Page 37 - Haematologica Vol. 109 - July 2024

REVIEW ARTICLE - Minimal residual disease in multiple myeloma R. Szalat et al.
Bone marrow MRD assessment requires repeated invasive procedures, and is limited by the known patchy nature of MM and the possibility of EMD that are inherent to the disease. Therefore, alternative approaches including blood or imaging-based MRD evaluation assessment have been developed.24
Blood-based minimal residual disease assessment
Circulating plasma cells
The presence of circulating plasma cells (CPC) in periph- eral blood (PB) can be detected in most MM patients and is associated with poor prognosis. Different methods have been used to assess the presence of CPC. The standard EuroFlow NGF is reliable and requires a small volume of blood. Other methods are available that use a plasma cell enrichment method, which requires a larger blood sample, and is more sensitive but also more complex. NGF has been used to identify and track CPC in MM patients with interesting results. However, while CPC detection appears to be a powerful prognostic factor, CPC is unlikely to be a good MRD marker. Indeed, a comparison between NGF in BM and PB after therapy in a real-world case series of 137 patients showed that 40% of patients achieving blood- based MRD negativity had BM MRD-positive disease, strongly suggesting that blood NGF-based MRD evaluation is a less sensitive MRD marker than BM MRD.22,25,26
Circulating cell-free DNA for minimal residual disease assessment
Circulating cell-free DNA (cfDNA)-based methods, often referred to as ‘liquid biopsy’, allow tracking genomic aber- rations such as tumor mutations, copy number aberration or translocation present in circulating cfDNA isolated from blood plasma.27-29 Multiple studies showed a high concor- dance of somatic mutations and copy-number alterations between BM and cfDNA of patients with MM.30-33 However, the low level of circulating tumor DNA is a significant chal- lenge and most current methods are not sensitive enough. Ultradeep targeted sequencing has significantly improved the detection of cfDNA, but its sensitivity relates to the number of tumor mutations available to track and has so far only been evaluated in few clinical studies. In a study which compared blood and BM evaluation with NGS and cfDNA in 42 patients, there was only 49% consistency and poor correlation between the two methods. Similar to CPC detection, BM MRD was more often positive and suggest- ed lack of sensitivity of the cfDNA approach.34 Novel and more sensitive methods are needed before cfDNA can be utilized as a standard approach.
Single-cell RNA sequencing
Single-cell RNA sequencing (scRNA-seq) is another powerful technology widely used in research. It allows transcriptomic analysis at a single cell level and can detect rare malig- nant cells.35 Ongoing research is investigating whether this approach could even allow the selection of therapy based on transcriptomic features and clonal heterogeneity.36,37 However, its availability, its relative complex workflow, re- producibility and cost are significant challenges that need to be addressed before it can be considered for use in clinical practice.38 This approach is also limited by the fact that it can currently only evaluate a certain number of cells, far fewer than with flow- or NGS MRD-based assessment methods. Therefore, the lack of detection of malignant cells would not necessarily correspond to negative MRD, and scRNAseq appears more as a potential complimentary method that may help tailor therapy to target MRD positive cells rather than to determine MRD status.
Mass spectrometry methods
Instead of tracking the malignant residual plasma cell, mass spectrometry (MS) methods have been applied to detect the monoclonal immunoglobulin produced by the malignant plasma cells. This very sensitive method can detect the presence of a monoclonal immunoglobulin at a much lower concentration than standard serum immunofixation. MS can also distinguish therapeutic monoclonal antibodies from myeloma monoclonal immunoglobulin, and can identify post-translational modifications relevant for patients with monoclonal gammopathy of clinical significance, including light chain (AL) amyloidosis.39-41 Several MS methods have been developed. Matrix-assisted laser desorption/ionization MS (MALDI-TOF MS) and the MASS-FIX assay have been shown to be particularly efficient, and both more sensitive and more specific than serum protein electrophoresis and immunofixation.40,42,43 Another MS approach was initially developed by the Binding Site company in collaboration with the Mayo Clinic. The Mayo Clinic went on to develop their own approach using a commercial assay, while the Binding Site assay, which has more selective probes, was sold to ThermoFisher and is used in individual labs. Methods using liquid chromatography-tandem mass spectrometry (LC-MS/MS) are currently under clinical investigation and are not yet FDA-approved. Other MS methods focusing on quantifying unique clonotypic peptides (MS-MRD) derived from the variable region of the monoclonal immunoglob- ulin by enzymatic digestion followed by LC-MS/MS have been developed and provide great sensitivity. MS-MRD demonstrated a 1,000-fold higher sensitivity compared to serum protein electrophoresis (SPEP) and can be used to monitor patients with MRD.40,44-46 While some preliminary reports suggest that MS may be a strong predictor of PFS,47 additional studies are needed to incorporate these assays into clinical practice. In particular, the persistence of the monoclonal immunoglobulin in the serum in the context
Haematologica | 109 July 2024
2051