title: Rapid solid-phase extraction method to quantify [(11)C]-verapamil, and its [(11)C]-metabolites, in human and macaque plasma.
abstract: BACKGROUND
  P-glycoprotein (P-gp), an efflux transporter, is a significant barrier to drug entry into the brain and the fetus. The positron emission tomography (PET) ligand, [(11)C]-verapamil, has been used to measure in vivo P-gp activity at various tissue-blood barriers of humans and animals. Since verapamil is extensively metabolized in vivo, it is important to quantify the extent of verapamil metabolism in order to interpret such P-gp activity. Therefore, we developed a rapid solid-phase extraction (SPE) method to separate, and then quantify, verapamil and its radiolabeled metabolites in plasma.
  
  
  METHODS
  Using high-performance liquid chromatography (HPLC), we established that the major identifiable circulating radioactive metabolite of [(11)C]-verapamil in plasma of humans and the nonhuman primate, Macaca nemestrina, was [(11)C]-D-617/717. Using sequential and differential pH elution on C(8) SPE cartridges, we developed a rapid method to separate [(11)C]-verapamil and [(11)C]-D-617/717. Recovery was measured by spiking the samples with the corresponding nonradioactive compounds and assaying these compounds by HPLC.
  
  
  RESULTS
  Verapamil and D-617/717 recovery with the SPE method was >85%. When the method was applied to PET studies in humans and nonhuman primates, significant plasma concentration of D-617/717 and unknown polar metabolite(s) were observed. The SPE and the HPLC methods were not significantly different in the quantification of verapamil and D-617/717.
  
  
  CONCLUSIONS
  The SPE method simultaneously processes multiple samples in less than 5 min. Given the short half-life of [(11)C], this method provides a valuable tool to rapidly determine the concentration of [(11)C]-verapamil and its [(11)C]-metabolites in human and nonhuman primate plasma.
authors.affiliation: Department of Pharmaceutics, University of Washington, Box 357610, Seattle, WA 98195, USA. jash@u.washington.edu
authors.forename: Jashvant D
authors.initials: JD
authors.lastname: Unadkat
authors.forename: Francisco
authors.initials: F
authors.lastname: Chung
authors.forename: Lucy
authors.initials: L
authors.lastname: Sasongko
authors.forename: Dale
authors.initials: D
authors.lastname: Whittington
authors.forename: Sara
authors.initials: S
authors.lastname: Eyal
authors.forename: David
authors.initials: D
authors.lastname: Mankoff
authors.forename: Ann C
authors.initials: AC
authors.lastname: Collier
authors.forename: Mark
authors.initials: M
authors.lastname: Muzi
authors.forename: Jeanne
authors.initials: J
authors.lastname: Link
mesh_terms: D000818:Animals; D002250:Carbon Radioisotopes; D002851:Chromatography, High Pressure Liquid; D005260:Female; D006801:Humans; D008254:Macaca nemestrina; D049268:Positron-Emission Tomography; D052616:Solid Phase Extraction / Q000379:methods*; D014700:Verapamil / Q000097:blood* / Q000378:metabolism
chemical_list: D002250:Carbon Radioisotopes; D014700:Verapamil