Predictive Value of Clonal Plasma Cells in Autografts Assessed By Next Generation Flow Cytometry on Pre-Transplant Bone Marrow MRD in Multiple Myeloma Patients

Background: High-dose melphalan followed by autologous stem cell transplantation (ASCT) represents the standard of care for multiple myeloma (MM) patients. However, the clinical significance of minimal residual disease (MRD) in hematopoietic cell grafts (gMRD) remains unclear. Several studies have associated gMRD positivity with worse progression-free survival (PFS), overall survival, and post-ASCT response (Kostopoulos et al., 2021; Pasvolsky et al., 2023). Nonetheless, comparative research investigating the relationship between gMRD and pre-ASCT MRD levels in bone marrow (BM) is still missing, although gMRD assessment could potentially spare patients from this increasingly common and invasive procedure.

Aims: To elucidate the clinical significance of gMRD evaluation using next generation flow cytometry (NGF) and to compare gMRD with pre-ASCT BM MRD assessment.

Methods: This single-center study included 99 newly diagnosed MM patients treated with induction therapy followed by CD34+ cell mobilization, apheresis, high-dose melphalan, and ASCT between 2019 and 2024. Grafts and paired pre-ASCT/D+100 BMs were processed according to Euroflow SOP for bulk lysis, stained for a standardized MM panel, and analyzed according to NGF MRD standards to reach the limit of detection (LOD) 0.0002% (2 x 10^-6) (Flores Montero et al., 2017). BMs contaminated with peripheral blood were excluded from the analysis.

Results: The median age of patients in the cohort was 66 years and 57% were males. ISS stages: 34% I, 36% II, 29% III. Cytogenetics: 77% standard risk, 18% high risk, 5% NA. The median LOD was 0.0002% for gMRD (n = 99), as well as paired pre-ASCT BM (n = 76) and D+100 BM (n = 92). The median follow-up from the time of gMRD assessment was 29.9 months.

In total, 44% (44/99) of grafts were MRD+ with a median level of 0.0076% (range 0.0003-0.79%) of clonal plasma cells (cPCs). Levels of gMRD ≥10^-4, <10^-4 to ≥10^-5, and <10^-5 to ≥2 x 10^-6 were respectively detected in 48%, 32%, and 20% of positive samples. 79% (60/76) of patients were MRD+ in pre-ASCT BM and 62% (57/92) were MRD+ at D+100. There were no significant differences in the main characteristics between gMRD+ and gMRD- subgroups except for ISS I being more prevalent in gMRD- (p = 0.04). The median PFS of the whole cohort has not been reached yet; although, in line with published data, the 2-year PFS was significantly worse for gMRD+ patients compared to gMRD- (64% vs 82%, p = 0.043). Furthermore, the number of gMRD+ patients decreased with a better post-induction response (CR = 19%, 4/21; VGPR = 33%, 14/43; PR = 74%, 23/31).

Importantly, patients who were gMRD+ were always MRD+ in the pre-ASCT BM (100%, 32/32; p < 0.001), and 83% (34/41) of patients with gMRD+ remained MRD+ in BM even at D+100 (p < 0.001). A strong correlation was confirmed between MRD levels of all (MRD+/-) paired grafts and pre-ASCT BMs (r = 0.74, p < 0.001), with a median 2-log increase in MRD levels between paired MRD+ grafts and pre-ASCT BMs (median infiltration, range: 0.01%, 0.0003-0.79% vs 1.1%, 0.0128%-21%; p < 0.001). Moreover, the gMRD+ group also had a median 2-log higher pre-ASCT MRD level compared to the gMRD- group (0.021% vs. 1.1%; p < 0.001).

Conclusion: This study, for the first time, outlines the association between easily accessible and non-invasive gMRD and pre-ASCT BM MRD assessment by NGF. Our data suggest that virtually all gMRD+ patients are also MRD+ in the pre-ASCT BM. Moreover, gMRD results provide a reliable estimate of BM MRD level with a median 2-log increase in cPC infiltration. Overall, our data imply that invasive pre-ASCT BM MRD evaluation might be omitted in gMRD+ patients, which is becoming highly relevant in the current era when the majority of patients reach deep responses after induction.

Acknowledgements: This work was supported by the European Union project LERCO (No. CZ.10.03.01/00/22_003/0000003), by the Operational Programme Just Transition; SALVAGE project CZ.02.01.01/00/22_008/0004644, supported by OP JAK, with co-financing from the EU and the State Budget.