Three-year follow-up of treatment-naive and previously treated patients with Waldenstrom macroglobulinemia (WM) receiving single-agent zanubrutinib.

Background: Inhibitors of Bruton tyrosine kinase (BTK) have established therapeutic activity in patients with WM. Zanubrutinib, a potent and selective BTK inhibitor was evaluated in a phase 1/2 study in treatment-naive (TN) and relapsed/refractory (R/R) patients with WM. Method(s): Patients had TN or R/R WM and required treatment as per International Workshop on WM (IWWM) criteria. Treatment consisted of oral zanubrutinib at 160 mg twice daily (n = 50) or 320 mg once daily (n = 23) until disease progression or unacceptable toxicity. Efficacy endpoints included the proportion of patients achieving a complete response (CR) or very good partial response (VGPR) in accordance with IWWM-6 criteria. Efficacy analyses were conducted on the 73 patients evaluable (24 TN, 49 R/R). Result(s): Between September 2014 and August 2018, 77 patients with WM (24 TN and 53 R/R) began treatment with zanubrutinib (55% aged > 65 years; 21% aged > 75 years). At a median follow up of 32.7 months, 73% remain on treatment. Reasons for treatment discontinuation included adverse events (AE) in 13% (only one related), disease progression (10.4%), and other (3.9%). Results are presented for TN and R/R combined. The overall response rate was 96% and VGPR/CR rate was 45%. The rates of VGPR/CR increased over time; 22% at 6 mos, 33% at 12 months and 45% at 24 months. Three-year progression-free survival (PFS) was 81%, and overall survival (OS) was 85%. The most commonly reported AEs were upper respiratory tract infection (52%), contusion (33%, all grade 1) and cough (22%). AEs of interest include neutropenia (18.2%), major hemorrhage (4%), atrial fibrillation/flutter (5%), and grade 3 diarrhea (3%). Conclusion(s): Long-term follow up with continued zanubrutinib treatment demonstrated deep and durable responses in the majority of WM patients. The rates of VGPR/CR increased with prolonged therapy. Disease progression was uncommon. The safety profile of long-term zanubrutinib therapy in these patients w