title: Sulfasalazine is a potent inhibitor of the reduced folate carrier: implications for combination therapies with methotrexate in rheumatoid arthritis.
abstract: OBJECTIVE
  To investigate whether interactions of sulfasalazine (SSZ) with reduced folate carrier (RFC), the dominant cell membrane transporter for natural folates and methotrexate (MTX), may limit the efficacy of combination therapy with MTX and SSZ in patients with rheumatoid arthritis.
  
  
  METHODS
  Human RFC-(over)expressing CEM cells of T cell origin were used to analyze the effect of SSZ on the RFC-mediated cellular uptake of radiolabeled MTX and the natural folate leucovorin. Moreover, both cells with and those without acquired resistance to SSZ were used to assess the antiproliferative effects of MTX in combination with SSZ.
  
  
  RESULTS
  Transport kinetic analyses revealed that SSZ was a potent noncompetitive inhibitor of RFC-mediated cellular uptake of MTX and leucovorin, with mean +/- SD K(i) (50% inhibitory concentration) values of 36 +/- 6 microM and 74 +/- 7 microM, respectively. Consistent with the inhibitory interaction of SSZ with RFC, a marked loss of MTX efficacy was observed when MTX was coadministered with SSZ: up to 3.5-fold for CEM cells in the presence of 0.25 mM of SSZ, and >400-fold for SSZ-resistant cells in the presence of 2.5 mM of SSZ. Importantly, along with diminished efficacy of MTX, evidence for cellular folate depletion was obtained by the demonstration of an SSZ dose-dependent decrease in leucovorin accumulation.
  
  
  CONCLUSIONS
  At clinically relevant plasma concentrations, interactions of SSZ with RFC provide a biochemical rationale for 2 important clinical observations: 1) the onset of (sub)clinical folate deficiency during SSZ treatment, and 2) the lack of additivity/synergism of the combination of SSZ and MTX when these disease-modifying antirheumatic drugs are administered simultaneously. Thus, when considering use of these drugs in combination therapies, the present results provide a rationale both for the use of folate supplementation and for spacing administration of these drugs over time.
authors.affiliation: Department of Rheumatology, Vrije Universiteit Medisch Centrum, Amsterdam, The Netherlands. g.jansen@vumc.nl
authors.forename: Gerrit
authors.initials: G
authors.lastname: Jansen
authors.forename: Joost
authors.initials: J
authors.lastname: van der Heijden
authors.forename: Ruud
authors.initials: R
authors.lastname: Oerlemans
authors.forename: Willem F
authors.initials: WF
authors.lastname: Lems
authors.forename: Ilan
authors.initials: I
authors.lastname: Ifergan
authors.forename: Rik J
authors.initials: RJ
authors.lastname: Scheper
authors.forename: Yehuda G
authors.initials: YG
authors.lastname: Assaraf
authors.forename: Ben A C
authors.initials: BA
authors.lastname: Dijkmans
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000378:metabolism; D018501:Antirheumatic Agents / Q000008:administration & dosage / Q000493:pharmacokinetics / Q000494:pharmacology* / Q000627:therapeutic use; D001172:Arthritis, Rheumatoid / Q000188:drug therapy; D001692:Biological Transport / Q000187:drug effects; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004351:Drug Resistance; D004359:Drug Therapy, Combination; D005492:Folic Acid / Q000378:metabolism; D006801:Humans; D002955:Leucovorin / Q000493:pharmacokinetics; D049990:Membrane Transport Modulators; D026901:Membrane Transport Proteins / Q000037:antagonists & inhibitors* / Q000378:metabolism; D008727:Methotrexate / Q000493:pharmacokinetics / Q000627:therapeutic use; D058978:Reduced Folate Carrier Protein; D012460:Sulfasalazine / Q000008:administration & dosage / Q000494:pharmacology* / Q000627:therapeutic use
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D018501:Antirheumatic Agents; D049990:Membrane Transport Modulators; D026901:Membrane Transport Proteins; D058978:Reduced Folate Carrier Protein; C548699:SLC19A1 protein, human; D012460:Sulfasalazine; D005492:Folic Acid; D002955:Leucovorin; D008727:Methotrexate