Peripherally acting enkephalin analogues. 2. Polar tri- and tetrapeptides.

The design, synthesis, and biological activity of a series of D-Arg2-enkephalin-derived tetrapeptide amides and tripeptide aralkylamides are reported. These polar analogues were designed to be excluded from the central nervous system with their action thus limited to peripheral opioid receptors. The effects of the nature of the aromatic ring, aryl ring substitution, and aralkylamine chain length on activity were investigated; in a number of cases the N-terminal amino group of Tyr1 was converted to a guanidino group to further increase hydrophilicity. The peptides were all synthesized by classical solution methodology. The opioid activity of the peptides was assessed in vitro on the guinea pig ileum and their antinociceptive activity was determined in vivo in chemically induced writhing models (peripheral activity) and in the hot-plate test (central activity), in rodents. That the analgesic effects were predominantly mediated in the periphery was demonstrated by antagonism of antinociception by the peripheral opioid antagonist N-methylnalorphine and by comparison of the activities in the writhing and hot-plate tests. As a class, the tetrapeptides were more potent than the tripeptides; N alpha-amidination generally increased activity. A number of compounds exhibited very potent opioid activity and had the desired pharmacological profile, indicating a high degree of peripheral selectivity.

Effect of BW443C81, a novel opioid, on non-cholinergic bronchoconstrictor responses and neurogenic plasma extravasation in the guinea pig.

The novel, peripherally acting opioid peptide, BW443C81, which attenuates airway sensory nerve impulses, was examined on non-cholinergic (NC) constrictor responses in vitro and in vivo and neurogenic plasma extravasation in vivo in guinea-pig airways. Non-cholinergic contractions of guinea pig isolated bronchi, evoked by electrical field stimulation, were concentration-dependently inhibited by BW443C81 and morphine (10 nmol/1-100 mumol/l). In anaesthetised, artificially ventilated guinea pigs, frequency-related NC bronchoconstrictor responses evoked by antidromic electrical stimulation of the vagus nerves were reduced by BW443C81 (100 micrograms/kg/min i.v. infusion) and morphine (1 mg/kg i.v.). Neurogenic plasma extravasation produced by bilateral electrical vagal nerve stimulation in spontaneously breathing, anaesthetised guinea pigs was also inhibited by BW443C81 (1 mg/kg i.v.). The inhibitory effects of BW443C81 were reversed/prevented by naloxone. BW443C81 inhibits NC bronchoconstrictor responses and neurogenic plasma extravasation in guinea pig airways, consistent with its previously described mu-opioid receptor-mediated inhibitory action on airway sensory nerve function.