Human Adenosine Receptor Subtypes Research Articles

The Small Antitumoral Immune Response Modifier Imiquimod Interacts with Adenosine Receptor Signaling in a TLR7- and TLR8-Independent Fashion

Imiquimod, a small-molecule immune response modifier of the imidazoquinoline family, has shown profound antitumoral and antiviral efficacy both in vitro and in clinical applications in vivo. It has been demonstrated that this activity is mediated through the Toll-like receptor (TLR)7- and TLR8-signaling cascade resulting in the secretion of proinflammatory cytokines and, consecutively, induction of a tumor-directed cellular immune response. In addition, imiquimod exerts a direct proapoptotic activity in tumor cells. We demonstrate here that imiquimod induces activation of the transcription factor NF-kappaB and the downstream production of proinflammatory cytokines in the absence of TLR7 and TLR8. In Chinese hamster ovary cells stably transfected with the human adenosine receptor subtypes, we then show in radioligand-binding competition experiments that imiquimod binds to adenosine receptors at concentrations relevant in clinical settings, with highest affinities to the A(1) and A(2A) subtypes. The effect on the receptor-mediated activation of adenylyl cyclase was also studied, and these experiments revealed that imiquimod acts as an adenosine receptor antagonist. In addition, imiquimod had an inhibitory effect on adenylyl cyclase activity downstream from the receptor. Finally, using transformed human keratinocytes, we provide experimental evidence that imiquimod and A(2A) adenosine receptor-specific compounds similarly induce proinflammatory cytokines in the absence of immune cells. Thus, imiquimod appears to suppress an important feedback mechanism of inflammation by antagonism of adenosine receptor-dependent increase of cAMP and a concomitant receptor-independent inhibition of cAMP production. These novel mechanisms presumably act synergistic with the positive induction of proinflammatory cytokines and can, at least in part, explain the profound inflammation observed in some patients in vivo.

Synthesis, Biological and Modeling Studies of 1,3‐Di‐n‐propyl‐2,4‐dioxo‐6‐methyl‐8‐(substituted) 1,2,3,4‐Tetrahydro[1,2,4]‐triazolo[3,4‐f]‐purines as Adenosine Receptor Antagonists.

AbstractFor Abstract see ChemInform Abstract in Full Text.

New 2-Arylpyrazolo[4,3-c]quinoline Derivatives as Potent and Selective Human A3 Adenosine Receptor Antagonists

In this paper we report the synthesis and biological evaluation of a new class of 2-phenyl-2,5-dihydro-pyrazolo[4,3-c]quinolin-4-ones as A(3) adenosine receptor antagonists. We designed a new route based on the Kira-Vilsmeier reaction for the synthesis of this class of compounds. Some of the synthesized compounds showed A(3) adenosine receptor affinity in the nanomolar range and good selectivity as evaluated in radioligand binding assays at human (h) A(1), A(2A), A(2B), and A(3) adenosine receptor subtypes. We introduced several substituents on the 2-phenyl ring. In particular substitution at the 4-position by methyl, methoxy, and chlorine gave optimal activity and selectivity 6c (K(i)hA(1), A(2A)>1000 nM, EC(50)hA(2B)>1000 nM, K(i)hA(3) = 9 nM), 6d (K(i)hA(1), A(2A)>1000 nM, EC(50)hA(2B)>1000 nM, K(i)hA(3) = 16 nM), 6b (K(i)hA(1), A(2A) >1000 nM, EC(50)hA(2B)>1000 nM, K(i)hA(3) = 19 nM). In conclusion, the 2-phenyl-2,5-dihydro-pyrazolo[4,3-c]quinolin-4-one derivatives described herein represent a new family of in vitro selective antagonists for the adenosine A(3) receptor.

Synthesis, biological and modeling studies of 1,3-di- n-propyl-2,4-dioxo-6-methyl-8-(substituted) 1,2,3,4-tetrahydro [1,2,4]-triazolo [3,4-f]-purines as adenosine receptor antagonists

A new series of potential adenosine receptor antagonists with a [1,2,4]-triazolo-[3,4-f]-purine structure bearing at the 1 and 3 position n-propyl groups have been synthesized, and their affinities at the four human adenosine receptor subtypes (A 1, A 2A, A 2B and A 3) have been evaluated. In this case the presence of n-propyl groups seems to induce potency at the A 2A and A 3 adenosine receptor subtypes as opposed to our previously reported series bearing methyl substituents at the 1 and 3 positions. In particular the non-acylated derivative 17 showed affinity at these two receptor subtypes in the micromolar range. Indeed, preliminary molecular modeling investigations according to the experimental binding data indicate a modest steric and electrostatic antagonist-receptor complementarity.

