title: Agonists and antagonists induce different palonosetron dissociation rates in 5-HT₃A and 5-HT₃AB receptors.
abstract: Palonosetron is a potent 5-HT₃ receptor antagonist with a unique structure and some unusual properties. Here we explore the properties of palonosetron at heterologously expressed 5-HT₃A and 5-HT₃AB receptors. We used receptors expressed in HEK293 cells, and functionally analysed them using a membrane potential sensitive dye in a Flexstation, which revealed IC₅₀s of 0.24 nM and 0.18 nM for 5-HT₃A and 5-HT₃AB receptors respectively. Radioligand binding studies with [(3)H]palonosetron revealed similar Kds: 0.34 nM for 5-HT3A and 0.15 nM for 5-HT₃AB receptors. Kinetic studies showed palonosetron association and dissociation rates were slightly faster in 5-HT₃AB than 5-HT₃A receptors, and for both subtypes dissociation rates were ligand-dependent, with antagonists causing more rapid dissociation than agonists. Similar ligand effects were not observed for [(3)H]granisetron dissociation studies. These data support previous studies which show palonosetron has actions distinct to other 5-HT3 receptor antagonists, and the slow rates observed for agonist induced dissociation (t₁/₂ > 10 h) could at least partly explain the long duration of palonosetron effects in vivo.
authors.affiliation: Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB2 1QW, UK. sl120@cam.ac.uk
authors.forename: Sarah C R
authors.initials: SC
authors.lastname: Lummis
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Thompson
mesh_terms: D017829:Granisetron / Q000494:pharmacology; D057809:HEK293 Cells; D006801:Humans; D020128:Inhibitory Concentration 50; D007546:Isoquinolines / Q000494:pharmacology*; D000077924:Palonosetron; D021122:Protein Subunits / Q000235:genetics / Q000378:metabolism; D011812:Quinuclidines / Q000494:pharmacology*; D011869:Radioligand Assay; D044406:Receptors, Serotonin, 5-HT3 / Q000235:genetics / Q000378:metabolism*; D058827:Serotonin 5-HT3 Receptor Agonists / Q000494:pharmacology*; D058831:Serotonin 5-HT3 Receptor Antagonists / Q000494:pharmacology*; D014162:Transfection; D014316:Tritium
chemical_list: D017829:Granisetron; D007546:Isoquinolines; D000077924:Palonosetron; D021122:Protein Subunits; D011812:Quinuclidines; D044406:Receptors, Serotonin, 5-HT3; D058827:Serotonin 5-HT3 Receptor Agonists; D058831:Serotonin 5-HT3 Receptor Antagonists; D014316:Tritium
keywords: 5-HT; 5-hydroxytryptamine (serotonin); Allosteric binding site; FlexStation assays; HEK; IC(50); K(d); Radioligand binding; Serotonin receptor; Site-directed mutagenesis; affinity constant; concentration of antagonist required for half-maximal inhibition; human embryonic kidney