MALDI-TOF Mass Spectrometry in Serum for the Follow-up of Newly Diagnosed Multiple Myeloma Patients Treated with Daratumumab-Based Combination Therapy

Introduction

Mass spectrometry-based methods have been shown to be more sensitive for detecting monoclonal proteins (M-protein) in serum compared to current electrophoretic techniques, serum protein electrophoresis (SPEP) and immunofixation (IFE). In particular, MALDI-TOF mass spectrometry (MALDI-TOF MS) may soon replace these techniques for the routine monitoring of multiple myeloma (MM) patients due to its relatively low cost and high throughput. In this study, we evaluate the performance of MALDI-TOF MS in the follow up of newly diagnosed multiple myeloma (NDMM) patients treated with a daratumumab-based combination therapy. We report our findings compared to SPEP and IFE results and discuss the advantages and disadvantages of the technique in the serial analysis of patients.

Patients and Methods

Twenty-seven NDMM patients treated with daratumumab-based combination therapy were included in this study; median age 57 years (range 33-79 years) and 52% were males. Each patient had 10 time points of follow-up: baseline, day 15 of cycle 1, the first day of each cycle from cycle 2 to cycle 8, and at the end of treatment (EOT). All samples were analyzed in a blinded fashion by MALDI-TOF MS. First, immunoglobulins were purified from serum using magnetic beads specific for IgG and IgA heavy chains or kappa and lambda light chains. Immunoglobulins were eluted from the beads and the light chains and heavy chains were separated by adding a reducing agent. Purified samples were analyzed using a Microflex LT MALDI-TOF mass spectrometer (Bruker). Samples taken at baseline were used to identify the mass to charge ratio (m/z) of the M-protein which served as a surrogate marker in the analysis of subsequent samples. MALDI-TOF MS results were compared to SPEP, IFE and the kappa/lambda free light chain (κ/λ) ratio.

Results

At baseline, IFE and MALDI-TOF MS were positive for all 27 patients while SPEP was negative for M-protein in 2 patients. Different M-protein isotypes were observed including 3 free kappa, 1 free lambda, 15 IgG kappa, 3 IgG Lambda, 3 IgA kappa and 2 IgA lambda. The κ/λ ratio was abnormal for 26/27 patients. Twenty-three patients completed the 8 cycles of treatment.

During the follow-up, 14 of the 23 patients remained positive until the EOT by MALDI-TOF MS. Regarding these patients, 3 were negative by SPEP and IFE at the EOT. Nine of the 23 patients became negative by MALDI-TOF MS in a median time of 5 cycles (range 2- 8). Among these 9 patients, 1 reached a complete response (CR) and 6 reached stringent CR in a median time of 3 cycles (range cycle 2 - EOT). The 2 patients that did not reach CR but were negative by MALDI are suspected to have a false positive IFE result. These patients' IgG kappa M-protein overlaps with daratumumab on IFE and the Hydrashift assay (Sebia) was unavailable at the time of analysis. In these cases, MALDI provided better specificity compared to IFE as the M-protein could be distinguished from daratumumab based on m/z. However, daratumumab could not always be distinguished from the M-protein at some timepoints for some patients. The patient that still had an abnormal κ/λ ratio but was negative by MALDI had κ light chain MM. MALDI-TOF MS may be less sensitive for the detection of free light chains in serum. We observed differences between the M-spike intensity of the heavy- and light-chain specific purifications especially when the M-protein was at low levels. This may be due to differences in the polyclonal background for each purification reaction and will affect the sensitivity of M-protein detection.

Conclusions

This study is important because it helps to understand the performance of MALDI-TOF MS in the follow-up of MM patients under therapy. The use of serial samples allowed us to characterize patterns of immune markers longitudinally in relation to given therapy. The m/z ratio at baseline is a key for the interpretation during the follow-up and to avoid interference with other monoclonal immunoglobulins, like daratumumab, for example. When more than one monoclonal immunoglobulin is present, their relative concentration, not just their m/z values, is important for distinguishing two different peaks. MALDI-TOF MS is useful for monitoring patients under therapy because it provides higher specificity and sensitivity than electrophoretic methods. This may be especially important in clinical trials and in accurately defining CR and sCR.