Monitoring M-Protein, Therapeutic Antibodies, and Polyclonal Antibodies in a Multiparametric Mass Spectrometry Assay Provides Insight into Therapy Response Kinetics in Patients with Multiple Myeloma

Abstract

Background/Objectives: Multiple Myeloma (MM) is a hematologic malignancy caused by clonally expanded plasma cells that produce a monoclonal immunoglobulin (M-protein), a personalized biomarker. Recently, we developed an ultra-sensitive mass spectrometry method to quantify minimal residual disease (MS-MRD) by targeting unique M-protein peptides. Therapeutic antibodies (t-Abs), key in MM treatment, often lead to deep and long-lasting responses. However, t-Abs can significantly decrease the total polyclonal immunoglobulin (Ig) levels which require supplemental IgG infusion. Here, we demonstrate the simultaneous monitoring of M-proteins, t-Abs, and polyclonal Ig-titers using an untargeted mass spectrometry assay, offering a comprehensive view of therapy response. Methods: Sera collected between 2013 and 2024 from four patients and cerebrospinal fluid (CSF) from one patient who received various t-Abs were analyzed with MS-MRD. M-protein sequences were obtained with a multi-enzyme de novo protein sequencing approach. Unique peptides for M-proteins and t-Abs were selected based on linearity, sensitivity, and slope coefficient in serial dilutions. Ig constant regions were monitored using isotype-specific peptides. Results: The MS-MRD multiplex analysis provided detailed information on drug concentrations and therapy response kinetics. For example, in two patients with refractory disease over five lines of therapy, the MS-MRD analysis showed that the deepest responses were achieved with bispecific t-Ab (teclistamab) treatment. M-protein and t-Ab were also detectable in the CSF of one patient with MS-MRD. Conclusions: This proof-of-concept study shows that the multiplex monitoring of the M-protein, any t-Ab combination, and all Ig-isotypes within one mass spectrometry run is feasible and provides unique insight into therapy response kinetics.

Keywords: bispecific antibody; minimal residual disease; multiple myeloma; therapeutic antibody; therapeutic drug monitoring; therapy response kinetics.

Figure 1

Dynamic M-protein, t-Ab, and total IgG levels in the sera of four patients and CSF samples of one patient. The different therapy lines are indicated by blue and white colored areas that are numbered above each graph. The details of each line of therapy are shown in Table 1. The LoD for each M-protein is indicated by the orange dashed line. For patients 2 and 4, routine M-protein monitoring was performed using SPEP. For patients 1 and 3, FLC assay was used to routinely monitor the M-protein. FLC status is indicated on the x-axis of patients 1 and 3. For patient 4, follow-up sera and CSF samples were available from the 4th line of therapy. ASCT: autologous stem cell transplantation; Dara: daratumumab; Isa: isatuximab; IVIg: intravenous immunoglobulins; Talq: talquetamab; Tec: teclistamab; Toci: tocilizumab; CSF: cerebrospinal fluid; LOD: limit of detection.