Clinical and Molecular Results of the Phase II Brb (Bendamustine, Rituximab and Bortezomib) Trial of the Fondazione Italiana Linfomi (FIL) for Relapsed/Refractory Waldenström Macroglobulinemia Patients

BACKGROUND: Standard rituximab plus chemotherapy salvage treatment has shown moderate activity in patients with relapsed/refractory Waldenström's macroglobulinemia (RR-WM), with 18-months progression free survival (18-PFS) of about 50%. On behalf of the Fondazione Italiana Linfomi (F.I.L.) we designed a multicenter phase II study to assess the efficacy of a combination of bendamustine, rituximab and bortezomib (BRB) in improving these results. METHODS and PATIENTS: This single-arm phase II study tested the hypothesis that 18-PFS is at least 65%. The required sample size was 38 patients (alpha=0.10; beta=0.25; minimum follow up=24 months). Treatment plan was: rituximab 375 mg/m2 intravenously on day 1 followed by intravenously bendamustine 90 mg/m2 on day 1 and 2 and subcutaneous bortezomib 1.3 mg/m2 on day 1, 8, 15 and 22, every 28 days for 6 months. Patients with RR-WM after first line of therapy were enrolled in 18 F.I.L. centers, from October 2014 to November 2017. In the last 23 patients MYD88^L265Pwas tested by the recently described droplet digital PCR (ddPCR) assay both on bone marrow (BM) and peripheral blood (PB) samples, both at baseline (as mutational screening) and at the end of treatment (for minimal residual disease purposes, MRD). RESULTS: At the time of analysis, 29 patients completed the six cycles of therapy, six patients stopped therapy for toxicity, two patients died and one had just finished therapy and was not yet evaluated. 18-PFS was 84% (95%CI: 61-94%), with two progressions and two deaths without evidence of progression (one cerebrovascular accident during the fifth cycle and one pulmonary embolism at three months follow up). On an intention-to-treat analysis (N=37), overall response rate was 70%, (N=26) including 4 (11%) complete, 11 (30%) very good partial, 10 (27%) partial responses and 1 (3%) minimal response according to IWM response criteria. Overall, treatment was well tolerated, the most common adverse events of any grade included 13 patients (34%) experiencing grade 3-4 neutropenia, especially in cycle 4 (leading in four cases to treatment discontinuation). Peripheral nervous system toxicity was observed in five patients (13%; 4 of grade 1-2 and 1 of grade 3-4), with no discontinuations. Serious adverse events were observed only in three patients, mainly rash, all resolved. All the 23 patients assessed for MYD88^L265Pat baseline scored positive in BM, while only 18/23 (78%) in PB, prospectively confirming the risk of false negative results when only PB of rituximab pre-treated patients is analyzed. Among the 21 patients monitored for MRD after treatment 5 scored MRD negative in BM and 13 in PB, highlighting the deep activity of the BRB regimen in clearing the disease. CONCLUSIONS: Among patients with RR-WM after first line of therapy, BRB regimen is a well-tolerated salvage treatment, resulting in high rates of PFS at 18 months. Moreover, the deep anti-tumor activity of this regimen is highlighted by the promising rates of both clinical and molecular responses. More complete and mature results will be presented during the meeting. (ClinicalTrials.gov number: NCT02371148).