title: Palonosetron exhibits unique molecular interactions with the 5-HT3 receptor.
abstract: BACKGROUND
  Palonosetron is a 5-HT(3)-receptor antagonist (5-HT(3)-RA) that has been shown to be superior to other 5-HT(3)-RAs in phase III clinical trials for the prevention of acute, delayed, and overall chemotherapy-induced nausea and vomiting. The improved clinical efficacy of palonosetron may be due, in part, to its more potent binding and longer half-life. However, these attributes alone are not sufficient to explain the results with palonosetron. We sought to elucidate additional differences among 5-HT(3)-RAs that could help explain the observations in the clinic.
  
  
  METHODS
  Receptor site saturation binding experiments were performed with [3H] palonosetron, [3H] granisetron, and [3H] ondansetron to obtain the corresponding Scatchard analyses and Hill coefficients. Diagnostic equilibrium binding experiments and kinetic dissociation experiments were conducted to examine competitive versus potential allosteric interactions between ondansetron, granisetron and palonosetron and the 5-HT(3) receptor. Finally, the long-term effect of the three antagonists on receptor function as measured by Ca2+ influx in HEK 293 cells expressing the 5-HT(3)-receptor was compared.
  
  
  RESULTS
  Analyses of binding isotherms using both Scatchard and Hill plots suggested positive cooperativity for palonosetron and simple bimolecular binding for both granisetron and ondansetron. Equilibrium diagnostic tests discriminated differential effects of palonosetron on [3H] ligand binding indicating that palonosetron was an allosteric antagonist whereas granisetron and ondansetron were competitive antagonists. Using dissociation rate strategies, palonosetron was shown to be an allosteric modifier that accelerated the rate of dissociation from the receptor of both granisetron and ondansetron. Differences in the binding mode of palonosetron to the 5-HT(3) receptor were shown to have an impact on receptor function. In these experiments, cells were incubated with each antagonist, followed by infinite dilutions and dissociation for 2.5 h; cells previously incubated with either granisetron or ondansetron showed calcium-ion influx similar to control cells that had not been exposed to a 5-HT(3) receptor antagonist. In contrast, substantial inhibition of calcium-ion influx was observed in cells that had been incubated with palonosetron.
  
  
  CONCLUSIONS
  Palonosetron exhibited allosteric binding and positive cooperativity when binding to the 5-HT(3) receptor. Palonosetron also triggered functional effects that persisted beyond its binding to the 5-HT(3) receptor at the cell surface. Differences in binding and effects on receptor function may be relevant to the unique beneficial actions of palonosetron. To our knowledge, this is the first report showing palonosetron's interaction with the 5-HT(3) receptor at the molecular level, clearly differentiating it from other 5-HT(3)-RAs.
authors.affiliation: Research and Translational Development, MGI PHARMA INC, 6611 Tributary St., Baltimore, MD 21224-6515, USA. camilo.rojas@mgipharma.com
authors.forename: Camilo
authors.initials: C
authors.lastname: Rojas
authors.forename: Marigo
authors.initials: M
authors.lastname: Stathis
authors.forename: Ajit G
authors.initials: AG
authors.lastname: Thomas
authors.forename: Ed B
authors.initials: EB
authors.lastname: Massuda
authors.forename: Edward B
authors.initials: EB
authors.lastname: Massuda
authors.forename: Jesse
authors.initials: J
authors.lastname: Alt
authors.forename: Jie
authors.initials: J
authors.lastname: Zhang
authors.forename: Edward
authors.initials: E
authors.lastname: Rubenstein
authors.forename: Ed
authors.initials: E
authors.lastname: Rubenstein
authors.forename: Silvia
authors.initials: S
authors.lastname: Sebastiani
authors.forename: Sergio
authors.initials: S
authors.lastname: Cantoreggi
authors.forename: Solomon H
authors.initials: SH
authors.lastname: Snyder
authors.forename: Barbara
authors.initials: B
authors.lastname: Slusher
mesh_terms: D000818:Animals; D000932:Antiemetics / Q000378:metabolism*; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D002460:Cell Line; D017829:Granisetron; D007546:Isoquinolines / Q000378:metabolism*; D017294:Ondansetron; D000077924:Palonosetron; D020250:Postoperative Nausea and Vomiting / Q000517:prevention & control; D011812:Quinuclidines / Q000378:metabolism*; D011869:Radioligand Assay; D044406:Receptors, Serotonin, 5-HT3 / Q000378:metabolism*; D058831:Serotonin 5-HT3 Receptor Antagonists; D014162:Transfection
chemical_list: D000932:Antiemetics; D002118:Calcium; D017829:Granisetron; D007546:Isoquinolines; D017294:Ondansetron; D000077924:Palonosetron; D011812:Quinuclidines; D044406:Receptors, Serotonin, 5-HT3; D058831:Serotonin 5-HT3 Receptor Antagonists