Aspen: long-term follow-up results of a phase 3 randomized trial of zanubrutinib (zanu) vs ibrutinib (ibr) in patients (pts) with waldenstrom macroglobulinemia (wm).

Background: ZANU is a potent and selective next-generation Bruton tyrosine kinase inhibitor (BTKi) designed to have greater affinity to BTK while minimizing off-target inhibition of TEC-and EGFR-family kinases. ASPEN (NCT03053440) is a randomized, open-label, phase 3 study comparing ZANU with the first-generation BTKi IBR in pts with WM. We present data with a median follow-up of 43 months. Aim(s): To compare the efficacy and safety of ZANU vs IBR in pts with MYD88 mutant (MYD88mut) WM and ZANU in pts with wild-type MYD88 (MYD88wt) WM. Method(s): Pts with MYD88mut WM were assigned to cohort 1 and randomized 1:1 to receive ZANU 160 mg twice daily or IBR 420 mg once daily. Pts with MYD88wt were assigned to cohort 2 and received ZANU 160 mg twice daily until disease progression. Randomization was stratified by CXCR4 mutational status by Sanger sequencing and lines of prior therapy (0, 1-3, or >3). All pts gave informed consent. The primary endpoint was proportion of pts achieving very good partial response or better (VGPR + complete response [CR]). Primary analysis occurred at 19 months median follow-up, and final analysis is planned to occur ~4 years after the first pt enrolled. Result(s): A total of 201 pts (102 ZANU; 99 IBR) were enrolled in cohort 1 and 28 pts in cohort 2. Baseline characteristics in cohort 1 differed between pts treated with ZANU vs IBR in CXCR4 mutations by next-generation sequencing (32% vs 20%, or 33 of 98 vs 20 of 92 available samples, respectively) and pts aged >75 years (33% vs 22%, respectively). Median duration of treatment was 42 months (ZANU) and 41 months (IBR), with 67% and 58% remaining on treatment, respectively. The VGPR+CR rate by investigator was 36% with ZANU vs 22% with IBR (descriptive p = 0.02) in cohort 1, and 31% in cohort 2. One pt in cohort 2 obtained a CR. In pts with wild-type (65 ZANU; 72 IBR) or mutant CXCR4 (33 ZANU; 20 IBR) from cohort 1, VGPR+CR rates with ZANU vs IBR were 45% vs 28% (p = 0.04) and 21% vs 5% (p = 0.