The 30-month OS was 94% with ibrutinib and 92% with placebo. There were 30 patients in the placebo arm that crossed over to receive ibrutinib. As assessed by the independent review committee, response rates were significantly higher with ibrutinib-rituximab (overall response rate, 92% vs. 47%). The major response rate (complete response, very good partial response, or partial response) was higher in the ibrutinib arm (72% vs. 32%). Mutation status did not affect the response rate or quality of response. Among those with at least a partial response, the median duration of response was not reached in the ibrutinib group, as compared with a median duration of response of 21.2 months in the placebo group. Serum IgM response was greater and more rapid with ibrutinib compared to placebo. Furthermore, transient increases in serum IgM levels, or “IgM flare,” was seen less frequently with the addition of ibrutinib (8% vs. 47%). No patient receiving ibrutinib required plasmapheresis. Hemoglobin response was seen more frequently with ibrutinib (73% vs. 41%).
Grade 3 or higher adverse events (AE) were seen in 60% of patients in each group. Hypertension (13% vs. 4%) and atrial fibrillation (12% vs. 1%) occurred more commonly in the ibrutinib group compared with placebo. Serious AEs were seen more frequently with ibrutinib compared to placebo (43% vs. 33%). Atrial fibrillation of any grade occurred in 15% of patients receiving ibrutinib; however, 27% of these patients had a history of atrial fibrillation prior to enrollment. Bleeding occurred more frequently with ibrutinib; however, the vast majority of these were grade 1 or grade 2. Major bleeding occurred in 3 patients in each arm. No fatal adverse events were noted in the ibrutinib group, while 3 patients in the placebo group experienced a fatal event. Discontinuation rates were similar in both arms (5% vs. 4%). Dose reduction of ibrutinib occurred in 13 patients.
Conclusion. The combination of ibrutinib and rituximab reduced the risk of disease progression by 80% compared with rituximab alone. This combination should be considered as a standard treatment option for patients with symptomatic Waldenström macroglobulinemia.
Commentary
Waldenström macroglobulinemia is a B-cell lymphoma characterized by infiltrating IgM producing clonal lymphoplasmacytic cells. Observation remains the preferred approach to asymptomatic patients; however, the presence of clinical symptoms including anemia, hyperviscosity, fatigue, or other constitutional symptoms should prompt initiation of therapy. Given the relative lack of large studies to define standard treatment strategies, rituximab monotherapy has frequently been used, with response rates of approximately 40% to 50%.2,3 Complete responses to single-agent rituximab have not been reported. Ibrutinib is an oral Bruton tyrosine kinase (BTK) inhibitor that has shown high response rates in the relapsed setting in previous studies. A study of single-agent ibrutinib in patients with relapsed disease showed overall and major response rates of 90% and 73%, respectively.4 The 2-year PFS was 69%. Additionally, such studies have suggested higher response rates in patients with mutated MYD88 genotype. This data led to the approval of ibrutinib for rituximab-refractory disease. In the treatment-naive setting, at least a minor response was seen in all patients (n = 30) in a small cohort treated with ibrutinib.5
In the reported trial, the combination of ibrutinib plus rituximab resulted in a more robust and durable response than single-agent rituximab, with significantly prolonged PFS. Of note, the response was similar for both treatment-naive and relapsed, rituximab-sensitive patients. Interestingly, a transient increase in serum IgM level was not seen in those treated with combination ibrutinib-rituximab. Improvements in PFS and response rates were independent of IPSS score. Previous studies have suggested that response to ibrutinib is related to MYD88 and CXCR4 mutational status. For example, in a phase 2 trial of ibrutinib in previously treated patients with symptomatic disease, major response rates for MYD88 L265P/CXCR WT, MYD88 L265P/CXCR4 WHIM, and MYD88 WT/CXCR4 WT groups were 91%, 62%, and 29%, respectively.4 In the current study, however, responses with ibrutinib-rituximab were seen across all genotypes at similar rates. Furthermore, PFS did not differ based on mutational status.