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fncel-13-00124 March 26, 2019 Time: 18:47 # 1
REVIEW
published: 28 March 2019
doi: 10.3389/fncel.2019.00124
Edited by:
David Blum,
INSERM U1172 Centre de Recherche
Jean-Pierre Aubert, France
Reviewed by:
Sergi Ferre,
Intramural Research Program (NIDA),
United States
Francisco Ciruela,
University of Barcelona, Spain
Patrizia Popoli,
Istituto Superiore di Sanità (ISS), Italy
*Correspondence:
Kenneth A. Jacobson
kennethJ@niddk.nih.gov;
kajacobs@helix.nih.gov
Received: 08 January 2019
Accepted: 13 March 2019
Published: 28 March 2019
Citation:
Jacobson KA, Tosh DK, Jain S
and Gao Z-G (2019) Historical
and Current Adenosine Receptor
Agonists in Preclinical and Clinical
Development.
Front. Cell. Neurosci. 13:124.
doi: 10.3389/fncel.2019.00124
Historical and Current Adenosine
Receptor Agonists in Preclinical and
Clinical Development
Kenneth A. Jacobson*, Dilip K. Tosh, Shanu Jain and Zhan-Guo Gao
Molecular Recognition Section, Laboratory of Bioorganic Chemistry, National Institute of Diabetes and Digestive and Kidney
Diseases, National Institutes of Health, Bethesda, MD, United States
Adenosine receptors (ARs) function in the body’s response to conditions of pathology
and stress associated with a functional imbalance, such as in the supply and demand of
energy/oxygen/nutrients. Extracellular adenosine concentrations vary widely to raise or
lower the basal activation of four subtypes of ARs. Endogenous adenosine can correct
an energy imbalance during hypoxia and other stress, for example, by slowing the heart
rate by A1AR activation or increasing the blood supply to heart muscle by the A2AAR.
Moreover, exogenous AR agonists, antagonists, or allosteric modulators can be applied
for therapeutic benefit, and medicinal chemists working toward that goal have reported
thousands of such agents. Thus, numerous clinical trials have ensued, using promising
agents to modulate adenosinergic signaling, most of which have not succeeded.
Currently, short-acting, parenteral agonists, adenosine and Regadenoson, are the only
AR agonists approved for human use. However, new concepts and compounds are
currently being developed and applied toward preclinical and clinical evaluation, and
initial results are encouraging. This review focuses on key compounds as AR agonists
and positive allosteric modulators (PAMs) for disease treatment or diagnosis. AR
agonists for treating inflammation, pain, cancer, non-alcoholic steatohepatitis, angina,
sickle cell disease, ischemic conditions and diabetes have been under development.
Multiple clinical trials with two A3AR agonists are ongoing.
Keywords: purinergic signaling, adenosine receptors, inflammation, pain, CNS
INTRODUCTION
Adenosine receptors (ARs) are G protein-coupled receptors (GPCRs) that sense an imbalance of
demand and supply of energy/oxygen/nutrients. Extracellular adenosine concentrations rise in
response to hypoxia and other stress, to act upon four subtypes of ARs (A1AR, A2AAR, A2B AR, and
A3AR) (Fredholm et al., 2001;Chen et al., 2013). As shown with mice lacking all four AR subtypes,
extracellular adenosine is mainly a sensor of tissue damage or danger, rather than a homeostatic
regulator under baseline conditions (Xiao et al., 2019). Elevated adenosine can correct an energy
imbalance during distress of an organ, for example by slowing the heart rate by A1AR activation
or increasing the blood supply to heart muscle by the A2AAR (Borea et al., 2016). However, there
are conditions in which chronic adenosine overproduction can be harmful, leading to increased
inflammation, fibrosis, cytokine release, brain dopamine depletion, and kidney damage (Borea
et al., 2017). Moreover, exogenous AR agonists, antagonists, or allosteric modulators can be applied
for therapeutic benefit, and thousands of such agents have been reported by medicinal chemists
working toward that goal (Jacobson and Gao, 2006;Kiesman et al., 2009). Clinically important
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effects of adenosine also include suppression of the immune
response, glomerular filtration, seizures and pain (Fredholm
et al., 2001;Cekic and Linden, 2016;Antonioli et al., 2018).
Adenosine 50-triphosphate (ATP) released from cells under
stress conditions or damage is the source of much of the
extracellular adenosine. There is typically a basal level of AR
stimulation, especially for A1AR, A2AAR and A3AR at low
nM concentrations, while A2BAR activation generally occurs at
higher (µM) adenosine concentrations (Fredholm et al., 2001).
Therefore, AR antagonists have distinct biological effects in vivo.
Purinergic signaling is also to be considered in the larger context
of ligand (ATP)-gated P2X receptors or G protein-coupled P2Y
receptors that respond to extracellular purine and pyrimidine
mono- and di-nucleotides (Burnstock and Boeynaems, 2014).
There are many approaches to small molecule therapeutics
based on experimental modulators of ARs (Chen et al., 2013;
Borea et al., 2018). Numerous subtype-selective agonists and
antagonists have been introduced, both as pharmacological
probes and as clinical candidate molecules, and representative
compounds are described here. AR knockout (KO) mice are
increasingly used to validate in vivo results with agonists
and antagonists, which can actually have variable selectivity
(Carlin et al., 2018). This review covers both directly acting
AR modulators, i.e., agonists, and several positive allosteric
modulators (PAMs). It describes compounds that have been in
human trials, for both therapeutics and diagnostics, and some
compounds for which clinical trials have only been contemplated.
Other reviews cover the use of enzymatic or transport inhibitors
to increase – such as inhibitors of adenosine kinase or adenosine
deaminase that deplete adenosine levels, or uptake inhibitors –
or to decrease, such as inhibitors of ectonucleotidases, the levels
of endogenous nucleosides (Boison, 2013;Allard et al., 2017;
Vuerich et al., 2019). These enzymes are often upregulated in
inflammatory states (Boison, 2013;Haskó et al., 2018).
AR AGONISTS FOR CLINICAL
DEVELOPMENT
Agonists of the A1AR, A2AAR and A3AR have been the
subject of preclinical and clinical evaluation (Figures 1, 2 and
Tables 1,2). A2BAR agonist development is the most limited
among the ARs, and agonists for this AR subtype have not
yet entered clinical trials. There is also much controversy
about whether A2BAR agonists or antagonists would be more
useful clinically (Eisenstein et al., 2015;Sun and Huang, 2016;
Gao and Jacobson, 2017).
Each AR can signal through multiple pathways, and different
agonists may show a signaling preference, i.e., bias. For example,
the A2BAR acts through Gs, Gq, or Gi, depending on the tissue,
receptor density and final measure of activity (Gao et al., 2018).
The possibility of biased signaling of AR agonists or PAMs could
provide a means to increase selectivity for a particular tissue or
condition (Gao et al., 2011;Baltos et al., 2017;Gao et al., 2018;
Vecchio et al., 2018).
One limitation of AR agonists for human therapeutics is
that their target GPCRs might be subject to agonist-induced
desensitization as shown for all four ARs in cell lines (Mundell
and Kelly, 2011). However, in other cases, depending on multiple
pharmacological factors, a prolonged agonist exposure did not
lead to in vivo desensitization of the desired effect (Little et al.,
2015), even with a full A3AR agonist. Two possible approaches to
circumvent GPCR desensitization are the use of partial agonists
in cases where the desired effect is maintained, such as in A1AR
antiarrhythmic activity (Elzein and Zablocki, 2008;Voors et al.,
2017), and the use of PAMs (Vincenzi et al., 2014). Partial
agonists are also known to display tissue or organ selectivity,
because of the differential receptor expression level (Van der
Graaf et al., 1999). Thus, when spare receptors are present,
i.e., overexpression or high level of endogenous expression, a
partial agonist could be fully efficacious in tissues where there
is a receptor reserve and activation of only a fraction of the
receptors suffices.
