title: Absorption rate limit considerations for oral phosphate prodrugs.
abstract: OBJECTIVE
  To evaluate the potential of phosphate ester prodrugs to significantly improve the absorptive flux of poorly soluble parent drugs.
  
  
  METHODS
  Absorptive transport studies of parent drugs and their prodrugs were carried out in Caco-2 cells. Prodrugs of parent drugs with variable aqueous solubilities were tested: Hydrocortisone-phosphate/Hydrocortisone, Fosphenytoin/phenytoin, TAT-59/DP-TAT-59, and Entacapone phosphate/Entacapone. Additional absorption studies were carried out in rats.
  
  
  RESULTS
  Absorptive fluxes of DP-TAT-59 and phenytoin increased 9.8 or 3.3-fold after dosing TAT-59 and 500 microM fosphenytoin, respectively. Hydrocortisone's flux did not increase with hydrocortisone-phosphate at 100 microM. Permeability of the highly lipophilic and protein bound compound, DP-TAT-59, was significantly increased with serosal albumin. No permeability increase was observed for the other drugs with albumin. Entacapone phosphate failed to improve the flux of entacapone compared to an entacapone solution, but the prodrug solution did yield higher entacapone plasma levels in rats when compared with an entacapone suspension.
  
  
  CONCLUSIONS
  Ideal phosphate prodrug candidates are characterized by high permeability and low solubility (BCS Class II drugs). For low dose BCS Class II drug candidates, however, no biopharmaceutical advantage may be gained. Phosphate prodrugs of parent drugs with limited permeability may fail. When screening highly lipophilic parent drugs transport studies should be done with albumin.
authors.affiliation: The University of Michigan, College of Pharmacy, 428 Church Street, Ann Arbor, Michigan 48109, USA.
authors.forename: Tycho
authors.initials: T
authors.lastname: Heimbach
authors.forename: Doo-Man
authors.initials: DM
authors.lastname: Oh
authors.forename: Lilian Y
authors.initials: LY
authors.lastname: Li
authors.forename: Markus
authors.initials: M
authors.lastname: Forsberg
authors.forename: Jouko
authors.initials: J
authors.lastname: Savolainen
authors.forename: Jukka
authors.initials: J
authors.lastname: Leppänen
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Matsunaga
authors.forename: Gordon
authors.initials: G
authors.lastname: Flynn
authors.forename: David
authors.initials: D
authors.lastname: Fleisher
mesh_terms: D000284:Administration, Oral; D000465:Algorithms; D000818:Animals; D019540:Area Under Curve; D001693:Biological Transport, Active; D018938:Caco-2 Cells; D002396:Catechols / Q000493:pharmacokinetics; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D006801:Humans; D006854:Hydrocortisone / Q000493:pharmacokinetics; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007408:Intestinal Absorption / Q000502:physiology*; D007413:Intestinal Mucosa / Q000378:metabolism; D007583:Jejunum / Q000378:metabolism; D008297:Male; D009570:Nitriles; D010477:Perfusion; D010539:Permeability; D010672:Phenytoin / Q000031:analogs & derivatives* / Q000737:chemistry / Q000493:pharmacokinetics; D011355:Prodrugs / Q000493:pharmacokinetics*; D051381:Rats; D017208:Rats, Wistar; D013629:Tamoxifen / Q000031:analogs & derivatives* / Q000737:chemistry / Q000493:pharmacokinetics
chemical_list: D002396:Catechols; D009570:Nitriles; D011355:Prodrugs; D013629:Tamoxifen; C064635:TAT 59; C071192:entacapone; D010672:Phenytoin; C043114:fosphenytoin; D006854:Hydrocortisone