Olsalazine and 6-mercaptopurine-related bone marrow suppression: a possible drug-drug interaction

Clin Pharmacol Ther. 1997 Oct;62(4):464-75. doi: 10.1016/S0009-9236(97)90125-9.


A patient with refractory Crohn's disease had two separate episodes of bone marrow suppression while receiving 50 to 75 mg 6-mercaptopurine a day and 1000 to 1750 mg olsalazine a day. This adverse reaction necessitated dose reduction of 6-mercaptopurine on the first occasion and withdrawal of 6-mercaptopurine and olsalazine on the second occasion. The patient's red blood cell thiopurine methyltransferase (TPMT) activity was 1.2 U per milliliter red blood cells (low normal range) and her TPMT genotype was wild-type sequence for all known alleles of TPMT that result in low TPMT enzyme activity. In vitro enzyme kinetic studies confirmed the hypothesis that olsalazine and olsalazine-O-sulfate are potent noncompetitive inhibitors of recombinant human TPMT. We suggest that the patient's relatively low baseline level of TPMT activity was inhibited by olsalazine and olsalazine-O-sulfate, leading to decreased clearance of 6-mercaptopurine and its accumulation. This ultimately increased intracellular 6-thiopurine nucleotide levels to toxic concentrations, which caused bone marrow suppression.

Publication types

  • Case Reports
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adolescent
  • Aminosalicylic Acids / adverse effects*
  • Aminosalicylic Acids / therapeutic use
  • Anti-Inflammatory Agents / adverse effects*
  • Anti-Inflammatory Agents / therapeutic use
  • Bone Marrow / drug effects*
  • Crohn Disease / drug therapy
  • Crohn Disease / immunology*
  • Drug Interactions
  • Humans
  • Immunosuppressive Agents / adverse effects*
  • Immunosuppressive Agents / therapeutic use
  • Mercaptopurine / adverse effects*
  • Mercaptopurine / therapeutic use
  • Prednisone / adverse effects*
  • Prednisone / therapeutic use


  • Aminosalicylic Acids
  • Anti-Inflammatory Agents
  • Immunosuppressive Agents
  • Mercaptopurine
  • olsalazine
  • Prednisone