Positron emission tomography (PET) for in vivo imaging of
ARs has been extensively explored (van Waarde et al., 2018).
PET imaging could potentially be essential in drug discovery and
clinical development of AR modulators, to determine the target
engagement and to assess the role of endogenous adenosine.
Although most high affinity ligands designed for in vivo AR
imaging by PET have been antagonists, some agonists have also
been labeled with positron-emitting 18F, 76Br or 11C isotopes
(Kiesewetter et al., 2009;Guo et al., 2018).
A1AR and A2AAR are examples of the growing list of
GPCRs for which physically determined three-dimensional
structures with a bound agonist are known (Lebon et al., 2011;
Xu et al., 2011). This understanding facilitates the structure-
based design of novel AR agonists (Tosh et al., 2019). Several
structures determined by X-ray crystallography and cryo-
electron microscopy contain a G-protein or G-protein fragment,
which is more representative of the active state AR structures
(Draper-Joyce et al., 2018;García-Nafría et al., 2018). NMR
studies of assigned A2AAR Trp and Gly residues are informative
about microswitches involved in the signal propagation from
bound agonist toward the intracellular region for G protein
binding (Eddy et al., 2018). There can be multiple active
conformations of a given GPCR, and agonist-induced changes
are nuanced with respect to both structure and signaling. Thus,
direct biophysical methods for receptor characterization can
impact agonist design.
The following examples of AR drugs and molecular probes
are arranged according to their target receptor: A1(1–20,39,
40), A2A (21 –29), A2B (30), A3(31 –38,41). It has also
been suggested that coactivation of two AR subtypes might be
beneficial, such as both A1AR and A3AR in cardioprotection
(Jacobson et al., 2005).
Early AR Agonists
Adenosine 1
Arrhythmias
Many pathophysiological conditions including hypoxia and
ischemia may cause arrhythmias. Adenosine is considered as an
endogenous antiarrhythmic agent partly due to its endogenous
anti-ischemic property (Kiesman et al., 2009;Szentmiklosi
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FIGURE 1 | Adenosine (1), a non-selective AR agonist, and its derivatives as A1AR-selective agonists, including nucleosides (2–16) and non-nucleosides (17–20).
et al., 2015). As described earlier, infused adenosine (under
the name of Adenocard, approved in 1989, Olsson, 2003) and
its precursor ATP have long been used for the treatment of
paroxysmal supraventricular tachycardia (PSVT) (Pelleg et al.,
2012;Szentmiklosi et al., 2015). The antiarrhythmic action of
adenosine has been suggested to occur via the A1AR in the
sinoatrial and atrioventricular nodes, which leads to modification
of AV nodal conduction (Kiesman et al., 2009). A1AR activation
is known to induce opening of ATP-sensitive potassium channels
(Fredholm et al., 2001;Jacobson and Gao, 2006). Adenosine
infusion (under the name of Adenoscan, approved in 1995) is
used for myocardial perfusion imaging (MPI), through its short-
lived A2AAR activation, to dilate the coronary artery (Olsson,
2003). Many recent clinical trials in various cardiovascular and
ischemic conditions have utilized adenosine as an approved drug
being tested for new uses (Table 1). For example, a clinical
trial of adenosine to reverse left ventricular impairment in
Takotsubo syndrome (Galiuto et al., 2010) was initiated, but
terminated in 2018.
Dermatological conditions
Adenosine applied topically has been used to promote hair
growth and skin health (Abella, 2006;Faghihi et al., 2013). For the
treatment of androgenetic alopecia, adenosine (0.75% solution)
displayed efficacy similar to minoxidil but was preferred by
patients because of the response speed and its quality. Applied
in cosmetic preparations (0.1% cream or 1% dissolvable film) for
2 months, adenosine significantly improved skin smoothness and
reduced facial wrinkles.
Epilepsy
Adenosine is considered an endogenous antiseizure agent
and also attenuates epileptogenesis by an epigenetic
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FIGURE 2 | A2AAR- (21–29) and A2B AR- (30) selective agonists.
mechanism, i.e., nuclear adenosine reduces DNA methylation
(Boison, 2013). Elevated adenosine indirectly inhibits DNA
methyltransferases through two stages of enzymatic product
inhibition, beginning with S-adenosylhomocysteine hydrolase
(SAH). In the epileptic brain there is an overexpression of
adenosine kinase in astrocytes, which depletes adenosine both
inside and outside the cell. Because peripheral adenosine
itself is rapidly removed from circulation in seconds, drug
delivery systems are needed to raise its brain concentration.
Experimental use of an adenosine-releasing silk brain
implant was found to be an efficacious form of adenosine
augmentation therapy (AAT) for treating refractory epilepsy
in a rat model of kindling epileptogenesis (Szybala et al.,
2009). Adenosine was encapsulated in microspheres,
which were embedded into nanofilm-coated silk fibroin
scaffolds. The polymer was introduced surgically into the
infrahippocampal cleft, and when monitored for 10 days,
demonstrated a lack of convulsive seizures. This protective
effect was antagonized by an A1AR-selective antagonist
8-cyclopentyl-1,3-dipropylxanthine (DPCPX).
AMP (Adenosine 50-Monophosphate) and ATP
(Adenosine 50-Triphosphate)
Asthmatic drug testing
Ectonucleotidases readily cleave the naturally occurring
50-phosphoesters of adenosine 50-phosphates to produce
adenosine in situ. Adenosine 50-monophosphate (AMP) is used
diagnostically in inhalation challenge testing (Basoglu et al.,
2005;Isogai et al., 2017). AMP readily forms adenosine in situ
and thus most of its actions occur through ARs, but not all of
its in vivo effects arise from AR activation (Carlin et al., 2018;
Xiao et al., 2019).
Cancer
Adenosine 50-triphosphate plays a physiological role in
obstructive airway disease to increase inflammation and
to induce bronchoconstriction and cough (Pelleg et al.,
2016). ATP has been administered in humans intravenously
in clinical trials for the treatment of both cachexia in
cancer and the cancer itself (Rapaport et al., 2015). The
working hypothesis was that by increasing the intracellular
ATP pools in erythrocytes, the energy balance could be
restored. Alternatively, an action on P2 receptors was
considered, but the extracellular ATP pools were elevated
only briefly in patients with advanced solid tumors.
Nevertheless, with the ubiquitous presence of ectonucleotidases,
one main action of ATP would be through increased
adenosine acting at ARs.
Metrifudil 2
Glomerulonephritis
Many adenosine derivatives found in early studies (1960s
and 1970s) to be biologically active at what were later
termed the A1AR and A2AAR, contain bulky, hydrophobic
substitution at the adenine N6position (Olsson, 2003;
Jacobson and Gao, 2006;Mantell et al., 2010). This
describes two potent agonists, Metrifudil 2and R-PIA (R-
N6-phenylisopropyladenosine (structure not shown), which
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TABLE 1 | Representative recent clinical trials of AR agonists and an A1AR PAM (data from ClinicalTrials.gov, accessed 12-28-2018).
