Research
ON-GOING PROJECTS
1. Regulator of G protein Signaling proteins (RGS) and opioid analgesia
Regulators of G protein Signaling (RGS) proteins are a family of accessory proteins that act as negative
modulators of cell signaling, including signaling downstream of the mu-opioid receptor. Thus, knockdown of RGS protein activity should increase mu-opioid receptor signaling. However, our work suggests that while RGS proteins serve to switch-off some signaling pathways they paradoxically, increase intracellular calcium release. Thus, RGS activity appears to control the balance of signaling pathways. We suggest that RGS proteins act as “signal callers” and as such direct signaling downstream of mu-opioid receptors along certain pathways, thus potentially altering physiological outcomes. In this regard, using transgenic mice expressing a Galpha protein that is insensitive to the action of RGS proteins we have shown diverse effects on opioid-mediated antinociception that is dependent on the opioid agonist and pain assay employed. These findings raise a series of fundamental questions: Are clinically-relevant pain models also regulated by RGS proteins? Are the diverse responses across the antinociceptive tests due to differential effects of RGS proteins on signaling in areas related to pain control? What is the role of other neurotransmitter systems that also use Galpha proteins? Since males and females differ in sensitivity to opioids and certain RGS proteins are regulated by female sex hormones we are also interested to know how sex affects the action of RGS proteins on opioid signaling and behavior.
This study is in collaboration with Susan Ingram’s Lab at the Oregon Health Sciences University
(http://www.ohsu.edu/xd/research/research-expertise/researchers/index.cfm?personid=2260).
2. Allosteric modulation of the mu-opioid receptor
Morphine and related opioid drugs exert their effects by acting at the orthosteric site on the mu-opioid receptor,
i.e. the site where the endogenous opioid peptides bind. Recent advances in knowledge of GPCRs, and the realization that their function may be controlled by compounds binding at a separate, allosteric site on the receptors provides promise for attaining improved pharmacotherapies. In collaboration with Bristol-Myers Squibb we have identified the first small molecule positive allosteric modulator (PAM) that acts at mu-opioid receptors. Mu-PAMs could fundamentally change opioid analgesia. They have the potential to be used as adjuncts to morphine and so reduce the level of morphine required to afford analgesia. Perhaps more importantly, Mu-PAMs could be effective alone to enhance the activity of endogenous opioid peptides released, for example, under conditions of stress. This would preserve the temporal and spatial characteristics of neuronal signaling and avoid compensatory mechanisms that are induced by chronic mu-opioid receptor activation. We are further exploring these modulators along several avenues including characterization of the allosteric binding site on the receptor using X-ray crystallography and mutagenesis, development of structure activity relationships and study of the in vivo pharmacology of the mu-PAMs.
This project is in collaboration with the Showalter Lab in Medicinal Chemistry in the College of Pharmacy (https://pharmacy.umich.edu/people/showalh), the Kobilka Lab at Stanford (http://med.stanford.edu/kobilkalab/), the Filizola lab at Mount Sinai, NY (http://filizolalab.org/) and the Canals Lab at Monash (http://monash.edu/pharm/research/areas/drug-discovery/laboratories/canals.html)
3. Mixed function opioid peptides and peptidomimetics as potential analgesics
Activation of the mu-opioid receptor with morphine, while simultaneously blocking the delta opioid receptor, is reported to result in analgesia with reduced development of tolerance and dependence.
Thus there is therapeutic benefit in developing single chemical entities that have the potential to interact with both mu and delta opioid receptors simultaneously, but with different efficacies. Using receptor structural models and ligand docking we have developed a peptide (KSK103) with nM affinity for both receptors, but with agonist action at mu-receptors and antagonist action at delta receptors, and VRP26 a glycosylated analog of KSK103, which retains the KSK103 profile and is antinociceptive after peripheral administration in mouse models. We are currently examining alternate glycosylated analogs of KSK103 to improve upon the bioavailability of VRP26 as well as developing simpler structural analogs. In parallel we are studying “peptidomimetics” that retain the key structural elements of KSK103 on a non-peptide scaffold giving more ‘drug-like’ molecules.
This project is in collaboration with the Mosberg Lab in Medicinal Chemistry, College of Pharmacy (http://mosberglab.phar.umich.edu/) and the Jutkiewicz Lab in Pharmacology (https://sites.google.com/a/umich.edu/jutkiewicz-lab/).
4. Development of Pharmacotherapies for relapse prevention
Current treatments for drug addiction are only marginally effective. Approximately 70% of treated addicts relapse within the first year following treatment, while the majority of drug addicts take more than one addictive drug at any one time (poly-drug abuse). The aim of this project is to provide a pharmacotherapy to help prevent relapse to drug taking within the poly-drug using community. Our approach is based on findings that a
combination of the mu-opioid receptor partial agonist buprenorphine with the opioid antagonist naltrexone has been shown to be effective in significantly preventing relapse in the treatment of opioid dependence in patients who also use cocaine. However, pharmacokinetic incompatibility means buprenorphine and naltrexone are not amenable to formulation as a single product and it is not desirable to develop a product for currently abstinent addicts in which buprenorphine is taken separately to, or can be separated from the antagonist due to the risk of precipitating relapse and/or of buprenorphine being diverted. In addition, the resulting pharmacology of the combination product it is not known. We propose that a single chemical entity mimicking the buprenorphine/naltrexone combination will have broad utility in preventing relapse, will be safe to use within the recovering addict community and will answer the question of which pharmacological profile is best able to prevent relapse. To this end we are developing molecules that possess potent kappa- and delta-receptor antagonism, low mu-receptor agonism and with differing degrees of nociceptin receptor agonism. Receptor interactions of potential medications are defined in vitro with the ultimate designation of success measured as an ability to prevent relapse driven by cue, stress and/or drug in a rat model of reinstatement to cocaine or opioid self-administration.
This work is in collaboration with the Husbands Medicinal Chemistry Lab at the University of Bath, UK (http://www.bath.ac.uk/pharmacy/contacts/academics/stephen_husbands/)
Contact Information: Dr. John Traynor | Email: jtraynor@umich.edu | Phone: 734.647.7479 | Fax: 734.763.4450