title: Relationship between 6-mercaptopurine dose and 6-thioguanine nucleotide levels in patients with inflammatory bowel disease.
abstract: BACKGROUND
  6-Thioguanine nucleotides (6-TGN) are active metabolites of azathioprine (AZA) and 6-mercaptopurine (6MP). Higher remission rates have been observed in patients with higher 6-TGN levels. However, many physicians prescribe AZA/6MP using milligrams per kilogram (mg/kg) dosing regimens without measuring 6-TGN levels. The aim of this study was to examine the association between 6MP dose and 6-TGN levels.
  
  
  METHODS
  We conducted a cross-sectional study of patients treated for inflammatory bowel disease (IBD) with AZA or 6MP, whose 6-TGN levels were measured. Patients with low or intermediate thiopurine methyl transferase (TPMT) activity were excluded. AZA dose was converted to 6MP equivalents. The relationship between dose and 6-TGN levels was assessed with the Spearman correlation coefficient. We used logistic regression to assess the relationship between dose and 6-TGN levels of >230 pmol/8 x 10(8).
  
  
  RESULTS
  In this study, 155 patients met our inclusion criteria (median dose, 1.01 +/- 0.40; range, 0.61-1.41 mg/kg). There was a weak correlation between 6-TGN levels and the absolute dose (rho = 0.18, P = 0.04) and the dose in mg/kg (rho = 0.19, P = 0.03). The correlation between mg/kg dosage and 6-TGN levels was slightly stronger in those using concomitant 5-aminosalicylate (5-ASA) medications (rho = 0.24, P = 0.02). Compared with <1.0 mg/kg per day, doses of > or =1.5 mg/kg per day were strongly associated with 6-TGN levels of >230 pmol/8 x 10(8) RBC (adjusted odds ratio [OR] = 3.7, 95% confidence interval [CI] = 1.1-11.8). However, only 37% of patients receiving > or =1.0 mg/kg per day had 6-TGN levels of >230 pmol/8 x 10(8) RBC.
  
  
  CONCLUSIONS
  6MP dose is weakly associated with 6-TGN levels. The use of standard mg/kg dosing regimens will result in low 6-TGN levels in most patients.
authors.affiliation: Department of Medicine, University of Pennsylvania, Philadelphia, PA 19104-6021, USA.
authors.forename: Ahmed
authors.initials: A
authors.lastname: Morales
authors.forename: Sandhya
authors.initials: S
authors.lastname: Salguti
authors.forename: Chuan L
authors.initials: CL
authors.lastname: Miao
authors.forename: James D
authors.initials: JD
authors.lastname: Lewis
mesh_terms: D000328:Adult; D001682:Biological Availability; D003430:Cross-Sectional Studies; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D007166:Immunosuppressive Agents / Q000008:administration & dosage / Q000493:pharmacokinetics* / Q000494:pharmacology; D015212:Inflammatory Bowel Diseases / Q000188:drug therapy*; D008297:Male; D015122:Mercaptopurine / Q000008:administration & dosage / Q000493:pharmacokinetics* / Q000494:pharmacology; D013866:Thioguanine / Q000097:blood*
chemical_list: D007166:Immunosuppressive Agents; D015122:Mercaptopurine; D013866:Thioguanine