Receptor Condition Compound Years Phase, NCT# Reference
A1Neuropathic pain 12014–2018 2, 00349921 Zylka, 2011
SVTa12011–2017 −, 01495481 Chrysostomou et al., 2013
Perioperative pain 12006 2, 00298636 Jin and Mi, 2017
Pediatric heart transpl. 12015–2018 1, 02462941 Flyer et al., 2017
Takotsubo syndrome 12016–2018 2, 02867878 Galiuto et al., 2010
Paroxysmal AF 12017 2, 03032965 Letsas et al., 2017
Glaucoma 42015–2016 3, 02565173 Myers et al., 2016
Neuropathic pain 92002–2003 2, 00376454 Elzein and Zablocki, 2008
T2D 10 ∼2009 1,−Staehr et al., 2013
AF 13 2008–2014 2, 00713401 Corino et al., 2015
AF 14 2015–2018 2, 00040001 Elzein and Zablocki, 2008
Stable angina 17 2007–2011 2, 00518921 Tendera et al., 2012
AF 17 2008 2, 00568945 Kiesman et al., 2009
Heart failureb19 2014–2016 2, 02040233 Lam et al., 2018
Heart failurec19 2016–2018 2, 02580851 Voors et al., 2017
Posthepetic neuralgia 39 2008–2012 2, 00809679 Miao et al., 2018
A2A CAD (MPI) 21 2005–2009 3, 00208312 Zoghbi and Iskandrian, 2012
Sickle cell anemia 21 2013–2018 2, 01788631 Field et al., 2014, 2017
Lung transplant 21 2017−1, 03072589 Sharma et al., 2016
IHD (MRI) 21 2011–2012 1, 00881218 Lasley, 2018
Pulmonary hypertens. 21 2014–2018 −, 02220634 Palani and Ananthasubramaniam, 2013
Heart transplant (MRI) 21 2017−4, 03102125 Palani and Ananthasubramaniam, 2013
BBB defect 21 2015–2018 1, 02389738 Jackson et al., 2018
Diabetic nerve pain 22 2007–2014 2, 00452777 Knezevic et al., 2015
Diabetic foot ulcers 23 2006–2012 2, 00318214 Montesinos et al., 2015
CAD 24 2009–2012 3, 00944294 Zoghbi and Iskandrian, 2012
CAD (SPECT-MPI) 25 2009–2012 3, 00990327 Zoghbi and Iskandrian, 2012
CAD (MPI) 25 2011–2012 3, 01313572 Zoghbi and Iskandrian, 2012
COPD 28 2007–2013 2, 00430300 Mantell et al., 2010
Healthy subjects (PK) 29 2012–2017 1, 01640990 Allen et al., 2013
A2B- A3− − Sun and Huang, 2016
Rheumatoid arthritis 31 2016−3, 02467762 Fishman et al., 2012
Plaque psoriasis 31 2017−3, 03168256 David et al., 2016;
Glaucoma 31 2009–2015 2, 01033422 Jacobson and Civan, 2016
Dry eye disease 31 2010–2015 3, 01235234 Avni et al., 2010
NASH 32 2016−2, 02927314 Fishman et al., 2018
HCC 32 2008–2015 1/2, 00790218 Stemmer et al., 2013
HCC 32 2014−2, 02128958 Stemmer et al., 2013
Chronic HCV 32 2008–2015 1/2, 00790673 Bar-Yehuda et al., 2008
aComparison to dexmedetomidine. bWith preserved ejection fraction. cWith reduced ejection fraction. SVT, supraventricular tachycardia; AF, atrial fibrillation; CAD,
coronary artery disease; MPI, myocardial perfusion imaging; MRI, magnetic resonance imaging; SPECT, single-photon emission computed tomography; BBB, blood
brain barrier; IHD, ischemic heart disease; PK, pharmacokinetics; NASH, nonalcoholic steatohepatitis; HCC, hepatocellular carcinoma; HCV, hepatitis C virus.
are moderately A1AR-selective. Metrifudil (60 mg oral
dose) was one of the earliest synthetic adenosine agonists
to be studied in humans (Schaumann and Kutscha, 1972;
Wilbrandt et al., 1972), along with R-PIA (Schaumann
et al., 1972). Following the discovery that vasodilator
dipyridamole (equilibrative adenosine transporter ENT1
inhibitor) was active against glomerulonephritis by an
adenosinergic mechanism, Metrifudil was applied to its
treatment, which resulted in limited improvement in
three patients. However, no subsequent trials of Metrifudil
or R-PIA were reported, probably because of their
cardiovascular side effects.
A1AR-Selective Agonists
Trabodenoson (INO-8875, PJ-875) 4
Glaucoma
A1AR partial agonist Trabodenoson (INO-8875) 4, which
is a 50-nitrate ester derived from CPA 3, was in advanced
clinical trials as an ophthalmic formulation for primary
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TABLE 2 | AR Binding affinity of selected AR agonists described here (human, if not specified; p, pig; r, rat; m, mouse).a,b
Compound pKiA1AR pKiA2AAR pKiA3AR
1 Adenosine 7.0 6.5 6.5
2 Metrifudil 7.22 (r) 7.62 (r) 7.33, 7.46 (r)
3 CPA 8.64, 9.66 (m) 6.10, 6.09 (m) 7.37, 6.27 (m)
4 Trabodenoson 9.0 ND ND
5 CHA 8.62 5.86 7.14
6 SDZ WAG994 7.64 (p), 7.12 (r) 4.64 (p), 5.24 (r) ND
7 Cl-ENBA 9.29, 9.70 (m) 5.87 5.89, 5.62 (m)
8 MRS7469 8.67, 9.43 (m) 5.45 4.97, 6.05 (m)
10 GS 9667 7.92 <5<6
11 GR79236X 8.51 (r) 5.89 (r) ND
13 Tecadenoson 8.52 ND ND
14 Selodenoson 8.22 ND ND
15 NNC 21-0136 8.33 (r) 5.89 (r) ND
16 MRS5474 7.30 5.40 6.33
17 Capadenoson 8.85 <5<5
18cNeladenoson 10.0 6.17 <5.52
20 MMPD 9.31, 9.68 (r) 7.15, 7.28 (r) <5
21 Regadenoson <5, 8.11 (m) 6.34, 7.11 (m) <5, <5 (m)
22 BVT.115959 6.81 6.01 6.81
23 Sonedenson <5 6.31 ND
24 Binodenoson 4.32 6.57 <4
25 Apadenson 7.11 9.30 7.35
26 Evodenoson 7.24 9.15 6.60
27cUK-371104 6.99 7.70 <6
28 UK-432097 ND 8.40 ND
29 GW328276X 6.05 8.63 8.38
30 Bay 60-6583 6.41, 6.45 (m) <5, <5 (m) 6.94, 6.87 (m)
31 Piclodenoson 7.29, 7.79 (m) 5.50, 5.54 (m) 8.74, 9.66 (m)
32 Namodenoson 6.66 5.27 8.85
33 LJ-529 6.71 6.65 9.42
34 CP-532,903 6.05 (m) <5 (m) 8.05 (m)
35 CP-608,039 5.14 <4.3 8.24
36 MRS5698 <5<5 8.52
37 MRS5980 <5<5 9.15
aReferences: Knutsen et al., 1999;Gao et al., 2003;Kiesman et al., 2009;Alnouri et al., 2015;Meibom et al., 2017;Baraldi et al., 2018;Jacobson et al., 2018;Tosh
et al., 2019.bA2BAR affinity (pKi; h, unless noted): 1, 4.8; 18, 7.10 (pEC50 ); 21,<5; 23,<5; 25,<6; 26,<6; 27, 5.36 (pEC50); 29, 8.30; 30, 6.94, 7.00 (r), 6.87 (m); 31,
4.96; 34,<5 (m). cpEC50 in cAMP assays. ND, not determined.
open-angle glaucoma and ocular hypertension. However, the
development was terminated in 2017 following a Phase 3
trial due to failure to achieve its primary endpoint (Jacobson
and Civan, 2016). Trabodenoson and its congeners were
also considered for treatment of arrhythmias (Elzein and
Zablocki, 2008; see below). Another analog containing
a 50-nitrate ester, but with a N6-3-aminotetrahydrofuryl
group was found to lose its nitro group in vivo to
form the parent full agonist, which was associated
with side effects.
