Treatment of Relapsed/Refractory Waldenström Macroglobulinemia Patients: Final Clinical and Molecular Results of the Phase II Brb (Bendamustine, Rituximab and Bortezomib) Trial of the Fondazione Italiana Linfomi (FIL)
Authors: Benevolo Giulia , Ferrero Simone , Villivà Nicoletta , Castiglione Anna , Monaco Federico , Boccomini Carola , Cabras Maria Giuseppina , Califano Catello , Casula Paolo , Cavallo Federica , Conconi Annarita , Drandi Daniela , Gaidano Gianluca , Gregorini Anna Ines , Felici Stefano , Ferrante Martina , Mannina Donato , Molinari Anna Lia , Musto Pellegrino , Musuraca Gerardo , Merli Michele , Sartori Roberto , Tani Monica , Varettoni Marzia , Vitolo Umberto , Orsucci Lorella 
Published: November 2021

Abstract
Introduction
Symptomatic patients with relapsed/refractory Waldenström Macroglobulinemia (RR-WM) treated with standard rituximab plus chemotherapy as second-line salvage therapy, generally show a 18-months progression free survival (PFS) of about 50%. On behalf of the Fondazione Italiana Linfomi, a multicenter phase II study was designed to assess whether a combination of bendamustine, rituximab and bortezomib (BRB) (EudraCT Number:2013-005129-22) could be considered a promising new treatment in this setting.
Patients and Methods
This single-arm phase II study tested the hypothesis that 18-months 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 provided: 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 (6 cycles).
MYD88 L265P and CXCR4 S338X mutations were tested by ddPCR in bone marrow (BM), plasma and peripheral blood (PB) samples, both at baseline (as mutational screening) and at the end of treatment (for minimal residual disease purposes, MRD).
Results
Median age was 66.8 years (8 patients were older than 75 years). Many patients had features of advanced disease such as cytopenia (anemia 71%, thrombocytopenia 20%), systemic symptoms (40%) and symptomatic splenomegaly (24%). Sixteen (42%) patients had at least one comorbidity, mostly cardiovascular disease (21%) or metabolic disorders (16%), such as diabetes mellitus.
Thirty patients completed six cycles, 7 patients stopped therapy for toxicity and 1 for progressive disease. Overall response rate at the end of therapy was 82%, including 4 (11%) complete, 15 (39%) very good partial, 12 (32%) partial responses according to IWM response criteria.
At 18, 24, and 30 months PFS was 84% (95% CI 68-92%), 81% (95%CI 65-91) and 79% (95%CI 62-89) respectively. At 18 months OS was 92% (95%CI 77-97%) and no deaths were observed between 18 and 30 months. Nineteen patients (50%) experienced grade ≥3 hematological toxicity, mainly thrombocytopenia, 12 patients (31.5%) developed grade ≥3 extra-hematological toxicity of which only one cutaneous toxicity related to bendamustine. Bortezomib-related nervous system disorders were observed in 6 patients (5 of grade 1-2 and 1 of grade 3), with no discontinuations.
Mutational data were available for 21 patients: all patients scored MYD88 L265P in BM, 18/19 (95%) in plasma and only 18/21 (86%) in PB, prospectively confirming the risk of false negative results when only PB of rituximab pre-treated patients is analyzed. CXCR4 S338X was detected only in one patient at baseline. MRD negativization rates after treatment differed across investigated tissues: in detail, 5/17 (29%) in BM, 6/14 (43%) in plasma and 12/16 (75%) in PB. Overall, a good concordance was observed between BM and plasma (Cohen's kappa= 0.714), suggesting the possibility of avoiding BM aspiration for mutational screening and MRD analysis.
Conclusion
The final results of FIL BRB phase II trial showed that BRB regimen, used as second-line therapy, is an effective and well-tolerated salvage treatment for RR-WM patients. The deep anti-tumor activity of the novel combination is highlighted by an absolute increase of PFS rate in comparison to historical controls (30-months PFS of 79%), as well as by high rates of clinical response, with an ORR (CR+VGPR+PR) of 82% (95%CI 66-92). Moreover, MRD monitoring showed promising efficacy of BRB regimen in clearing the residual disease.

Disclosures
Benevolo: Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Janssen: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees; Amgen: Speakers Bureau. Ferrero: Morphosys: Research Funding; Servier: Speakers Bureau; EUSA Pharma: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Janssen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Gilead: Research Funding, Speakers Bureau; Incyte: Membership on an entity's Board of Directors or advisory committees; Clinigen: Membership on an entity's Board of Directors or advisory committees. Cavallo: ROCHE: Membership on an entity's Board of Directors or advisory committees; Servier: Speakers Bureau; Gilead: Speakers Bureau. Gaidano: Abbvie: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Astrazeneca: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Beigene: Membership on an entity's Board of Directors or advisory committees; Incyte: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Musuraca: janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; roche: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; incyte: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees. Varettoni: AstraZeneca: Membership on an entity's Board of Directors or advisory committees; beigene: Membership on an entity's Board of Directors or advisory committees; janssen: Membership on an entity's Board of Directors or advisory committees; roche: Membership on an entity's Board of Directors or advisory committees. Vitolo: Gilead: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Novartis: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Kite: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau.