A Bottom-Up Liquid Chromatography–Tandem Mass Spectrometry Method for Therapeutic Drug Monitoring of Infliximab Method Development, Comparison With 2 Enzyme-Linked Immunosorbent Assay Methods, and Evaluation of Anti-Drug Antibody Interference

Context.—Therapeutic drug monitoring is recommended to optimize infliximab use and improve outcome in chronic inflammatory disorders. Objective.—To describe a simple and affordable liquid chromatography–tandem mass spectrometry (LC-MS/MS) method to measure infliximab in serum. Design.—Infliximab was measured using winged stable isotope–labeled peptides as internal standards. Linearity, lower limit of measuring interval, limit of detection, precision, accuracy, carryover, and ion suppression were evaluated. Method comparison against 2 enzyme-linked immunosorbent assay (ELISA) methods (Remsima Monitor and IDKmonitor Infliximab) and anti-drug antibody (ADA) interference were evaluated using clinical specimens from inflammatory bowel disease patients (N = 237). Results.—Analytical run time and sample preparation time were 5 minutes per sample and 3 hours per batch, respectively. Analytical measurement interval and limit of detection were 0.50 to 50.0 lg/mL (R² = 0.998) and 0.25 lg/mL, respectively. The intraday and interday imprecision percentage coefficients of variation were less than 6.1%. Accuracy was 94.2% to 98.7%. No significant ion suppression or carryover was observed. Infliximab concentrations measured by LC-MS/MS showed good agreement with those measured by Remsima Monitor (mean percentage difference, 5.7%; 95% CI, 21.2% to 12.6%) but were markedly lower than those measured by IDKmonitor (232.6%; 235.8% to 229.4%), demonstrating significant bias between ELISAs. Although a good agreement between LC-MS/MS and ELISA was observed for ADA-negative samples (23.5%; 212.8% to 5.9%), a significant bias was observed for ADA-positive samples (13.6%; 1.7% to 25.6%). Conclusions.—This simple, fast, and affordable LC-MS/MS method for infliximab quantitation could improve standardization of infliximab quantitation and optimization of infliximab use in patients with high-titer ADA.