CHA 5
Hypothermia
The use of A1AR agonist CHA 5for inducing therapeutic
hypothermia has been proposed (Jinka et al., 2015). However,
it is likely that the hypothermic effect of CHA in rodents
may be through both a peripheral A3AR, and a central A1AR
(Carlin et al., 2017).
SDZ WAG994 6
Diabetes and obesity
A1AR is highly expressed in adipose tissue and involved in
triglyceride (TG) storage. Breakdown of TG and the subsequent
increase in plasma free fatty acids (FFA) results in development
of insulin resistance in peripheral organs. Insulin resistance is
associated with obesity and development of type 2 diabetes
(Antonioli et al., 2015). Adenosine-mediated A1AR activation
or A1AR overexpression in adipocytes results in suppression of
lipolysis and reduction of plasma FFA. Various adenosine analogs
have been developed for their potential intervention in diabetes.
N6-Cyclopentyladenosine (CPA, 3) reduced FFA and cholesterol
levels and increased glycogen synthesis in skeletal muscle in
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streptozotocin (STZ) diabetic rats (Cheng et al., 2000). Thus,
A1AR agonists may have beneficial effects on glucose utilization
by peripheral tissues to lower plasma glucose.
Typically, both 20- and 30-hydroxyl groups are important for
nucleoside recognition in the AR binding sites. SDZ WAG994
6is a structurally unusual 20-O-methyl A1AR agonist that did
not display hemodynamic side effects in humans. However,
SDZ WAG994 increased PR interval, consistent with A1AR
activation in the AV node and its suggested use in tachycardia
(Jacobson and Knutsen, 2001).
SDZ WAG994 6was also under development as an
antilipolytic agent for treating diabetes (Ishikawa et al., 1998).
Oral administration of SDZ WAG994 and another A1AR agonist
Selodenoson (RG14202, 14) in STX-treated rats decreased FFA,
TG levels and heart rate in a dose-dependent manner (Cox
et al., 1997). A human study revealed that RPR749 (structure not
shown), was capable of decreasing circulating FFA levels and thus
may be beneficial in treating hyperlipidemia (Shah et al., 2004).
These compounds have therapeutic potential for the treatment of
cardiovascular and metabolic disorders.
Cl-ENBA 7 and MRS7469 8
Hypothermia and pain
Several A1AR agonists, Cl-ENBA 7and MRS7469 8, were
recently reported to activate in the mouse A1AR in the brain
when administered peripherally, without comparable peripheral
A3AR activation (Carlin et al., 2017;Tosh et al., 2019). Cl-
ENBA 7is a highly A1AR-selective adenosine agonist that is
used as a pharmacological probe (Tosh et al., 2019), and its
efficacy in pain models demonstrated (Luongo et al., 2012).
It consists of a mixture of two diastereoisomers, and thus its
in vivo A1AR target engagement is complicated. However, when
administered intraperitoneally in mice, it activated the central
A1AR preferentially over the peripheral A3AR in mast cells,
due partly its being a full agonist for the A1AR but a partial
agonist of low efficacy or an antagonist at the A3AR (Carlin et al.,
2017). This is consistent with a report that S-N6-endo-norbornyl-
adenosine (S-ENBA) is a full agonist at the A1AR but low-efficacy
partial agonist at the A3AR (Gao et al., 2003).
Like Cl-ENBA, MRS7469 8is a highly selective agonist that
activated the central A1AR preferentially when administered
peripherally, leading to A1AR-dependent hypothermia and
locomotor depression (Tosh et al., 2019). When administered
icv. (52 µg/kg) it caused intense hypothermia. Thus, it crosses
the blood brain barrier (BBB) sufficiently, given its high affinity
(Ki0.37 nM at mouse A1AR), to activate the A1AR. Moreover,
MRS7469 with a non-chiral N6group is a pure diastereoisomer,
which is advantageous for in vivo studies.
GW493838 9
Pain
The A1AR is involved in depressant and protective functions in
the brain and spinal cord, including suppressing pain (Sawynok,
1998;Zylka, 2011;Giorgi and Nieri, 2013). Intrathecal opioids
induce local adenosine release (Eisenach et al., 2004), and
exogenously applied adenosine and other A1AR agonists and
PAMs reduce pain (Eisenach et al., 2013;Vincenzi et al., 2014).
A1AR agonist GW493838 9, which is an adenosine analog highly
modified at N6and 50positions, showed efficacy in animal models
of pain (Imlach et al., 2015). However, GW493838 (50 mg oral
dose) failed to show significant efficacy in a clinical trial for
treatment of chronic pain (diabetic pain). Also, a meta-analysis
of patient postoperative pain in clinical data showed no analgesic
effect of adenosine (Jin and Mi, 2017).
CVT-3619 (GS-9667) 10, GR79236 11 and ARA 12
Diabetes
Increasing evidence indicates a crucial role of A1AR in
the regulation of insulin sensitivity and glucose homeostasis
especially in metabolically active organs such as adipose
tissue, liver and skeletal muscle, which are related to diabetes
mellitus (Peleli and Carlstrom, 2017). It has been convincingly
demonstrated that A1AR is critical for regulation of lipid
metabolism, and thus A1AR agonists have been proposed for the
treatment of type II diabetes (T2D) and obesity (Antonioli et al.,
2018). The white adipocyte A1AR inhibits lipolysis. Curiously,
a functional A2AAR activates thermogenic brown adipose tissue
(BAT) as indicated using human PET imaging (Lahesmaa et al.,
2018), and A2AAR agonists might prove beneficial in metabolic
conditions (Tozzi and Novak, 2017). Cold exposure in human
subjects reduced PET ligand binding in BAT, indicative of
elevated local adenosine release.
Several A1AR agonists, GR79236 11, ARA 12, and CVT-3619
10, have been in clinical trials for T2D due to their ability to
increase insulin sensitivity (Bigot et al., 2004;Kiesman et al., 2009;
Staehr et al., 2013). However, development of full agonists, such
as GR79236 and ARA, was not successful due to cardiovascular
side effects (Elzein and Zablocki, 2008). Although both full and
partial agonists may lower non-esterified fatty acid levels, it is
suggested that partial agonists may improve insulin sensitivity
without producing severe cardiovascular side effects (Elzein
and Zablocki, 2008). A single dose of ARA given to healthy
individuals during a phase I clinical trial reduced plasma FFA
levels, but individuals developed tolerance to the drug (Zannikos
et al., 2001). The A1AR partial agonist CVT-3619 (GS-9667) has
been reported to lower FFA in both healthy and obese subjects
without showing evidence of A1AR desensitization (Staehr et al.,
2013), but oral doses of ≥300 mg were required to see the
FFA effect. The individual benefits of GS-9667 and sitagliptin
[Januvia, an inhibitor of DPP4 (dipeptidyl peptidase 4)] on
glucose and lipid homeostasis were enhanced in combination
(Ning et al., 2011).