Synthesis and biological evaluation of novel N6-[4-(substituted)sulfonamidophenylcarbamoyl]adenosine-5'-uronamides as A3 adenosine receptor agonists.

A new series of 1-deoxy-1-[(6-(4-(substituted-aminosulfonyl)phenyl)amino)carbonylamino-9H-purin-9-yl]-N-ethyl-beta-D-ribofuranuronamides (83-102) have been synthesized and tested at the human A3 adenosine receptor subtype. All the derivatives described in this work displayed affinity versus this receptor in the nanomolar range and good selectivity versus A1 adenosine receptor subtype, confirming that the p-sulfonamido moiety positively affected the activity of the molecules. The best substituents at the sulfonamido nucleus were found to be small alkyl groups, like methyl, isopropyl, ethyl, or allyl moieties (compounds 96-100), whereas monosubstitution at the amino group led to a decrease in A3 affinity values. The selectivity versus A1 adenosine receptor subtype is increased when the amino group in the sulfonamido core is represented by a hydrogenated heterocyclic ring like piperidine, morpholine, or pyrroline. Bulky groups, like adamantane and alkyl chains with more than four carbon atoms, are detrimental for the affinity and the selectivity of the A3 adenosine receptor agonists described here.

3H]OSIP339391, a selective, novel, and high affinity antagonist radioligand for adenosine A2B receptors.

Until recently, the characterization of adenosine A(2B) receptors has been hampered by the lack of high affinity radioligands. This study describes the synthesis and in vitro characterization of the radiolabeled derivative of OSIP339391, a novel, potent, and selective pyrrolopyrimidine A(2B) antagonist. OSIP339391 had a selectivity of greater than 70-fold for A(2B) receptors over other human adenosine receptor subtypes. The radiolabel was introduced by hydrogenation of the acetylenic precursor with tritium gas resulting in the incorporation (on average) of three tritium atoms in the molecule, yielding a ligand with specific activity of 87Ci/mmol (3.2TBq/mmol). Using membranes from HEK-293 cells expressing the human recombinant A(2B) receptor, [3H]OSIP339391 was characterized in kinetic, saturation, and competition binding experiments. From the association and dissociation rate studies, the affinity was 0.41nM and in close agreement with that found in saturation binding experiments (0.17nM). In competition, binding studies using 0.5nM [3H]OSIP339391, the affinity of a range of agonists and antagonists was consistent with previously reported data. Thus, [3H]OSIP339391 is a novel, selective, and high affinity radioligand that can be a useful tool in the further exploration and characterization of recombinant and endogenous adenosine A(2B) receptors.

Adenosine receptor agonists: synthesis and binding affinity of 2- (aryl)alkylthioadenosine derivatives

The synthesis of a new series of 2-(aryl)alkylthio derivatives of N-ethylcarboxamido adenosine (NECA) is described, in comparison with the corresponding derivatives of adenosine. Binding studies (A1, A2A, and A3) and adenylyl cyclase activity (A2B) at human adenosine receptor subtypes stably transfected in Chinese hamster ovary (CHO) cells showed that the 2- (aryl)alkylthioadenosine derivatives are more potent than the corresponding NECA derivatives at A1 receptors, while the NECA derivatives possess highest affinity at both A2A and A3 receptors. Thus, the 2-(1-pentyl)thioadenosine (7) with a Ki A1 = 91 nM, the 2-phenylethylthioNECA (18) with a Ki A2A = 24 nM, and the 2-phenylmethylthioadenosine (11) with a Ki A3 = 68 nM, could be useful tools to be modificated in order to find very potent and selective agonists for the human adenosine receptor subtypes.

Pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine derivatives as adenosine receptor antagonists. Influence of the N5 substituent on the affinity at the human A 3 and A 2B adenosine receptor subtypes: a molecular modeling investigation.

