A phase 3 clinical trial in patients with chronic lymphocytic leukemia (CLL) or small lymphocytic leukemia (SLL) has been stopped early after significant benefit was seen with ibrutinib (Imbruvica, Pharmacyclics/Janssen).
The novel drug, a first-in-the class inhibitor of Bruton's tyrosine kinase, is currently awaiting approval for use in CLL (with a decision expected from the US Food and Drug Administration before the end of February); it was approved in November 2013 for use in mantle cell lymphoma.
Ibrutinib has stirred up considerable excitement in hematology circles, with experts describing it as a "turning point" in the treatment of CLL and "a step change" in the treatment of mantle cell lymphoma.
The trial that has just been stopped because of benefit, known as RESONATE, was a phase 3 study conducted at more than 70 clinical sites across 10 countries. It involved 391 patients with relapsed or refractory CLL or SLL who had received at least 1 previous therapy. A head-to-head comparison trial, it pitched the oral drug ibrutinib against the intravenous drug ofatumumab (Arzerra, GlaxoSmithKline), which was approved for CLL in 2009.
According to a press release from Pharmacyclics, an interim analysis of this trial showed that patients on ibrutinib had a statistically significant improvement in progression-free survival (the primary end point of the study), as well as in overall survival (a secondary end point), when compared with ofatumumab.
No further details were given, and the results are due to be presented at an upcoming meeting.
As a result of this finding, the Independent Data Monitoring Committee recommended that the trial be stopped and that any patients on ofatumumab be offered treatment with ibrutinib.
Earlier Trial Now Published
An earlier trial with ibrutinib showing "encouraging" efficacy in elderly patients with previously untreated CLL has just been published in the January issue of the Lancet Oncology.
The results come from a phase 1b/2 study, conducted in the United States, of 29 patients with CLL and 2 patients with SLL who had previously not been treated. (This was a part of a larger study — the greater part of this study was conducted in patients who had been previously treated, and these results were reported last year.)
All patients were at least 65 years of age, and most patients (74%) were at least 70 years old. They all received oral ibrutinib once daily at a dose of 420 mg (some initially received the higher dose of 840 mg, but this was discontinued after comparable activity of the 2 doses was shown).
After a median follow-up of 22.1 months, a complete response was reported in 4 patients (13%), a partial response in 17 (55%), and a nodular partial response in 1 (3%). The overall objective response rate was 71% (22 of 31 patients).
The authors, led by Susan O'Brien, MD, from the University of Texas M.D. Anderson Cancer Center in Houston, say the trial shows that ibrutinib is well tolerated and effective in older patients with CLL, and support the continued assessment of this drug in this population of patients.
Notably Good Toxicity, But Questions Remain
In an accompanying comment, Jennifer Brown, MD, from the Dana-Farber Cancer Institute in Boston, comments that the toxicity profile of single-agent ibrutinib was "notably good, with low myelosuppression and few infections," and this makes it "potentially very appealing as a therapeutic option, especially for elderly patients."
An interesting feature of ibrutinib activity is that most patients have prolonged stable remissions (i.e., persistent disease, rather than complete remissions), Dr. Brown comments. However, this persistence of disease does raise the concern that resistant clones can emerge over time, she notes, adding that this has been reported in patients who have relapsed on ibrutinib (J Clin Oncol.2013;31[15 suppl]:abstr 7014).
An additional concern is that this may result in Richter's transformation, in which CLL is transformed into an aggressive lymphoma, which can be fatal, she adds. This has also been reported in patients who have relapsed on ibrutinib (in 1 patient in this current study, and in 7 of 11 patients in the other part of this trial).
These concerns emphasize some of the remaining questions on how to best use ibrutinib, and in particular if it may be best to use the drug in combination with an agent that produces deeper remissions, Dr. Brown adds.
The only agent so far to have shown an early definitive overall survival benefit in CLL is rituximab (Rituxan, Genentech/Roche), and hence a combination of ibrutinib with rituximab is very appealing — and trials of this combination are in progress, she notes.
Despite the unanswered questions, there is hope that ibrutinib, which is the first of several extremely active novel agents coming out of development, will lead to "striking benefit for patients in the coming years," Dr. Brown comments.
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