# Design, Syntheses, and Pharmacological Evaluations of Core Ring Expanded Fentanyl Analogues as Potential Counteracting Agents Against Fentanyl Induced Respiratory Depression

## TL;DR

Researchers designed new fentanyl-like compounds that can reverse the dangerous effects of fentanyl overdose, including respiratory depression.

## Contribution

The study introduces ring-expanded fentanyl analogs as potent mu opioid receptor antagonists with central nervous system activity.

## Key findings

- 15 compounds effectively blocked synthetic opioid antinociception in vivo.
- Compound 53 showed the highest potency against fentanyl and morphine with favorable pharmacokinetics.
- Compound 53 reversed fentanyl-induced respiratory depression in whole-body plethysmography tests.

## Abstract

The escalating synthetic opioid crisis necessitates novel
treatments,
especially for fentanyl overdose. This study presents 84 ring-expanded
fentanyl analogs, replacing its piperidine core with 4-azepane and
5-azocane structures. In vivo antagonism studies
identified 15 compounds that effectively blocked synthetic opioid
antinociception. Further dose–response analysis identified
four potent antagonists (16, 46, 53, and 69) against both fentanyl and morphine. Notably,
Compound 53 demonstrated the highest potency with AD50 of 2.02 mg/kg against morphine and 4.02 mg/kg against fentanyl.
Compound 53 exhibits a favorable pharmacokinetic profile,
including moderate human metabolic stability, low efflux, and efficient,
sustained CNS penetration, making it a promising centrally acting
MOR antagonist candidate. Significantly, whole-body plethysmography
confirmed that compound 53 reversed fentanyl-induced
respiratory depression. These results suggest that expanding the core
ring structure of fentanyl is a promising strategy to develop potent
mu opioid receptor antagonists to inhibit both antinociception and
respiratory depression offering potential solutions to fentanyl overdose.