A new series of pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidines bearing various substituents at both the N5-pyrimidinyl and N8-pyrazolyl positions have been synthesized, and their binding affinities at the four human adenosine receptor subtypes (hA(1), hA(2A), hA(2B), and hA(3)) have been evaluated. All the described compounds contain arylacetyl moieties at the N5 position and arylalkyl substituents at the N8 position. Surprisingly, all the compounds present their most potent affinities at the hA(2B) adenosine receptor with a range of selectivities against the other subtypes. When bulky groups are present simultaneously at the N5 and N8 positions (e.g., compound 9), the best selectivity for the hA(2B) receptor was observed (K(i)(hA(1)) = 1100 nM; K(i)(hA(2A)) = 800 nM; K(i)(hA(2B)) = 20 nM; K(i)(hA(3)) = 300 nM, K(i)(hA(1)/A(2B)) = 55, K(i)(hA(2A)/A(2B)) = 40, K(i)(hA(3)/hA(2B)) = 15). To understand the molecular significance of these results, we compared the putative TM (transmembrane) binding motif of compound 9 on both hA(2B) and hA(3) receptors. From our docking studies, compound 9 fits neatly inside the TM region of the hA(2B) receptor but not in the corresponding hA(3) region, illustrating significant differences between the two subtypes. The study herein presented permits an understanding of why the bioisosteric replacement of an -NH, present in previously reported hA(3) receptor antagonists, with a -CH(2) group at the N5 position induces such large differences in hA(2B)/hA(3) affinity. In the molecular structure of the hA(3) receptor, two residues, Ser243 (TM6) and Ser271 (TM7), create a hydrophilic region, which seems to permit a better accommodation of the phenylurea series into this putative hA(3) binding site than the phenylacetyl series.

New strategies for the synthesis of A 3 adenosine receptor antagonists

New A 3 adenosine receptor antagonists were synthesized and tested at human adenosine receptor subtypes. An advanced synthetic strategy permitted us to obtain a large amount of the key intermediate 5 that was then submitted to alkylation procedures in order to obtain the derivatives 6– 8. These compounds were then functionalised into ureas at the 5-position (compounds 9– 11, 18 and 19) to evaluate their affinity and selectivity versus hA 3 adenosine receptor subtype; in particular, compounds 18 and 19 displayed a value of affinity of 4.9 and 1.3 nM, respectively. Starting from 5, the synthetic methodologies employed permitted us to perform a rapid and a convenient divergent synthesis. A further improvement allowed the regioselective preparation of the N 8-substituted compound 7. This method could be used as an helpful general procedure for the design of novel A 3 adenosine receptor antagonists without the difficulty of separating the N 8-substituted pyrazolo[4,3- e]1,2,4-triazolo[1,5- c]pyrimidines from the corresponding N 7-isomers.

Medicinal chemistry of adenosine A2A receptor agonists.

The search for potent and selective A(2A) adenosine receptor agonists has been particularly fruitful in the early nineties. A series of 2-amino, 2-alkoxy, 2-alkythio-, 2-alkynyl-, and 2-alkenyl-derivatives of adenosine (Ado, 1) and N-ethylcarboxamidoadenosine (NECA, 30) have been synthesized and tested mainly on different model of rat A(1) and A(2A) receptor subtypes. From these studies some ligands, such as CGS 21680 (33), HENECA (42), and (S)-PHPNECA (46b), showed to possess high A(2A) affinity combined with good A(2A) vs A(1) selectivity. More detailed characterization of these ligands at the four cloned human adenosine receptor subtypes revealed that none of the prototypical adenosine receptor agonists exhibits at the same time high affinity and selectivity for the human A(2A)AR subtype. Both NECA and CGS 21680, which are avalaible as radioligands for this subtype, have lower affinity at human than at rat receptor. The 2-alkynylNECA derivatives HENECA an PHPNECA showed high affinity also at human A(3) receptors. In particular, (S)-PHPNECA displayed K(i)s in the low nanomolar range at A(1), A(2A), and A(3)subtypes and an EC(50) of 220 nM at human A(2B) receptor. On the other hand, it is now well known that the coronary vasodilation induced by Ado in different species is mediated by activation of A(2A)AR and a compound capable of producing coronary vasodilation through activation of A(2A)AR, but that is devoid of A(1)- and A(1)-agonist activity would have advantage over Ado for use in myocardial perfusion imaging studies. Other potential therapeutic applications of selective A(2A)AR agonists are as anti-aggregatory, anti-inflammatory, anti-psychotic, and anti-Huntington's disease agents. This review is aimed at presenting a complete overview of the medicinal chemistry development of A(2A) adenosine receptor agonists and at stressing the strong need for more selective ligands at A(2A) human subtype.

