysis and three-dimensional quantitative structure–activity relationship (3D-QSAR) approaches. The binding analysis reveals that the pyrimidine derivatives are anchored in TM2, 3, 5, 6 and 7 of A2A AR by the aromatic stacking and hydrogen bonding interactions. The key residues involving Phe168, Glu169, and Asn253 stabilize the monocyclic and bicyclic cores of inhibitors. The thermodynamic analysis by molecular mechanics/Poisson Boltzmann surface area (MM-PBSA) approach also confirms the reasonableness of the binding modes. In addition, the ligand-/receptor-based comparative molecular similarity indices analysis (CoMSIA) models of high statistical significance were generated and the resulting contour maps correlate well with the structural features of the antagonists essential for high A2A AR affinity. A minor/bulky group with negative charge at C2/C6 of pyrimidine ring respectively enhances the activity for all these pyrimidine derivatives. Particularly, the higher electron density of the ring in the bicyclic derivatives, the more potent the antagonists. The obatined results might be helpful in rational design of novel candidate of A2A adenosine receptor antagonist for treatment of Parkinson's disease.

ChemInform Abstract: Synthesis and Biological Activity of Tricyclic Cycloalkylimidazo‐, Pyrimido‐ and Diazepinopurinediones.

AbstractMost of the new compounds exhibit adenosine A2a receptor affinity, with compounds (IIIb), (VIa) and (VIII) to be the most potent ones.

Predictive 3D‐Quantitative Structure‐Activity Relationship for A1 and A2A Adenosine Receptor Ligands

AbstractThe use of QSAR applications to develop adenosine receptor (AR) antagonists is not so common. A library of all xanthine derivatives, obtained at the Department of Technology and Biotechnology of Drugs, was created. Sixty‐three active adenosine A1 receptor ligands and one hundred thirty nine active adenosine A2A receptor ligands were used for 3D‐QSAR investigation. The 3D‐QSAR equations with a high predictive power in estimating the binding affinity values of potential A1 and A2A ARs ligands were derived. For the first time, hybrid shape‐property descriptors were used in 3D‐QSAR for xanthine ARs ligands. The obtained models were characterized by a high regression and cross‐validation coefficients. Two types of the model validation were tested – dividing the library into the training set for model development and external set for model validation and increasing the number of library components and checking the model by cross‐validated regression coefficient. The analysis of the results depicts that for the A1 AR binding activity it is important for ligands to possess R1‐propyl substituents along with the phenyl or benzyl substituents bearing halogen atom and phenethyl moiety. For A2A AR affinity it could be favorable to introduce phenethyl or phenyl substituent connected with the tricyclic ring by the alkoxy chain. The nature of R1 group may not significantly affect the A2A AR affinity. High predictive power of the equations suggests their use for further development of adenosine receptor antagonists within xanthine derivatives.

