Synthesis and Biological Characterization of Cyclic Disulfide-Containing Peptide Analogs of the Multifunctional Opioid/Neuropeptide FF Receptor Agonists That Produce Long-Lasting and Nontolerant Antinociception

Abstract

In a previously described chimeric peptide, we reported that the multifunctional opioid/neuropeptide FF (NPFF) receptor agonist <b>0</b> (BN-9) produced antinociception for 1.5 h after supraspinal administration. Herein, four cyclic disulfide analogs containing l- and/or d-type cysteine at positions 2 and 5 were synthesized. The cyclized analogs and their linear counterparts behaved as multifunctional agonists at both opioid and NPFF receptors <i>in vitro</i> and produced potent analgesia without tolerance development. In comparison to <b>0</b>, cyclized peptide <b>6</b> exhibited sevenfold more potent μ-opioid receptor agonistic activity <i>in vitro</i>. Interestingly, the cyclized analog <b>6</b> possessed an improved stability in the brain and an increased blood-brain barrier permeability compared to the parent peptide <b>0</b> and produced more potent analgesia after supraspinal or subcutaneous administration with improved duration of action of 4 h. In addition, antinociceptive tolerance of analog <b>6</b> was greatly reduced after subcutaneous injection compared to fentanyl, as was the rewarding effect, withdrawal reaction, and gastrointestinal inhibition.