title: Determination of unbound fraction of pazopanib in vitro and in cancer patients reveals albumin as the main binding site.
abstract: BACKGROUND
  Pazopanib exhibits wide inter-patient pharmacokinetic variability which may contribute to differences in treatment outcome. Unbound drug concentrations are believed to be more relevant to pharmacological responses than total concentrations. Thus it is desirable to evaluate pazopanib binding on plasma proteins and different factors potentially affecting this process.
  
  
  METHODS
  An equilibrium dialysis method coupled with UPLC-MS/MS assay has been optimized and validated for the determination of pazopanib unbound fraction (fu%) in human plasma. Pazopanib binding in the plasma of healthy volunteers and in isolated protein solutions was investigated. The unbound fraction was determined for 24 cancer patients treated daily with pazopanib.
  
  
  RESULTS
  We found that pazopanib was extensively bound in human plasma (>99.9 %) with a mean fu% value of 0.0106 ± 0.0013 % at 40 μg/mL. Protein binding was concentration independent over a clinically relevant range of concentrations. In isolated protein solutions, pazopanib at 40 μg/mL was mainly bound to albumin (40 g/L) and to a lesser extent to α1-acid glycoprotein (1 g/L) and low density lipoproteins (1.2 g/L), with a mean fu% of 0.0073 ± 0.0022 %, 0.992 ± 0.44 % and 7.4 ± 1.7 % respectively. Inter-patient variability (CV%) of fu% in cancer patients was limited (27.2 %). A correlation was observed between individual unbound fraction values and albuminemia.
  
  
  CONCLUSIONS
  Pazopanib exhibits extensive binding to plasma proteins in human plasma. Variable albumin concentrations, frequently observed in cancer patients, may affect pazopanib unbound fraction with implications for inter-patient variability in drug efficacy and toxicity.
authors.affiliation: Department of Pharmacology, Institut Claudius-Regaud, IUCT-Oncopole, 1, avenue Irène Joliot-Curie, Toulouse, F-31059, France.|EA4553 Université Toulouse III Paul-Sabatier, Toulouse, France.
authors.forename: Diane-Charlotte
authors.initials: DC
authors.lastname: Imbs
authors.affiliation: Department of Pharmacology, Institut Claudius-Regaud, IUCT-Oncopole, 1, avenue Irène Joliot-Curie, Toulouse, F-31059, France.|EA4553 Université Toulouse III Paul-Sabatier, Toulouse, France.
authors.forename: Marie-Noelle
authors.initials: MN
authors.lastname: Paludetto
authors.affiliation: Centre Léon-Bérard, Lyon, France.
authors.forename: Sylvie
authors.initials: S
authors.lastname: Négrier
authors.affiliation: Faculty of Medical Sciences, Newcastle University, Newcastle upon Tyne, UK.
authors.forename: Helen
authors.initials: H
authors.lastname: Powell
authors.affiliation: Department of Pharmacology, Institut Claudius-Regaud, IUCT-Oncopole, 1, avenue Irène Joliot-Curie, Toulouse, F-31059, France.|EA4553 Université Toulouse III Paul-Sabatier, Toulouse, France.
authors.forename: Thierry
authors.initials: T
authors.lastname: Lafont
authors.affiliation: EA4553 Université Toulouse III Paul-Sabatier, Toulouse, France.
authors.forename: Melanie
authors.initials: M
authors.lastname: White-Koning
authors.affiliation: Department of Pharmacology, Institut Claudius-Regaud, IUCT-Oncopole, 1, avenue Irène Joliot-Curie, Toulouse, F-31059, France. chatelut.etienne@iuct-oncopole.fr.|EA4553 Université Toulouse III Paul-Sabatier, Toulouse, France. chatelut.etienne@iuct-oncopole.fr.
authors.forename: Etienne
authors.initials: E
authors.lastname: Chatelut
authors.affiliation: Department of Pharmacology, Institut Claudius-Regaud, IUCT-Oncopole, 1, avenue Irène Joliot-Curie, Toulouse, F-31059, France.|EA4553 Université Toulouse III Paul-Sabatier, Toulouse, France.
authors.forename: Fabienne
authors.initials: F
authors.lastname: Thomas
mesh_terms: D000970:Antineoplastic Agents / Q000493:pharmacokinetics* / Q000627:therapeutic use; D001665:Binding Sites; D001798:Blood Proteins / Q000378:metabolism; D016022:Case-Control Studies; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D006801:Humans; D066298:In Vitro Techniques; D007191:Indazoles; D009369:Neoplasms / Q000188:drug therapy / Q000378:metabolism*; D011485:Protein Binding; D011743:Pyrimidines / Q000493:pharmacokinetics* / Q000627:therapeutic use; D012709:Serum Albumin / Q000378:metabolism*; D013449:Sulfonamides / Q000493:pharmacokinetics* / Q000627:therapeutic use; D053719:Tandem Mass Spectrometry / Q000379:methods
chemical_list: D000970:Antineoplastic Agents; D001798:Blood Proteins; D007191:Indazoles; D011743:Pyrimidines; D012709:Serum Albumin; D013449:Sulfonamides; C516667:pazopanib
keywords: Albumin; Pazopanib; Plasma unbound fraction; α1-acid glycoprotein