PET imaging of adenosine receptors
Adenosine is an endogenous purine nucleoside, composed of nucleobase adenine and β-D-ribose moieties. It has a central role in cell metabolism and signalling as such and as the backbone for ATP (adenosine triphosphate) and cAMP (cyclic adenosine monophosphate).
Adenosine is rapidly catabolized by adenosine deaminases to inosine, which can be further deribosylated by purine nucleoside phosphorylase to hypoxantine. Adenosine deaminase isoform 1 (ADA1) is found in most cells, including blood cells. Isoform 2 (ADA2) is found in macrophages and in the plasma. Adenosine kinase in red blood cells converts adenosine into adenine nucleotides.
Adenosine is produced both intracellularly and extracellularly. Intracellular adenosine production and release is tightly regulated, and it is not stored in vesicles. Extracellular adenosine is produced by degradation of ATP, ADP, and AMP, by ecto-phosphatases, especially during inflammation, ischemia, hypoxia, and acute tissue injury. Methotrexate (MTX) increases adenosine release by inhibition of purine metabolism.
Adenosine cannot freely pass across the cell and vesicular membranes, but it needs to be transported by nucleoside transporters, which are present in most cells, including erythrocytes. Endothelial cells have a very active transport system for adenosine, and also high rate of metabolism, converting adenosine to inosine, hypoxanthine, xanthine, uric acid, and other compounds. Dipyridamole elevates extracellular adenosine concentrations by inhibiting equilibrative nucleoside transporters (ENTs). ENTs are highly expressed in the central nervous system. Concentrative nucleoside transporters (CNTs) are Na+-dependent transporters, found in most tissue types. Interstitial adenosine concentration is normally in the range 20-200 nM, and its extracellular half-life is short, a few seconds at most.
In the brain adenosine modulates the action of neurotransmitters, and affects both neuronal and glial cell functions; both cell types can release adenosine. Adenosine modulates the release and uptake of glutamate. Adenosine increases the permeability of the blood-brain barrier. Outside the brain, adenosine has paracrine function; adenosine and adenosine agonists attenuate inflammation, and induce vasodilation and angiogenesis in the cardiovascular system. Cellular signalling occurs through adenosine receptors (ARs).
All four adenosine receptor subtypes (A1, A2A, A2B, and A3) are G protein-coupled receptors (GPCRs), either increasing or decreasing intracellular cAMP levels by affecting adenylate cyclase activity. A1 and A3 receptor subtypes can also stimulate K+ channels and inhibit voltage-dependent Ca2+ channels. Like other GPCRs, adenosine receptors have a single polypeptide chain, forming three extracellular and three intracellular loops. ARs can form oligomers, also with other receptors than just the AR subtypes, including purinoceptors (P2), dopamine, and glutamate receptors, which affects also the kinetics of PET radiopharmaceuticals.
Adenosine receptors are also referred to as purine receptors, or P1 purinoceptors. The other purinoceptors (P2) are more activated by ATP and other substrates than adenosine.
Adenosine has the highest affinity to the A1R and A2AR, intermediate affinity to A3R, and lowest affinity to A2BR. Adenosine activates A1 and A2A receptors already in nanomolar concentrations.
Selective agonists and antagonists have been developed for each of the AR subtypes. PET radiopharmaceuticals for each AR subtype have also been developed, although only radiopharmaceuticals for A1R and A2AR are routinely used in human studies. Agonists, including endogenous agonist adenosine, lead to the activation of all AR subtypes, and subsequent regulation by desensitization and trafficking (Mundell and Kelly, 2011). In addition to AR agonists and antagonists, also allosteric modulators have been introduced.
The A1R is expressed throughout in the body, with the highest densities in the brain. Receptor density is high in the striatum and thalamus, moderate in the cerebral cortex and pons, and low in the cerebellum, midbrain and brain stem. Adenosine binding to both pre- and postsynaptic A1Rs has an inhibitory effect in the brain. A1Rs are also present on microglia, astrocytes, and oligodendrocytes.
