Clinical Significance of Sensitive Flow-MRD Monitoring in Elderly Multiple Myeloma Patients on the Pethema/GEM2010MAS65 Trial

The genetic heterogeneity of multiple myeloma (MM) makes it unlikely that established or novel chemotherapy could be equally effective in all genetic subgroups. Therefore, genetics alone is insufficient to fully capture different disease outcomes, and there is growing body of evidence showing that detection of minimal residual disease (MRD), using immunophenotypic or molecular-based approaches, also provides powerful independent prognostic information particularly among transplant-eligible patients. However, it is perhaps in elderly MM, the major patient subgroup and in which optimal balance between efficacy and toxicity is critical, that sensitive response assessment could help to tailor patients’ treatment.

Here, we used for the first time sensitive 8-color multidimensional flow cytometry (cut-off of 10^-5) to monitor MRD among elderly MM patients included in the PETHEMA/GEM2010MAS65 trial (sequential chemotherapy with 9 cycles of bortezomib-melphalan-prednisone (VMP) followed by 9 cycles of lenalidomide-low dose dexamethasone (Rd), or alternating cycles of VMP and Rd up to 18 cycles). A single 8-color antibody combination (CD45-PacB/CD138-OC515/CD38-FITC/CD56-PE/CD27-PerCPCy5.5/CD19-PECy7/CD117-APC/CD81-APCH7) was used to detect phenotypically aberrant clonal plasma cells (PCs), and MRD-negativity was defined when <20 clonal PCs were detected among ≥2.000.000 leukocytes (<0.001%). MRD assessment was centralized in three PETHEMA/GEM laboratory-cores, cytometrists were blinded to all clinical data, and results were prospectively uploaded into a locked intranet dataset. Median follow-up of the series was 27 months, and time-to-progression (TTP) / overall survival (OS) was measured from the moment of MRD assessment.

First, we evaluated the MRD status at cycle 9 of chemotherapy (n=117), and no significant differences were observed for MRD-negative rates between the sequential vs alternating regimens (23% vs 25%; P = .86). However, when we focused on patients in complete response (CR; n=41) and compared the quality of CR achieved in each arm according to patients’ MRD status, we found significantly higher frequencies of MRD-negative rates after the sequential vs alternating schema (75% vs 40%; P = .03). Patients in CR attaining MRD-negativity at cycle 9 showed a significantly prolonged TTP (100% vs 41% at 2-years; P = .001) as well as OS (100% vs 71% at 2-years; P= .03) as compared to patients in CR but with persistent MRD cells.

To understand the kinetics of MDR response with sequential vs alternating 18 cycles of chemotherapy, we focused on 72 patients with paired Flow-MRD assessments at cycles 9 and 18. No MRD-negative patients at cycle 9 turned into MRD-positive at cycle 18; however, 21% of MRD-positive patients at cycle 9 became MRD-negative at cycle 18, with no significant differences between rates of transformation after sequential vs alternating regimens (P = .23). At the end of cycle 18, MRD-negative rates among patients randomized to the sequential vs alternating schema were of 48% vs 31% (P = .08), and the quality of CR (according to patients’ MRD status) was slightly but not significantly superior in the sequential vs alternating arm (66% vs 48%; P = .16). Again, patients in CR at cycle 18 attaining MRD-negativity showed superior TTP as compared to those in CR with persistent MRD: TTP at 2-years of 83% vs 56% (P= .06).

We also compared the impact of Flow-MRD among cytogenetically defined standard- and high-risk [+1q, t(4;14), t(14;16), and/or del(17p)] patient subgroups (n=125). As expected, standard-risk patients attaining MRD-negativity had significantly prolonged TTP as compared to MRD-positive patients (94% vs 58% at 2-years; P = .035); however, also high-risk cytogenetic patients achieving Flow-CR showed significantly superior TTP (median not reached vs 10 months; P= .001).

In summary, we unravel the clinical impact of sensitive Flow-MRD monitoring (10^-5) among elderly MM patients in which attaining MRD-negativity, particularly early in therapy, translated into virtually relapse-free intervals at 2-years. In parallel, we also show the value of sensitive MRD kinetics to understand the benefit of additional (sequential or alternating) chemotherapy to further reduce MRD levels, as well as the significance of Flow-MRD among cytogenetically defined standard- and high-risk patents.