SAN DIEGO – A recurrent mutation in BCL2, the therapeutic target of venetoclax (Venclexta), appears to be a major contributor to drug resistance in patients with chronic lymphocytic leukemia (CLL), investigators reported.
The mutation has been detected in some patients with CLL up to 2 years before resistance to venetoclax actually develops, said lead author Piers Blombery, MBBS, from the Peter MacCallum Cancer Center in Melbourne.
“We have identified the first acquired BCL2 mutation developed in patients clinically treated with venetoclax,” he said in a late-breaking oral abstract session at the annual meeting of the American Society of Hematology.
The mutation, which the investigators have labeled BCL2 Gly101Val, “is a recurrent and frequent mediator of resistance and may be detected years before clinical relapse occurs,” he added.
The paper was published online in Cancer Discovery (2018 Dec 4. doi: 10.1158/2159-8290.CD-18-1119) to coincide with the presentation at ASH.
Despite the demonstrated efficacy of venetoclax as continuous therapy in patients with relapsed or refractory CLL, the majority of patients experience disease progression, prompting the investigators to explore molecular mechanisms of secondary resistance.
To do this, they analyzed paired samples from 15 patients with CLL, enrolled in clinical trials of venetoclax, collected both before the start of venetoclax therapy and at the time of disease progression.
In seven of the patients, they identified a novel mutation that showed up at the time of progression, but was absent from the pre-venetoclax samples. The mutation first became detectable from about 19 to 42 months after the start of therapy and preceded clinical progression by as much as 25 months, the investigators found.
They pinned the mutation down to the BH3-binding groove on BCL2, the same molecular site targeted by venetoclax. They found that the mutation was not present in samples from 96 patients with venetoclax-naive CLL nor in any other B-cell malignancies. Searches for references to the mutation in both a cancer database (COSMIC) and a population database (gnomAD) came up empty.
In other experiments, they determined that cell lines overexpressing BCL2 Gly101Val are resistant to venetoclax, and that in the presence of venetoclax in vitro, BCL2 Gly101Val-expressing cells have a growth advantage, compared with wild type cells.
Additionally, they showed that the mutation results in impaired venetoclax binding in vitro.
“BCL2 Gly101Val is observed subclonally, implicating multiple mechanisms of venetoclax resistance in the same patient,” Dr. Blombery said.
In an interview, Dr. Blombery said that the identification of the resistance mutation is a strong rationale for using combination therapy to treat patients with relapsed or refractory CLL to help prevent or attenuate selection pressures that lead to resistance.
The investigators were supported by the Wilson Center for Lymphoma Genomics, Snowdome Foundation, National Health Medical Research Council, Leukemia and Lymphoma Society, Leukemia Foundation, Cancer Council of Victoria, and Australian Cancer Research Foundation. Dr. Blombery reported having no relevant disclosures.
SOURCE: Blombery P et al. ASH 2018, Abstract LBA-7.