In a previously described chimeric
peptide, we reported that the
multifunctional opioid/neuropeptide FF (NPFF) receptor agonist 0 (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 in vitro and produced potent analgesia without tolerance
development. In comparison to 0, cyclized peptide 6 exhibited sevenfold more potent μ-opioid receptor
agonistic activity in vitro. Interestingly, the cyclized
analog 6 possessed an improved stability in the brain
and an increased blood–brain barrier permeability compared
to the parent peptide 0 and produced more potent analgesia
after supraspinal or subcutaneous administration with improved duration
of action of 4 h. In addition, antinociceptive tolerance of analog 6 was greatly reduced after subcutaneous injection compared
to fentanyl, as was the rewarding effect, withdrawal reaction, and
gastrointestinal inhibition.