# Dual Opioid–Neuropeptide FF Small Molecule Ligands Demonstrate Analgesia with Reduced Tolerance Liabilities

## TL;DR

A new type of painkiller that combines opioid and neuropeptide FF effects shows strong pain relief with less tolerance and fewer side effects.

## Contribution

Dual-action opioid–NPFF ligands demonstrate analgesia with reduced tolerance and side effects compared to morphine.

## Key findings

- Compound 22b showed dose-dependent antinociception with an ED50 of 6.88 nmol.
- 22b prevented NPFF-induced hyperalgesia and avoided morphine-like side effects.
- Repeated treatment with 22b caused significantly less tolerance than morphine.

## Abstract

Neuropeptide FF (NPFF) receptor antagonists prevent morphine-mediated antinociceptive tolerance, and compounds with dual mu opioid receptor (MOR) agonist and NPFF antagonist activity produce antinociception without tolerance. Compounds synthesized showed affinities in radioligand competition binding assays in the nM and µM range at the opioid and NPFF receptors, respectively, and displayed substitution-dependent functional profiles in the [35S]GTPγS functional assay. From six compounds screened in vivo for antinociception and ability to prevent NPFF-induced hyperalgesia in mouse warm water tail withdrawal tests, compound 22b produced dose-dependent MOR-mediated antinociception with an ED50 value (and 95% confidence interval) of 6.88 (4.71–9.47) nmol, i.c.v., and also prevented NPFF-induced hyperalgesia. Meanwhile, 22b did not demonstrate the respiratory depression, hyperlocomotion, or impaired intestinal transit of morphine. Moreover, repeated treatment with 22b produced a 1.6-fold rightward shift in antinociceptive dose response, significantly less acute antinociceptive tolerance than morphine. Evaluated for microsomal stability in vitro and in vivo pharmacokinetic profile, 22b showed suitable microsomal stability paired in vivo with a large apparent volume of distribution and a clearance smaller than the hepatic flow in rats, suggesting no extra-hepatic metabolism. In conclusion, the present study confirms that dual-action opioid–NPFF ligands may offer therapeutic promise as analgesics with fewer liabilities of use.