Efficacy and Safety of Single-Agent Ibrutinib in Patients with Mantle Cell Lymphoma Who Progressed after Bortezomib Therapy

Introduction: Despite recent advances, mantle cell lymphoma (MCL) remains difficult to treat with frequent chemoresistance in the relapsed or refractory setting. Ibrutinib, a first-in-class, once-daily, oral covalent inhibitor of Bruton’s tyrosine kinase, demonstrated durable single-agent efficacy in a previous phase 2 study of patients with MCL who had received 1 to 5 prior therapies (Wang M, et al. N Engl J Med. 2013;369:507-516). In that study, the investigator-assessed overall response rate was 68% (complete response rate, 21%). The current study reports on the efficacy and safety of single-agent ibrutinib specifically in patients with MCL who had received a rituximab-containing regimen and had progressed after at least 2 cycles of bortezomib therapy.

Methods: In this phase 2, multicenter, single-arm study, patients received 560 mg/day oral ibrutinib continuously until progressive disease or unacceptable toxicity. The primary end point was the overall response rate (ORR) in response evaluable patients, as assessed by an Independent Review Committee (IRC). Secondary end points, also assessed by IRC, included duration of response (DoR), progression-free survival (PFS), overall survival (OS), and safety.

Results: 120 patients in this international multicenter study were enrolled. The median age was 67.5 years, ranging from 35 to 85 years with 62.5% ≥ 65 years. Most patients had stage IV disease at study entry (77.5%), and 9.2% were reported as blastoid variant (per investigator). 76.3% of patients had an intermediate or high risk simplified MIPI score, and 52.5% had bulky disease (longest diameter ≥ 5 cm). Forty two (35.0%), 67 (55.8%) and 11 patients (9.2%) had an ECOG score of 0, 1 and 2, respectively. The median number of prior lines of systemic therapy was 2 (range 1-8 lines) with almost half of the patients (47.5%) receiving 3 or more prior lines of therapy. Overall, 33% of patients had received prior stem cell transplantation. At the time of clinical cut-off for the primary analysis (29 April, 2014), median follow-up was 14.9 months with median treatment duration of 8 months (range: 0.5-20.9 months). The main reasons for treatment discontinuation were disease progression in 53 patients (44.2%) and an adverse event (AE) in 8 patients (6.7%). The ORR for response evaluable patients was 62.7% (95% confidence interval [CI]: 53.7%-71.8%) with a complete response rate of 20.9%. Subgroup analysis suggested that the ORR was independent of age, gender, geographic region, number of prior lines of therapies, baseline extranodal disease, simplified MIPI score, bulky disease, and stage of MCL. Median DoR by IRC was 14.9 months and the median time to first response was 2.1 months, ranging from 1.3 to 6.3 months. Median PFS was 10.5 months and 47% of the patients remained progression-free at 1 year. The OS rate at 18 months was 61%. The most common AEs were fatigue (any grade, 43.3%; grade 3 or 4, 3.3%) and diarrhea (any grade, 42.5%; grade 3 or 4, 2.5%). The most common grade 3 or higher AEs were neutropenia (20.8%), thrombocytopenia (13.3%), and pneumonia (12.5%). Any-grade hemorrhagic events were reported in 45 patients (37.5%), including 3 (2.5%) with major hemorrhagic events. The median time to initial hemorrhagic event was 84 days (range 1-515 days), with a median duration of 22 days (95% CI: 8-31 days). Atrial fibrillation was reported in 13 patients (10.8%), which was grade 3 or 4 in 6 patients (5%). AEs led to dose reductions in 8 patients (6.7%).

Conclusion: Single agent ibrutinib is highly efficacious and well tolerated, with an acceptable toxicity profile in patients with MCL who progressed after rituximab-containing chemotherapy and bortezomib therapy. These results are consistent with previous ibrutinib studies, with no new safety signals.