title: Effects of aminosalicylates on thiopurine S-methyltransferase activity: an ex vivo study in patients with inflammatory bowel disease.
abstract: BACKGROUND
  Based on in vitro experiments using recombinant human thiopurine S-methyltransferase this enzyme is inhibited by sulfasalazine (sulphasalazine) and 5-aminosalicylate. Thus, during treatment with azathioprine or mercaptopurine, both metabolized by thiopurine S-methyltransferase, sulfasalazine or 5-aminosalicylate could modify the action of azathioprine/mercaptopurine.
  
  
  OBJECTIVE
  To examine whether this interaction is effective under ex vivo conditions.
  
  
  METHODS
  In 18 azathioprine-free patients and in 12 patients on azathioprine the inhibitory potential of sulfasalazine, 5-aminosalicylate and its metabolite (Ac-5-aminosalicylate) was assessed by ex vivo measurement of thiopurine S-methyltransferase in red blood cells.
  
  
  RESULTS
  According to concentration response curves mean IC50 values (microm) for sulfasalazine, 5-aminosalicylate and Ac-5-aminosalicylate have been calculated in three groups of azathioprine-free patients and variable basal levels of thiopurine S-methyltransferase activity (very high, normal and intermediate). In all three groups sulfasalazine was the strongest inhibitor (IC50: 9-17 microm) if compared with 5-aminosalicylate (129-236) and Ac-5-aminosalicylate (58-74). In patients on azathioprine similar IC50 values have been calculated.
  
  
  CONCLUSIONS
  Comparing human plasma concentrations of sulfasalazine (15-77 microm), 5-aminosalicylate (3-14 microm) and Ac-5-aminosalicylate (8-18 microm) with the IC50 values one can assume that only sulfasalazine would have the potential to inhibit thiopurine S-methyltransferase in vivo. However, the therapeutic impact should be proved by clinical studies.
authors.affiliation: Dr Margarete Fischer-Bosch Institute of Clinical Pharmacology, Stuttgart, Germany.
authors.forename: H
authors.initials: H
authors.lastname: Xin
authors.forename: C
authors.initials: C
authors.lastname: Fischer
authors.forename: M
authors.initials: M
authors.lastname: Schwab
authors.forename: U
authors.initials: U
authors.lastname: Klotz
mesh_terms: D000293:Adolescent; D000328:Adult; D000636:Aminosalicylic Acids / Q000627:therapeutic use*; D004347:Drug Interactions; D005260:Female; D006801:Humans; D015212:Inflammatory Bowel Diseases / Q000188:drug therapy*; D020128:Inhibitory Concentration 50; D008297:Male; D008780:Methyltransferases / Q000378:metabolism*; D008875:Middle Aged
chemical_list: D000636:Aminosalicylic Acids; D008780:Methyltransferases; C022745:thiopurine methyltransferase