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

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.