PROJECT SUMMARY Pain is a major global health problem, causing a significant economic and social burden. Most clinically used opioid drugs are -opioid receptor (MOR) agonists with liabilities of tolerance, physical dependence and addiction mediated by this receptor. Centrally acting kappa-opioid receptor (KOR) agonists also inhibit pain, but without abuse potential and without the adverse side effects associated with MOR agonists. Antidepressants that target neurotransmitters serotonin and norepinephrine relieve pain in patients with chronic neuropathic pain syndromes. However, KOR agonists and antidepressants are not perfect. KOR agonists developed to date exhibit undesirable dysphoria and aversion, while antidepressants have not been successful in all pain conditions. Thereby, there is a large unmet need for innovative therapies for pain management. The overall goal of the proposed research is to develop novel peptide analgesics, for use along the entire continuum of care, as an alternative to the existing treatment options for acute and chronic pain. Peptides have long been recognized for their roles in the normal function of the central nervous system (CNS), making them particularly attractive candidates for the discovery and development of novel pain medications. Conventional methods of drug administration such as oral or intravenous are inefficient in delivering peptides to the brain. Both the blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCB) restrict the transport of these therapeutic agents from blood into the CNS. Alternatively, intranasal (i.n.) delivery may be a route of administration that would allow therapeutic peptides to bypass the BBB and BCB and directly enter the brain. To address the need for novel therapeutics to treat acute and chronic pain syndromes, we designed new cyclic peptide-based analgesics suitable for i.n. delivery that combine selective KOR agonist and antidepressant pain reliving mechanisms. Combining KOR agonist activity with simultaneous delivery of norepinephrine and/or serotonin may improve pain management when more than one physiological mechanism or system is implicated and may potentially minimize side effects associated with the KOR activation. The proposed cyclic peptides represent a new class of analgesics structurally distinct from the commonly prescribed pain medications. To further validate our approach for the design of novel cyclic peptide-based analgesics we propose to: (a) further improve the analgesic properties of the OL-based cyclic peptides and minimize or eliminate undesired side effects by modifying amino acid sequences using rational synthetic approach based on our previous studies (Aim 1), (b) assess metabolic stability and nose-to-brain transit of selected peptides in mice (Aim 2); (c) assess the therapeutic potentials of selected peptides in well-established pain rodent models and identify lead OL-based cyclic peptides (2 peptides) with improved analgesic activit...