Key Points

  • Ibrutinib can result in durable disease control in hairy cell leukemia patients who are not expected to benefit from purine analogues.

  • The safety profile of ibrutinib in hairy cell leukemia is similar to other diseases with cytopenias and risk of frequent infections.

Hairy cell leukemia is a rare B-cell malignancy where there is a need for novel treatments for patients who do not benefit from purine analogues. Ibrutinib, an oral agent targeting Bruton tyrosine kinase in the B-cell receptor signaling pathway, is highly effective in several malignancies. Its activity in HCL was unknown. Therefore, we conducted a multisite phase 2 study (NCT01841723) of oral ibrutinib in patients with either relapsed classic or variant hairy cell leukemia. The primary outcome measure was the overall response rate at 32 weeks with response at 48 weeks and best response during treatment also assessed. Key secondary objectives were characterization of toxicity and determination of progression-free and overall survival. Thirty-seven patients were enrolled (24 at 420mg, 13 at 840mg). The median duration of follow-up was 3.5 years (range 0-5.9). The overall response rate at 32 weeks was 24% which increased to 36% at 48 weeks. The best overall response rate was 54%. The estimated 36-month progression-free and overall survivals were 73% and 85%, respectively. The most frequent adverse events were diarrhea (59%), fatigue (54%), myalgia (54%), and nausea (51%). Hematologic adverse events were common with anemia (43%), thrombocytopenia (41%), and neutropenia (35%). Ibrutinib can be safely administered to hairy cell leukemia patients with objective responses and results in prolonged disease control. While the initial primary outcome objective of the study was not met, the observation of objective responses in heavily pretreated patients coupled with a favorable progression-free survival suggest that ibrutinib may be beneficial in these patients.

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