The vasodilatory effects of adenosine A2A receptor agonists make them useful as cardiac stress agents in imaging but have made it difficult to exploit these molecules' potent anti-inflammatory effects.1 Researchers in Germany have dissociated the anti-inflammatory effects of adenosine A2A receptor agonists from their hemodynamic effects and are determining a specific inflammation-related indication to pursue with the molecule.2

Activation of the adenosine A2A receptor (ADORA2A) on endothelial cells promotes vasodilation, and activation of the receptor on immune cells inhibits inflammatory processes. The vasodilatory effects become dose-limiting side effects in the inflammation setting by causing hypotension, which in turn triggers a compensatory increase in heart rate and cardiac output.

Thus, Jürgen Schrader and colleagues at the Heinrich Heine University of Duesseldorf and the University of Bonn have been looking at the endogenous adenosine signaling pathway for molecular targets that can be exploited to dissociate the vasodilatory effects of ADORA2A agonists from their anti-inflammatory effects.

The search led the group to ecto-5ʹ-nucleotidase (NT5E; NT; CD73), which dephosphorylates adenosine monophosphate (AMP) to form adenosine3,4 (see "Effects of adenosine A2A receptor agonism"2A receptor agonism").

Schrader, a professor of physiology and head of the Department of Molecular Cardiology at the Heinrich Heine University of Duesseldorf, said the group was drawn to CD73 because of the enzyme's expression under normal and inflammatory conditions.

"We've previously observed that whenever there is inflammation, immune cells dramatically increase their expression of both CD73 and the A2A receptor," he told SciBX. "So our idea is why not make a phosphorylated A2A receptor agonist prodrug that remains biologically inactive until it is locally activated by CD73 at sites of inflammation."

In 2009, the researchers published data on a series of AMP derivatives and showed in vitro that the molecules were accepted as substrates by mouse Cd73 and dephosphorylated into the corresponding ADORA2A agonist.5

Now, the group has shown in vivo that one of the lead derivatives from the 2009 study, 2-cyclohexylethylthio-AMP (chet-AMP), is selectively activated at sites of inflammation and has negligible vasodilatory effects.

In mouse models of collagen-induced arthritis, delivery of chet-AMP via implanted osmotic minipumps decreased joint inflammation compared with no treatment. Importantly, plasma chet-AMP concentrations that had an anti-inflammatory effect were in the subnanomolar range and about 100-fold lower than the concentrations required for vasodilation.

Results were published in Science Translational Medicine.

Inflammatory history lessons

The prodrug strategy could help overcome a key barrier that has caused some ADORA2A agonists to flounder in the inflammation setting.

"The hemodynamic effects of A2A receptor agonists limit their dosing to the point where they are no longer effective at resolving inflammation, and this has contributed to the failure of several such compounds," said Schrader.

Examples of discontinued ADORA2A agonists include GW328267X from GlaxoSmithKline plc and UK-432097 from Pfizer Inc. Both compounds were being developed to treat chronic obstructive pulmonary disorder (COPD) but were discontinued in Phase II trials due to poor efficacy.

At least two ADORA2A agonists are still in clinical trials for inflammation-related indications. BVT.115959, which is being developed by CBT Development Ltd., Ergomed Group and Swedish Orphan Biovitrum AB, is in Phase II testing to treat diabetic neuropathic pain. Lexiscan regadenoson is being evaluated in an investigator-led Phase I trial to treat sickle cell disease-associated pain crisis.

Holger Eltzschig, a professor of anesthesiology, medicine, cell biology and immunology at the University of Colorado Denver School of Medicine, said targeting ADORA2A with a specific agonist could have an advantage over drugs that work via adenosine liberation, such as methotrexate. The specific compounds, he said, could be more effective at activating the receptor and may not be affected by pathways that rapidly shut down adenosine signaling.

Gilead Sciences Inc. and Astellas Pharma Inc. already market Lexiscan as a cardiac stress agent for use in myocardial perfusion imaging (MPI).

Despite the multitude of ADORA2A agonists, Schrader cautioned that the group's prodrug strategy cannot be generalized to any ADORA2A agonist. "The phosphorylated prodrug needs to be compatible as a substrate for CD73," he told SciBX.

