title: Quantitation of infliximab using clonotypic peptides and selective reaction monitoring by LC-MS/MS.
abstract: OBJECTIVE
  Although therapeutic concentrations of infliximab (chimeric IgG1 kappa) are associated with improved clinical prognosis, clinical laboratory methods for monitoring are limited. Therefore, we aimed to develop a LC-MS/MS method to measure infliximab in serum.
  
  
  METHODS
  Infliximab was measured using isotope-labeled peptides and horse IgG as a surrogate internal standard. After trypsin digestion, peptides were separated by reverse-phase C8 LC and detected by MS/MS on an ABSciex API 5000; analyte-to-internal standard peak area ratios were used for quantitation. Sera from patients receiving infliximab were collected at different time points in treatment and compared with a commercially available ECLIA method.
  
  
  RESULTS
  The linear dynamic range of the assay was 1-100 μg/mL (R(2)>0.998); both intra- and inter-assay imprecisions were <20%. Patients undergoing infliximab therapy had trough concentrations of 8.5 ± 8.8 μg/mL (mean ± SD), which substantially increased 48-72 h after infusion (77 ± 40 μg/mL), then fell after 28-32 days (15 ± 11 μg/mL). A comparison of LC-MS/MS and ECLIA methods demonstrated a slope of 0.967 (95% CI: 0.894-1.034, r=0.970).
  
  
  CONCLUSIONS
  We have demonstrated the ability to quantify infliximab in patients using clonotypic peptides. This approach has the potential to be quickly adaptable to other monoclonal antibodies and to expand the availability of testing for this class of therapeutics in routine clinical practice.
authors.affiliation: Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN, USA.
authors.forename: Maria Alice V
authors.initials: MA
authors.lastname: Willrich
authors.affiliation: Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN, USA.
authors.forename: David L
authors.initials: DL
authors.lastname: Murray
authors.affiliation: Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN, USA.
authors.forename: David R
authors.initials: DR
authors.lastname: Barnidge
authors.affiliation: Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN, USA.
authors.forename: Paula M
authors.initials: PM
authors.lastname: Ladwig
authors.affiliation: Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN, USA. Electronic address: snyder.melissa@mayo.edu.
authors.forename: Melissa R
authors.initials: MR
authors.lastname: Snyder
mesh_terms: D000893:Anti-Inflammatory Agents / Q000097:blood*; D018501:Antirheumatic Agents / Q000097:blood*; D002853:Chromatography, Liquid; D006801:Humans; D007074:Immunoglobulin G; D000069285:Infliximab / Q000097:blood*; D010455:Peptides; D053719:Tandem Mass Spectrometry
chemical_list: D000893:Anti-Inflammatory Agents; D018501:Antirheumatic Agents; D007074:Immunoglobulin G; D010455:Peptides; D000069285:Infliximab
keywords: Infliximab; Mass spectrometry; Monoclonal antibody therapeutics; Selective reaction monitoring