Tecadenoson 13 and Selodenoson 14
Arrhythmias
Despite its demonstrated A1AR-dependent beneficial effect in
PSVT, adenosine is known to cause atrial fibrillation (AF)
in about 15% of patients by decreasing the refractory period
of the atrium and causes other adverse effects related to
the activation of other AR subtypes (Glatter et al., 1999).
Thus, extensive efforts have been made in developing selective
A1AR agonists as anti-arrhythmic agents (Mason and DiMarco,
2009). A1AR full agonists Tecadenoson 13 (Corino et al.,
2015), Selodenoson 14 and Trabodenoson 4(Kiesman et al.,
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2009;Mason and DiMarco, 2009) have been under development.
However, full agonists are known to cause tachyphylaxis,
presumably due to A1AR desensitization. Tecadenoson has
been in a Phase 3 trial for the termination of supraventricular
tachycardia (SVT) (Elzein and Zablocki, 2008;Mason and
DiMarco, 2009), but its development was discontinued in 2009.
A clinical safety study of Tecadenoson for the treatment of AF
was performed, but its clinical development was also curtailed.
NNC-21-0136 15
Stroke
NNC-21-0136 15 is an A1AR selective agonist that was designed
for neuroprotection. Its hemodynamic effects were minimal,
revealing a brain-protective effect in stroke models (Knutsen
et al., 1999;Jacobson and Knutsen, 2001), but it did not
enter human testing.
MRS5474 16
Seizures and depression
A1AR agonists are of interest in the CNS for their anxiolytic,
antinociceptive, antidepressant and antiseizure properties, and
behavioral results with A1AR KO mice support the use of
A1AR in this context. Unfortunately, many adenosine derivatives
display minimal ability to cross the BBB (Schaddelee et al., 2005;
Tosh et al., 2019). 40-Truncated nucleosides, thionucleosides
and methanocarba-nucleosides (containing a [3.1.0]bicyclohexyl
ring system) were originally characterized as A3AR low-efficacy,
selective partial agonists. However, an N6-dicyclopropylmethyl
group present in MRS5474 16 substantially shifts the selectivity
toward A1AR, especially in mouse (204-fold compared to mouse
A3AR), compared to other a-branched N6groups (Tosh et al.,
2012, 2019;Carlin et al., 2017). The small molecular weight
(376), polar surface area (93 Å2) and number of H-bond donor
groups (3) positioned MRS5474 for potential brain application.
MRS5474 showed antidepressant activity in a mouse model that
was mediated by homer1 protein in the medial prefrontal cortex,
and upregulation of homer1 by an AR agonist was lost in A1AR
KO mice (Serchov et al., 2015). Nevertheless, it also activated a
peripheral mA3AR (Carlin et al., 2017).
Capadenoson 17 and Neladenoson 18
Angina
Anti-ischemic effect of A1AR agonists has been demonstrated
in animal studies, but clinical successes are lacking, and
more relevant clinical models are needed (Borea et al., 2016;
Lasley, 2018).
Most known AR agonists are adenosine derivatives, but two
classes of pyridine-derived agonists are known (Guo et al., 2018).
The non-nucleoside A1AR agonist Capadenoson 17 (BAY68-
4986), having an atypical 3,5-dicyanopyridine structure, was
evaluated in patients with stable angina using an oral dose of
4 mg, once daily (Kiesman et al., 2009;Tendera et al., 2012).
However, Capadenoson was withdrawn from clinical trials for
angina and for AF.
Heart failure
Rather than full agonists, an A1AR partial agonist Neladenoson
18 (in the form of a dipeptide ester prodrug 19) is now being
tested in patients with chronic heart failure (Greene et al., 2016;
Dinh et al., 2017;Meibom et al., 2017;Voors et al., 2017).
Compared with Capadenoson (Baltos et al., 2017), Neladenoson
is a more selective partial agonist for A1AR. Neladenoson has
been shown to improve cardiac function without producing
bradycardia, atrioventricular blocks, or undesirable effect on
blood pressure (Meibom et al., 2017;Voors et al., 2017).
The rationale for using partial A1AR agonists is based on
the observation that the activation of myocardial A1ARs by
partial agonists protects cardiac function related to ischemia
and reperfusion injury without producing severe side effects
(Albrecht-Küpper et al., 2012;Voors et al., 2017). A multiple dose
study of Neladenoson (BAY 1067197) (ParSiFAL, 5 – 40 mg oral
dose, once daily) in heart failure is ongoing.
MMPD 20
Imaging
Numerous A1AR ligands have been in development for potential
use in diagnosis of various conditions, such as depression,
Parkinson’s disease, Alzheimer’s disease, epilepsy, ischemia, and
sleep disorders. The A1AR is highly expressed in many brain
regions, such as the hippocampus, neocortex, thalamus and basal
ganglia (Fredholm et al., 2001). In vivo imaging of A1AR in the
human brain is therefore an attractive approach for diagnosis,
and various AR agonists and antagonists have been developed
for PET brain imaging (van Waarde et al., 2018). Although
varied AR subtype selectivities and agonist efficacies are seen with
the class of atypical 3,5-dicyanopyridine ligands, partial agonist
MMPD 20 was recently shown to be highly A1AR selective, with
16% maximal human A1AR activation, and suitable for the PET
imaging in the rat brain (Guo et al., 2018). It has a relatively
low molecular weight (373) and polar surface area (108 Å2),
which allows it to cross the BBB. Typical ribose-containing A1AR
agonists have limited utility to be administered therapeutically for
CNS treatment due to their low degree of brain uptake from the
periphery (Schaddelee et al., 2005).
A2AAR-Selective Agonists
Regadenoson 21
Imaging
Regadenoson 21 (CVT-3146, Lexiscan) is a moderately selective,
short acting A2AAR agonist that is administered i.v. for MPI
(Palani and Ananthasubramaniam, 2013). It was first approved
as a pharmacologic stress agent in 2008. At present, it is the only
synthetic AR agonist that is approved for human use, although
it is not highly potent or selective for the A2AAR. Nevertheless,
Regadenoson’s A2AAR selectivity is higher in human than in
mouse, in which it is actually 10-fold A1AR selective compared to
A2AAR (Carlin et al., 2018). The availability of an FDA-approved
new chemical entity (NCE) allows it to be tested in diverse clinical
trials for cardiovascular treatment and diagnosis (>60 currently
listed in ClinicalTrials.gov, accessed 12-31-2018).
Sickle cell disease
In addition to their diagnostic application in MPI, A2AAR
agonists have been considered for treatment of inflammation
(Cekic and Linden, 2016) and sickle cell disease (SCD, Field et al.,
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2014). A2AAR activation in natural killer T (iNKT) cells is anti-
inflammatory as demonstrated in a transgenic mouse model of
SCD. However, A2BAR activation in erythrocytes is predicted to
have a harmful effect in SCD. The effects of Regadenoson 21 as an
A2AAR agonist in SCD patients were evaluated in a clinical trial,
but there was no statistically significant benefit (Field et al., 2017).
Lung transplantation
Murine lung ischemia reperfusion injury occurring via NADPH
oxidase 2 (NOX2) and IL-17 is also attenuated by A2AAR agonist
26 (Sharma et al., 2016), which has led to an ongoing clinical trial
of Regadenoson 21 in lung transplantation. The safety of using
Regadenoson for MPI in patients with mild to moderate COPD
and asthma was established (Golzar and Doukky, 2014).
