Background
Phase 3, open-label, randomized controlled trial (ASPEN). N=229 patients with WM: Cohort 1 = 201 patients with confirmed MYD88 L265P mutation; Cohort 2 = 28 patients with MYD88 wild-type or indeterminate. Patients were treatment-naive or had received ≥1 prior line. Zanubrutinib is a next-generation, highly selective BTK inhibitor with full-cycle BTK occupancy and minimal off-target activity.
Interventions and follow up
Arm A: Zanubrutinib 160 mg PO twice daily until progression or unacceptable toxicity
Arm B: Ibrutinib 420 mg PO once daily until progression or unacceptable toxicity
Primary endpoint: ≥VGPR rate (Cohort 1, MYD88 L265P)
Median follow-up: 19.4 months
Arm B: Ibrutinib 420 mg PO once daily until progression or unacceptable toxicity
Primary endpoint: ≥VGPR rate (Cohort 1, MYD88 L265P)
Median follow-up: 19.4 months
Results
≥VGPR (Cohort 1): 28.4% vs 19.2% (zanubrutinib vs ibrutinib), P=.09 — not significant
ORR (Cohort 1): 94.1% vs 92.7%
MR or better (Cohort 1): 76.5% vs 71.3%
Atrial fibrillation/flutter: 2.5% (zanubrutinib) vs 15.3% (ibrutinib), P=.002
ORR (Cohort 1): 94.1% vs 92.7%
MR or better (Cohort 1): 76.5% vs 71.3%
Atrial fibrillation/flutter: 2.5% (zanubrutinib) vs 15.3% (ibrutinib), P=.002
Adverse events
Overall: Grade ≥3 AEs 59.8% (zanubrutinib) vs 61.7% (ibrutinib); discontinuation due to AEs 17.6% vs 21.1%
Cardiovascular: Atrial fibrillation any grade 2.5% vs 15.3% (markedly lower with zanubrutinib); hypertension grade ≥3 3.9% vs 5.6%
Hematologic/bleeding: Neutropenia grade ≥3 15.7% (zanubrutinib) vs 6.7% (ibrutinib); major hemorrhage 5.9% vs 5.6%
Cardiovascular: Atrial fibrillation any grade 2.5% vs 15.3% (markedly lower with zanubrutinib); hypertension grade ≥3 3.9% vs 5.6%
Hematologic/bleeding: Neutropenia grade ≥3 15.7% (zanubrutinib) vs 6.7% (ibrutinib); major hemorrhage 5.9% vs 5.6%
Conclusions
Zanubrutinib did not meet the primary endpoint of superior ≥VGPR rate in MYD88 L265P WM (P=.09), but demonstrated a markedly lower rate of atrial fibrillation (2.5% vs 15.3%), establishing zanubrutinib as the preferred BTK inhibitor in WM on the basis of its superior cardiac safety profile. Response depth favored zanubrutinib numerically, with deepening responses on extended follow-up.
Key Limitations
The primary endpoint (≥VGPR rate) was not met — the regulatory approval and clinical preference for zanubrutinib rests largely on the safety advantage rather than proven efficacy superiority. Open-label design introduces response assessment bias. Median follow-up of 19.4 months is short for a chronic disease with prolonged responses. The MYD88 wild-type cohort (N=28) is underpowered. Head-to-head PFS comparison with longer follow-up would strengthen the evidence base.
Clinical Context
FDA approved zanubrutinib for WM in 2021; the ASPEN head-to-head data confirmed its safety advantage over ibrutinib. Despite not meeting its primary endpoint, zanubrutinib has become the preferred BTK inhibitor for WM at most centers given its cardiac safety profile. Ibrutinib + rituximab (iNNOVATE) and zanubrutinib monotherapy are both approved; combination data for zanubrutinib + rituximab are emerging. ESMO-MCBS: not assigned.
References
References: Tam CS et al, Blood 2020 (primary analysis)