17-Cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-(4′-pyridylcarboxamido)morphinan (NAP) Modulating the Mu Opioid Receptor in a Biased Fashion

Abstract

Mounting evidence has suggested that G protein-coupled receptors can be stabilized in multiple conformations in response to distinct ligands, which exert discrete functions through selective activation of various downstream signaling events. In accordance with this concept, we report biased signaling of one C6-heterocyclic substituted naltrexamine derivative, namely 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-(4′-pyridylcarboxamido)morphinan (NAP) at the mu opioid receptor (MOR). NAP acted as a low efficacy MOR partial agonist in the G protein-mediated [35S]GTPγS binding assay, whereas it did not significantly induce calcium flux or β-arrestin2 recruitment. In contrast, it potently blocked MOR full agonist-induced β-arrestin2 recruitment and translocation. Additionally, NAP dose-dependently antagonized MOR full agonist-induced intracellular calcium flux and β-arrestin2 recruitment. Further results in an isolated organ bath preparation confirmed that NAP reversed the morphine-induced reduction in colon motility. Ligand docking and dynamics simulation studies of NAP at the MOR provided more supporting evidence for biased signaling of NAP at an atomic level. Due to the fact that NAP is MOR selective and preferentially distributed peripherally upon systemic administration while β-arrestin2 is reportedly required for impairment of intestinal motility by morphine, biased antagonism of β-arrestin2 recruitment by NAP further supports its utility as a treatment for opioid-induced constipation.

Don’t stress about CRF: assessing the translational failures of CRF1antagonists

“…Similarly, the structural manner of binding the receptor may be important; perhaps small molecules that bind differentially to certain residues of the atypical allosteric binding site (e.g., compare MTIP vs. CP-376395; Xu et al 2015) may slow the antagonist’s escape kinetics (Bai et al 2014) or direct its anti-signaling pathway bias (Suen et al 2014; Zhang et al 2016). Alternatively, a small molecule that binds to the orthosteric (agonist)-binding site, rather than to the atypical, deep allosteric binding site that is bound by all clinically-evaluated CRF 1 antagonists to date (see Zorrilla and Koob, 2010; Hollenstein et al 2013; Hausch 2013) may yield different pharmacological effects.…”

Mu-Opioid receptor biased ligands: A safer and painless discovery of analgesics?

“…For example, GPCRs have high homology within the transmembrane (TM) domains (approximately 60% of sequence identity), but less in the extracellular regions, a feature that contributes to the selectivity between different ligands [24], particularly at extracellular loop 3 (ECL3) and the extracellular ends of TM6 and TM7 [25]. Differences in intracellular regions are involved in downstream signaling for the activation of different pathways, where conformational changes in TM7, helix 8 (H8), and ECL2 might have a role in β-arrestin recruitment, while changes in TM3, TM5, and TM6, as well as in the intracellular loop 2 (ICL2), might be related to G protein-mediated signaling [26]. This downstream preference is the basis of functional selectivity.…”

“…Unlike the biased MOR agonists, NAP was characterized as a biased MOR antagonist rather than a weak partial agonist by Zhang et al [26]. NAP has a morphine-like structure and exhibited weak partial agonist potency (23% of DMAGO) in a [ 35 S]GTPγS binding assay with no apparent recruitment of β-arrestin-2 [56].…”

Biased G Protein-Coupled Receptor Signaling: New Player in Modulating Physiology and Pathology

“…Moreover, it has no apparent analgesic effect due to its low efficacy in activating G protein-mediated signaling with no apparent effect on β-arrestin2 recruitment. However, its therapeutic potential lies in its ability to antagonize MOR full agonist-induced intracellular calcium flux and β-arrestin2 recruitment (Zhang et al ., 2016). NAP dose-dependently restored the morphine-impaired intestinal motility without precipitating significant withdrawal of symptoms and thus held great promise in the treatment of opioid-induced constipation (Yuan et al ., 2012) (Table 1).…”