Several PET radiopharmaceuticals for A1R have been introduced, including [18F]CPFPX and [11C]MPDX.
The density of A2A receptors is high in the basal ganglia, especially in putamen. Expression in cerebral cortex, particularly in the frontal lobe, is lower, and very low in cerebellum and brainstem. A2ARs are also found on endothelial cells and microglia, playing a role in neuroinflammation and neurodegenerative diseases (Rissanen et al., 2013; Vuorimaa et al., 2017). A2AR can form complexes with dopamine D2 and mGluR5 receptors in the brain.
A2ARs are also present in peripheral organs like heart, lungs, liver, kidneys, spleen, and thymus. A2AR expression is high in platelets and leukocytes, endothelial cells, and vascular smooth muscle. Expression is strongly upregulated at sites of inflammation. Adenosine induces collagen synthesis and fibrosis via A2ARs.
A2AR is the most abundant AR in human and murine brown adipose tissue (BAT). Adenosine and A2AR agonists activate BAT and induces the browning of white adipose tissue (Gnad et al., 2014). Adenosine administration increases perfusion in BAT even more than cold exposure (Lahesmaa et al., 2018). Endogenous adenosine under cold exposure leads to desensitization and trafficking of A2ARs and competes with receptor-specific radioligands, as seen by decreased binding of [11C]TMSX (Lahesmaa et al., 2018).
Several PET radiopharmaceuticals for A2AR have been introduced (Khanapur et al., 2013; van Waarde et al., 2018), including xanthine derivative [11C]TMSX (also known as [11C]KF18446), and non-xanthine radioligands [11C]SCH442416, [11C]preladenant, [18F]MNI-444, and [18F]FLUDA.
A2B receptors are expressed widely in the body, but densities in the brain in neuronal and glial cells are low. Macrophages and lymphocytes express A2BR, suggesting its role in regulation of inflammation. A2BR in macrophages may affect insulin sensitivity in the tissues (Johnston-Cox et al., 2014). Because the affinity of A2BR for adenosine is low, the receptor may be activated only under pathological conditions, such as inflammation, where both the expression of A2BR and adenosine levels are increased.
A2BRs may be involved in adipose tissue metabolism, since A2BR expression is associated with BMI (Johnston-Cox et al., 2012), and adenosine is one of the regulators of adipogenesis (Eisenstein & Ravid, 2014).
PET radiopharmaceuticals for A2BR are being developed (Petroni et al., 2016).
A3 receptors are expressed widely in the body, but densities in the brain, heart and kidneys are low. Lung, liver, and immune cells express high levels of A3R mRNA in humans. A3R can promote both pro- and anti-inflammatory responses. Generally, inhibition of A1 and A3 receptors reduces and inhibition of A2 receptors increases oxidative stress.
In contrast to other AR subtypes, the structure of A3 receptor is highly variable among mammals, leading to different pharmacological profiles of the species homologs. Also tissue distribution and expression levels are different among species. This should be taken into account when developing PET radiopharmaceuticals in animal studies for use in humans.
Caffeine, theobromine, theophylline and paraxanthine are nonselective adenosine receptor antagonists. Caffeine is metabolised in the liver into theobromine, theophylline, and paraxanthine. The compounds stimulate CNS and enhance physical performance. Caffeine consumption is associated with reduced risk for Parkinson's disease, and may reduce risk for Alzheimer's disease. Caffeine increases basal metabolic rate, and reduces risk of non-alcoholic fatty liver disease.
Caffeine and related compounds are found in coffee, tea, and chocolate, and is the most widely used psychoactive substance in the world.
Caffeine use should be carefully controlled in adenosine receptor studies. Elmenhorst et al. (2012) observed up to 50% occupancy of A1R using FCPFPX. Due to the interactions between adenosine and dopamine systems caffeine also increases dopamine D2/D3 receptor availability (Volkow et al., 2015). Caffeine increases GFR and is a substrate for Cytochrome P450 1A2 (CYP1A2), and may thus affect the plasma clearance and metabolism of radiotracers.
