Discovery of Novel Delta Opioid Receptor (DOR) Inverse Agonist and Irreversible (Non-Competitive) Antagonists

The delta opioid receptor (DOR) is a crucial receptor system that regulates pain, mood, anxiety, and similar mental states. DOR agonists, such as SNC80, and DOR-neutral antagonists, such as naltrindole, were developed to investigate the DOR in vivo and as potential therapeutics for pain and depression. However, few inverse agonists and non-competitive/irreversible antagonists have been developed, and none are widely available. This leaves a gap in our pharmacological toolbox and limits our ability to investigate the biology of this receptor. Thus, we designed and synthesized the novel compounds SRI-9342 as an irreversible antagonist and SRI-45128 as an inverse agonist. These compounds were then evaluated in vitro for their binding affinity by radioligand binding, their functional activity by ³⁵S-GTPγS coupling, and their cAMP accumulation in cells expressing the human DOR. Both compounds demonstrated high binding affinity and selectivity at the DOR, and both displayed their hypothesized molecular pharmacology of irreversible antagonism (SRI-9342) or inverse agonism (SRI-45128). Together, these results demonstrate that we have successfully designed new inverse agonists and irreversible antagonists of the DOR based on a novel chemical scaffold. These new compounds will provide new tools to investigate the biology of the DOR or even new potential therapeutics.

(-)-Reboxetine inhibits muscle nicotinic acetylcholine receptors by interacting with luminal and non-luminal sites

The interaction of (-)-reboxetine, a non-tricyclic norepinephrine selective reuptake inhibitor, with muscle-type nicotinic acetylcholine receptors (AChRs) in different conformational states was studied by functional and structural approaches. The results established that (-)-reboxetine: (a) inhibits (±)-epibatidine-induced Ca(2+) influx in human (h) muscle embryonic (hα1β1γδ) and adult (hα1β1εδ) AChRs in a non-competitive manner and with potencies IC50=3.86±0.49 and 1.92±0.48 μM, respectively, (b) binds to the [(3)H]TCP site with ~13-fold higher affinity when the Torpedo AChR is in the desensitized state compared to the resting state, (c) enhances [(3)H]cytisine binding to the resting but activatableTorpedo AChR but not to the desensitized AChR, suggesting desensitizing properties, (d) overlaps the PCP luminal site located between rings 6' and 13' in the Torpedo but not human muscle AChRs. In silico mutation results indicate that ring 9' is the minimum structural component for (-)-reboxetine binding, and (e) interacts to non-luminal sites located within the transmembrane segments from the Torpedo AChR γ subunit, and at the α1/ε transmembrane interface from the adult muscle AChR. In conclusion, (-)-reboxetine non-competitively inhibits muscle AChRs by binding to the TCP luminal site and by inducing receptor desensitization (maybe by interacting with non-luminal sites), a mechanism that is shared by tricyclic antidepressants.

Discovery of heterobicyclic templates for novel metabotropic glutamate receptor subtype 5 antagonists

Investigation of a series of heterobicyclic compounds with essential pharmacophoric features of the metabotropic glutamate receptor 5 (mGluR5) antagonists MPEP and MTEP provided novel structural templates with sub-micromolar affinities at the mGluR5.