Development of Drugs Acting at Adenosine Receptors

National Institute Of Diabetes And Digestive And Kidney Diseases

Investigators

Linked publications & trials

Abstract

Our laboratory has been one of the most active sources of new AR ligands that have therapeutic potential, and we are careful to cover promising discoveries with patent applications to safeguard the interest of public health and to facilitate translational medicine. Furthermore, we have designed and synthesized many ligands that are used widely in research. Synthetic selective ligands that either mimic the action of adenosine (i.e. agonists) in a fashion specific at a single subtype, or suppress it (i.e. antagonists), can be applied with great benefit in models of disease states, and therefore such agents are being explored as potential pharmaceuticals. For example, selective AR antagonists can benefit patients suffering from neurodegenerative diseases. On the other hand, AR agonists are more applicable to treating other chronic diseases, e.g. in inflammatory diseases and cardioprotection. Thus, we are constantly searching for the most viable disease targets and translational opportunities for our technology. Our collaborations include studies of the role of ARs in the central nervous system, inflammatory/immune system, cardiovascular system, skeletal muscle, and other systems throughout the body. We have combined in vitro and machine learning approaches to search for adenosine receptor ligands, an approach that could be readily applied to other GPCR targets, identifying several existing molecules with previously unknown AR activity that could be a starting point for future molecule designs. A structural understanding of the molecular recognition and activation of ARs is an important component of our studies that leads to novel ligands. The more that is known about the 3-dimensional structure of the target receptor, the easier it is to design appropriate ligands. In that regard, we have made significant advances in the approaches to structure-based discovery of small molecular ligands of ARs. The rational design of AR ligands was greatly advanced by the recent elucidation of both the agonist-bound (activated) and antagonist-bound (inactive) states of the A2AAR. We have collaborated for X-ray crystallography of membrane-bound proteins with the lab of Prof. Ray Stevens of the Scripps Research Inst., to report the first X-ray structure of an agonist-bound A2AAR. We are now using this structure advantageously for in silico screening to discover new chemically diverse ligands and to modify existing ligands in ways made possible only through a detailed understanding of molecular recognition and activation of the receptor. This computerized process to discover new potential pharmaceuticals still requires chemical synthesis of target compounds to validate the structural hypotheses. We collaborate with Matt Eddy to use nuclear magnetic resonance to follow the signaling path within the A2A AR, for example the role of anionic lipids in the activation pathway. Our novel selective A3AR agonists and antagonists containing a methanocarba (bicyclo3.1.0hexane) ribose-ring substitution constrained in the receptor-preferred North (N) conformation have enhanced pharmacological profiles. Also, at the A1AR, (N)-methanocarba nucleosides were truncated to eliminate 5-CH2OH with partial or full retention of agonism. Truncated derivatives have more drug-like physical properties; this approach is appealing for preclinical development of nucleoside analogues. Our ongoing program to develop selective A3AR agonists has resulted in advanced clinical trials of two nucleosides. Therapeutic interests related to selective ligands for the A3AR are anti-inflammatory, anti-ischemic (e.g. in the heart, brain, lungs, and skeletal muscle) and anticancer, by molecular mechanisms that entail modulation of the Wnt and the NF-kB signal transduction pathways. The A3AR is overexpressed in inflammatory and cancer cells, while low expression is found in normal cells, rendering the A3AR as a potential therapeutic target. Currently, A3AR agonists discovered in our lab (IB-MECA and Cl-IB-MECA) are already in advanced clinical trials; MRS4322, recently entered a clinical trial for stroke, and is also intended for traumatic brain injury. If successful, it would fill an unmet medical need. We study the inhibition of ABC transporters by A3 agonists and synthesizing nucleosides that are selective for that action. We designed a masked, photocleavable derivative of A3 agonist MRS5698 that can be used as a light-activated prodrug for skin conditions. When administered systemically, it was protective in a mouse model of IL-23 induced psoriasis, but only when the lesion area was irradiated with blue light. This is one of the first examples of light-directed delivery of a potent GPCR ligand selectively to the skin to act as an anti-inflammatory agent. Sterically constrained ((N)-methanocarba) adenosine derivatives were nanomolar full agonists of the A3AR and highly selective (>3000-fold). Combined 2-arylethynyl-N6-3-chlorobenzyl substitutions preserved A3AR affinity/selectivity (e.g., 3,4-difluorophenylethynyl full agonist MRS5698). We discovered rationally designed macrocyclic derivatives of small molecular GPCR agonists, i.e. closing a ring as predicted by molecular modeling to preserve high A3 agonist affinity. There were differences between Cl-IB-MECA and the macrocycles in the spectrum of signaling pathways. In collaboration with Daniela Salvemini, we have shown encouraging protective activity of A3AR agonists in models of chronic neuropathic pain, including visceral pain. A3AR agonists suppress or prevent the development of chronic neuropathic pain in mice following chronic constriction injury or cancer chemotherapeutic agents (by both central and peripheral mechanisms). Highly selective A3 agonists MRS5698 and MRS5980 were more potent and efficacious than morphine. If used therapeutically, A3 agonists could facilitate the life-saving use of cancer chemotherapy, which often has to be limited or discontinued because of severe side effects such as pain. A3 agonists in combination with opioids reduce opioid side effects to tolerance, withdrawal and opioid-induced hyperalgesia.In collaboration with Laura Lucarini, we identified a new beneficial activity of A3AR agonists, potential treatment of lung fibrosis. Another promising application of AR ligands is the use of A3AR antagonists for topical application in the treatment of glaucoma, including A3AR antagonists from our lab. Blocking A2A and or A2B receptors with antagonists is being explored to boost cancer immunotherapy. Our collaboration with Lak Shin Jeong demonstrated that a thiophene modification at the C8 position in the common adenine scaffold converted an A2AAR agonist into an antagonist. We discovered that protein kinase C (PKC) both enhances and activates the A2B adenosine receptor. Thus, anticancer A2B antagonists may act in two ways, blocking the effects of local adenosine and PKC (also an anticancer target for inhibitors). A means of lowering adenosine in the tumor microenvironment is to inhibit the enzyme CD73, and we have discovered novel nucleotide inhibitors of CD73. We have discovered positive allosteric modulators (PAMs) of the A3AR that magnify the beneficial effects of the endogenous adenosine released during stress conditions. These effects of the PAMs would be event-specific within the body, i.e. the PAM would have no biological effect on its own unless adenosine was locally elevated. In collaboration with John Auchampach (Med. Coll. Wisconsin), we recently expanded the range of heterocyclic PAMs of the A3AR and demonstrated that they have an unexpected site of action: on the intracellular side of the receptor. A1 agonist are cerebroprotective and anticonvulsant agents. Most known A1 receptor agonists display unacceptable side effects due to their peripheral action, but MRS5474 is efficacious without these side effects.