Novel molecular targets of dezocine and their clinical implications

Abstract

Background: Although dezocine is a partial μ-opioid receptor agonist, it is not a controlled substance. Thus, the characterization of the molecular targets of dezocine is critical for scientific and clinical implications. The goal of this study is to characterize molecular targets for dezocine and determine their implications.

Methods: A binding screen for dezocine was performed on 44 available receptors and transporter proteins. Functional assays for the novel targets were performed along with computation calculations to locate the binding site. A G protein activation study was performed for the human κ opioid receptor to determine whether dezocine is a κ-antagonist. Data are presented as mean ± standard error.

Results: The affinities for dezocine were 3.7 ± 0.7 nM for the μ receptor, 527 ± 70 nM for the δ-receptor, and 31.9 ± 1.9 nM for the κ-receptor. Dezocine failed to induce G protein activation with κ-opioid receptor and concentration dependently inhibited κ-agonist (salvinorin A and nalbuphine)-induced receptor activation, indicating that dezocine is a κ-antagonist. Two novel molecular targets (norepinephrine transporter and serotonin transporter) were identified. Dezocine concentration-dependently inhibited norepinephrine and serotonin reuptake in vitro. The half maximal inhibitory concentrations (expressed as pIC50) were 5.68 ± 0.11 for norepinephrine transporter and 5.86 ± 0.17 for serotonin transporter. Dezocine occupied the binding site for known norepinephrine transporter and serotonin transporter inhibitors.

Conclusions: The unique molecular pharmacological profile of dezocine as a partial μ-receptor agonist, a κ-receptor antagonist, and a norepinephrine and serotonin reuptake inhibitor (via norepinephrine transporter and serotonin transporter) was revealed. These discoveries reveal potentially important novel clinical implications and drug interactions of dezocine.

Figure 1

The structures of the ligands used in this study to probe the pharmacological properties of dezocine are listed. All structures were obtained from public domain without further modification or verification by a chemist except salvinorin A and JDTic. Salvinorin A (http://commons.wikimedia.org/wiki/File:Salvinorin-A_structure.png) and JDTic (http://commons.wikimedia.org/wiki/File:JDTic_cas_361444-66-8.svg) are obtained from Wikimedia Commons and are licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Figure 2

A) Dezocine (magenta) overlaps with beta-Funaltrexamine (orange,) a mu receptor antagonist and the ligand found in the crystal structure of the mu opioid receptor (4DKL¹³, in the binding pocket. Polar interaction with ASP147 and TYR326 is predicted. B) Dezocine (magenta) overlaps with JDTic (orange), a kappa receptor antagonist and the ligand found in the crystal structure of the kappa opioid receptor (4DJH) ¹⁴, in the binding pocket. Some of interacting residues (ASP 138, TYR 139 and MET 142) are colored in yellow. A nitrogen in dezocine might hydrogen bond with oxygen atoms of ASP 138 (distance colored in magenta, 2.6 Å, or in green, 2.8 Å).

Figure 3

A) Nalbuphine and salvinorin A, full agonists of kappa opioid receptors, concentration dependently activate the G protein in the presence of kappa receptor. Dezocine fails to induce any G protein activation, indicating receptor antagonism. Based on the activity of the G protein in its presence, nor-binaltorphimine is an inverse agonist of kappa opioid receptor. B) G protein was pre-activated with a full agonist (Nalbuphine, 250 nM or Salvinorin A, 20 nM) and then increasing amounts of dezocine was added. Dezocine inhibited the agonist effects of nalbuphine and salvinorin A concentration-dependently with a total blockage at high concentration, confirming the kappa receptor antagonism effect of dezocine. The relationship is plotted using the following model: Y=Bottom + (Top-Bottom)/(1+10^((LogEC50-X)*HillSlope)).

Figure 4

A) Norepinephrine reuptake is dose dependently inhibited by both dezocine and nisoxetine. However, inhibition is weaker in the case of dezocine. B) Serotonin reuptake is concentration dependently inhibited in the presence of dezocine and nisoxetine with comparable potency. The relationship is plotted using the following model: Y=Bottom + (Top-Bottom)/(1+10^((LogEC50-X)*HillSlope)).

Figure 5

Docking result of dezocine and nisoxetine in the model of norepinephrine transporter (NET). Dezocine (magenta) shares the same binding site of nisoxetine (cyan), a NET inhibitor, as indicated by the close overlap. Dezocine is located in close proximity to TRP103, TYR127, GLU281, and LEU368 which are all colored in yellow.

Figure 6

A) Dezocine (magenta) sits in the preformed ligand binding pocket for selective serotonin reuptake inhibitors in the model of human serotonin transporter. The key interacting residues lining the pocket (Y95, D98, I172, Y176, F335, F341, and S438) are colored in yellow. This binding pocket has been demonstrated to be the binding site for many important clinical drugs such as fluoxetine, sertraline, and amitriptyline. B) Dezocine (magenta) shares the same binding pocket and overlap well with desiprimine (orange), the ligand in the LeuT crystal structure (2QJU).