MTX concentrations not helpful in dose adjustment

The monitoring of methotrexate polyglutamate (MTXGlu) concentrations is unlikely to be helpful in guiding dose adjustment for long-term MTX therapy but red blood cell folate levels might, New Zealand researchers report. Contrary to previous reports, the study of 192 patients receiving oral MTX found no positive association between MTXGlu concentrations and disease activity. The researchers said their finding was unexpected but may reflect differences in established versus early disease, selection of patients tolerant to MTX, dosage increases in MTX resistant patients, RBC folate status, or differences in intracellular drug metabolism. However, somewhat unexpectedly, the researchers found a positive correlation between MTXGlu and the SJC, TJC, M-HAQ score and DAS28, which persisted when adjusting for factors such as age, and duration of RA. “The clinical significance of these results is uncertain, but they suggest that there is a group of patients in whom disease remains active despite higher MTX dosages and higher RBC MTXGlu concentrations,” the researchers wrote in Arthritis and Rheumatism. Consistent with previous studies, the researchers found that red blood count folate levels influenced disease activity. “Our data suggest that RBC folate levels may be a more important factor than MTXGlu concentrations for disease control in patients receiving MTX.” They suggested that a long-term prospective study examining the relationship between MTX dosage, MTXGlu concentrations, RBC folate status and effects, adverse effects after commencing treatment would clarify the situation. Arthritis and Rheumatism 2010; 62:359-368....