Tuned-Affinity Bivalent Ligands for the Characterization of Opioid Receptor Heteromers.

Opioid receptors, including the mu and delta opioid receptors (MOR and DOR) are important targets for the treatment of pain. Although there is mounting evidence that these receptors form heteromers, the functional role of the MOR/DOR heteromer remains unresolved. We have designed and synthesized bivalent ligands as tools to elucidate the functional role of the MOR/DOR heteromer. Our ligands (L2 and L4) are comprised of a compound with low affinity at the DOR tethered to a compound with high affinity at the MOR, with the goal of producing ligands with "tuned affinity" at MOR/DOR heteromers compared to DOR homomers. Here we show that both L2 and L4 demonstrate enhanced affinity at MOR/DOR heteromers compared to DOR homomers, thereby providing unique pharmacological tools to dissect the role of the MOR/DOR heteromer in pain.