Real-World Experience with Minimal Residual Disease Testing with Next Generation Flow Cytometry and Functional Imaging in Multiple Myeloma

Background:

Minimal residual disease (MRD) diagnostics in multiple myeloma (MM) are gaining increasing importance to determine response depth beyond complete remission (CR) since novel agents have shown to induce high rates of deep clinical responses. Moreover, recent reports indicated combining functional imaging with next generation flow cytometry (NGF) could be beneficial in predicting clinical outcome. This applies in particular to the subset of patients suffering from relapsed/refractory multiple myeloma (RRMM) who tend to show a higher incidence of residual focal lesions despite serological response. Here, we report our institutions experience with implementing both functional imaging and NGF-guided MRD diagnostics in clinical practice.

Methods:

Our study included patients with newly diagnosed multiple myeloma (NDMM) and RRMM achieving VGPR, CR or sCR. Bone marrow aspirates were obtained for MRD-testing according to IMWG 2016 criteria. Samples were collected between July 2019 and July 2020 and analyzed with NGF (according to EuroFlow^TM guidelines) at a sensitivity level of 10^-5. Results were compared to functional imaging obtained with positron emission tomography (PET) and diffusion-weighted magnetic resonance imaging (DW-MRI). High-risk disease was defined as presence of deletion 17p, translocation (14;16) or (4;14).

Results:

We included 66 patients with NDMM (n=39) and RRMM (n=27) who achieved VGPR or better. In patients with RRMM the median number of treatment lines was 2 (range 2-11). Fifteen patients suffered from high-risk disease. Median age at NGF diagnostics was 64 years (range 31-83).

Among patients achieving VGPR (n=27), CR (n=10) and sCR (n=29) seventeen (26%) were MRD-negative by NGF testing. CR or better was significantly associated NGF MRD-negativity (p=0.04). Notably, rates of NGF MRD-negativity were similar among patients with NDMM (28%) and RRMM (26%). Even some heavily pretreated patients who underwent ≥ 4 lines of therapy achieved MRD-negativity on NGF (2 of 9).

Functional imaging was performed in 46 (70%) patients with DW-MRI (n=22) and PET (n=26). Median time between NGF and imaging assessment was 2 days (range 0-147). Combining results from imaging and NGF, 12 out of 46 (26%) patients were MRD-negative with both methods (neg/neg). Three patients displayed disease activity as measured with both, imaging and NGF (pos/pos). Twenty-nine of the remaining patients were MRD-positive only according to NGF (pos/neg), while two patients were positive on imaging only (neg/pos). More patients demonstrated combined MRD-negativity on NGF and imaging (neg/neg) in the NDMM setting than in RRMM (32% versus 19%). We also observed that 30% of the patients with high-risk genetics showed MRD-negativity on both imaging and NGF. Of note, none of the patients with very advanced disease (≥4 previous lines) was MRD-negative on both techniques.

Conclusion

In the clinical routine, MRD diagnostics could be used to tailor maintenance and consolidation approaches for patients achieving deep responses by traditional IMWG criteria. Our real-world experience highlights that MRD-negativity can be achieved in patients suffering from high-risk disease and also in late treatment lines, supporting its value as endpoint for clinical trials. However, our data also support MRD diagnostics to be combined with functional imaging at least in the RRMM setting to rule out residual focal lesions. Future studies using MRD for clinical decision-making are highly warranted.