Purine nucleosides bearing 1-alkynyl chains as adenosine receptor agonists.

The synthesis and the pharmacological activity of alkynyl derivatives of adenosine (Ado) and N-ethylcarboxamidoadenosine (NECA), that have been tested on adenosine receptors from different sources, have been reviewed. Most of compounds have been characterized in the last ten years by using radioligand binding assays on rat brain membranes and functional studies on different animal models. More recently, the four human adenosine receptor subtypes have been stably transfected into Chinese hamster ovary (CHO) cells allowing for comparative studies in a similar cellular background, utilizing radioligand binding studies (A(1), A(2A), A(3)) or adenylate cyclase activity assays (A(2B)). From the whole pattern of studies the following structure-activity relationships have been drown: The activities of 2-alkynylAdos resulted slightly higher at A(1) and lower at A(3) and A(2B) subtypes than the corresponding NECA derivatives, whereas the affinities at A(2A) subtype are similar for the two series of nucleosides. The presence of a methyl group on N(6) of the 2-alkynyladenosines, inducing a contemporary increase in affinity at the human A(3) receptor and a decrease at the other subtypes, resulted in a relevant increase in A(3) selectivity. In particular, 2-phenylethynyl-N(6)-methylAdo showed an A(3) affinity in the low nanomolar range (K(i) A(3) = 3.4 nM), and about 500 fold A(1)/A(3) and about 2500 fold A(2A)/A(3) selectivity. The presence of a hydroxyl group in some alkynyl side chains led to potent inhibitors of platelet aggegation induced by ADP. -Introduction of particular substituents, such as the racemic 2-phenylhydroxypropynyl group, both in adenosine and in NECA analogues, led to highly potent, non selective agonists at all the four subtypes. For the potency at A(2B) receptor it seems to be very important the type of alkynyl chain in 2-position and the presence of the carboxyamido group on the sugar; in fact, the (S)-2-phenylhydroxypropynylNECA [(S)-PHPNECA, EC(50) A(2B) = 220 nM] proved to be one of the most potent A(2B) agonist reported so far. The introduction of alkynyl chain in 8-position of adenosine led to very selective ligands for the A(3) receptor subtype. These nucleosides behave as adenosine antagonists, since they do not stimulate basal [(35)S]GTPgammaS binding, but inhibit NECA-stimulated binding.

7-Substituted 5-amino-2-(2-furyl)pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidines as A2A adenosine receptor antagonists: a study on the importance of modifications at the side chain on the activity and solubility.

It was demonstrated in the early 1990s that adenosine exerts many physiological functions through the interaction with four different receptors, named A1, A2A, A2B, and A3. In the past few years, our group has been involved in the development of A2A antagonists, which led to the synthesis of SCH 58261 (1), the first potent and selective adenosine A2A antagonist, which has been widely used as a reference compound. In this paper, we present an extended series of pyrazolotriazolopyrimidines synthesized with the aim to investigate the influence of the substitutions on the pyrazole ring. The choice of the substituents was based on their capability to improve water solubility while retaining high affinity and selectivity at the human A2A adenosine receptor subtype. In this series, some structural characteristics that are important for activity, i.e., tricyclic structure, free amino group at 5-position, furan ring, and substituent at 7-position on the pyrazole moiety, have been maintained. We focused our attention on the nature of the phenyl ring substituent to improve water solubility. Following this strategy, we developed new compounds with good affinity and selectivity for A2A adenosine receptors, such as 8d (K(i) 0.12 nM; hA1/hA2A ratio = 1025; R(m) = 2.8), 8h (K(i) 0.22; hA1/hA2A ratio = 9818; R(m) = 3.4), 8i (K(i) 0.18 nM; hA1/hA2A ratio = 994; R(m) = 2.8), 8k (K(i) 0.13 nM; hA1/hA2A ratio = 4430; R(m) = 3.6), and 14b (K(i) 0.19 nM; hA1/hA2A ratio = 2273; R(m) = 2.7). All the new synthesized compounds have no significant interaction with either A2B or A3 receptor subtypes. This new series of compounds deeply enlightens some structural requirements to display high affinity and selectivity for the A2A adenosine receptor subtype, although our goal of identifying new compounds with increased water solubility was not completely achieved. On this basis, other strategies will be devised to improve this class of compounds with a profile that appears to be promising for treatment of neurodegenerative disorders, such as Parkinson's disease.

