In vitro pharmacological characterization of mixed mu/NOP receptor agonists

Pain is an unpleasant sensory and emotional experience that is associated with actual or potential tissue damage or described in such terms. Currently, pain is a worldwide burden, and, in the process of pain treatment, patients experience various drug side effects. Therefore, researchers continue to develop medications for pain without side effects. Opioids are one of the leading options for treating severe acute pain. Most opioid effects are mediated by the mu-opioid receptor, a class A G protein-coupled receptor (GPCR). Mu-opioid receptors activation can also lead to respiratory depression, sedation, constipation, nausea, reward/euphoria, and dependence/withdrawal. Intriguingly, the simultaneous activation of the mu and the nociceptin / orphanin FQ (N/OFQ) peptide (NOP) receptor leads in animal models of pain to a significant increase in therapeutic index. GPCRs function is perpetrated by their interaction with heterotrimeric G proteins and signaling modulated upon β-arrestin recruitment. Importantly, over the last 15 years, research has shown that opioid adverse effects may be caused by the recruitment of β-arrestin, whereas signaling downstream of G protein activation generates antinociception. Therefore, opioid "biased agonists” capable of mediating a selective G protein activation, rather than β-arrestins’, might represent a very intriguing opportunity for painkiller development. Combining biased agonism and promiscuous mu and NOP receptors activation will very likely empower our toolbox of pain treatments. Consequently, this thesis aimed to study mu and NOP receptors’ simultaneous activating agents with a higher propensity to foster G protein stimulation rather than β-arrestin’s. Here, by applying a bioluminescence resonance energy transfer (BRET) approach, applied to study receptor-transducer interaction, we compared the in vitro pharmacological activities of a series of peptide and nonpeptide mu and NOP agonists generated with different chemistry approaches. Results obtained will improve the knowledge of a simultaneous, albeit G protein selective, mu/NOP activation. Further in vivo studies will, in perspective, corroborate the value of this prototype of innovative pharmacology.