For your inflammation

Schrader said the group's key step forward will be to select an appropriate inflammation-related indication to pursue with chet-AMP.

Bruce Cronstein, a professor in the Department of Medicine and Pathology and the Department of Biochemistry and Molecular Pharmacology at the New York University Langone Medical Center, thinks the ADORA2A agonist prodrug could be applied in chronic inflammation settings because it may be selectively activated at sites of inflammation.

Cronstein suggested that the researchers develop their compound in indications for which marketed anti-inflammatory drugs are not particularly effective. "Where they may want to head is to conditions such as psoriatic arthritis and ankylosing spondylitis," he told SciBX.

Joel Linden, professor of inflammation biology at the La Jolla Institute for Allergy & Immunology, liked the idea that chet-AMP could be used to create high local concentrations of an active ADORA2A agonist at sites of inflammation and thinks it is a clever and appealing approach to limit the exposure of other tissues to the active form of the compound.

However, he noted that because the prodrug compound is phosphorylated, it would most likely need to be delivered intravenously and in a hospital setting, which could make it difficult to develop for chronic inflammatory conditions.

Linden therefore thinks chet-AMP would be best suited for acute inflammation, such as treating acute flare-ups in patients with rheumatoid arthritis (RA).

Linden wanted to see additional examples of ADORA2A agonists that incorporate the CD73-activated prodrug strategy described by the researchers. He added that it also will be a good idea to determine whether chet-AMP could be formulated for oral delivery, or as a subcutaneous injection or implant.

Cronstein added that it will be important to show that chet-AMP is a substrate for human CD73 and to demonstrate that there would be no acute effects on blood pressure and bleeding in humans.

Finally, Eltzschig wanted to know whether the prodrug strategy could be applied to an ADORA2B agonist, as such compounds also are known to have potent anti-inflammatory effects.

Schrader said the group now is looking at CD73 and ADORA2A expression patterns in mouse models of other inflammation-related conditions, such as ischemia/reperfusion injury and sepsis, to help determine whether chet-AMP could be useful in such settings.

"If we could show in these other disease models that our A2A receptor prodrug works equally well as known A2A receptor agonists but without the hemodynamic side effects, it could considerably broaden its application in inflammation," he told SciBX.

The Heinrich Heine University of Duesseldorf and the University of Bonn have cofiled a patent covering phosphorylated ADORA2A agonists. The work is available for licensing.

Lou, K.-J. SciBX 5(34); doi:10.1038/scibx.2012.887
Published online Aug. 30, 2012


1.   Haskó, G. et al. Nat. Rev. Drug Discov. 7, 759-770 (2008)

2.   Flögel, U. et al. Sci. Transl. Med.; published online Aug. 8, 2012; doi:10.1126/scitranslmed.3003717
Contact: Jürgen Schrader, Heinrich Heine University of Duesseldorf, Duesseldorf, Germany

3.   Deaglio, S. et al. J. Exp. Med. 204, 1257-1265 (2007)

4.   Kobie, J.J. et al. J. Immunol. 177, 6780-6786 (2006)

5.   El-Tayeb, A. et al. J. Med. Chem. 52, 7669-7677 (2009)


      Astellas Pharma Inc. (Tokyo:4503), Tokyo, Japan

      CBT Development Ltd., Cambridge, U.K.

      Ergomed Group, Frankfurt, Germany

      Gilead Sciences Inc. (NASDAQ:GILD), Foster City, Calif.

      GlaxoSmithKline plc (LSE:GSK; NYSE:GSK), London, U.K.

      Heinrich Heine University of Duesseldorf, Duesseldorf, Germany

      La Jolla Institute for Allergy & Immunology, La Jolla, Calif.

      New York University Langone Medical Center, New York, N.Y.

      Pfizer Inc. (NYSE:PFE), New York, N.Y.

      Swedish Orphan Biovitrum AB (SSE:SOBI), Stockholm, Sweden

      University of Bonn, Bonn, Germany

      University of Colorado Denver School of Medicine, Denver, Colo.