This document provides an overview of adenosine receptors and their therapeutic applications. It discusses the four types of adenosine receptors (A1, A2A, A2B, A3), their distribution, mechanisms of action, and effects. Several therapeutic areas are mentioned where adenosine receptors play a role, such as respiratory disease, CNS disorders, cardiovascular conditions, and inflammation. Drugs currently used in the clinic that act on adenosine receptors are also outlined, as well as some drugs in clinical trials that target specific receptor subtypes.

1. denosine
denosine receptors
Dr. Jervin Mano, MD
A

2. Overview
Introduction
Adenosine
Purinergic receptors
Adenosine receptors
Classification
Therapeutic targets
Drugs in clinic
Drugs under CT
Summary

3. Purinergic receptors
Present in most organ systems
Extracellular purines (Adenosine, ADP, ATP)
Extracellular pyrimidines (Uridine, UDP, UTP)
Mediate biological effects via P rs
 Classification :

4. Purinergic receptorsPurinergicreceptors
P1(Adenosine)
A1
A2
A2A
A2B
A3
P2 (ATP, ADP, UDP, UTP)
P2X
P2Y(ATP)
GPCR
Ligand gated ion
channel

5. Adenosine
 Metabolite of ATP
 Naturally occurring endogenous purine nucleoside
 Adenosine is made up of adenine attached to a ribose.
 Chemical name: 6-amino-9-beta-D-ribofuranosyl-9-H-purine.
 Plasma adenosine level : 0.04 - 0.2 µmol.
 Short t 1/2 : 10s
 Functions of adenosine:
 Neuromodulator, cellular energy transfer, signal of inflammation,
ischaemic preconditioning.

6. Adenosine-
Neurotransmitter??
 It is neither stored in synaptic vesicles nor it acts exclusively on
synapses
 Release & uptake: mediated by bidirectional nucleoside transporter
 Neuromodulator:
 Presynaptic: controls NT release
 Postsynaptic: hyper/depolarizes the neurons
 Non synaptic: regulates the effect on glial cells
 Characteristics of NT:
 Adenosine producing enzymes present in synapse
 Actions via interaction with Rs
 Actions can be blocked by specific antagonist
 Actions can be terminated by reuptake and metabolic syndrome

7. Metabolism

8. Adenosine receptors (P1)
GPCR
Endogenous ligand – Adenosine
Classification:
A 1
A2A
A2B
A 3

9. A1 receptors
 Distribution: Brain, SC, atria
 Superclass: GPCR – Gi
 Effector mechanism:
 Inhibition of cAMP
 Increased K+ permeability
 Inhibition of Ca2+ channels
 Activation of phospholipase C
 Agonist: N6-Cyclopentyladenosine, N(6)-cyclohexyladenosine
 Antagonist:
 Non-specific: Caffeine, Theophylline
 Specific: Bamifylline, Rolofylline

10. Effects of the receptors
CVS:
 Negative dromotropy
 Negative chronotropy
 Negative inotropy
 Inhibits pacemaker and L-type calcium currents
CNS:
 Decrease neurotransmitter release
 Sedation
 Anticonvulsant effects

11. Effects of the receptors
RENAL:
 Inhibits renin release
 Increase Na reabsorption in PCT
 Vasoconstriction of afferent arteriole-reduces GFR
Metabolic:
 Inhibits lipolysis
 Increases insulin sensitivity

12. A2A receptors
Distribution: Brain, Heart, Blood.
Superclass: GPCR – Gs
Effector mechanisms:
Activates adenylate cyclase → ↑ cAMP
Activates phospholipases C
↓Intracellular Ca levels
Agonist: Regadenoson, Limonene
Antagonist: Preladenant, Istradefylline, Vipadenant

13. Effects of the receptors
CVS:
Coronary and peripheral vasodilation
Inhibits platelet aggregation
CNS:
Modulate the neurotransmission of GABA,
Ach, glutamate → Motor activity

15. A2B receptors
 Distribution: GIT, GUT, lungs
 Superclass: GPCR – Gs
 Effector mechanisms:
 Activates adenylate cyclase → ↑ cAMP
 ↑Intracellular Ca levels
 Adenosine has low affinity
 Agonist: N-ethylcarboxamidoadenosine,
 Antagonist: MRS-1706

16. Pulmonary:
Vasodilation
Mast cell release of IL-8

18. A3 receptors
Distribution: Kidney, testis, mast cells, eosinophills
Superclass: GPCR – Gi/Gq
 Effector mechanisms:
 Inhibits adenylate cyclase → cAMP
 Activates phospholipases C&D
 Influx of Ca & release from stores
Agonist: N methyl adenosine, IB-MECA, Inosine
Antagonist: Theophylline, KF- 26777

