New reduced peptide bond substance P agonists and antagonists: effects on smooth muscle contraction

Backbone cyclization of the C-terminal part of substance P. Part 1: The important role of the sulphur in position 11

Novel backbone-to-side chain and backbone-to-backbone cyclic analogues of substance P (SP) were prepared by solid-phase synthesis and screened for biological activity. An analogue containing a thioetherlactam ring between positions 9 and 11 showed an EC50 value of 20 nM toward the neurokinin 1 (NK-1) and was inactive toward the NK-2 and NK-3 receptors. On the other hand, in a multiple backbone cyclic peptide library of similar analogues, in which the sulphur was excluded from the ring, very low activity was detected. The activity was re-evaluated and was found to be even lower (EC50 = 0.11 mM) than the previously published data. These results indicate that the thioether moiety has a crucial role in receptor activation. The results also show tolerance of the NK-1 receptor, but not NK-2 or NK-3, to cyclization of the C-terminal portion of the SP6-11 hexapeptide.

Synthesis of potent agonists of substance P by replacement of Met11 with Glu(OBzl) and N-terminal glutamine with Glp of the C-terminal hexapeptide and heptapeptide of substance P

The analogues [Glp6,Glu(OBzl)11]SP(6-11) and [Glp5,Glu(OBzl)11]SP(5-11) of the C-terminal hexapeptide and heptapeptide of Substance P have been synthesized by conventional solution methods. In each analogue the N-terminal glutamine has been replaced by pyroglutamic acid, while the COOCH2C6H5 ester group has replaced the SCH3 group of the Met11 side chain. The in vitro activity of both analogues has been determined on three biological preparations: guinea pig ileum (GPI), rat vas deferens (RVD) and rat portal vein (RPV). The results showed that both analogues are highly potent and selective agonists on GPI through the NK-1 receptor. They are more potent than SP itself, with 1.54 and 1.25 respective values of relative potency on GPI. Their selectivity has been studied by utilizing atropine-treated guinea pig ileum (GPI+At). The analogues showed low activity on RVD and RPV tissues, which represent NK-2 and NK-3 monoreceptor assay, respectively.

Pharmacological assessment of spantide II analogues

We have studied the structure-activity relationship of a series of tachykinin receptor antagonists based on spantide II. Fifteen novel peptides were tested for their ability to antagonize the electrically evoked tachykinin receptor-mediated response in the isolated rabbit iris sphincter muscle. Substitution or deletion of one to three amino acids in the spantide II sequence caused significant changes in biological activity. Eight of the novel analogues were found to be as potent as or more potent than spantide II and some were found to have better water solubility. We tested the selectivity for different tachykinin receptors of spantide II and two of the eight most potent analogues. They all interacted with tachykinin NK1 (rabbit jugular vein) and tachykinin NK2 (rabbit pulmonary artery) receptors with pA2 values of about 6.5-7.5 at the NK1 receptor and of 5.9-7.2 at the NK2 receptor, while being inactive at the tachykinin NK3 receptor (rat portal vein). Spantide II and the novel analogues were without effect on electrically evoked cholinergic responses of the isolated rabbit iris sphincter and on electrically evoked sympathetic responses of the guinea-pig vas deferens; moreover, they were without local anaesthetic-like effects on action potentials of the frog sciatic nerve, which suggests that they do not produce a general neurosuppressive effect. They were as effective as or slightly less effective than spantide II in causing histamine release from rat peritoneal mast cells.

Reduced gastric acid inhibitory effect of a pGIP(1-30)NH2 fragment with potent pancreatic amylase inhibitory activity

Gastric inhibitory polypeptide (GIP) strongly stimulates insulin secretion in the presence of glucose and also stimulates somatostatin release from gastric mucosa. It was reported recently that both stimulatory activities can be dissociated by removing the C-terminal 12 amino acid residues. Since insulin and somatostatin are involved in regulation of exocrine pancreatic and gastric secretion in rats, we compared the inhibitory effects of pGIP and the pGIP(1-30)NH2 fragment on pancreatic amylase and gastric acid secretion. pGIP(1-30)NH2 displayed full activity on inhibition of bombesin (BN)-stimulated amylase release relative to GIP itself, but was about 10-fold less potent in inhibiting gastric acid secretion. These results suggest that the receptors involved in these two events have quite different ligand binding requirements and that more specific analogues of GIP can be designed which should be of value in elucidating the physiological roles of this hormone.