title: A monovalent agonist of TrkA tyrosine kinase receptors can be converted into a bivalent antagonist.
abstract: BACKGROUND
  Receptor tyrosine kinases (RTK) act through dimerization. Previously it was thought that only bivalent ligands could be agonistic, whereas monovalent ligands should be antagonistic. This notion changed after the demonstration that monovalent ligands can be agonistic, including our report of a small molecule monovalent ligand "D3" that is a partial agonist of the NGF receptor TrkA. A bivalent "D3-linker-D3" was expected to increase agonism.
  
  
  METHODS
  Dimeric analogs were synthesized and tested in binding, biochemical, and biological assays.
  
  
  RESULTS
  One analog, 1-ss, binds TrkA with higher affinity than D3 and induces or stabilizes receptor dimers. However, 1-ss exhibited antagonistic activity, through two mechanisms. One mechanism is that 1-ss blocks NGF binding, unlike D3 which is non-competitive. Inhibition of NGF binding may be due to the linker of 1-ss filling the inter-receptor space that NGF traverses before docking. In a second mechanism, 1-ss acts as a pure antagonist, inhibiting NGF-independent TrkA activity in cells over-expressing receptors. Inhibition is likely due to 1-ss "freezing" the TrkA dimer in the inactive state.
  
  
  CONCLUSIONS
  Dimerization of an RTK can result in antagonism, through two independent mechanisms.
  
  
  CONCLUSIONS
  we report a small molecule monovalent agonist being converted to a bivalent antagonist.
authors.affiliation: Lady Davis Institute-Jewish General Hospital, Pharmacology and Therapeutics, Oncology and the Cancer Center. McGill University, Canada.
authors.forename: Fouad
authors.initials: F
authors.lastname: Brahimi
authors.forename: Jing
authors.initials: J
authors.lastname: Liu
authors.forename: Andrey
authors.initials: A
authors.lastname: Malakhov
authors.forename: Shafinaz
authors.initials: S
authors.lastname: Chowdhury
authors.forename: Enrico O
authors.initials: EO
authors.lastname: Purisima
authors.forename: Ljubica
authors.initials: L
authors.lastname: Ivanisevic
authors.forename: Antoine
authors.initials: A
authors.lastname: Caron
authors.forename: Kevin
authors.initials: K
authors.lastname: Burgess
authors.forename: H Uri
authors.initials: HU
authors.lastname: Saragovi
mesh_terms: D000818:Animals; D006801:Humans; D051379:Mice; D041681:NIH 3T3 Cells; D020932:Nerve Growth Factor / Q000037:antagonists & inhibitors / Q000378:metabolism / Q000494:pharmacology; D009842:Oligopeptides / Q000737:chemistry / Q000494:pharmacology*; D011485:Protein Binding / Q000187:drug effects; D047428:Protein Kinase Inhibitors / Q000737:chemistry / Q000494:pharmacology*; D055503:Protein Multimerization / Q000187:drug effects*; D020917:Receptor, trkA / Q000819:agonists / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism
chemical_list: C502240:NGF protein, human; D009842:Oligopeptides; D047428:Protein Kinase Inhibitors; D020932:Nerve Growth Factor; D020917:Receptor, trkA