19. Effects of the receptors
Pulmonary:
Mast cell release of allergic mediators

20. Distribution and expression of P1 receptors
Expression
level
A1 receptors A2A
receptors
A2B receptors A3 receptors
High
expression
Brain (cortex,
hippocampus,
cerebellum),
spinal cord, eye,
adrenal gland,
atria
Blood
platelets,
olfactory bulb
Spleen,
thymus,
leukocytes
Cecum, colon,
bladder
Testis (rat),
mast cells (rat)
Intermediat
e
expression
Other brain
regions, skeletal
muscles, liver,
kidney, adipose
tissue
Heart, lung,
blood vessels,
peripheral
nerves
Lung, blood
vessels, eye,
mast cells
Cerebellum,
hippocampus
Low
expression
Lungs (but
probably higher in
bronchi),
pancreas
Other brain
regions
Adipose tissue,
adrenal gland,
brain, kidney
Thyroid, most
of brain
adrenal gland,
spleen, liver,

21. Adenosine receptors
RECEP
TOR
GENE MECHANISM AGONIST ANTAGONIST
A 1 ADORA 1 G i - ↓cAMP N 6
Cyclopentoaden
osine
Caffeine,
theophylline
A 2 a ADORA 2 a G s - ↑cAMP Regadenoson Caffeine,
theophylline,
istradefylline
A 2 b ADORA 2 b G s - ↑cAMP 5 N
Ethylcarboxamid
oadenosine
Theophylline,
CVT-6883, MRS-
1706
A 3 ADORA 3 G i - ↓cAMP N methyl
adenosine,
IB-MECA
Theophylline,
MRS-1191,
1220,1334

22. Therapeutic
interventions

23. Respiratory system
Bronchial asthma:
Stable form of adenosine – AMP - broncho
constriction
Adenosine levels increased in BAL fluid and cause
hyperresponsiveness in airways
Adenosine receptor antagonist-theophylline &
selective A1 receptor antagonist- bamiphylline

24. Respiratory system
Refractory primary pulmonary hypertension:
Routine treatment
Adenosine infusion for refractory cases
Mechanism-Pulmonary vasodilation by A2
receptors
Increase AMP-vascular smooth muscle relaxation
Stimulation of K+ ATPase channel

25. CNS
Anaesthesia and intensive care:
Adenosine blocks nociceptive transmission
IV adenosine infusion-analgesia at 50-70 mg/kg/min
Reduces ischaemic pain
Adenosine infusion during GA- good recovery and
sustained post op pain relief
Superior to remifentanil

26. CNS
Epilepsy:
 Astrogliosis - hallmark of epileptogenesis
 Adenosine-inhibitory modulator of brain activity
 Activation of A1 receptors
 Adenosine kinase- major metabolic enzyme
predominantly expressed in adult brain
 ADK inhibitor GP 515 tested-reduced kindled seizures in
rats

27. CNS
Parkinsons disease:
A2a receptor antagonist-
Istradefylline in phase III trial
KF 17837
KW 6002

28. CVS
Ischaemia/reperfusion injury:
Ischaemic preconditioning(IPC)
Adenosine theory-
 After 5 min of cardiac ischaemia interstitial adenosine rises
double the time
 Adenosine receptor stimulation reduces myocardial damage
following ischaemia
Adenosine reduces ischaemia by-
 Improved tissue perfusion
 Anti inflammatory action
 Direct intracellular effect

29. CVS
Hypolipidemic agent:
Elevated TG levels-independent risk factor of CAD
RPR 749-potent and selective A1 agonist for
hypertriglyceridemia

30. Platelet
Platelets are rich in A2A receptors
Adenosine -> increases cAMP -> decreased plt
aggregation
Dipyridamole inhibits adenosine reuptake –
increased levels – decreased plt aggregation,
vasodilation

31. GIT
Inflammatory bowel disease:
 Role of adenosine in inflammation
 A2a and A3 receptors
 Reduces leucocyte infiltration and proinflammatory
cytokines
 IB-MECA - A3 agonist tested in DNBS induced colitis in
mice

32. Bacterial Sepsis:
Acadesine, adenosine precursor injected iv in
mongrel pigs following soft tissue injury and shock
72 hrs later E.coli 0111:B4 endotoxin given, effects
observed were:
Reduction in endotoxin induced hypoxia and
pulmonary hypertension
Reduction in fluid requirement
Reduction in mortality and prolonged survival time

