No abstract

μ opioid receptor (MOR) agonists have been widely applied for treating moderate to severe pain. However, numerous adverse effects have been associated with their application, including opioid-induced constipation (OIC), respiratory depression, and addiction. On the basis of previous work in our laboratory, NAP, a 6β-N-4′-pyridyl substituted naltrexamine derivative, was identified as a peripheral MOR antagonist that may be used to treat OIC. To further explore its structure–activity relationship, a new series of NAP derivatives were designed, synthesized, and biologically evaluated. Among these derivatives, NFP and NYP significantly antagonized the antinociception effect of morphine. Whereas NAP acted mainly peripherally, its derivatives NFP and NYP actually can act centrally. Furthermore, NFP produced significantly lesser withdrawal symptoms than naloxone at similar doses. These results suggest that NFP has the potential to be a lead compound to treat opioid abuse and addiction.

No abstract

The 6β-N-heterocyclic naltrexamine derivative, NAP, has been demonstrated to be a peripherally selective mu opioid receptor modulator. To further improve peripheral selectivity of this highly potent ligand, its pyridal ring was quaterinized with benzyl bromide to produce BNAP. In radioligand binding assay, the Ki of BNAP for MOR was 0.76 ± 0.09 nM and was >900 fold more selective for MOR than DOR. The Ki for KOR was 3.46 ± 0.05 nM. In [35S]GTPγS ligand stimulated assay, BNAP showed low agonist efficacy with 14.6% of the maximum response of DAMGO with an EC50 of 4.84 ± 0.6 nM. However, unlike its parent compound NAP, BNAP displayed partial agonist activity at KOR with % maximum response at 45.9 ± 1.7% of U50,488H. BNAP did not reverse morphine-induced antinociception when administered subcutaneously but did antagonize when administered intracerebroventricularly. BNAP antagonized morphine-induced contractions of the circular muscle in mice colon. BNAP inhibition of field-stimulated contractions in longitudinal muscle strips for the guinea-pig ileum were also blocked by nor-BNI, a kappa opioid receptor antagonist. BNAP induced inhibition of acetic acid induced abdominal stretching in chronic morphine treated mice. These findings suggest that BNAP is a dual MOR antagonist/KOR agonist and may have functional use in irritable bowel patients.

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.