Project Summary This proposal is in response to PAR-20-092: Development of Medications to Prevent and Treat Opioid Use Disorders and Overdose (UG3/UH3) (Clinical Trial Optional). We propose to develop novel MOR antagonists carrying 1-phenethyl-N-phenylpiperidin-4-amine skeleton to effectively and specifically reverse the acute toxicity of fentanyl and its analogs. The fentanyls are a large family of synthetic opioids and are prominent on the list of opioid abuse and addiction due to the fact that some of them are up to 10,000-fold more potent than morphine. Because of their high potency and longer half-life than naloxone, the front-line treatment for fentanyl overdose, multiple infusions and high doses of naloxone may be required during reversal procedure. Another fentanyl addiction treatment, naltrexone, has been reported with patient non-compliance and unwanted side effects. Recently we have identified a novel molecular mechanism of fentanyl binding and activation on its target protein, the MOR, through systematic computational chemistry and biochemistry studies. More importantly, we identified a new fentanyl derivative, phenylfentanil, as a potent antagonist of the MOR. Phenylfentanil shares the same structural skeleton, i.e. 1-phenethyl-N-phenylpiperidin-4-amine, with fentanyl. It carries reasonably high affinity to the MOR and acts as a neutral antagonist on the receptor based on recent studies. Accordingly, we plan to apply these findings to further characterize phenylfentanil and its derivatives with similar pharmacological characteristics both in vitro and in vivo, and pursue preclinical development on these leads as novel reversal agents against the acute toxicity of fentanyl. In the UG3 phase, we plan to pursue two specific aims in order to define novel leads with reasonable potency and pharmacokinetics profiles as fentanyl reversal agents while in the UH3 phase, we propose to conduct IND-enabling studies on the most promising candidate and eventually file an IND with the FDA by the end of the funding period.