Pyrazolo[4,3‐e]1,2,4‐triazolo[1,5‐c]pyrimidine derivatives as adenosine receptor ligands: A starting point for searching A2B adenosine receptor antagonists

AbstractA series of novel pyrazolo[4,3‐e]1,2,4‐triazolo[1,5‐c]pyrimidine substituted at N8 pyrazole nitrogen and at the 5‐amino group was synthesized and their affinities to all four cloned human adenosine receptor subtypes were evaluated by competition binding assays using [3H]‐DPCPX (A1 and A2B), [3H]‐SCH 58261 (A2A), and [3H]‐MRE3008‐F20 (A3) as radioligands. In particular, the structural requirements necessary for adenosine A2B receptor recognition were investigated. These preliminary results confirm that the free amino group at the 5‐position confers better A2B affinity, while the effect of the chain at nitrogen pyrazole nucleus, the 8‐position, seems to be preferred with respect to the corresponding N7 regioisomers. The introduction of an aminoacyl chain at the 5‐amino group produces better selectivity for A2B receptors vs. A2A, but the compounds still remain more potent for A1 and a significant decrease (about 5‐fold) of affinity at A2B receptors was observed. Surprisingly, with these polar chains a good affinity at human A3 adenosine receptors was also detected. The most interesting compounds in binding studies proved to be potent antagonists (nM range) in functional assays, measuring the inhibition of cAMP production induced by 100 nM NECA. Drug Dev. Res. 53:225–235, 2001. © 2001 Wiley‐Liss, Inc.

Adenosine receptors and their ligands.

The regulatory actions of adenosine are mediated via four subtypes of G protein-coupled receptors distinguished as A1, A2A, A2B and A3 receptors. Their presence on basically every cell makes them an interesting target for the pharmacological intervention in many pathophysiological situations. A large number of ligands have been synthesized over the last two decades and provide agonists and antagonists that are more or less selective for the known receptor subtypes. In addition, many radioligands are available in tritiated or radioiodinated form. The comparative pharmacological characterization of all four human adenosine receptor subtypes revealed that some of the compounds thought to be selective from data in other species have unexpected potencies at human receptors. As a result, compounds that exhibit high affinity to only one subtype are an exception. Although the selection of ligands is immense, it is less than satisfying for most subtypes of adenosine receptors.

ChemInform Abstract: Synthesis of Di‐ and Trisubstituted Adenosine Derivatives (V) and (VIII) and Their Affinities at Human Adenosine Receptor Subtypes.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Synthesis of Di- And Tri-Substituted Adenosine Derivatives and Their Affinities at Human Adenosine Receptor Subtypes

The synthesis of 2-(hex-l-ynyl)adenosine derivatives substituted at the N6- and/or 5′-position was carried out on the basis that 2-(hex-l-ynyl)adenosine-5′-N-ethyluronamide (HENECA, 2) showed good affinity and different degree of selectivity for rat adenosine receptors. All new compounds were tested in radioligand binding and adenylyl cyclase assays with recently cloned human A1, A2A, A2B, and A3 adenosine receptors.

2-Substituted N-ethylcarboxamidoadenosine derivatives as high-affinity agonists at human A3 adenosine receptors.

A number of 2-substituted 5'-N-ethylcarboxamidoadenosine (NECA) derivatives was investigated for their affinity and selectivity at human A3 adenosine receptors. The compounds were tested in radioligand competition studies and modulation of adenylyl cyclase activity on membranes from CHO cell lines stably transfected with the four human adenosine receptor subtypes. In binding studies the most potent compound, 2-(3-hydroxy-3-phenyl)propyn-1-yl-NECA (PHPNECA), exhibited a subnanomolar affinity for A3 adenosine receptors with a Ki value of 0.4 nM. As opposed to the limited A3 selectivity of PHPNECA, a 100-fold selectivity compared to both A1 and A2A receptors was found for 2-(2-phenyl)ethynyl-NECA (PENECA; Ki 6 nM). The EC50 values for activation of adenylyl cyclase via A2A adenosine receptors were in good agreement with the respective Ki values from binding experiments. In contrast, IC50 values for A1 and A3 receptor-mediated inhibition of adenylyl cyclase were shifted to higher values compared to the respective affinities determined in radioligand competition studies. Similar discrepancies between binding and functional data have been observed for the inhibitory A1 adenosine receptor in previous studies. Therefore, the same A3 selectivity of PENECA compared to A1 receptors was found in binding and adenylyl cyclase inhibition whereas the selectivity compared to A2A receptors that was detected in ligand binding was obscured in the functional assay. The series of compounds presented in this study identifies 2-substitution of the purine system as a promising target for the development of A3-selective high-affinity ligands.

