B-180 The EXENT® Solution Provides Evidence for High Prevalence of Multiple M-proteins in Monoclonal Gammopathies
Authors: Allen S , Birtwistle J , Berlanga O , Malin G , Lakos G 
Published: September 2023

Abstract

Background
Monoclonal gammopathies (MG) are characterized by excessive production of monoclonal immunoglobulin (M protein), typically consisting of a heavy chain (HC) and a light chain (LC), or a LC only. In some patients, however, more than one restricted band may be present on serum protein electrophoresis. These additional M proteins may be present at initial diagnosis or emerge later following observation or treatment, especially autologous stem cell transplantation. Immunofixation electrophoresis may not be able to differentiate between two bands of the same isotype or other sources of an additional M protein, such as polymerization, glycosylation or therapeutic monoclonal antibody. Mass spectrometry (MS) has recently emerged as an analytically sensitive technology for the detection of M proteins. The EXENT® solution (in development by The Binding Site, part of Thermo Fisher Scientific) uses immuno-purification of immunoglobulins, followed by matrix-assisted laser desorption ionization time of flight (MALDI-ToF) MS analysis. The automated, algorithm-driven analysis of the resulting mass spectra determines the isotype, the molecular mass and the quantity of M proteins. The molecular mass of the M protein serves as a “fingerprint” and allows for easy differentiation between multiple M proteins. The goal of this cross-sectional study was to assess the prevalence of biclonal and multiclonal M proteins in monoclonal gammopathy of undetermined significance (MGUS), smoldering multiple myeloma (SMM), multiple myeloma (MM), Waldenstrom macroglobulinemia (WM) and AL amyloidosis with mass spectrometry.

Methods
Diagnostic (baseline) samples from 271 patients (35 MGUS, 66 SMM, 125 MM, 31 WM and 14 AL amyloidosis) were analyzed with EXENT® using the EXENT® Immunoglobulin Isotypes reagents. A second M protein was considered distinct from the primary M component if it had a different HC or LC component, or, if it had the same isotype, but had a different molecular mass. Samples producing an additional high-mass peak attributed to LC glycosylation have not been considered biclonal.

Results
The EXENT® solution has identified a second M protein in 201 (74%) samples, although the second M protein was below the lower limit of the measuring interval in 107 (39%) cases, and only 36 (13.5%) was ≥0.2 g/L: 5 out of 35 in MGUS, 10 out of 66 in SMM, 13 out of 125 in MM, 8 out of 31 in WM and none in AL amyloidosis. Among these, the second M protein had the same isotype in 17 cases and contained a different HC or LC or both in 19 cases. IgG was the most prevalent; the distribution of kappa and lambda LC was 11:13. More than two M proteins were identified in 120 (44%) patients, in 12 (4.5%) cases with a concentration of ≥0.2 g/L.

Conclusion
The prevalence of two or more M proteins was higher than the previously reported 3%–6%. The higher prevalence is likely the result of the ability of the system to detect small monoclonal peaks that have previously gone unrecognized by traditional methods, and to distinguish between M proteins of the same isotype based on molecular mass.