Achievement of Minimal Residual Disease Negativity By Multiparameter Flow Cytometry Is an Important Therapeutic Endpoint in Patients with Relapsed/Refractory B-Cell Acute Lymphoblastic Leukemia Receiving Salvage Treatment

Background: Minimal residual disease (MRD) assessment by multiparameter flow cytometry (MFC) is prognostic for survival in newly diagnosed patients (pts) with acute lymphoblastic leukemia (ALL). The significance of achieving MRD negativity in the relapsed/refractory setting is less clear.

Methods: Between 6/2010 and 5/2015, we identified 130 adult pts with relapsed/refractory B-cell ALL treated at our institution with either inotuzumab ozogamicin (n=75), blinatumomab (n=20) or mini-hyper-CVD plus inotuzumab ozogamicin (HCVD+InO; n=35) in either salvage 1 (S1; n=68) or salvage 2 (S2; n=62). MRD by MFC was assessed on remission bone marrow specimens at the time of achievement of CR/CRp/CRi. The MRD assay used a 15-marker, 6-color panel with a sensitivity of ≤0.01%.

Results: Of the initial 130 pts, 78 (60%) achieved morphological response with a median time to response of 30 days (range, 13-99 days) and are the subject of this analysis. Of the 78 responding pts, 41 (53%) received inotuzumab, 11 (14%) blinatumomab, and 26 (33%) HCVD+ino. 46 pts (59%) were in S1 and 32 (41%) in S2. The median number of cycles to best response was 1 (range, 1-3). MRD negativity was achieved in 41 pts (53%). MRD negativity rates for pts in CR, CRp, and CRi were 57%, 53%, and 16%, respectively. Among pts who achieved remission, MRD negativity was achieved in 17 pts (41%) with inotuzumab, 8 (73%) with blinatumomab, and 16 (62%) with HCVD+InO (P=0.10). 26 pts (57%) in S1 and 15 (47%) in S2 became MRD-negative (P=0.40).

The median follow-up duration was 27 months (range, 6-55 months). The median event-free survival (EFS) was 12 months in pts who achieved MRD negativity vs. 6 months in those who remained MRD-positive (P=0.09). The median overall survival (OS) was 17 months versus 9 months, respectively (P=0.18). Among pts in S1, achieving MRD negativity was associated with a longer EFS (median 18 months versus 7 months; 2-year EFS rate 46% versus 17%; P=0.06; Figure 1A) and OS (median 27 months versus 9 months; 2-year OS 52% versus 36%; P=0.15; Figure 1B). EFS and OS were similar in S2 regardless of MRD response. As expected, among pts who achieved MRD negativity, those in S1 had longer EFS (median 18 months vs. 5 months; P=0.001) and OS (median 27 months vs. 7 months; P=0.01) compared to those in S2. In contrast, for pts who remained MRD-positive, EFS and OS were similar regardless of salvage status (P=0.41 and P=0.39, respectively).

In a 2-month landmark analysis of 64 pts, survival >2 years was observed in all groups of pts regardless of salvage treatment, salvage status or MRD status. 42 (66%) of the pts in this analysis underwent allogeneic stem cell transplantation (alloSCT). EFS and OS did not significantly differ between pts who did or did not undergo alloSCT, although a clear trend for improved long-term survival with alloSCT was observed. Among pts who achieved MRD negativity, the median EFS was 17 months and 12 months, and 2-year EFS rates were 46% and 28% for pts who underwent alloSCT vs. those who did not (P=0.24). The median OS was 24 months and 23 months, and 2-year OS rates were 55% and 46%, respectively (P=0.41). Pts who achieved MRD negativity after S1 treatment and then underwent alloSCT had the best outcomes. Of the 22 pts who achieved MRD negativity after S1 treatment, the median EFS for pts who underwent alloSCT (n=14) compared to those who did not (n=8) was not reached vs. 18 months, and the median OS was not reached vs. 27 months, respectively (P=0.28 for both). Among the 14 pts who achieved MRD negativity after S1 treatment and subsequently underwent alloSCT, 10 (71%) are still alive with a median follow-up of 24 months (range, 5-55 months).

Conclusions: In patients with relapsed/refractory ALL, achievement of MRD negativity is associated with improved outcomes. Patients with relapsed/refractory ALL who achieve MRD negativity in S1 can achieve excellent long-term survival, especially if alloSCT is performed.

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