Red blood cell methotrexate polyglutamate concentrations in inflammatory bowel disease

Methotrexate (MTX) use in inflammatory bowel disease (IBD) is complicated by unpredictable efficacy and toxicity. In rheumatoid arthritis, total and individual red blood cell MTX polyglutamates (RBC MTXGlu1-5) correlate with disease activity and possibly toxicity, and therefore may be useful in guiding treatment. It is unclear if this applies in IBD. The aim of this pilot study was to measure total and individual RBC MTXGlu1-5 concentrations in patients with IBD to see if these relate to efficacy and adverse effects. RBC MTXGlu1--5 concentrations were measured on three occasions in 18 patients with IBD receiving MTX at a constant dose for 3 or more months. The intrapatient variability, expressed as the coefficient of variation, of RBC MTXGlu1-5 concentrations at steady state was determined, and disease activity and adverse effects were assessed against concentrations. The intrapatient coefficients of variation of individual MTXGlu1-5 varied from 12% to 27%. In Crohn's disease, higher RBC MTXGlu4&5 concentrations correlated with worse disease activity (r = 0.42 and 0.53, respectively, P