New substituted 9-alkylpurines as adenosine receptor ligands

In the present study an investigation of the structure–activity relationships in 9-ethylpurine derivatives, aimed at preparing A 1, A 2A, A 2B, and A 3 selective adenosine receptor antagonists, was undertaken. Our synthetic approach was to introduce various substituents (amino, alkoxy and alkynyl groups) into the 2-, 6-, or 8-positions of the purine ring. The starting compounds for each series of derivatives were respectively: 2-iodo-9-ethyladenine ( 9), obtained from 2-amino-6-chloropurine ( 5); 9-ethyl-6-iodo-9 H-purine ( 11), 8-bromo-9-ethyl-adenine ( 3) and 8-bromo-9-ethyl-6-iodo-9 H-purine ( 13), obtained from 9-ethyl-adenine ( 2). The synthesized compounds were tested in in vitro radioligand binding assays at A 1, A 2A, and A 3 human adenosine receptor subtypes. Due to the lack of a suitable radioligand the affinity of the 9-ethyladenine derivatives at A 2B adenosine receptors was determined in adenylyl cyclase experiments. In general, the series of 9-ethylpurine derivatives exhibited a similar pharmacological profile at A 1 and A 2A receptors whereas some differences were found for the A 3 and the A 2B subtypes. 8-Bromo-9-ethyladenine ( 3) showed higher affinity for all receptors in comparison to the parent compound 2, and the highest affinity in the series for the A 2A and A 2B subtypes ( K i=0.052 and 0.84 μM, respectively). Analyzing the different substituents, a phenethoxy group in 2-position ( 10a) gave the highest A 2A versus A 2B selectivity (near 400-fold), whereas a phenethylamino group in 2- and 6-position ( 10b and 12b, respectively) improved the affinity at A 2B receptors, compared to the parent compound 2. The presence of a hexynyl substituent in 8-position led to a compound with good affinity at the A 3 receptor ( 4d, K i=0.62 μM), whereas (ar)alkynyl groups are detrimental for the potency at the A 2B subtype. These differences give raise to the hope that further modifications will result in the development of currently unavailable leads with good affinity and selectivity for A 2B adenosine receptors.

Localization of the adenosine A1 receptor subtype gene (ADORA1) to chromosome 1q32.1

Adenosine, acting through its receptors, exerts effects on almost all organ systems, influencing a diversity of physiological responses, including the inhibition of neurotransmitter release, the modulation of cardiac rhythmicity and contractility, and the potentiation of IgE-dependent mediator release. Adenosine receptors belong to the G protein-coupled receptor superfamily, a class of cell-surface receptors that, when activated, couple to a heterotrimeric G protein complex to effect signal transduction. Molecular cloning and subsequent pharmacological and biochemical analyses have led to the identification of four different subtypes of adenosine receptor. The A3 receptor has been localized to chromosome 3 in the mouse by interspecific backcross analysis, suggesting a human chromosomal localization of 1p13 from known mouse-human linkage homologies. We have previously mapped the A2b adenosine receptor subtype to chromosome 17p11.2-p12 using fluorescence in situ hybridization (FISH) and PCR-based screening of somatic cell hybrid DNAs. A previous report has concluded that the Al and A2a receptor subtypes are localized on chromosome 22q11.2-q13.1 and 11q11-q13, respectively, but conflicts with that of MacCollin et al., who have mapped the A2a gene to chromosome 22. In this report, we show that the human A1 adenosine receptor subtype does not map to chromosome 22q11.2-q13.1, but is instead localized onmore » chromosome 1q32. 13 refs., 1 fig.« less