Therapeutic drug monitoring of methotrexate and its metabolites in the red blood cells and mononuclear cells of patients with rheumatoid arthritis

Objective: to assess the time course of changes in the concentration of methotrexate (MTX) and its main metabolites in the red blood cells (RBC) and mononuclear cells (MNC) of patients with rheumatoid arthritis (RA), by taking into account individual characteristics (age, statin therapy, and smoking).

Patients and methods. The investigation enrolled 33 MTX-treated patients (mean age 53.2±11.7 years) with RA, who underwent therapeutic drug monitoring to measure the RBC and MNC concentrations of free MTX and MTX polyglutamates (MTXPGs) with 2, 3, and 4 glutamate residues (MTXPG 2–4) in using tandem chromatomass spectrometry after 4, 12, and 24 weeks of therapy.

Results and discussion. Following 12 weeks, the concentration of MTXPG4 in the MNC was higher in patients taking statins, while that of MTX and MTXPG2 in the RBC were significantly lower than in smokers. At 24 weeks, older patients were observed to have a higher MTX level and a lower MTXPG4 concentration in the RBC.

Conclusion. After 24 weeks of therapy, the RBC concentration of MTPG4 was lower and that of MTX was higher in older patients than in others, which confirms data on a slower MTX metabolism in the elderly. The use of statins is likely to have a positive impact on the accumulation of MTXPG. There is a statistically significantly lower RBC concentration of MTXPG in at 12 weeks of therapy.

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