title: Role of kidney-specific organic anion transporters in the urinary excretion of methotrexate.
abstract: BACKGROUND
  High-dose folinic acid is used to accelerate methotrexate elimination to avoid renal toxicity of the drug. The present study was carried out to examine the role of the renal organic anion transporters OAT-K1 and OAT-K2 in the urinary excretion of methotrexate, especially in the methotrexate-folinic acid rescue therapy.
  
  
  METHODS
  Madin-Darby canine kidney cells stably expressing OAT-K1 and OAT-K2 were used for the in vitro transport study; 5/6 nephrectomized rats were used to detect changes in mRNA expression levels of OAT-K1 and OAT-K2 and to evaluate methotrexate pharmacokinetics under conditions of renal insufficiency.
  
  
  RESULTS
  Methotrexate efflux mediated by these transporters in stable transfectants was stimulated in the presence of extracellular folic acid and folinic acid, suggesting that they could serve as anion exchangers to enhance the apical efflux of methotrexate. The mRNA expression levels of OAT-K1 and OAT-K2 were markedly diminished after 5/6 nephrectomy, but those of multidrug resistance associated protein 2, which could transport methotrexate, were maintained. Renal clearance of methotrexate was markedly decreased in 5/6 nephrectomized rats compared with that in sham-operated rats. Additional folinic acid treatment resulted in a significant increase in methotrexate renal clearance in sham-operated rats but not in 5/6 nephrectomized rats.
  
  
  CONCLUSIONS
  The decreased expressions of OAT-K1 and OAT-K2 may be attributable to the longer exposure to methotrexate and ineffective folinic acid rescue. In terms of contributing to patient safety, renal clearance of methotrexate, especially folinic acid-stimulated tubular secretion of the drug via these transporters, would be a key factor in methotrexate therapy.
authors.affiliation: Department of Pharmacy, Kyoto University Hospital, Kyoto, Japan.
authors.forename: A
authors.initials: A
authors.lastname: Takeuchi
authors.forename: S
authors.initials: S
authors.lastname: Masuda
authors.forename: H
authors.initials: H
authors.lastname: Saito
authors.forename: T
authors.initials: T
authors.lastname: Doi
authors.forename: K
authors.initials: K
authors.lastname: Inui
mesh_terms: D000818:Animals; D027321:Anion Transport Proteins; D002352:Carrier Proteins / Q000378:metabolism*; D002460:Cell Line; D004285:Dogs; D066298:In Vitro Techniques; D007678:Kidney Glomerulus / Q000378:metabolism*; D007684:Kidney Tubules / Q000378:metabolism*; D002955:Leucovorin / Q000378:metabolism; D008297:Male; D008565:Membrane Proteins / Q000378:metabolism; D008727:Methotrexate / Q000378:metabolism* / Q000652:urine; D009392:Nephrectomy; D027361:Organic Anion Transporters; D012333:RNA, Messenger / Q000032:analysis / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar
chemical_list: D027321:Anion Transport Proteins; D002352:Carrier Proteins; D008565:Membrane Proteins; D027361:Organic Anion Transporters; D012333:RNA, Messenger; C101140:Slc21a4 protein, rat; D002955:Leucovorin; D008727:Methotrexate