Caffeine, and caffeine withdrawal, affect cerebral perfusion (Field et al., 2003).
- Quantification of A1R with [11C]MPDX
- Quantification of A2AR with [11C]TMSX
- PET imaging of P2 purinoceptors
- Binding potential
Aherne CM, Kewley EM, Eltzschig HK. The resurgence of A2B adenosine receptor signaling. Biochim Biophys Acta 2011; 1808: 1329-1339.
Bauer A, Ishiwata K. Adenosine receptor ligands and PET imaging of the CNS. Handb Exp Pharmacol. 2009; 193: 617-642.
Borea PA (ed.): A3 Adenosine Receptors - from Cell Biology to Pharmacology and Therapeutics. Springer, 2010.
Borea PA, Varani K, Gessi S, Merighi S, Vincenzi F (eds.): The Adenosine Receptors. Springer, 2018. ISBN 978-3-319-90808-3.
Borea PA, Varani K, Vincenzi F, Baraldi PG, Tabrizi MA, Merighi S, Gessi S. The A3 adenosine receptor: history and perspectives. Pharmacol Rev. 2015; 67: 74-102.
Burnstock G, Verkhratsky A: Purinergic Signalling and the Nervous System. Springer, 2012. doi: 10.1007/978-3-642-28863-0.
Chen J-F, Elzschig HK, Fredholm BB. Adenosine receptors as drug tragets - what are the challenges? Nat Rev Drug Discov. 2013; 12(4): 265-286.
Chen JF, Lee CF, Chern Y. Adenosine receptor neurobiology: overview. Int Rev Neurobiol. 2014; 119: 1-49.
Elmenhorst D, Bier D, Holschbach M, Bauer A. Imaging of adenosine receptors. In: Dierckx RAJO et al. (eds.): PET and SPECT of Neurobiological Systems, Springer, 2014, p 181-198.
Fredholm BB, IJzerman AP, Jacobson KA, Linden J, Müller CE. International union of basic and clinical pharmacology. LXXXI. Nomenclature and classification of adenosine receptors - an update. Pharmacol Rev. 2011; 63: 1-34.
Gessi S, Merighi S, Sacchetto V, Simioni C, Borea PA. Adenosine receptors and cancer. Biochim Biophys Acta 2011; 1808: 1400-1412.
Gnad T, Scheibler S, von Kügelgen I, Scheele C, Kilić A, Glöde A, Hoffmann LS, Reverte-Salisa L, Horn P, Mutlu S, El-Tayeb A, Kranz M, Deuther-Conrad W, Brust P, Lidell ME, Betz MJ, Enerbäck S, Schrader J, Yegutkin GG, Müller CE, Pfeifer A. Adenosine activates brown adipose tissue and recruits beige adipocytes via A2A receptors. Nature 2014; 516(7531): 395-399. doi: 10.1038/nature13816.
Gomes CV, Kaster MP, Tomé AR, Agostinho PM, Cunha RA. Adenosine receptors and brain diseases: neuroprotection and neurodegeneration. Biochim Biophys Acta 2011; 1808: 1380-1399.
Griffith DA, Jarvis SM. Nucleoside and nucleobase transporter systems of mammalian cells. Biochim Biophys Acta 1996; 1286(3): 153-181.
Headrick JP, Peart JN, Reichelt ME, Haseler LJ. Adenosine and its receptors in the heart: regulation, retaliation and adaptation. Biochim Biophys Acta 2011; 1808: 1413-1428.
Heinonen I, Kemppainen J, Kaskinoro K, Peltonen JE, Sipilä HT, Nuutila P, Knuuti J, Boushel R, Kalliokoski KK. Effects of adenosine, exercise, and moderate acute hypoxia on energy substrate utilization of human skeletal muscle. Am J Physiol Regul Integr Comp Physiol. 2012; 302(3): R385-R390.
