Graduate Research Assistant Purdue University, Indiana, United States
Yazan Meqbil (Purdue University)| Hongyu Su (Purdue University)| Robert Cassell (Purdue University)| Anna Gutridge (Purdue University)| Benjamin Cummins (Purdue University)| Lan Chen (Purdue University)| Richard van Rijn (Purdue University, Purdue University, Purdue University)
The δ opioid receptor (δOR), a class A G-protein coupled receptor, has emerged as an attractive therapeutic target for alcohol and substance use disorders, chronic pain, migraine. depression, anxiety, and ischemic insult. However, currently no δOR modulator has been approved for clinical use. An increasing number of studies have demonstrated the ability of biased signaling via Gi/o-protein or β-arrestin recruitment to improve therapeutic window. For the δOR as well as other opioid receptors, β-arrestin 2 recruitment has been associated with negative consequences, however several studies have suggested a potential beneficial role for β-arrestin 1 recruitment and signal transduction. To identify possible novel δOR agonists with strong β-arrestin 1 recruitment we performed a small scale (~5000 compounds) high-throughput screen of CNS-focused drug libraries using a δOR/β-arrestin 1 PathHunter assay. Through the screen we identified a novel δOR chemotype notably devoid of a basic nitrogen-atom, a common feature in most opioids and part of interacting with Asp128 in TM3 of the δOR. Pharmacological assessment for affinity, cAMP inhibition and β-arr recruitment revealed that the hit compound possessed micromolecular affinity and potency and 10-fold selectivity over µ- and kORs. However, the compound did not display a significant bias for the β-arrestin 1 isoform over β-arrestin 2. In the presence of the endogenous δOR peptide Leu-enkephalin, the hit compound acted as an orthosteric agonist. Molecular dynamic simulations using Desmond (Schrödinger suite) further confirmed the ligand preferentially occupying the orthosteric binding pocket. A structure activity relationship (SAR) by catalog with 14 analogs did not produce any improvements in δOR potency in the cAMP assay.
In conclusion, a relatively small drug library screen revealed a δOR selective compound with a novel chemotype. Molecular dynamics were performed to obtain a possible binding mode of the hit, which can be used to propose future modifications to improve the potency and affinity of the hit. Furthermore, our results provide support for the use of β-arrestin 1 PathHunter cells to screen for novel δOR compounds that could exhibit unique pharmacology at the δOR and will contribute to the elucidation of δOR signaling cascades and potentially lead to δOR therapeutics.