Glioblastoma
A2AAR agonists transiently increase BBB permeability (Kim
and Bynoe, 2016), and this is being evaluated as a novel
pharmacological approach to drug delivery to the brain.
Regadenoson was tested clinically in an attempt to raise the
concentration of the anticancer drug temozolomide in the brain
interstitium, determined using microdialysis in glioblastoma
patients (Jackson et al., 2018).
Spongosine (BVT.115959, CBT-1008) 22 and Other
Naturally Occurring AR Agonists
Pain
Numerous other A2AAR agonists were studied in preclinical
testing or clinical trials prior to the approval of Regadenoson.
Among the first such agonists was the simple 2-methoxy
derivative of adenosine, spongosine (BVT.115959) 22, a marine
natural product (García et al., 2018). Actually, spongosine is
slightly selective for and equipotent at the human A1AR and
A3AR. It was shown to be effective in a clinical trial for
diabetic neuropathic pain (7 mg oral dose, 3X daily), which was
terminated because the company discontinued small molecule
research (Knezevic et al., 2015).
Inflammation
Activation of the A2AAR by endogenous adenosine provides
benefit in animal models of inflammation and rheumatic disease,
for example in rat models of osteoarthritis (Cronstein and
Sitkovsky, 2017;Haskó et al., 2018). Other naturally occurring
adenosine or deoxyadenosine derivatives have been applied
as AR agonists. Polydeoxyribonucleotide (PDRN, structure
not shown), of molecular weight 80–200 KD and extracted
from trout or salmon sperm, is degraded by plasma DNA
nucleases or cell membrane-bound nucleases giving rise to
nucleosides and nucleotides. It is asserted that the degraded
products pharmacologically activate the A2AAR, based on
antagonism by the relatively weak and non-selective A2AAR
antagonist 3,7-dimethyl-1-propargylxanthine (DMPX). PRDN’s
therapeutic effects include tissue repairing, anti-ischemic, and
anti-inflammatory, making it suitable in regenerative medicine
and for treating diabetic foot ulcers. Topically applied PDRN
was in a clinical trial for reducing inflammation to promote
wound healing in cases of diabetic foot ulcers (Squadrito et al.,
2017). Also, topically applied PDRN significantly reduced pain
and increased joint function in an animal model of osteoarthritis
and increased neurogenesis in a spinal cord injury model
(Irrera et al., 2018).
Sonedenoson (MRE-0094) 23 and Binodenoson
(WRC-0470, MRE-0470) 24
Imaging
Many adenosine derivatives that proved to be A2AAR-selective
agonists have bulky, hydrophobic substitution at the C2
position of adenine (Jacobson and Gao, 2006). A2AAR agonist
Binodenoson 24 (≤1.5 µg/kg, i.v.) administered for MPI of
patients with coronary artery disease did not cause the side effect
of bronchoconstriction (Murray et al., 2009).
Wound healing
Sonedenoson (MRE-0094) 23 was effective in the treatment of
poorly healing wounds in animal models (Victor-Vega et al.,
2002), an A2AAR-agonist effect later found to be dependent
on tissue plasminogen activator (Montesinos et al., 2015).
A Phase 2 clinical trial of Sonedenoson administered as a
topical gel for diabetic foot ulcers had poor enrollment and was
terminated in 2008.
Apadenoson (ATL-146e, BMS 068645) 25 and
Evodenoson (ATL-313, DE-112) 26
Imaging
Apadenoson 25 (Rieger et al., 2001;Zoghbi and Iskandrian,
2012) was in several clinical trials for MPI and SCD, which has
a component of hypoxia. Apadenoson contains a labile ester
moiety, which is cleaved in vivo to limit its duration of action. Its
more stable, urethane-containing congener Evodenoson (ATL-
313) 26, was developed as a candidate drug for treating multiple
myeloma (Rickles et al., 2010;van Waarde et al., 2018).
UK-371104 27 and UK-432097 28
Pulmonary inflammation
Intratracheal administration of A2AAR agonist UK-371104 27,
with sterically bulky N6and C2 substituents, in anesthetized
guinea pig, inhibited the capsaicin-induced bronchoconstriction
without affecting blood pressure (Trevethick et al., 2008). Thus,
additional lung-focused A2AAR agonists were explored for
treating lung inflammation.
UK-432097 28 is a selective A2AAR agonist that was in a
failed clinical trial for COPD (Mantell et al., 2010), although
its pharmacology is comparable to its preceding congener, UK-
371104 27. UK-432097 as an inhaled dry powder was not
efficacious in human trials (discontinued in 2008), possibly due
to its agonist activity at the A1and A3ARs, and/or its high MW
(778), and multiple H-bond donor (7) and acceptor (13) groups
reduced its bioavailability, even when administered directly in the
lungs by inhalation. However, it displays a favorably slow off-rate
from the receptor, which has been suggested to contribute to its
sustained agonist effects (Hothersall et al., 2017). The extended
N6and C2 substituents of UK-432097 and its congeners interact
with A2AAR extracellular regions to impede their dissociation.
The bulky N6and C2 substitutions of UK-432097 enabled the
structural determination of its A2AAR complex (Xu et al., 2011).
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GW328267X 29
Asthma and allergy
A dual A2AAR agonist and A3AR antagonist GW328267X 29
failed to show efficacy in a clinical trial for asthma (inhaled)
and allergic rhinitis (intranasal) (Trevethick et al., 2008), despite
its anti-inflammatory efficacy in animal models. Its structure is
unusual in that it contains an ethyl-tetrazole group at the ribose
40position, thus contributing to its dual action at the two AR
subtypes (Trevethick et al., 2008). Its side effects (hypotension,
tachycardia) even when administered by inhalation were dose-
limiting in the clinical trials. However, intravenous infusion
of GW328267X in humans (52 µg/kg, over 5.5 h) resulted in
a mechanism-related tachycardia that was ascribed to A2AAR
activation in the carotid bodies, which was not alleviated upon
prolonged agonist exposure. The lack of tachyphylaxis leading
to prolonged tachycardia was not acceptable (Allen et al., 2013),
and its clinical testing was discontinued. The translational failure
of A2AAR agonists is likely due to their limited selectivity,
especially their agonist activity at the A1AR. It has been suggested
that A1AR antagonists and A2AAR agonists may have beneficial
effects for asthma (Gao and Jacobson, 2017).
A2BAR-Selective Agonist
BAY 60-6583 30
Ischemia, inflammation, diabetes, asthma, and cancer
Although there are no A2BAR agonists currently in clinical
evaluation, animal models suggest its activation might result
in beneficial effects in acute lung injury, ischemia and vascular
leakage (Eltzschig et al., 2003;Eltzschig, 2009). The non-
nucleoside (3,5-dicyanopyridine) agonist BAY 60-6583 30 has
been used as an in vitro and in vivo pharmacological probe,
although its degree of efficacy and its species dependence of
affinity/selectivity can vary (Gao et al., 2014). In some models,
including insulin release in MIN6 mouse insulinoma cells,
the compound was reported to act as an A2BAR antagonist.
Therefore, highly selective and reliably efficacious A2BAR
agonists are still lacking. Moreover, the signaling pathways
activated or inhibited by the nominally Gs-coupled A2BAR are
complex and involve multiple G proteins (Gao et al., 2018).
