title: In vivo antinociception of potent mu opioid agonist tetrapeptide analogues and comparison with a compact opioid agonist-neurokinin 1 receptor antagonist chimera.
abstract: BACKGROUND
  An important limiting factor in the development of centrally acting pharmaceuticals is the blood-brain barrier (BBB). Transport of therapeutic peptides through this highly protective physiological barrier remains a challenge for peptide drug delivery into the central nervous system (CNS). Because the most common strategy to treat moderate to severe pain consists of the activation of opioid receptors in the brain, the development of active opioid peptide analogues as potential analgesics requires compounds with a high resistance to enzymatic degradation and an ability to cross the BBB.
  
  
  RESULTS
  Herein we report that tetrapeptide analogues of the type H-Dmt1-Xxx2-Yyy3-Gly4-NH2 are transported into the brain after intravenous and subcutaneous administration and are able to activate the μ- and δ opioid receptors more efficiently and over longer periods of time than morphine. Using the hot water tail flick test as the animal model for antinociception, a comparison in potency is presented between a side chain conformationally constrained analogue containing the benzazepine ring (BVD03, Yyy3: Aba), and a "ring opened" analogue (BVD02, Yyy3: Phe). The results show that in addition to the increased lipophilicity through amide bond N-methylation, the conformational constraint introduced at the level of the Phe3 side chain causes a prolonged antinociception. Further replacement of NMe-D-Ala2 by D-Arg2 in the tetrapeptide sequence led to an improved potency as demonstrated by a higher and maintained antinociception for AN81 (Xxx2: D-Arg) vs. BVD03 (Xxx2: NMe-D-Ala). A daily injection of the studied opioid ligands over a time period of 5 days did however result in a substantial decrease in antinociception on the fifth day of the experiment. The compact opioid agonist-NK1 antagonist hybrid SBCHM01 could not circumvent opioid induced tolerance.
  
  
  CONCLUSIONS
  We demonstrated that the introduction of a conformational constraint has an important impact on opioid receptor activation and subsequent antinociception in vivo. Further amino acid substitution allowed to identify AN81 as an opioid ligand able to access the CNS and induce antinociception at very low doses (0.1 mg/kg) over a time period up to 7 hours. However, tolerance became apparent after repetitive i.v. administration of the investigated tetrapeptides. This side effect was also observed with the dual opioid agonist-NK1 receptor antagonist SBCHM01.
authors.affiliation: Department of Organic Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium.
authors.forename: Karel
authors.initials: K
authors.lastname: Guillemyn
authors.forename: Patrycja
authors.initials: P
authors.lastname: Kleczkowska
authors.forename: Alexandre
authors.initials: A
authors.lastname: Novoa
authors.forename: Bart
authors.initials: B
authors.lastname: Vandormael
authors.forename: Isabelle
authors.initials: I
authors.lastname: Van den Eynde
authors.forename: Piotr
authors.initials: P
authors.lastname: Kosson
authors.forename: Muhammad Faheem
authors.initials: MF
authors.lastname: Asim
authors.forename: Peter W
authors.initials: PW
authors.lastname: Schiller
authors.forename: Mariana
authors.initials: M
authors.lastname: Spetea
authors.forename: Andrzej W
authors.initials: AW
authors.lastname: Lipkowski
authors.forename: Dirk
authors.initials: D
authors.lastname: Tourwé
authors.forename: Steven
authors.initials: S
authors.lastname: Ballet
mesh_terms: D000700:Analgesics / Q000494:pharmacology; D000818:Animals; D004305:Dose-Response Relationship, Drug; D007275:Injections, Intravenous; D008024:Ligands; D051379:Mice; D008810:Mice, Inbred C57BL; D009020:Morphine / Q000008:administration & dosage / Q000494:pharmacology; D064729:Neurokinin-1 Receptor Antagonists; D059225:Nociception / Q000187:drug effects*; D009842:Oligopeptides / Q000737:chemistry / Q000494:pharmacology*; D018847:Opioid Peptides / Q000008:administration & dosage / Q000819:agonists* / Q000494:pharmacology; D018040:Receptors, Neurokinin-1 / Q000378:metabolism; D017450:Receptors, Opioid, mu / Q000819:agonists* / Q000378:metabolism; D011994:Recombinant Proteins / Q000737:chemistry / Q000494:pharmacology*; D013997:Time Factors
chemical_list: D000700:Analgesics; D008024:Ligands; D064729:Neurokinin-1 Receptor Antagonists; D009842:Oligopeptides; D018847:Opioid Peptides; D018040:Receptors, Neurokinin-1; D017450:Receptors, Opioid, mu; D011994:Recombinant Proteins; D009020:Morphine