ATLANTA – More than one-fifth of patients with relapsed or refractory mantle cell lymphoma had a complete response to single-agent therapy with experimental ibrutinib, and another two-fifths had a partial response, investigators reported.
"Colleagues, this is the highest response rate ever reported, ever achieved by one single drug in the history of relapsed mantle cell lymphoma," Dr. Michael Wang told attendees at the annual meeting of the American Society of Hematology.
Dr. Wang’s evident excitement about the data came exactly 1 year to the day after he announced preliminary results of the phase II trial at ASH 2011. At that time, the drug was known only as PCI-32765.
Those early data showed that ibrutinib, an oral inhibitor of Bruton’s tyrosine kinase (BTK) expressed in several hematologic malignancies, induced complete response in 16% and partial response in 53% of patients evaluated at that time for a combined overall response rate of 69%.
The drug demonstrated efficacy against bulky disease, and its effects in early studies appeared to be independent of the MCL [Mantle Cell Lymphoma] International Prognostic Index (MIPI) score.
At this year’s meeting, Dr. Wang of the University of Texas M.D. Anderson Cancer Center, Houston, reported that in an efficacy cohort of 110 patients, 22% had a complete response and 46% a partial response. Among patients who had previously been treated with bortezomib (Velcade), 23% had a complete response, and 49% had a partial response. In bortezomib-naive patients the response rates were 21% and 44%, respectively.
At 9.2 months of follow-up (data cutoff Sept. 21, 2012), the median duration of response had not been reached. Median progression-free survival was 13.9 months.
Ibrutinib was well tolerated
BTK, an essential element of the B-cell antigen receptor–signaling pathway, is expressed in several hematologic malignancies, including lymphoma and chronic lymphocytic leukemia, for which positive clinical trial data were also presented at the meeting. Ibrutinib blocks receptor signaling and induces apoptosis, as well as mantle cell migration and adhesion. It has been shown in in vitro studies to block pERK, pJNK, and NF-KappaB pathways in MCL cell lines.
Dr. Wang and colleagues at 18 U.S. and European centers enrolled patients with confirmed overexpression of cyclin D1 or the 11;14 translocation and measurable disease. The patients had not been able to achieve at least a partial response to prior therapy, or had disease progression following their most recent treatment regimen. All had at least one, but not more than five prior lines of therapy, and adequate end-organ function, and Eastern Cooperative Oncology Group performance status of 2 or lower.
The drug was generally well tolerated, he said, with neutropenia, thrombocytopenia, and anemia being the most frequent hematologic toxicities, and diarrhea, fatigue, nausea, and respiratory tract infections being the most common nonhematologic adverse events.
Dr. Wang noted that longer follow-up of data on 51 patients in the cohort that he presented at ASH 2011 show improvement in complete response rates. The initial rates among bortezomib-naive, bortezomib experienced, and all patients were 16%, 15%, and 16%, respectively, at a median of 3.7 months. In the current follow-up of these patients, however (median 14.7 months), the complete response rates had improved to 40%, 38%, and 39%, respectively, with respective overall response rates of 77%, 71%, and 75%. He called the gradual increase in complete response rates the "phenomenon of incremental response."
Phenomenon of incremental response
Dr. Andrew D. Zelenetz of Memorial Sloan-Kettering Cancer Center, New York, said after Dr. Wang’s presentation that the "phenomenon of incremental response" Dr. Wang described is not really a phenomenon at all.
"The reason you’re giving the drug continuously is that you expect to see a better response," he said. "It’s not unique to this drug, but in rituximab it’s seen when you stop the drug, in lenalidomide it’s seen when you stop the drug, and in radioimmunotherapy it’s seen when you stop the drug. So incremental response is a well described phenomenon in lymphomas," Dr. Zelenetz said.
"This has been just an interim analysis our data. We look forward to updating you with the final results of this clinical trial with excitement, caution, and confidence," Dr.Wang said.
The study was supported by Pharmacyclics. Dr. Wang is on the scientific advisory board of the company. Dr. Zelenetz had no relevant disclosures.