Mast cell A2BAR activation might be useful in the treatment
of asthma (Gao and Jacobson, 2017). In the intestines, kidney
and other organs, this receptor has an anti-ischemic effect
(Grenz et al., 2008;Hart et al., 2011). A2BAR activation is
predicted to have beneficial cardiovascular effects and maintain
the endothelial cell barrier (Eltzschig et al., 2003). BAY 60-6583
was shown to have protective effects in a model of myocardial
reperfusion injury (Tian et al., 2015). A2BAR activation leading
to the PI3K/Akt pathway is anti-inflammatory by shifting
macrophages to an M2 phenotype. Pre-ischemic administration
of BAY 60-6583 stimulated leukocyte PI3K/Akt in the mouse
spleen to reduce myocardial reperfusion injury in an IL-10-
dependent manner (Ni et al., 2018).
A2BAR activation might also be useful in treating T2D
and atherosclerosis, and preventing vascular lesions due to
smooth muscle cell proliferation after angioplasty (Koupenova
et al., 2012;Sun and Huang, 2016). A2BAR KO mice
displayed increased fatty liver pathology, tissue inflammation
and insulin resistance due to the lack of this receptor in
macrophages (Johnston-Cox et al., 2014). A2BAR activation
reduced inflammation and macrophage activation resulting from
FFA (Csóka et al., 2014). The receptor was highly upregulated in
mice subjected to a high-fat diet (HFD), and A2BAR KO mice on
this diet developed obesity and insulin resistance. BAY 60-6583
administered for 4 weeks HFD restored endocrine function and
reduced inflammation. However, A2BAR gene expression was
found to be elevated in cases of human gestational diabetes, but
this observation did not establish whether an A2BAR agonist or
antagonist would be more beneficial (Wojcik et al., 2014).
Although blocking the A2BAR is considered a target in
conjunction with cancer immunotherapy, its activation also has
been reported to reduce proliferation of cancer cells (Koussémou
et al., 2018). A2BAR activation led to ERK1/2 dephosphorylation
and reduced cell proliferation through inhibition of the MAPK
signaling pathway in the MDA-MB-231 breast cancer cell line.
A3AR-Selective Agonists
IB-MECA 31
Autoimmune inflammatory diseases
A3AR agonists display anti-inflammatory and anticancer effects
in various in vivo disease models (Cronstein and Sitkovsky,
2017;Jacobson et al., 2018). A3AR agonists stimulate chemotaxis
in neutrophils through the leading edge, which could be pro-
inflammatory. However, systemic A3AR agonist administration
could actually have an anti-inflammatory effect by inhibiting
neutrophil chemotaxis because of the non-directional agonist
exposure (Chen et al., 2006).
IB-MECA (CF101, Piclodenoson) 31, the first moderately
selective A3agonist (Gallo-Rodriguez et al., 1994), is being
developed for the treatment of autoimmune anti-inflammatory
diseases, including rheumatoid arthritis (RA) and psoriasis (both
in Phase 3) (Fishman et al., 2012). In Phase 2 trials, its action
in RA and psoriasis compared favorably to existing treatments
for those conditions, but it did not display serious adverse
effects, as do the current treatments. In a comparison of 1, 2,
and 4 mg oral IB-MECA doses in a 12-week Phase 2 psoriasis
trial, the greatest patient improvement was observed with the
2 mg dose (Fishman et al., 2012). Similarly in a Phase 2 RA
trial, the middle (1 mg, compared to 0.1 and 4 mg) oral dose
achieved the highest responses. Peripheral blood mononuclear
cells (PBMCs) from psoriasis patients showed elevated A3AR
expression. IB-MECA inhibited proliferation and formation of
IL-17 and IL-23 in a human keratinocyte cell line (Cohen et al.,
2018). IB-MECA was previously in Phase 2 clinical trials for dry
eye disease and glaucoma, 1 mg and 2 mg, respectively (oral, twice
daily), which failed to demonstrate efficacy (Avni et al., 2010;
Jacobson and Civan, 2016).
Cl-IB-MECA 32
Liver diseases
Cl-IB-MECA 32, was initially shown to display a higher
A3AR agonist selectivity than IB-MECA at the rat ARs.
However, at the mA3AR, IB-MECA is more potent and selective
than Cl-IB-MECA (Carlin et al., 2017). Cl-IB-MECA (CF102,
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Namodenoson) is being developed for the treatment of liver
conditions, including hepatocellular carcinoma (HCC) and non-
alcoholic steatohepatitis (NASH) (Fishman et al., 2018). A3AR
agonists have apoptotic and anticancer effects in vivo induced
by Wnt signaling deregulation (Bar-Yehuda et al., 2008). The
US Food and Drug Administration and the European Medicines
Agency granted fast track designation to 32 for the treatment of
liver cancer. Cl-IB-MECA (up to a 25 mg oral dose) increased
the median overall survival in patients with advanced HCC
by 7.8 months in patients (Stemmer et al., 2013), which was
improved over the current treatment. There were no serious
adverse effects or dose-limiting toxicity. Secondarily, the trial
examined using the A3AR as a predictive marker of the
CF102 clinical response. The use of A3AR to prevent cytokine
release syndrome in cancer immunotherapy has been proposed
(Cohen and Fishman, 2019).
An anti-steatotic effect of Cl-IB-MECA in an HFD mouse
model of NASH, induced by STZ administered 2 days after birth,
was mediated via a molecular mechanism leading to decreased
α-smooth muscle actin (αSMA, a marker of pathological
fibroblasts) and cytokeratin 18 (CK-18, a predictor of NASH
severity). A Phase 2 trial of CF-102 (12.5 and 25 mg oral doses,
twice daily) for NASH treatment is underway.
Skeletal muscle protection
A3AR agonists, including Cl-IB-MECA, protect skeletal muscle
in ischemic models in a phospholipase C-b2/b3-dependent
manner (Zheng et al., 2007).
CP-608,039 34 and CP-608,039 35
Cardioprotection
DeNinno et al. (2003) reported that CP-608,039 35 is a
highly A3AR selective and water-soluble agonist that was
being evaluated for the prevention of perioperative myocardial
ischemic injury. This follows numerous other reports showing
that A3AR activation protects ischemic cardiomyocytes by
preconditioning (Lasley, 2018).
Recently, selective A3AR deletion in mouse cardiomyocytes
was used to demonstrate that activation by selective agonist CP-
532,903 (34) of a myocardial A3AR provides ischemic tolerance
that is dependent on KATP channels (Wan et al., 2019). This study
resolves a long-standing controversy by showing that a protective
A3AR is present in adult ventricular cardiomyocytes, although
expressed at very low levels (copy number of 85 per 100 ng total
RNA versus 12,830 for the A1AR).
MRS5698 36
Pain
As noted above, A1AR agonists and PAMs are already under
consideration for pain treatment. The efficacy of A3AR agonists
for chronic pain was first explored depth in 2011 (Chen et al.,
2012). The approach of using A3AR agonists for pain treatment
was initially controversial, as A3AR activation had been described
in earlier review papers as an uninteresting target or even an anti-
target for pain relief (Nascimento et al., 2012;Janes et al., 2016).
Reasons for this premature characterization were: truly selective
A3AR agonists were not initially available and the A3AR causes
release of inflammatory mediators, e.g., histamine and serotonin,
from peripheral mast cells in rodents, but not human and other
species (Auchampach et al., 1997;Leung et al., 2014;Gao and
Jacobson, 2017). These mediators can contribute to inflammation
in mouse and rat, and Sawynok (1998) concluded that A3AR
activation induces pain and paw oedema. However, consistent
with the now well-documented action of A3AR activation in
various chronic pain models, A3AR KO mice had a lower pain
threshold in the hind-paw hot-plate test (40% greater latency,
Fedorova et al., 2003). Curiously, there was no difference between
A3AR KO and WT mice in the acute pain response in the
tail-flick test.
