B-337 Mass Spectrometry to evaluate treatment response in patients with Multiple Myeloma. A comparative study versus Immunofixation
Authors: Agulló C , Contreras T , Puig N , Castro S , Barbosa N , Diaz A , Pérez J , San Miguel J F , Lahuerta J , Bladé J , Mateos M V 
Published: October 2024

Abstract

Background
The International Myeloma Working Group (IMWG) defines treatment response categories based on monoclonal protein (MP) changes by serum protein electrophoresis (SPE) and immunofixation (sIFE). Patients that after treatment remain positive by sIFE while SPE is negative, are categorized as very good partial response (VGPR) and, if sIFE becomes negative, patients would be considered in complete remission (CR). However, the interpretation of sIFE is qualitative and subjective. In this study we evaluate the discrepancies between sIFE and Mass Spectrometry (MS) to evaluate tumor burden and the accuracy of treatment response and it´s clinical impact in terms of progression free survival (PFS).

Methods
Patient samples were automatically prepared on the EXENT-iP®500 liquid handler, and sample spots were analysed using the EXENT-iX®500 MALDI-TOF mass spectrometer. Light chain spectral peaks were identified by EXENT-iQ® software and then quantified indirectly in combination with total immunoglobulins concentrations by immunoturbidimetry in the Optilite® platform (The Binding Site, part of Thermo Fisher Scientific). MPs were also identified and quantified by SPE and sIFE.

Results
From the 430 samples from the GEM12, GEM14 and GEM-CESAR clinical trials analyzed at pre-determined time points to evaluate treatment response (post-Induction, post-ASCT, post-cons, 1st and 2nd year after maintenance), MS identified the M protein in 45% of cases (193/430) and sIFE was positive in 32% (137/430). Combining the results of both methods, we found that MS and sIFE were concordant in 76% of cases (326/430), 26.28% IFE+/ MS+ and 49.53% IFE-/ MS-. Considering sIFE as a reference, the negative predictive value (NPV) of MS was 0.8987, with a sensitivity and a specificity of 0.8248 and 0.7270, respectively (p&lt;0.0001). Nevertheless, the positive predictive value (PPV) was 0.5855 which could be explained by the discordances observed between IFE and MS techniques (5.58% of patients IFE+/MS- and 18.6% of patients IFE-/MS+). Indeed, when the 137 patients in VGPR (IFE+ patients) were analyzed according to MS status we found that patients IFE+/ MS+ had a trend to an inferior mPFS in comparison to IFE+/MS- (5.96 years vs not reached). These discordances were even more noticeable when comparing the 293 patients in CR (IFE- patients) with MS status, since IFE-/ MS+ patients had a mPFS of 4.65 years vs mPFS not reached for IFE-/ MS- patients (p=0.0001).

Conclusions
Our data confirms that IFE sensitivity limitations can affect the evaluation of treatment response in patients with MM, being more noticeable when the presence of MP band in sIFE is not clear because is very fuzzy or oligoclonal bands appear. Consequently, heterogeneity in interpretation practices across labs is a major cause of misattributed response categories, especially on those patients achieving treatment responses &gt;VGPR. In an era of highly effective MM treatments that induce quick and deep responses, we believe IMWG response criteria should be updated with an MS response category.