title: Impact of extracellular protein binding on passive and active drug transport across Caco-2 cells.
abstract: OBJECTIVE
  The objective of the study is to evaluate the mechanism behind alterations in passive and active transport of drugs in the presence of basolaterally applied extracellular protein using the Caco-2 cell model.
  
  
  METHODS
  The permeation across Caco-2 monolayers of two groups of compounds, transported either solely by passive diffusion or partly also by active transport in the secretory direction, was studied at two donor concentrations in the absence or presence of bovine serum albumin (BSA, 0-4%). Each group contained compounds with high or low protein binding (PB) capabilities and high or low absorption in humans (fraction absorbed, f(a)). The unbound fraction (f(u)) of each compound was determined by ultrafiltration.
  
  
  RESULTS
  The transport rate of the passively permeating compounds was the same in both apical-to-basolateral (absorptive) and basolateral-to-apical (secretory) directions in the absence of BSA. Basolaterally applied BSA increased the transport rate in the absorptive direction and decreased it in the secretory direction for all compounds, in direct proportion to the extent of PB. The efflux ratios for the actively transported compounds were reduced in the presence of BSA.
  
  
  CONCLUSIONS
  Basolaterally applied BSA, which mimics in vivo PB, alters both passive and active drug transport in the Caco-2 cell model. Using C(u) in the calculations of transport rate allowed elucidation of the different mechanisms behind these alterations. Our data also suggest that active secretory transport for highly protein-bound compounds might have less effect in vivo than predicted from traditional Caco-2 cell models (without BSA).
authors.affiliation: Department of Pharmacy, Uppsala University, Box 580, SE-751 23 Uppsala, Sweden. Sibylle.Neuhoff@gmx.de
authors.forename: Sibylle
authors.initials: S
authors.lastname: Neuhoff
authors.forename: Per
authors.initials: P
authors.lastname: Artursson
authors.forename: Ismael
authors.initials: I
authors.lastname: Zamora
authors.forename: Anna-Lena
authors.initials: AL
authors.lastname: Ungell
mesh_terms: D000465:Algorithms; D000818:Animals; D001693:Biological Transport, Active; D001798:Blood Proteins / Q000378:metabolism; D018938:Caco-2 Cells; D002417:Cattle; D005110:Extracellular Space / Q000378:metabolism*; D006801:Humans; D004364:Pharmaceutical Preparations / Q000378:metabolism*; D011485:Protein Binding; D019275:Radiopharmaceuticals; D012710:Serum Albumin, Bovine / Q000378:metabolism
chemical_list: D001798:Blood Proteins; D004364:Pharmaceutical Preparations; D019275:Radiopharmaceuticals; D012710:Serum Albumin, Bovine