title: Rate-limited steps of human oral absorption and QSAR studies.
abstract: OBJECTIVE
  To classify the dissolution and diffusion rate-limited drugs and establish quantitative relationships between absorption and molecular descriptors.
  
  
  METHODS
  Absorption consists of kinetic transit processes in which dissolution, diffusion, or perfusion processes can become the rate-limited step. The absorption data of 238 drugs have been classified into either dissolution or diffusion rate-limited based on an equilibrium method developed from solubility, dose, and percentage of absorption. A nonlinear absorption model derived from first-order kinetics has been developed to identify the relationship between percentage of drug absorption and molecular descriptors.
  
  
  RESULTS
  Regression analysis was performed between percentage of absorption and molecular descriptors. The descriptors used were ClogP, molecular polar surface area, the number of hydrogen-bonding acceptors and donors, and Abraham descriptors. Good relationships were found between absorption and Abraham descriptors or ClogP.
  
  
  CONCLUSIONS
  The absorption models can predict the following three BCS (Biopharmaceutics Classification Scheme) classes of compounds: class I, high solubility and high permeability; class III, high solubility and low permeability; class IV, low solubility and low permeability. The absorption models overpredict the absorption of class II, low solubility and high permeability compounds because dissolution is the rate-limited step of absorption.
authors.affiliation: Department of Chemistry, University College London, United Kingdom.
authors.forename: Yuan H
authors.initials: YH
authors.lastname: Zhao
authors.forename: Michael H
authors.initials: MH
authors.lastname: Abraham
authors.forename: Joelle
authors.initials: J
authors.lastname: Le
authors.forename: Anne
authors.initials: A
authors.lastname: Hersey
authors.forename: Chris N
authors.initials: CN
authors.lastname: Luscombe
authors.forename: Gordon
authors.initials: G
authors.lastname: Beck
authors.forename: Brad
authors.initials: B
authors.lastname: Sherborne
authors.forename: Ian
authors.initials: I
authors.lastname: Cooper
mesh_terms: D000284:Administration, Oral; D006801:Humans; D007408:Intestinal Absorption / Q000502:physiology*; D004364:Pharmaceutical Preparations / Q000378:metabolism*; D021281:Quantitative Structure-Activity Relationship; D012044:Regression Analysis
chemical_list: D004364:Pharmaceutical Preparations