Ryan Funk

Methotrexate (MTX) is the cornerstone of therapy in the treatment of rheumatoid arthritis (RA). However, its efficacy and toxicity are variable and remain unpredictable. Interindividual variation in the metabolism of MTX by the enzyme folyl polyglutamate synthetase (FPGS) has been associated with response variability in RA. In this work, we propose the development of a FPGS phenotyping assay that can be evaluated as a tool for the prediction of efficacy and toxicity in patients with RA prior to initiating MTX therapy. FPGS activity was measured in erythrocyte lysate by monitoring methotrexate polyglutamate (MTX + Glun) formation using ultra-performance liquid chromatography tandem–mass spectrometry (UPLC/MS/MS). Erythrocyte FPGS activity was measured in newly diagnosed RA (n = 35) and osteoarthritis (n = 7) patients. The enzymatic assay was optimized for measuring FPGS activity in 25 µL of packed erythrocytes over two hours. The coefficient of variation for intra- and inter-day analysis was found to be 5% and 12%, respectively. The method was used to measure FPGS enzyme kinetics, resulting in a mean (SD) Km of 30.3 (4.8) µM and a Vmax of 612 (193) pmol MTX + Glu2/h/mL of packed erythrocytes. Mean (SD) erythrocyte FPGS activity in patients with RA was found to be 445.93 (344.50) pmol MTX + Glu2/h/mL and with a 26-fold difference in the range (range: 83–2179 pmol MTX + Glu2/h/mL) whereas for patients with OA, it was found to be 409.80 (157.66) pmol MTX + Glu2/h/mL with a 3.5-fold difference in the range (range: 200.95–683.93 pmol MTX + Glu2/h/mL). Monitoring erythrocyte FPGS activity may be a feasible strategy of phenotyping for methotrexate efficacy and toxicity in patients with RA.