The two selective A3AR agonists in already clinical trials, IB-
MECA and Cl-IB-MECA, at high doses in vivo might interact
with other ARs, as has been observed in experimental models
(Fozard, 2010). A new series of C2-extended (N)-methanocarba
analogs displayed even greater A3AR selectivity, estimated to
be in the range of at least 10,000-fold in comparison to other
ARs (Jacobson et al., 2018). Among these agonists is MRS5698
33, which has been shown to reduce chronic neuropathic pain,
including oxaliplatin-induced neuropathic pain (Little et al.,
2015;Wahlman et al., 2018). MRS5698 has many drug-like
properties – it is non-toxic and relatively stable in vivo, except
that its oral bioavailability in the rat is only 5%F (Tosh et al.,
2015). Nevertheless, by various modes of administration it is
efficacious in pain models in vivo, including chronic constriction
injury-induced, chemotherapy-induced and cancer-induced. Its
ability to reduce chronic hyperalgesia is not affected by the A3AR-
induced histamine release observed in rodent but not human
mast cells (Carlin et al., 2016).
MRS5980 37
Pain
MRS5980 37 is a highly selective C2-arylethynyl (N)-
methanocarba A3AR agonist, which has been demonstrated
to be highly efficacious in in vivo pain models following
administered by oral gavage, with a protective effect lasting up to
3 h (Tosh et al., 2014;Janes et al., 2016), and its metabolomics
has been studied (Fang et al., 2015). The 2-chlorothienyl group
is stable in vivo, and the aryl alkyne group was shown to be
not highly reactive. MRS5980 and other A3AR agonist were
shown to indirectly block a pro-nociceptive N-type Ca2+
calcium channels, a proven target in controlling pain, and cell
excitability in the spinal cord dorsal horn (Coppi et al., 2019).
Thus, therapeutic application of A3AR agonists appears to be a
promising approach for treating pain of different etiologies.
LJ-529 33 and MRS4322 38
Stroke
von Lubitz et al. (1999) found that both A1AR and A3AR agonists
have cerebroprotective properties in a model of gerbil forebrain
ischemia. An (N)-methanocarba nucleoside MRS4322 38 was
proposed in a patent application (Korinek et al., 2018) as a
treatment for stroke and traumatic brain injury. The nucleoside
appears to act through the A3AR, to which it binds in the
µM range, because a selective A3AR antagonist propyl 6-ethyl-
5-((ethylthio)carbonyl)-2-phenyl-4-propylnicotinate (MRS1523)
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diminished the benefit of reduced stroke lesions. LJ-529 33
(40-thio-Cl-IB-MECA) is a selective A3adenosine agonist that
was shown to be protective in a rat stroke model and
inhibited brain migration of inflammatory cells (Choi et al.,
2011). However, platelet A2AAR activation by LJ-529 increased
the bleeding risk.
AR Allosteric Enhancers (PAMs)
PAMs of the A1AR and A3AR have been the subject of
preclinical and clinical evaluation (Figure 3 and Table 2).
PAMs, in principle, may remain silent until a large rise in
the extracellular adenosine occurs, at which time the PAM
would amplify adenosine’s action at a particular AR subtype.
Thus, PAMs are described as temporally and spatially specific
modulators (Gao et al., 2011). Also, PAMs tend to be more
subtype selective than orthosteric agonists, because their non-
canonical binding regions on the GPCRs are those that
have the most diverse sequences within the receptor family
(Vecchio et al., 2018).
T-62 39
Benzoylthiophenes are the earliest and most extensively studied
class of A1AR PAMs, and they have allosteric agonist properties
as well as enhancing the effect of other A1AR agonists (Vincenzi
et al., 2014;Jacobson and Gao, 2017). The benzoylthiophenes
Just as A1AR agonists have been considered for pain treatment,
a representative benzoylthiophene T-62 39 entered a clinical trial
for postherpetic neuropathic pain in 2008 that was discontinued
because of its lack of efficacy (Giorgi and Nieri, 2013). Also,
some patients displayed transient, elevated liver transaminases.
Nevertheless, T-62 reduced hypersensitivity in animal models
of neuropathic pain. The structural basis for recognition of
benzoylthiophene PAMs involving A1AR extracellular loops
(ELs) has been predicted using Gaussian accelerated molecular
dynamics (Miao et al., 2018).
TRR469 40
Pain
A later generation benzoylthiophene TRR469 40 was shown to
be an A1AR PAM that increases the affinity of A1AR agonists
(Vincenzi et al., 2014, 2016). It reduced pain, comparably to
morphine, in writhing and formalin tests and in chronic STZ-
induced diabetic neuropathy, and it displayed fewer behavioral
side effects than an A1AR orthosteric agonist. Furthermore, the
same A1AR PAM was anxiolytic in four behavioral models in the
mouse, in a manner comparable to the anxiolytic drug diazepam
(Vincenzi et al., 2016). This activity of 40, which was blocked
by a selective A1AR antagonist (DPCPX), is consistent with the
previously noted anxiolytic activity of A1AR agonists.
LUF6000 41
Inflammation and erectile dysfunction
LUF6000 41 is a imidazoquinolinamine A3AR allosteric enhancer
(PAM). It enhanced the maximal efficacy of A3AR agonists but
had no agonism on its own (Gao et al., 2011). Its interaction with
the A3AR was species-dependent (Du et al., 2018). Although it
was more efficacious in human, canine and rabbit than in rodent
species, it was shown to produce an anti-inflammatory effect in
rat models of adjuvant-induced arthritis and iodoacetate-induced
osteoarthritis and in a mouse model of concanavalin A-induced
liver inflammation (Cohen et al., 2014). The molecule is termed
CF602 and is on a translational path for treatment of erectile
dysfunction (Cohen et al., 2016).
CONCLUSION
The structural and pharmacological features of key AR
agonists and positive allosteric modulators (PAMs) have been
summarized, with an emphasis on molecules that have been
FIGURE 3 | A3AR- (31-38) selective agonists and allosteric enhancers (PAMs) of the A1AR (39,40) and A3AR (41).
Frontiers in Cellular Neuroscience | www.frontiersin.org 12 March 2019 | Volume 13 | Article 124
fncel-13-00124 March 26, 2019 Time: 18:47 # 13
Jacobson et al. Adenosine Receptor Agonists in Clinical Development
in humans or that were considered for human testing. From
the thousands of selective AR agonists or allosteric enhancers
reported, there are few translational successes. Many AR agonists
have been in clinical trials for disease treatment or diagnosis,
but only two are approved for human use, i.e., short-acting
agonists adenosine and Regadenoson. However, new concepts
and compounds are currently being developed and applied
toward preclinical and clinical evaluation, and initial results
are encouraging. AR agonists for treating inflammation, pain,
cancer, NASH, angina, sickle cell disease, ischemic conditions and
diabetes are under development. Multiple clinical trials with two
A3AR agonists are ongoing.
AUTHOR CONTRIBUTIONS
KJ organized the outline and wrote most of the text. DT
contributed to the writing and researching the topic. SJ
contributed to the writing and researching the topic. Z-GG
contributed to the writing and researching the topic.
ACKNOWLEDGMENTS
We thank the NIDDK Intramural Research Program for
funding (ZIA DK-31117).
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