FBNTI, a DOR-Selective Antagonist That Allosterically Activates MOR within a MOR–DOR Heteromer

Abstract

This report describes the unique pharmacological profile of FBNTI, a potent DOR antagonist that acts as a MOR agonist via an allosteric mechanism. Binding of FBNTI to opioid receptors expressed in HEK 293 cells revealed a 190-fold greater affinity for DOR (K i = 0.84 nM) over MOR (K i = 160 nM). In mice, intrathecal FBNTI produced potent antinociception (ED50 = 46.9 pmol/mouse), which was antagonized by selective MOR antagonists (CTOP, β-FNA). Autoantagonism of the MOR agonism by FBNTI was observed above the ED75 dose, suggesting antagonism of activated MOR. That FBNTI is devoid of agonism in DOR knockout mice is consistent with allosteric activation of the MOR protomer via FBNTI bound to within a MOR–DOR heteromer. This proposed mechanism is supported by calcium mobilization assays, which indicate that FBNTI selectively activates the MOR–DOR heteromer and functionally antagonizes the MOR protomer at >ED75. The unprecedented mode of MOR activation by FBNTI may be responsible for the lack of tolerance after intrathecal (i.t.) administration. FBNTI was highly effective upon topical administration to the ipsolateral hind paw in the Hargreaves assay (EC50 = 0.17 ± 0.08 μM) and without significant contralateral activity, suggesting a lack of systemic exposure.

MCRT, a multifunctional ligand of opioid and neuropeptide FF receptors, attenuates neuropathic pain in spared nerve injury model

“…Furthermore, quite opposite to the independence of opioid and NPFF receptors in colonic motility disorder, 20 its anti‐allodynia in SNI animals was fully antagonized by each antagonist of mu‐, delta‐opioid and NPFF receptors. Similar to the heterodimers of mu/delta receptor and mu/NPFF receptor, 35,36 the complete blocking by any of the antagonists might support the existence of mu/delta/NPFF receptor heteromer, which could be allosterically modulated by the antagonist occupancy of each protomer as previously hypothesized 37 . Unfortunately, the mechanism of MCRT was puzzled to make clear explanations as yet.…”

“…Similar to the heterodimers of mu/delta receptor and mu/NPFF receptor, 35,36 the complete blocking by any of the antagonists might support the existence of mu/delta/NPFF receptor heteromer, which could be allosterically modulated by the antagonist occupancy of each protomer as previously hypothesized. 37 Unfortunately, the mechanism of MCRT was puzzled to make clear explanations as yet.…”

Introduction to the Biochemistry of Pain Special Issue

“…A trio of papers then expands on natural products that may represent facile scaffolds for the development of safer and more effective drugs targeting opioid receptors. − The authors focus on the intriguing natural products salvinorin A (a selective k-opioid receptor agonist) and mitragynine, which is active ingredient of Kratom (Figure ). Combining recent high-resolution structural insights into opioid receptor actions − and molecular dynamics simulations, the authors show how insights into the molecular details of opioid receptor actions may accelerate the discovery of safer opioid analgesics.…”

“…Combining recent high-resolution structural insights into opioid receptor actions − and molecular dynamics simulations, the authors show how insights into the molecular details of opioid receptor actions may accelerate the discovery of safer opioid analgesics. Finally, Akgun and colleagues show how a δ-opioid-selective antagonist may exert allosteric actions at heterodimeric opioid receptors …”

“…Finally, Akgun and colleagues show how a δ-opioid-selective antagonist may exert allosteric actions at heterodimeric opioid receptors. 10 Given the rapid advances in structural determination of GPCR−transducer complexes by X-ray crystallography and cryo-electron microscopy (see refs 15 and 16 for recent examples) and the advent of ultra-large-scale computational docking approaches, 17 it is likely that structure-guided approaches will accelerate the discovery of safer analgesic agents. 18 We hope that you will enjoy this Biochemistry collection highlighting some of the latest advancements in the biochemistry of pain.…”