Drug abuse/addiction is a serious, chronically relapsing clinical condition with grave consequences for both individuals and public health. Cocaine is a major illicit drug of abuse, with over 5 million cocaine users in the U.S., yet there are no medications currently approved for the treatment of cocaine abuse and addiction. Thus, there is a pressing need to examine and develop new medical entities (NME) as therapeutics for the treatment of cocaine abuse and addiction. Cocaine enhances the signaling of dopamine, a neurotransmitter associated with the rewarding effects of this drug. Kappa opioid receptors (KOR) and their endogenous peptide agonists the dynorphins prominently modulate dopaminergic function. Moreover, dynorphin signaling is implicated in the response to stress, a key factor in the reinstatement of extinguished drug seeking behavior. Selective KOR antagonists can prevent stress-induced reinstatement of cocaine-seeking behavior in animal studies, and therefore have potential as maintenance medications to prevent relapse to cocaine abuse. However, the known nonpeptide selective KOR antagonists exhibit exceptionally long durations of activity (i.e. weeks after a single dose) which could complicate their therapeutic development. Thus, novel selective KOR antagonists with more finite durations of action could be important lead compounds for further development as potential therapeutic agents. We have identified a novel small cyclic peptide that selectively antagonizes KOR for a finite duration (hours) and prevents stress-induced reinstatement of extinguished cocaine-seeking behavior after oral administration. This cyclic peptide represents an important lead compound for the development of agents for the treatment of cocaine addiction and relapse to cocaine abuse. This proposal brings together a team of researchers with highly synergistic expertise to advance the development of this novel lead cyclic peptide, from the design and synthesis of analogs through their evaluation in preclinical models as potential treatments for cocaine addiction. The proposed research involves four specific aims: 1) the synthesis of analogs of the lead cyclic peptide and verification in initial in vitro assays of their KOR affinity, selectivity, antagonist activity and lack of neurotoxicity; 2) initial pharmacological evaluation in vivo in assays of agonist-induced antinociception in rodents to define KOR antagonist activity of the novel peptides, and analysis of their pharmacokinetic properties in vitro; 3) in vivo evaluation in rodent models of reward (conditioned place preference and self-administration assays) for therapeutic efficacy in preventing reinstatement of extinguished cocaine-seeking behavior, alongside tests for possible undesirable somatic and behavioral effects, and in vivo pharmacokinetic analysis; and 4) pharmacodynamic and pharmacokinetic analysis of optimized candidates in non-human primates, as a crucial translational step for support of potential later regulatory-based studies. This research is expected to produce candidates that can be rationally and productively advanced into late preclinical development as potential treatments for cocaine addiction and relapse.