Development of Gαi protein selective μ opioid receptor analgesics to limit side‐effects

Abstract

The clinical utility of opioids as analgesics is limited by their serious side effects, which include respiratory inhibition, gastrointestinal motility disorders, tolerance and addiction. Opioids act by binding to μ opioid receptors (MORs) to trigger G‐protein and β‐arrestin‐mediated signaling cascades. The Gαi class of G‐protein is thought to play an important role in transducing the analgesic actions of opioids. Gαi subunits include Gi1, Gi2, Gi3, Go and Gz. In addition, Gβγ subunits also contribute to the actions of opioids, and the many subtypes of Gβγ subunits adding yet more complexity to MOR‐mediated signaling.Gαi proteins show discrete tissue and brain distributions, each subunit has discrete enzymatic kinetics, and these subunits are differentially regulated by GTPase‐activating proteins (GAPs), Regulator of G protein (RGS) proteins, and other effectors. Moreover, findings from genetically modified mice suggest that disruption of specific Gαi proteins results in discrete patterns of altered responses to opioids. Based on these observations, we hypothesize that compounds can be developed to modulate specific Gi protein coupling to μ receptors, which will result in analgesic actions with less pronounced side‐effects.Currently, novel opioids are developed by structure‐based or Gqi5‐mediated calcium readout. Consequently, Gqi5 biased opioids are likely to be preferred. To identify functionally novel opioids, we constructed MOR/Gqi15, i35, o5 and z5 in HEK 293 cells. We profiled ten reference compounds: DAMGO, β‐endorphin, met‐enkephalin, morphine, hydrocodone, oxycodone, TRV130, PZM21, SR17018 and fentanyl in these cell lines. We found that the majority of opioids signal through all Gi proteins with similar potency and efficacy. However, a minority showed strong Gi protein bias. Finally, because neuronal excitability is mediated by Gβγ subunits acting at N‐type CaV and G protein gated inwardly‐rectifying potassium (GIRK) channels, we generated MOR/GIRK1/2 and MOR/CaV2.2 α1/α2δ1/β1 in HEK 293 cells. These lines will be used to identify molecules that influence Gβγ actions on the activity of these effector channels.These preliminary data demonstrate that opioids are likely signal in manner that can be biased to specific Gi proteins. This offers an opportunity to differentiate/discover compounds with unique pharmacodynamic profiles. Novel opioids that act through discrete Gi protein, or on Gβγ proteins, will facilitate greater understanding about the relationship between selective G‐protein coupling and opioid‐related side‐effects. We aim to leverage this new information to develop novel opioid analgesics with limited side‐effects.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.