Selectively Targeting Opioid Receptor Heterodimers 7. Project Summary/Abstract Opioid receptors are important targets for the treatment of acute and chronic pain indications and are one of the few targets currently subject to pharmacological intervention in the treatment of alcoholism. The opioid receptor system is comprised of three highly related receptors: the mu, delta, and kappa opioid receptors (MOR, DOR, and KOR respectively). Studies using knock-out animals have demonstrated that each of these receptors has a unique contribution to nociception and alcohol consumption. Despite more than 50 years of research, several mysteries remain as to the pharmacology of the opioid receptors. In particular, there are pharmacologically-defined subtypes of the MOR, DOR and KOR that exist in vivo that cannot be recapitulated in cell-based systems expressing a single receptor. Thus, it is extremely challenging to design better, more selective opioid drugs until the molecular nature of the pharmacological subtypes has been defined. We propose that heterodimerization of the opioid receptors could alter their pharmacology and explain the opioid subtypes. In particular, several lines of evidence suggest that DOR1 may be a heterodimer complex of MOR and DOR while DOR2 may be a homomer/monomer of DOR. Our preliminary data suggest that agonism of DOR1 reduces drinking and antagonism at DOR2 reduces drinking. Thus, our goal is to design new ligands that are agonists at DOR1 (MOR/DOR heterodimers) but antagonists at DOR2. We have designed a series of novel bivalent ligands that we predict may have these desired properties. We have designed our bivalent ligands to have novel function(s) on heterodimers that are distinct from their effects on homomers/monomers, due to their "tuned affinity". Specifically, each of our bivalent ligands features a high affinity compound tethered to a low affinity compound. We take this approach because one of the inherent drawbacks to "classical" bivalent ligands is that they are not selective for heterodimeric receptors. That is, each pharmacophore in classic bivalent ligands can interact with high affinity with its matching monomeric/homomeric receptor as well as with a receptor that is part of a heterodimer. In the two Specific Aims here, we will generate "tuned affinity" bivalent ligands and use them together with a unique set of tools, including cell lines and a complete set of opioid receptor knock out mice, to probe the functional role of the MOR/DOR heterodimers.
Here, we have designed several new tuned affinity bivalent opioid ligands that we believe will have novel pharmacologies due to their selective activity profile on MOR/DOR heterodimers. We will use these ligands to probe the existence and functional relevance of the MOR/DOR heterodimer.