*Article* **Identification of a Novel Delta Opioid Receptor Agonist Chemotype with Potential Negative Allosteric Modulator Capabilities**

**Yazan J. Meqbil 1,2 , Hongyu Su <sup>1</sup> , Robert J. Cassell <sup>1</sup> , Kendall L. Mores <sup>1</sup> , Anna M. Gutridge <sup>1</sup> , Benjamin R. Cummins <sup>3</sup> , Lan Chen <sup>4</sup> and Richard M. van Rijn 1,4,5,\***


**Abstract:** The δ-opioid receptor (δOR) holds great potential as a therapeutic target. Yet, clinical drug development, which has focused on δOR agonists that mimic the potent and selective tool compound SNC80 have largely failed. It has increasingly become apparent that the SNC80 scaffold carries with it potent and efficacious β-arrestin recruitment. Here, we screened a relatively small (5120 molecules) physical drug library to identify δOR agonists that underrecruit β-arrestin, as it has been suggested that compounds that efficaciously recruit β-arrestin are proconvulsant. The screen identified a hit compound and further characterization using cellular binding and signaling assays revealed that this molecule (R995045, compound **1**) exhibited ten-fold selectivity over µand κ-opioid receptors. Compound **1** represents a novel chemotype at the δOR. A subsequent characterization of fourteen analogs of compound **1**, however did not identify a more potent δOR agonist. Computational modeling and in vitro characterization of compound **1** in the presence of the endogenous agonist leu-enkephalin suggest compound **1** may also bind allosterically and negatively modulate the potency of Leu-enkephalin to inhibit cAMP, acting as a 'NAM-agonist' in this assay. The potential physiological utility of such a class of compounds will need to be assessed in future in vivo assays.

**Keywords:** chemotype; high-throughput screen; delta opioid receptor; allosteric modulation; beta-arrestin; molecular dynamics