The first second-generation adenosine drug enters the US market

On 10 April 2008, CV Therapeutics, Inc. of Palo Alto, California, USA, received approval from the US Food and Drug Administration (FDA) for the use of Lexiscan® (regadenoson; CVT-3146), an A2A adenosine receptor (A2AAdoR) agonist, as a pharmacological stress agent in conjunction with radionuclide myocardial perfusion imaging (MPI) in patients unable to undergo adequate exercise stress. Lexiscan® has a higher affinity for A2A adenosine receptors than adenosine and is a more potent coronary vasodilator. It preferentially dilates coronary arteries relative to other vascular beds [1], probably due to the high density of A2AAdoR in the coronary arteries and high receptor reserve at that site [2], resulting in maximal vasodilation induced by the activation of a small percentage of these receptors. MPI is commonly used in the clinical setting for the diagnosis of coronary artery stenosis. The rationale for the use of pharmacologic vasodilators in MPI is based on the differential blood flow response in normal vs. diseased coronary arteries, resulting in a coronary-steal phenomenon. The production of myocardial perfusion defects is reflected in nonhomogeneous distribution of a radiopharmaceutical, which is administered subsequent to the vasodilator agent, detected by a gamma camera, and quantified by a computer utilizing specific software. Adenosine and dipyridamole (an adenosine transport blocker that elevates extracellular adenosine levels) have previously been used as pharmacologic stressors; however, the use of both is associated with adverse effects caused by the activation of A1AdoR in addition to the A2AAdoR. For example, first-degree atrioventricular (AV) block occurs in approximately 10% and second- or third-degree AV block in approximately 4% of patients due to the inhibitory effect of adenosine on the AV nodal conduction [3]. Lexiscan® (Fig. 1), given as a bolus injection, increases coronary blood flow velocity by more than 2.5-fold above baseline for at least 2 min [4], which is ample time for the administration and distribution of a radiopharmaceutical. Fig. 1 Lexiscan® (CVT-3146): Adenosine, 2-[4-[(methylamino)carbonyl]-1H-pyrazol-1-yl]-, monohydrate Experience with regadenoson (Lexiscan®) has shown that it is well-tolerated, as effective as adenosine as a pharmacologic stressor in MPI, and associated with less side effects than adenosine [5]. Lexiscan® is approved 19 and 13 years after the approval of Adenocard® (intravenous adenosine) in 1989 for conversion of paroxysmal supraventricular tachycardia (PSVT) to sinus rhythm and Adenoscan® in 1995 as a pharmacological stress agent in MPI, respectively. The approval of Lexiscan® represents an important milestone in the adenosine research field. It is the first synthetic adenosine receptor ligand approved for medical use. The approval should serve as an impetus for the development of others adenosine and purine nucleotide analogs as novel diagnostic and therapeutic agents.

Tricyclic oxazolo[2,3-f]purinediones: potency as adenosine receptor ligands and anticonvulsants

Synthesis and physicochemical properties of 7-mono- and 6,7-disubstituted dihydrooxazolo-[3,2-f]purinediones are described. Oxazolo[2,3-f]purinediones were synthesized by cyclization of 8-bromotheophylline with oxiranes. The obtained compounds (1–22) were evaluated for their affinity at adenosine A1 and A2A receptors. They showed mainly adenosine A2A receptor affinity at low micromolar concentrations and A2A selectivity, for example, compound 9 with an octyl substituent at the oxazole ring displayed adenosine A2A receptor affinity (Ki=0.998μM) and at least 25-fold A2A versus A1 selectivity. This compound was less selective (5-fold) towards human recombinant A2B and A3 adenosine receptors. In this group of compounds active adenosine A1 receptor antagonists were also identified. Oxazolopurinediones were evaluated in vivo as anticonvulsants in MES and ScMet tests and examined for neurotoxicity in mice (ip). Compounds with long alkyl chains showed anticonvulsant activity in both tests (in 100 and 300mg/kg doses), accompanied by significant neurotoxicity. The anticonvulsant activity in rats (po) was higher and without signs of neurotoxicity. SAR and QSAR studies stressed the importance of lipophilic 7-substituents for both types of pharmacological activity. The volume of the substituent is, however, limited at the A2A AR, an n-octyl group being optimal.

Species differences in brain adenosine A1 receptor pharmacology revealed by use of xanthine and pyrazolopyridine based antagonists.

