Background
Phase 3, open-label, randomized controlled trial (RESONATE-2; PCYC-1115). N=269 treatment-naive CLL/SLL patients aged ≥65 years. Del(17p) patients were excluded (separate study). Randomized 1:1. Ibrutinib is a first-generation covalent BTK inhibitor; chlorambucil was the standard of care for elderly CLL at the time. Median age 73 years.
Interventions and follow up
Arm A: Ibrutinib 420 mg PO once daily continuously until progression or unacceptable toxicity
Arm B: Chlorambucil 0.5 mg/kg PO days 1 and 15 q28d, escalated to 0.8 mg/kg as tolerated, for up to 12 cycles
Primary endpoint: Progression-free survival (PFS)
Median follow up: 18.4 months (primary); up to 10 years (Burger Blood 2025)
Arm B: Chlorambucil 0.5 mg/kg PO days 1 and 15 q28d, escalated to 0.8 mg/kg as tolerated, for up to 12 cycles
Primary endpoint: Progression-free survival (PFS)
Median follow up: 18.4 months (primary); up to 10 years (Burger Blood 2025)
Results
Median PFS (primary): Not reached vs 18.9 months (ibrutinib vs Clb), HR 0.16 (95% CI 0.09–0.28), P<.001
Median PFS (10-yr update): 8.9 years (ibrutinib arm)
ORR: 86% vs 35%
OS (10-yr): 62% (ibrutinib) vs 57% (Clb); no significant difference
OS (primary): HR 0.16, P=.001 — favored ibrutinib at primary analysis
Median PFS (10-yr update): 8.9 years (ibrutinib arm)
ORR: 86% vs 35%
OS (10-yr): 62% (ibrutinib) vs 57% (Clb); no significant difference
OS (primary): HR 0.16, P=.001 — favored ibrutinib at primary analysis
Adverse events
Cardiovascular: Grade ≥3 atrial fibrillation 12%; hypertension grade ≥3 13%; cumulative AFib, atrial flutter, and hypertension rates increase with prolonged exposure.
Hemorrhage: Major hemorrhage 4%.
Infections: Pneumonia grade ≥3 6%; infections remained a concern throughout follow-up.
Tolerability: Discontinuation due to AEs 41% at 10 years.
Hemorrhage: Major hemorrhage 4%.
Infections: Pneumonia grade ≥3 6%; infections remained a concern throughout follow-up.
Tolerability: Discontinuation due to AEs 41% at 10 years.
Conclusions
Ibrutinib demonstrated dramatically superior PFS over chlorambucil in elderly CLL, with a 10-year median PFS of 8.9 years — the longest reported continuous BTK inhibitor follow-up. This established ibrutinib as the first effective alternative to chemoimmunotherapy in elderly CLL, transforming the treatment paradigm.
Key Limitations
Del(17p) patients were excluded, limiting generalizability. Chlorambucil is a weak comparator by modern standards; contemporary trials comparing ibrutinib to obinutuzumab-based combinations (iLLUMINATE) or fixed-duration venetoclax regimens (GLOW, CLL17) provide more relevant data. Long-term ibrutinib is complicated by cumulative toxicities (AFib, bleeding, hypertension), with 41% discontinuation at 10 years. No OS benefit at 10 years despite the PFS advantage, partly reflecting post-progression salvage. Next-generation BTKi (zanubrutinib) offers improved cardiac safety.
Clinical Context
RESONATE-2 established ibrutinib as the first BTKi to gain FDA approval for treatment-naive CLL (2016). Despite compelling PFS data, ibrutinib has been largely superseded in contemporary practice by acalabrutinib (ELEVATE-TN) and zanubrutinib (ALPINE, SEQUOIA) due to better tolerability, and by fixed-duration venetoclax combinations (CLL14, CLL17) offering treatment cessation. CLL17 subsequently demonstrated fixed-duration regimens non-inferior to continuous ibrutinib. ESMO-MCBS: 4.