Chronic pain effects 11.2% of Americans. Opioids are used for the treatment of pain but are associated with overdoses and addiction liability, which has led to the opioid epidemic. The need for a safer opioid-based analgesic led us to target the delta opioid receptor (δOR) as an alternate to the mu opioid receptor (µOR). Using a structure based approach we transformed a well-known δOR antagonist NTI into bitopic agonists through engagement of the sodium binding pocket, a surprising modulation of activity in view of the fact that for many Family A G protein-coupled receptors (GPCRs), the natural ligand sodium (Na⁺) acts as a negative allosteric modulator. Functional studies suggest that bitopic ligand interactions with the Na⁺ pocket control their efficacy and functional selectivity profiles for arrestins, and differential signaling can be achieved and is highly dependent on the length of the linkers used to engage the sodium binding pocket. We obtained single particle cryo-EM structures of δOR in complex with the two most promising bitopic agonists, C5-quino (2.8 Å) and C6-quino (2.8 Å), validating their design and highlighting key interactions between the guanidine group of bitopics and the Na⁺-binding pocket. In vivo, C6-quino displayed antinociception in a chronic pain model and was devoid of δOR related adverse effects like convulsions (seen with first generation δOR agonists) and hyperlocomotion, further strengthening the idea that targeting the Na⁺ site can provide an avenue for the design of safer analgesics. To the best of our knowledge this is only the second attempt to target the allosteric Na⁺ site (present in ~600 out of 719 Class A GPCR’s) at any GPCR which are targets for 40% of all FDA approved medications.

Cryo-EM structures of δOR in complex with <b>C5-quino</b> and <b>C6-quino </b>bitopic agonists

Cryo-EM structures of δOR in complex with C5-quino and C6-quino bitopic agonists