title: Determination of a human hepatic microsomal scaling factor for predicting in vivo drug clearance.
abstract: OBJECTIVE
  To determine a microsomal scaling factor for human liver suitable for prediction of in vivo drug clearance from in vitro data and to explore the role of inter-liver variability in this factor on the reported underprediction from microsomal parameters.
  
  
  METHODS
  Cytochrome P450 (henceforth P450) content in whole homogenates and microsomes from 38 donor livers was used to determine a microsomal scaling factor. In a subset (n = 20) of these preparations, individual P450 enzymes were examined by Western blotting and selective probe activities were determined.
  
  
  RESULTS
  The scaling factor from 38 livers averaged 40 mg microsomal protein per gram liver with a coefficient of variation of 31%. Western blotting experiments indicated that there was no P450 enzyme-specific trend in the distribution of individual P450 enzymes in liver microsomes relative to whole homogenate. Predictions based on an average scaling factor resulted in a satisfactory prediction of intrinsic clearance of three benzodiazepines similar to that obtained using individual factors for the same livers.
  
  
  CONCLUSIONS
  A value for human liver microsomal scaling of 40 mg microsomal protein per gram liver has been established. The reason for underprediction previously reported for 52 different drug substrates was not the use of an incorrect value for the scaling factor.
authors.affiliation: School of Pharmacy and Pharmaceutical Sciences, University of Manchester, Manchester, M13 9PL, UK.
authors.forename: Nancy
authors.initials: N
authors.lastname: Hakooz
authors.forename: Kiyomi
authors.initials: K
authors.lastname: Ito
authors.forename: Helen
authors.initials: H
authors.lastname: Rawden
authors.forename: Helen
authors.initials: H
authors.lastname: Gill
authors.forename: Lynn
authors.initials: L
authors.lastname: Lemmers
authors.forename: Alan R
authors.initials: AR
authors.lastname: Boobis
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Edwards
authors.forename: David J
authors.initials: DJ
authors.lastname: Carlile
authors.forename: Brian G
authors.initials: BG
authors.lastname: Lake
authors.forename: J Brian
authors.initials: JB
authors.lastname: Houston
mesh_terms: D001569:Benzodiazepines / Q000493:pharmacokinetics*; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D004353:Drug Evaluation, Preclinical / Q000379:methods; D006801:Humans; D008657:Metabolic Clearance Rate; D008862:Microsomes, Liver / Q000378:metabolism*; D015203:Reproducibility of Results
chemical_list: D001569:Benzodiazepines; D003577:Cytochrome P-450 Enzyme System