In humans, there are four opioid receptors: μ, δ, and κ (abbreviated as MOR, DOR, and KOR, respectively) and the nociception opioid receptor (NOR). Agonists of MOR (opioids) are commonly used for the treatment of acute and chronic pain, ailments that affect most humans at some point during their life. Unfortunately, current opioid medications such as morphine, oxycodone, and fentanyl, while potent in managing acute and chronic pain, are associated with increased risk of tolerance and complications such as respiratory depression, constipation, and addiction [1]. However, targeting alternative opioid receptor subtype, DOR, which has been identified as a therapeutic target for pain with reduced tolerance and adverse effects, offers a promising alternative for chronic pain treatment [2]. Positive allosteric modulators (PAMs) can offer further advantage by enhancing receptor activity while maintaining the natural spatiotemporal response, addressing the limitations associated with traditional orthosteric agonists, such as reduced efficacy over time and side effect profiles [3].
A structure of DOR bound to a novel PAM (MIPS3614) revealed a unique lipid-facing allosteric binding pocket. Informed by the structure, we generated analogs of MIPS3614 and receptor mutants designed to disrupt key interactions within the allosteric binding pocket. Through BRET-based functional assays, it was revealed that the oxygenated core of MIPS3614 is essential for its modulatory activity and that residues located in this allosteric pocket are important both for receptor expression and allosteric modulation of this compound. This study is the first to reveal an allosteric binding site at DOR and generate new DOR PAMs via a structure-based drug design approach. Through this, we enable the rational design of allosteric drugs targeting DOR that could be further developed as a new and improved class of analgesics.