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Review
. 2019 Apr 17:10:407.
doi: 10.3389/fphar.2019.00407. eCollection 2019.

A Review of the Therapeutic Potential of Recently Developed G Protein-Biased Kappa Agonists

Affiliations
Review

A Review of the Therapeutic Potential of Recently Developed G Protein-Biased Kappa Agonists

Kendall L Mores et al. Front Pharmacol. .

Abstract

Between 2000 and 2005 several studies revealed that morphine is more potent and exhibits fewer side effects in beta-arrestin 2 knockout mice. These findings spurred efforts to develop opioids that signal primarily via G protein activation and do not, or only very weakly, recruit beta-arrestin. Development of such molecules targeting the mu opioid receptor initially outpaced those targeting the kappa, delta and nociceptin opioid receptors, with the G protein-biased mu opioid agonist oliceridine/TRV130 having completed phase III clinical trials with improved therapeutic window to treat moderate-to-severe acute pain. Recently however, there has been a sharp increase in the development of novel G protein-biased kappa agonists. It is hypothesized that G protein-biased kappa agonists can reduce pain and itch, but exhibit fewer side effects, such as anhedonia and psychosis, that have thus far limited the clinical development of unbiased kappa opioid agonists. Here we summarize recently discovered G protein-biased kappa agonists, comparing structures, degree of signal bias and preclinical effects. We specifically reviewed nalfurafine, 22-thiocyanatosalvinorin A (RB-64), mesyl-salvinorin B, 2-(4-(furan-2-ylmethyl)-5-((4-methyl-3-(trifluoromethyl)benzyl)thio)-4H-1,2,4-triazol-3-yl)pyridine (triazole 1.1), 3-(2-((cyclopropylmethyl)(phenethyl)amino)ethyl)phenol (HS666), N-n-butyl-N-phenylethyl-N-3-hydroxyphenylethyl-amine (compound 5/BPHA), 6-guanidinonaltrindole (6'GNTI), and collybolide. These agonists encompass a variety of chemical scaffolds and range in both their potency and efficacy in terms of G protein signaling and beta-arrestin recruitment. Thus unsurprisingly, the behavioral responses reported for these agonists are not uniform. Yet, it is our conclusion that the kappa opioid field will benefit tremendously from future studies that compare several biased agonists and correlate the degree of signaling bias to a particular pharmacological response.

Keywords: G protein; antinociception; beta-arrestin; diphenethylamine; kappa opioid receptor; nalfurafine; side effects; signaling bias.

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Figures

FIGURE 1
FIGURE 1
Hypothesized benefits of G protein-biased κ opioid receptor agonists.
FIGURE 2
FIGURE 2
Chemical structures of ‘affinity-dominant’ and ‘efficacy-dominant’ G protein-biased κ opioid receptor agonists.
FIGURE 3
FIGURE 3
Nalfurafine and GR89,696, respectively potently and efficaciously recruit β-arrestin 2 recruitment following κOR activation. Dose-dependent recruitment of β-arrestin 2 to κOR following activation with nalfurafine (n = 3, red formula image), GR89,696 (n = 4, formula image), dynorphin A (n = 4, blue formula image), U50,488 (n = 4, ) and salvinorin A (n = 4, brown formula image) in PathHunter U2OS hOPRK1/β-arrestin-2 cells (DiscoverX, Fremont, CA, United States). Each concentration was tested in duplicate, and three independent dose-response curves were produced for each agonist as previously described (Chiang et al., 2016). Data is normalized to U50,488. Nalfurafine was purchased from AdooQ (Irvine, CA, United States) all other compounds were from Tocris (Minneapolis, MN, United States).
FIGURE 4
FIGURE 4
Difference between an “affinity-dominant” and “efficacy-dominant” G protein-biased agonist. An “affinity-dominant” κOR agonist has a higher affinity for the κOR conformation that activates G proteins than that recruits β-arrestin 2: see top left panel for an example of a κOR agonist that resembles RB-64, with a G protein-coupling EC50 potency of 5.5 nM (dotted line, pink formula image, G-protein) and EC50 potency for β-arrestin 2 recruitment of 550 nM and 100% efficacy (solid line, light blue formula image). In contrast an ‘efficacy-dominant” κOR agonist, that resembles HS666 with an EC50 potency for β-arrestin 2 recruitment of 550 nM, but 10% efficacy (solid line, blue formula image), only weakly recruits β-arrestin 2 even at high concentrations (top right panel). Consider the endogenous agonist dynorphin A, which recruits β-arrestin 2 at 100% efficacy (middle panel). At high concentrations the affinity-dominant agonist will displace dynorphin from the κOR, yet retain highly efficacious recruitment of β-arrestin 2 (bottom left panel). In contrast the efficacy-dominant agonist will reduce β-arrestin 2 recruitment efficacy once this type of agonists displaces dynorphin from the κOR (bottom right) panel.

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