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 immunosuppress ive properties of AZA/6-MP are mediated by the intracellular metabolism of 6-MP into its active metabolites, 6-thioguanine nucleotides (6TGN) and 6-me thylmercaptopurine (6-MMP). Preliminary studies have suggested that the red blood cell concentration of 6TGN (RBC 6TGN) is a potential guide to therap y. The aims of the study were to evaluate the RBC 6TGN concentrations in ad ult patients with CD under long-term AZA/6-MP therapy and to correlate it w ith response to treatment and haematological parameters. Methods: Twenty-ei ght CD patients treated for at least 3 months with AZA/6-MP were prospectiv ely studied. Patients were separated into three main groups: group 1 (n = 1 9), corresponding to quiescent CD receiving AZA (dose: 2.05 +/- 0.4 mg/kg/d ay for a mean of 28.6 +/- 25 months) or 6-MP (dose: 1.4 +/- 01 mg/kg/day fo r a mean of 7.5 +/- 3.5 months) alone; group 2 (n = 6), corresponding to qu iescent 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 ac tive CD on AZA (dose: 1.94 +/- 0.6 mg/kg/day for a mean of 31.3 +/- 35 mont hs) as the only treatment. An assessment was also made by merging groups I and 2 forming a larger group of patients (n = 25) defined by clinical remis sion and groups 2 and 3 forming a larger group of patients (n=9), non-compl ete responders with AZA/6-MP alone. Crohn disease index activity (CDAI), bl ood samples for full blood count and differential white cell count and meas urement of RBC 6TGN and 6-MMP concentrations were evaluated at inclusion an d at 6 months (n = 17). RBC 6TGN were measured using high performance liqui d chromatography (HPLC) on heparinized blood. Results: The baseline charact eristics of the three groups of patients were similar. There was no signifi cant difference among the three groups of patients regarding the dose and t he duration of immunosuppressive treatment. There was no significant differ ence between groups according to various parameters tested. Particularly, t he median RBC 6TGN concentration at inclusion was similar in the three grou ps 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 rema ined in remission and median RBC 6TGN concentration remained stable. No sid e effects were observed. Conclusions: There is, contrary to preliminary stu dies, a broad overlap in RBC 6TGN levels as well as for haematological para meters in patients in remission or not and responders or not to AZA/6-MP th erapy. This suggests, beside a variability in the metabolism of these drugs , the existence of complex mechanisms of action. Nevertheless, beside the u se of RBC 6TGN determination to confirm compliance to therapy, this dosage could be useful in non-responding patients, allowing, in absence of leukope nia, to increase the dose of AZA/6-MP safely.