Ho HTB, Wang J. (2014): The Nucleoside Transporters CNTs and ENTs. In: Drug Transporters: Molecular Characterization and Role in Drug Disposition, 2nd ed (eds You G, Morris ME), John Wiley & Sons, Inc., Hoboken, NJ. doi: 10.1002/9781118705308.ch7.
Ishiwata K, Kimura Y, de Vries EFJ, Elsinga PH. PET tracers for mapping adenosine receptors as probes for diagnosis of CNS disorders. Cent Nerv Syst Agents Med Chem. 2007; 7: 57-77.
Jacobson KA, Linden J (eds): Pharmacology of Purine and Pyrimidine Receptors. Academic Press, 2011. ISBN: 978-0-12-385526-8
Layland J, Carrick D, Lee M, Oldroyd K, Berry C. Adenosine: physiology, pharmacology, and clinical applications. JACC Cardiovasc Interv. 2014; 7(6): 581-591.
Merighi S, Borea PA, Gessi S. Adenosine receptors and diabetes: focus on the A2B adenosine receptor subtype. Pharmacol Res. 2015; 99: 229-236.
Mishina M, Ishiwata K. Adenosine receptor PET imaging in human brain. Int Rev Neurobiol. 2014; 119: 51-69.
Morelli M, Simola N, Wardas J (eds.): The Adenosinergic System - A Non-Dopaminergic Target in Parkinson's Disease. Springer, 2015.
Mori A (ed.): Adenosine Receptors in Neurology and Psychiatry. Academic Press, 2014.
Müller C, Jacobson KA. Xanthines as adenosine receptor antagonists. Handb Exp Pharmacol. 2011a; 200: 151-199.
Müller CE, Jacobson KA. Recent developments in adenosine receptor ligands and their potential as novel drugs. Biochim Biophys Acta 2011b; 1808: 1290-1308.
Mundell S, Kelly E. Adenosine receptor desensitization and trafficking. Biochim Biophys Acta 2011; 1808: 1319-1328.
Möser GH, Schrader J, Deussen A. Turnover of adenosine in plasma of human and dog blood. Am J Physiol. 1989; 256(4 Pt 1): C799-C806. doi: 10.1152/ajpcell.1989.256.4.C799.
Paul S, Elsinga PH, Ishiwata K, Dierckx RA, van Waarde A. Adenosine A1 receptors in the central nervous system: their functions in health and disease, and possible elucidation by PET imaging. Curr Med Chem. 2011; 18(31): 4820-4835.
Peleli M, Carlstrom M. Adenosine signaling in diabetes mellitus and associated cardiovascular and renal complications. Mol Aspects Med. 2017; 55: 62-74.
Ponnoth DS, Mustafa SJ. Adenosine receptors and vascular inflammation. Biochim Biophys Acta 2011; 1808: 1429-1434.
Preti D, Baraldi PG, Moorman AR, Borea PA, Varani K. History and perspectives of A2A adenosine receptor antagonists as potential therapeutic agents. Medicinal Res Rew. 2015; 35(4): 790-848.
Sun Y, Huang P. Adenosine A2B receptor: from cell biology to human diseases. Fron Chem. 2016; 4: 37.
van Waarde A, Dierckx RAJO, Zhou X, Khanapur S, Tsukada H, Ishiwata K, Luurtsema G, de Vries EFJ, Elsinga PH. Potential therapeutic applications of adenosine A2A receptor ligands and opportunities for A2A receptor imaging. Med Res Rev. 2018; 38(1): 5-56. doi: 10.1002/med.21432.
Wilson CN, Mustafa SJ (eds.): Adenosine receptors in health and disease. Springer, 2009.
Updated at: 2019-08-06
Created at: 2015-08-12
Written by: Vesa Oikonen