Therapeutic drug monitoring of azathioprine and 6-mercaptopurine metabolites in Crohn disease.

BACKGROUND 6-Mercaptopurine (6-MP) and its prodrug azathioprine (AZA) have proven efficacy in the treatment of Crohn disease (CD). The immunosuppressive properties of AZA/6-MP are mediated by the intracellular metabolism of 6-MP into its active metabolites, 6-thioguanine nucleotides (6TGN) and 6methylmercaptopurine (6-MMP). Preliminary studies have suggested that the red blood cell concentration of 6TGN (RBC 6TGN) is a potential guide to therapy. The aims of the study were to evaluate the RBC 6TGN concentrations in adult patients with CD under long-term AZA/6-MP therapy and to correlate it with response to treatment and haematological parameters. METHODS Twenty-eight CD patients treated for at least 3 months with AZA/6-MP were prospectively studied. Patients were separated into three main groups: group 1 (n = 19), corresponding to quiescent CD receiving AZA (dose: 2.05 +/- 0.4 mg/kg/day for a mean of 28.6 +/- 25 months) or 6-MP (dose: 1.4 +/- 01 mg/kg/day for a mean of 7.5 +/- 3.5 months) alone; group 2 (n = 6), corresponding to quiescent CD treated by AZA (dose: 2.14 +/- 0.5 mg/kg/day for a mean of 29.5 +/- 22 months) with oral steroids; and group 3 (n = 3), corresponding to active CD on AZA (dose: 1.94 +/- 0.6 mg/kg/day for a mean of 31.3 +/- 35 months) as the only treatment. An assessment was also made by merging groups 1 and 2 forming a larger group of patients (n = 25) defined by clinical remission and groups 2 and 3 forming a larger group of patients (n = 9), non-complete responders with AZA/6-MP alone. Crohn disease index activity (CDAI), blood samples for full blood count and differential white cell count and measurement of RBC 6TGN and 6-MMP concentrations were evaluated at inclusion and at 6 months (n = 17). RBC 6TGN were measured using high performance liquid chromatography (HPLC) on heparinized blood. RESULTS The baseline characteristics of the three groups of patients were similar. There was no significant difference among the three groups of patients regarding the dose and the duration of immunosuppressive treatment. There was no significant difference between groups according to various parameters tested. Particularly, the median RBC 6TGN concentration at inclusion was similar in the three groups of patients (166 (105-688), 183 (90-261) and 160 (52-194) pmol/8 x 10(8) RBC, respectively). The majority of patients had no detectable level of 6-MMP metabolite, except for 3 patients. There was also no difference between merging groups. Furthermore, there was no significant correlation between RBC 6TGN concentrations and the various biological parameters tested except for the mean erythrocyte volume. At 6 months, all patients of group 1 remained in remission and median RBC 6TGN concentration remained stable. No side effects were observed. CONCLUSIONS There is, contrary to preliminary studies, a broad overlap in RBC 6TGN levels as well as for haematological parameters in patients in remission or not and responders or not to AZA/6-MP therapy. This suggests, beside a variability in the metabolism of these drugs, the existence of complex mechanisms of action. Nevertheless, beside the use of RBC 6TGN determination to confirm compliance to therapy, this dosage could be useful in non-responding patients, allowing, in absence of leukopenia, to increase the dose of AZA/6-MP safely.