Quantification of total soluble BCMA from human serum in the presence of natural and therapeutic ligands by LC-MS/MS.

B-cell maturation antigen is highly expressed in myeloma cells and is an attractive target for multiple myeloma (MM) therapies. The extracellular domain of this protein is commonly shed from the plasma cell surface, releasing the soluble antigen that has been proposed as a potential therapeutic biomarker to assess disease burden and activity. Elevated levels of soluble BCMA can lead to target mediated drug disposition (TMDD), thereby interfering with anti-BCMA MM biotherapeutics. Additionally, the presence of natural and therapeutic ligands can increase the complexity and feasibility of biomarker assays used to monitor this soluble antigen. The LC-MS/MS method described here utilizes urea to disrupt ligand binding interactions followed by organic solvent protein precipitation to remove high molecular weight proteins, thereby enriching soluble BCMA from serum while eliminating interference from natural and therapeutic ligands. This assay has sufficient sensitivity to quantify total soluble BCMA in on-treatment samples with an analytical range of 3-1000 ng/mL from 100 μL of serum. The mean recovery across all QC levels in the presence of the natural ligands, APRIL and BAFF, as well as a therapeutic monoclonal antibody, was 101% ± 6.1%. Accuracy of this method across QC levels ranged from 91% to 110.4%. This method has been validated in a GLP compliant manner and the total sBCMA data generated using this assay was used to develop models to inform dosing in a clinical study for a novel monoclonal antibody being investigated to treat relapsed or refractory multiple myeloma.