1. The pharmacological profile of adenosine A1 receptors in human, guinea-pig, rat and mouse brain membranes was characterized in a radioligand binding assay by use of the receptor selective antagonist, [3H]-8-cyclopentyl-1,3-dipropylxanthine ([3H]-DPCPX). 2. The affinity of [3H]-DPCPX binding sites in rat cortical and hippocampal membranes was similar. Binding site affinity was higher in rat cortical membranes than in membranes prepared from guinea-pig cortex and hippocampus, mouse cortex and human cortex. pKD values (M) were 9.55, 9.44, 8.85, 8.94, 8.67, 9.39 and 8.67, respectively. The binding site density (Bmax) was lower in rat cortical membranes than in guinea-pig or human cortical membranes. 3. The rank order of potency of seven adenosine receptor agonists was identical in each species. With the exception of 5'-N-ethylcarboxamidoadenosine (NECA), agonist affinity was 3.5-26.2 fold higher in rat cortical membranes than in human and guinea-pig brain membranes; affinity in rat and mouse brain membranes was similar. While NECA exhibited 9.3 fold higher affinity in rat compared to human cortical membranes, affinity in other species was comparable. The stable GTP analogue, Gpp(NH)p (100 microM) reduced 2-chloro-N6-cyclopentyladenosine (CCPA) affinity 7-13.9 fold, whereas the affinity of DPCPX was unaffected. 4. The affinity of six xanthine-based adenosine receptor antagonists was 2.2-15.9 fold higher in rat cortical membranes compared with human or guinea-pig membranes. The rank order of potency was species-independent. In contrast, three pyrazolopyridine derivatives, (R)-1-[(E)-3-(2-phenylpyrazolo[1,5-a]pyridin-3-yl) acryloyl]-2-piperidine ethanol (FK453), (R)-1-[(E)-3-(2-phenylpyrazolo[1,5-a]pyridin-3-yl) acryloyl]-piperidin-2-yl acetic acid (FK352) and 6-oxo-3-(2-phenylpyrazolo[1,5-a]pyridin-3-yl)-1(6H)-pyridazinebutyric acid (FK838) exhibited similar affinity in human, guinea-pig, rat and mouse brain membranes. pKi values (M) for [3H]-DPCPX binding sites in human cortical membranes were 9.31, 7.52 and 7.92, respectively. 5. Drug affinity for adenosine A2A receptors was determined in a [3H]-2-[4-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamido ade nosine ([3H]-CGS 21680) binding assay in rat striatal membranes. The pyrazolopyridine derivatives, FK453, FK838 and FK352 exhibited pKi values (M) of 5.90, 5.92 and 4.31, respectively, compared with pKi values of 9.31, 8.18 and 7.57 determined in the [3H]-DPCPX binding assay in rat cortical membranes. These novel pyrazolopyridine derivatives therefore represent high affinity, adenosine A1 receptor selective drugs that, in contrast to xanthine based antagonists, exhibit similar affinity for [3H]-DPCPX binding sites in human, rat, mouse and guinea-pig brain membranes.

Pharmacokinetic modelling of the haemodynamic effects of the A2a adenosine receptor agonist CGS 21680C in conscious normotensive rats.

1. The aim of the present investigation was to determine the relationship between the blood concentration and haemodynamic effects of the adenosine A2a receptor agonist, CGS 21680C (the sodium salt of 2-p-(2-carboxyethyl)phenylethylamino-5'-N-ethylcarboxamidoadeno sin e) in conscious normotensive rats. 2. Chronically cannulated rats were randomly assigned to three groups which received 300, 1000 or 3000 micrograms kg-1 (0.56, 1.9 or 5.6 mumol kg-1) of CGS 21680C intravenously over 15 min. The mean arterial blood pressure (MAP) and heart rate (HR) were monitored continuously during the experiment and serial arterial blood samples were taken for analysis of drug concentration. 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. For each individual rat the reduction in mean arterial pressure was related to the blood concentration according to the sigmoidal Emax model. The concentration-effect relationships were consistent for the different treatment groups. The potency based on free drug concentrations (EC50,u) was 5.8 ng ml-1 (11 nM) (mean +/- s.e.; n = 19) and correlated well with the reported adenosine A2a receptor affinity (Ki 19 nM). In comparison with the reduction in blood pressure, CGS 21680C exhibited a greater potency for the reduction of the ratio MAP/HR. 4. It is concluded that estimates can be obtained for the potency and intrinsic activity of adenosine A2a receptor agonists in vivo by pharmacokinetic-pharmacodynamic analysis of mean arterial pressure data in a rat model. In future studies, total peripheral resistance may also be useful as a pharmacodynamic parameter for A24 activation, provided that possible changes of the stroke volume are also assessed.