Minimal Residual Disease Evaluation Using 8-Color Flow Cytometry Predicts Risk of Relapse in High-Risk and/or Young Myeloma Patients Who Receive Bortezomib Consolidation after Frontline Tandem Transplantation

Introduction: Multiple myeloma (MM) remains incurable with standard therapies. Allogeneic stem cell transplantation (alloSCT) is the only curative treatment for these patients. We hypothesized that bortezomib (BTZ) consolidation after tandem autologous stem cell transplantation (ASCT) and nonmyeloablative (NMA) alloSCT could improve quality of response while decreasing relapse and cGVHD. We also sought to determine prospectively the predictive value of bone marrow minimal residual disease (MRD) evaluation using a highly sensitive flow cytometry assay.

Methods: Newly diagnosed myeloma (NDMM) patients ≤65 years with high-risk (HR) features (based on cytogenetics, ISS 3 or plasma cell leukemia) or ≤50 year regardless of risk status with an 8/8 HLA matched donor are eligible to participate in this prospective trial. After a BTZ-based induction and ASCT, outpatient NMA alloSCT is performed with either fludarabine and cyclophosphamide (sibling donor) or fludarabine and TBI 2 Gy (unrelated donor) followed by peripheral blood stem cells. GVHD prophylaxis consists of tacrolimus and MMF. BTZ is initiated on day +120 post-alloSCT at 1.3 mg/m² every 2 weeks for 1 year. Response evaluation is based on IMWG criteria. Bone marrow MRD evaluation is performed on 10x10⁶ nucleated cells with highly sensitive (≥10^-5) next-generation flow cytometry using the 8-color EuroFlow protocol (CD45, CD38, CD138, CD56, CD19, CD27, CD81, CD117, CyIgκ and CyIgλ) before alloSCT, before BTZ and every 3 months for 2 years. Immunophenotypic complete response (iCR) is defined as stringent CR in addition to 2 consecutive negative MRD results. aGVHD and cGVHD are evaluated prospectively.

Results: As of June 29^th 2018, 37 patients have been enrolled with a median age of 53 (range: 35-64) years. ISS 3 is found in 43% and HR cytogenetics in 54% (5% del17p, 14% t(4;14), 22% gain 1q21 and 14% >1 HR cytogenetics). Induction consisted of CyBorD (81%) or VTD (19%) for a median of 4 (range: 4-7) cycles. Median times from induction to ASCT and from ASCT to alloSCT were 5.8 and 4.4 months, respectively. Sibling and unrelated donor transplants were performed in 43% and 57%, respectively. KPS and HCT-CI were 90 (range 80-100) and 1 (range 0-3), respectively. Median follow-up is 21 (range 0-39) months after alloSCT. Of enrolled patients, 34 have started BTZ and received 92.5% of planned doses, with no dose reduction needed for toxicity. Observed grade ≥3 non-hematologic toxicities possibly/related to BTZ included diarrhea (n=1), viral hemorrhagic cystitis (2 adenovirus, 1 BK) and nocardial brain abscesses (n=1). Cumulative incidences of grade II-IV and III-IV aGVHD were 26% and 11%. Incidences of all grade, moderate/severe and severe cGVHD at 24 months were 61%, 47% and 10%, respectively, with mostly mouth, skin and liver involvement. Compared to 27 historical controls who did not receive BTZ after tandem transplant, the incidence of moderate/severe cGVHD was much lower in BTZ recipients (47 vs 78%; p=0.002). After reviewing each target organ involvement, mouth and eye cGVHD were significantly less severe with BTZ. Three patients died, one from myeloma progression and 2 from grade III aGVHD, with a 24-month non-relapse mortality of only 8%. BTZ consolidation improved depth of response, increasing ≥CR rate from 64% to 85% and iCR rate from 25% to 59%, regardless of cytogenetic abnormalities (Table 1). Probability of progression-free survival (PFS) at 24 months was 65% (CI 95%: 42-81) while overall survival (OS) was 90% (CI 95%: 70-97; Fig. 1A). The cumulative incidence of progression at 24 months was 28%. Importantly, the presence of ≤50 myeloma cells in the bone marrow 10 months post-alloSCT (after 6 months of BTZ) was associated with a significantly lower probability of progression (15% versus 80%; p=0.03; Fig. 1B).

Conclusion: Tandem ASCT-NMA alloSCT followed by BTZ consolidation results in a remarkably high rate of ≥CR, including iCR. For the first time in allogeneic transplant recipients, MRD evaluation using the EuroFlow protocol demonstrates that identification of ≤50 myeloma cells in the bone marrow 10 months after alloSCT/6 months after BTZ seems predictive of a better outcome. If confirmed, this landmark could be used to design future therapeutic interventions in order to decrease the risk of relapse after tandem transplant. Finally, BTZ following alloSCT is safe and may contribute to decrease both incidence and severity of cGVHD.

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