33. Immunity
Adenosine accumulation and stimulation of (?A2)
receptors has been implicated in the
immunosuppression seen in critical illness
A3 receptor stimulation may inhibit tumour growth,
perhaps melanomas, colon or prostate carcinoma,
and lymphomas. Peripheral blood monocytes
produce G-CSF when stimulated by adenosine

34. Disease susceptibility
 Single nucleotide polymorphisms in the A1: Aspirin intolerant
asthma
 Alterations in ADORA1 gene: Change in infarct size in patients
with ischaemic infarct size
 SNPs in ADORA2A gene: Anxiety disoroders
 SNPs in ADORA2A gene: Stopping Mtx Rx for RhA due to
ADR(GIT) but not efficacy
 High expression of A2A Rs : Pts with unexplained syncope
 Up regulation of A2A Rs in endothelial cells: Lung cancer
 Dysfunctional A2B Rs : impotence

35. Drugs in clinic

36. Acupuncture
 Mechanical deformation of the skin by acupuncture needles
appears to result in the release of adenosine.
 A 2014 Nature Reviews Cancer review journal found that the
key mouse studies that suggested acupuncture relieves pain
via the local release of adenosine, which then triggered
close-by A1 receptors .
 The anti-nociceptive effect of acupuncture may be mediated
by the adenosine A1 receptor.

37. Methotrexate
 Methotrexate, which has strong anti-inflammatory properties,
inhibits the action of dihydrofolate reductase, leading to an
accumulation of adenosine.
 Adenosine-receptor antagonist caffeine reverses the anti-
inflammatory effects of methotrexate.

38. Adenosine
 Marked hyperpolarization and suppression of calcium-
dependent action potentials
 inhibits AV nodal conduction and increases the AV nodal
refractory period
 Use: paroxysmal supraventricular tachycardia
 Half life: 10 seconds
 ADR: Metallic taste, flushing, rashes, apprehension

39. Dipyridamole
 Dipyridamole inhibits the phosphodiesterase
 Adenosine reuptake inhibitor
 Uses:
 adjunct to coumarin anticoagulants in the prevention of
postoperative thromboembolic complications of cardiac valve
replacement
 Pulmonary hypertension
 Overdose: Rx – Xanthine derivatives

40. Caffeine
 Nonspecific adenosine Rs antagonist
 competitive nonselective phosphodiesterase inhibitor
 Bronchodilator, CNS stimulant, cardiac muscle stimulant and
a diuretic.
 Ischemic preconditioning
 Caffeine for apnoea of prematurity

41. Aminophylline
 Nonselective adenosine receptor antagonist.
 Competitive nonselective phosphodiesterase inhibitor
 Uses: Bronchodilator, Dipyrimadole overdose, anaphylactic
shock.

42. Drugs in trial

43. A1 partial agonists
 A1 agonists: Heart block, reduce pain, lipolysis
 Partial agonists: without heart block
 DRUGs:
 GW493838 – Peripheral neuropathic pain
 GS9667 – hypertriglyceridemia ass with DM
 Capadenoson – Phase II – AF

44. A2A agonists
 Vasodilation, reduce inflammation
 DRUGS
 Regadenoson: FDA 2008,
 Apadenoson
 BVT.115959: diabetic neuropathic pain

45. A3 agonists
 CF 101 (IB-MECA)
 Rheumatoid arthritis
 Glaucoma: phase II
 Dry eye disease: Phase III
 CF102
 Advanced HCC
 Chronic hepatitis C genotype 1
 Phase1-2

46. Selective A1 antagonists
 Rolofylline: PROTECT-2 : Heart failure
 Tonafylline
 KFM-19: Dementia, cognitive deficits
 8-cyclopentyl-1,3-dipropylxanthine(CFX) : Diuresis (K
sparing)

47. Selective A2a antagonists
Parkinson’s disease, drug addiction
Stimulate D2 receptors
Istradefylline: Phase III
Vipadenant: Parkinson’s : Monotherapy, adjunct:
{reclinical toxicological studies issues: Phase I
Preladenant: phase II

48. Selective A2B antagonists
Reversal insulin resistance in Type II DM
Inhibition of degranulation of human mast cells,
release of cytokines
Under development

49. Adenosine modulators
 Allosteric modulators for adenosine receptors may have potential
therapeutic advantage.
A1
 Allosteric enhancers - Antiarrhythmic and antilipolytic activity.
A2
 Not yet been developed
 Amiloride and analogues were demonstrated to be allosteric
inhibitors at A2A receptor
A3
 Allosteric enhancers are -useful against ischemic conditions or as
antitumor agents.

50. Adenosine precursors
5-AICAR
Precursor of adenosine
Protective effects in myocardial infarction
Acadesine - adenosine precursor