Significance: Opioids acting at the mu-opioid receptor (MOR) are the gold standard for pain relief. Unfortunately, these drugs have severe side effects, including addiction liability and respiratory depression. Consequently, safe pain management remains an unmet medical need. Here, we show that a positive allosteric modulator of MOR (a mu-PAM) does not directly activate MOR but produces analgesia in mouse models by enhancing the ability of endogenous opioid peptides, released in pain states, to activate MOR. Because the mu-PAM does not activate MOR directly, this raises the possibility of analgesia without the typical opioid side effects of addiction liability, respiratory depression, or constipation. The study provides proof of principle for mu-PAMs as an innovative and potentially safer approach to pain management.

Mitragynine and 7-hydroxymitragynine (7OH) are the major alkaloids mediating the biological actions of the psychoactive plant kratom. To investigate the structure–activity relationships of mitragynine/7OH templates, we diversified the aromatic ring of the indole at the C9, C10, and C12 positions and investigated their G-protein and arrestin signaling mediated by mu opioid receptors (MOR). Three synthesized lead C9 analogs replacing the 9-OCH₃ group with phenyl (4), methyl (5), or 3′-furanyl [6 (SC13)] substituents demonstrated partial agonism with a lower efficacy than DAMGO or morphine in heterologous G-protein assays and synaptic physiology. In assays limiting MOR reserve, the G-protein efficacy of all three was comparable to buprenorphine. 6 (SC13) showed MOR-dependent analgesia with potency similar to morphine without respiratory depression, hyperlocomotion, constipation, or place conditioning in mice. These results suggest the possibility of activating MOR minimally (G-protein E_max ≈ 10%) in cell lines while yet attaining maximal antinociception in vivo with reduced opioid liabilities.

Background and purpose: The κ receptor has a central role in modulating neurotransmission in central and peripheral neuronal circuits that subserve pain and other behavioural responses. Although κ receptor agonists do not produce euphoria or lead to respiratory suppression, they induce dysphoria and sedation. We hypothesized that brain‐penetrant κ receptor ligands possessing biased agonism towards G protein signalling over β‐arrestin2 recruitment would produce robust antinociception with fewer associated liabilities.Experimental approach: Two new diphenethylamines with high κ receptor selectivity, HS665 and HS666, were assessed following i.c.v. administration in mouse assays of antinociception with the 55°C warm‐water tail withdrawal test, locomotor activity in the rotorod and conditioned place preference. The [³⁵S]‐GTPγS binding and β‐arrestin2 recruitmentin vitroassays were used to characterize biased agonism.Key results: HS665 (κ receptor agonist) and HS666 (κ receptor partial agonist) demonstrated dose‐dependent antinociception after i.c.v. administration mediated by the κ receptor. These highly selective κ receptor ligands displayed varying biased signalling towards G protein couplingin vitro, consistent with a reduced liability profile, reflected by reduced sedation and absence of conditioned place aversion for HS666.Conclusions and implications: HS665 and HS666 activate central κ receptors to produce potent antinociception, with HS666 displaying pharmacological characteristics of a κ receptor analgesic with reduced liability for aversive effects correlating with its low efficacy in the β‐arrestin2 signalling pathway. Our data provide further understanding of the contribution of central κ receptors in pain suppression, and the prospect of dissociating the antinociceptive effects of HS665 and HS666 from κ receptor‐mediated adverse effects.