Characterization of Positive Allosteric Modulators of the Mu Opioid Receptor

Abstract

Activation of the mu-opioid receptor (MOR) is responsible for the beneficial analgesic actions of opioid drugs as well as their unwanted actions, including constipation, respiratory depression, and misuse liability. Opioid analgesics that act at MORs are the traditional standard for the clinical management of pain but come with significant risks for adverse events. Thus, an alternative approach for managing pain with improved safety remains an unmet need. One approach utilizes positive allosteric modulators (PAMs) of MOR that interact with a separate location on the receptor from endogenous and traditional (orthosteric) opioid drugs. Previous in vitro studies presented that PAMs enhance the potency and/or efficacy of orthosteric opioids. Additionally, PAMs have been shown to enhance endogenous opioid peptide-mediated antinociception with reduced development of side effects than traditional opioids in vivo.

One alternative approach suggests MOR agonists that preferentially signal to certain intracellular pathways (specifically a bias towards G-protein over -arrestin), might show an improved therapeutic index. Additionally, literature suggests that allosteric modulators can influence G-protein-coupled receptor (GPCR) ligand signaling profiles. However, studies have yet to examine this at MOR. In Chapter 2, I compare the ability of orthosteric MOR agonists to activate two signaling pathways; G-protein activation and β-arrestin recruitment in the absence or presence of two structurally distinct MOR PAMs, BMS-986187 or BMS-986122. Orthosteric agonists included in this study have been previously reported as neutral (morphine, methadone, and Met-Enkephalin), G-protein biased (SR17018), or potentially β-arrestin 2 biased (fentanyl). Both BMS-986187 and BMS-986122 shifted the G protein activation concentration-response curve to the left similarly across the MOR-agonists. In contrast, BMS-986187 enhanced the potency for β-arrestin recruitment ranging from 10-178 fold, whereas BMS-986122 elicited much smaller shifts with all orthosteric agonists (1-7 fold). In the absence of PAM, we report the signaling bias for DAMGO and methadone as neutral, morphine, Met-Enkephalin, and SR17018 as G protein, and fentanyl as β-arrestin 2 biased. BMS-986187 significantly enhanced the β-arrestin 2 bias for fentanyl as well as the G protein bias for SR17018. BMS-986122 reduced the β-arrestin 2 bias for fentanyl and increased the G protein bias for methadone. In addition, both PAMs increased receptor internalization induced by DAMGO, morphine, and methadone, but not fentanyl. Overall, these studies provide evidence that PAMs can differentially influence the degree and direction of downstream signaling of MOR depending on the orthosteric agonist present.

While previous studies determined the ability of PAMs to enhance endogenous opioid effects, we do not yet know the pharmacology of PAM function in the presence of exogenous opioids, for example morphine, in vivo. Furthermore, it is unknown if PAMs enhance all of the effects induced by exogenous opioids, such as antinociception, constipation, respiratory depression, and reward. If PAMs only enhanced the desired opioid-induced effects, they would be clinically applicable for opioid sparing, that is combining with opioid drugs to reduce the doses of opioids needed for clinical pain relief. In Chapter 3, I study the ability of MOR PAM, BMS-986122, to enhance the effects of three clinically relevant opioids, morphine, methadone, and fentanyl. I show that BMS-986122 enhances MOR opioid agonist-induced antinociception, but does not alter constipation or respiratory depression. Additionally, BMS-986122 shows a slight attenuation of fentanyl-induced reward. Overall, these data provide a rationale for further development of MOR PAMs to be used as opioid sparing agents in the clinic.