Dilution displacement enabled measurement of total ligand levels in the presence of an anti-ligand antibody using LBA

BACKGROUND: Measurement of soluble ligand targets of biotherapeutics can be challenging, especially when non-competing reagents are unavailable. This study aimed to develop a simple assay to quantify total soluble B-cell Maturation Antigen (sBCMA), including antibody-bound sBCMA, in serum when anti-BCMA antibody is present. RESEARCH DESIGN AND METHODS: A sample predilution strategy was coupled with a ligand-binding assay (LBA) to dissociate sBCMA from anti-BCMA antibody complexes and enable measurement of total sBCMA. RESULTS: A lower limit of quantitation (LLOQ) of 3 ng/mL in neat serum was achieved in the presence of clinically relevant concentrations of anti-BCMA antibody less than 30 µg/mL. sBCMA serum levels were elevated in patients with multiple myeloma (MM) (mean 198.3 ng/mL; range 3.3-2035.7 ng/mL) compared with healthy subjects (mean 12.4 ng/mL; range 2.9-18.4 ng/mL). CONCLUSIONS: This dilution-based approach enabled accurate measurement of total sBCMA despite circulating anti-BCMA antibody. However, this sample predilution approach may or may not have general applicability for similar bioanalytical purposes, when candidate biotherapeutic levels are moderate, assay is highly sensitive, and the target protein levels are substantially elevated, as described in this case.