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Definition
Substance P antagonists are substance P(SP) analogs retaining the amino acids in positions 2, 3, 4, 5, 6, 8 and 10 of substance P (Arg1 -Pro2 -Lys3 -Pro4 -Gln5 -Gln6 -Phe7 -Phe8 -Gly9 -Leu10 -Met11 -NH2), but having substitutions in positions 1, 7, 9 and 11 of substance P.

Structural Characteristics
Undecapeptides retaining the amino acids in positions 2, 3, 4, 5, 6, 8 and 10 of substance P (Arg1 -Pro2 -Lys3 -Pro4 -Gln5 -Gln6 -Phe7 -Phe8 -Gly9 -Leu10 -Met11 -NH2), but having substitutions in positions 1, 7, 9 and 11 of substance P have been discovered to have high antagonistic activity to block SP in biological systems. Exemplifying these potent antagonists is D-Arg1, D-Trp7,D-Trp9 -Leu11 -SP, which is an effective inhibitor and has high potency 1.

Mode of Action
Weak antagonists were obtained with a single modification, namely the replacement of Leu10 with trp in the sequences SP (6-11), SP (4-11) and SP (1-11). The affinity of octa and undecapeptide antagonists could be increased by using two (in positions 7 and 9 or 7 and 10) or 3 (in position 7, 9 and 10) substitutions of the natural residues with trp.  Affinity of antagonists was further increased by replacing Met11 with either Nle or Phe. These new compounds showed some selectivity, [pro4, trp7 ,9, Nle11 ]-SP (4-11) being more potent in the rabbit mesenteric vein than all other octapeptides described in the present study; on the other hand, [pro4, trp7 ,9,10,Phe11]-SP (4-11) was found to be the most potent antagonist of SP and related peptides in the guinea pig ileum and the guinea pig trachea. Both compounds were similarly active in the dog carotid artery. Undecapeptide antagonists, bearing the same structural modifications as the octapeptides, were found to be stimulant in the guinea pig trachea and relaxant in the dog carotid artery. The agonistic property was eliminated by repeated applications of the compounds in guinea pig tracheae, and therefore the compounds could be tested as antagonists. The undecapeptides were found to be much more active antagonists against kasinin and eledoisin than against SP and physalaemin. The data obtained with the octa and undecapeptide antagonists of SP have been used for identification and characterization of SP receptors in various smooth muscles. It appears that SP and related peptides may exert their numerous pharmacological effects by activating more than one receptor type 2.

Functions
These antagonists of SP are useful to elucidate some biological mechanisms of SP, and to treat inflammatory responses and to be novel analgesic agents for medical applications .

References

  1. Mizrahi J, Escher E, D'Orléans-Juste P, Regoli D (1984). Undeca- and octa-peptide antagonists for substance P, a study on the guinea pig trachea. Eur J Pharmacol., 99(2-3):193-202.
  2. Regoli D, Escher E, Mizrahi J (1984). Substance P--structure-activity studies and the development of antagonists. Pharmacology., 28(6):301-320.

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