Acquired BTKC481S and PLCG2 mutations led to ibrutinib resistance in chronic lymphocytic leukemia (CLL), investigators reported online in the Journal of Clinical Oncology.

These mutations preceded 85% of clinical relapses, appearing a median of 9.3 months beforehand, Jennifer A. Woyach, MD , and her associates from the Ohio State University, Columbus, concluded from a retrospective study of 308 patients. In a separate prospective study of 112 patients, acquired BTKC481S mutation and clonal expansion preceded all eight cases of relapse, they said. “Relapse of CLL after ibrutinib is an issue of increasing clinical significance,” they concluded. “We show that mutations in Bruton tyrosine kinase (BTK) and PLCG2 appear early and have the potential to be used as a biomarker for future relapse, suggesting an opportunity for intervention.”

Ibrutinib has transformed the CLL treatment landscape, but patients face poor outcomes if they relapse with Richter transformation or develop progressive disease. Past work has linked ibrutinib resistance to acquired mutations in BTK at the binding site of ibrutinib and in PLCG2 located just downstream. But the scope of ibrutinib resistance in CLL and key mutational players were unknown (J Clin Oncol. 2017. doi: 10.1200/JCO.2016.70.2282 ).

To fill that gap, the researchers retrospectively analyzed data from four sequential ibrutinib CLL trials at the Ohio State University. The separate prospective analysis involved analyzing the entire BTK and PLCG2 coding regions every 3 months.

In the retrospective study, patients had received a median of 3 and up to 16 prior therapies. Given the median follow-up period of 3.4 years, about 19% of patients experienced clinical relapse within 4 years of starting ibrutinib, the researchers estimated (95% confidence interval, 14%-24%). Deep sequencing by Ion Torrent (Life Technologies) identified mutations in BTKC481S and/or PLCG2, in 40 of 47 (85%) relapses. In 31 cases, BTKC481S was the sole mutation. Mutational burdens varied among patients, but generally correlated with CLL progression in peripheral blood versus primarily nodal relapse.

At baseline, 172 (58%) of retrospective study participants had complex cytogenetics, 52% had del(13q), 40% had del(17p), and 21% had MYC abnormality. Median age was 65 years (range, 26-91 years) and 70% of patients were female. Multivariable analyses linked transformation to complex karyotype (hazard ratio, 5.0; 95% CI, 1.5-16.5) and MYC abnormality (HR, 2.5; 95% CI, 1.0-4.7), and linked progressive CLL to age younger than 65 years, complex karyotype, and del(17)(p13.1).

Richter transformation usually occurred within 2 years of starting ibrutinib and had a cumulative 4-year incidence of 10%, the investigators also reported. Patients survived a median of only 3.9 months after stopping ibrutinib because of transformation. The cumulative rate of progressive CLL was higher (19.1%), but early progression was rare, and patients who stopped ibrutinib because of progression survived longer (median, 22.7 months).

In the prospective study, all eight patients with BTKC481S who had not yet clinically relapsed nonetheless had increasing frequency of this mutation over time, the investigators reported. Together, the findings confirm BTK and PLCG2 mutations as the key players in CLL resistance to ibrutinib, they stated. Perhaps most importantly, they reveal “a prolonged period of asymptomatic clonal expression” in CLL that precedes clinical relapse and provides a window of opportunity to target these cells with novel therapies in clinical trials, they wrote.

Given that ibrutinib was approved for use in relapsed CLL only 2 years ago, “We are likely just starting to see the first emergence of relapse in the community setting,” the researchers concluded. “Enhanced knowledge of both the molecular and clinical mechanisms of relapse may allow for strategic alterations in monitoring and management that could change the natural history of ibrutinib resistance.”

Funding sources included the D. Warren Brown Foundation, Mr. and Mrs. Michael Thomas, the Four Winds Foundation, the Leukemia and Lymphoma Society, Pelotonia, and the National Cancer Institute. Pharmacyclics also provided partial support. Dr. Woyach disclosed ties to Janssen, Acerta Pharma, Karyopharm Therapeutics, and MorphoSys, and a provisional patent related to C481S detection.


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