Folylpolyglutamate synthetase (FPGS) is a crucial enzyme in both cellular folate homeostasis and the intracellular retention of folate analogue drugs such as methotrexate (MTX), which is commonly used for the treatment of (pediatric) leukemia and the anchor drug in rheumatoid arthritis (RA) treatment. To date, assessment of FPGS catalytic activity relies on assays using radioactive substrates that are labor-intensive and require relatively large numbers of cells. Here, we describe a nonradioactive, ultra-high-performance liquid chromatography-tandem mass spectrometer (UHPLC-MS/MS)-based method allowing for sensitive and accurate measurements of FPGS activity in low cell numbers (ie, 1-2 × 10) of biological specimens, including leukemic blast cells of acute lymphoblastic leukemia patients and peripheral blood mononuclear cells of patients with RA.
The UHPLC-MS/MS assay was validated with 2 CCRF-CEM human leukemia cells, one proficient and one deficient in FPGS activity. Linearity of time and protein input were tested by measuring FPGS activity at 30-180 minutes of incubation time and 10-300 mcg protein extract. In addition, FPGS enzyme kinetic parameters were assessed.
The FPGS enzymatic assay showed a linear relation between FPGS activity and protein input (R ≥ 0.989) as well as incubation time (R ≥ 0.996). Moreover, the UHPLC-MS/MS method also allowed for evaluation of FPGS enzyme kinetic parameters revealing Km values for the substrates MTX and L-glutamic acid of 64 µmol/L and 2.2 mmol/L, respectively. The mean FPGS activity of acute lymphoblastic leukemia blast cells (n = 4) was 3-fold higher than that of CCRF-CEM cells and 44-fold and 88-fold higher than that of peripheral blood mononuclear cells from MTX-naive (n = 9) and MTX-treated RA patients (n = 6), respectively.
Collectively, given its sensitivity with low cell numbers and avoidance of radioactive substrates, UHPLC-MS/MS-based analysis of FPGS activity may be eligible for routine therapeutic drug monitoring of MTX in RA and leukemia for therapy (non)response evaluations.