SAN DIEGO-Identification of a genetic mutation that causes resistance to the targeted drug venetoclax in chronic lymphocytic leukemia (CLL) patients provides a potential biomarker of impending clinical relapse, according to a new study.
Venetoclax is a potent and highly selective B-cell leukemia/lymphoma 2 protein (BCL2) inhibitor approved for treatment of patients with previously treated CLL as monotherapy or in combination with rituximab. Studies show a complete remission rate of 20 percent with venetoclax monotherapy and from 20 to 50 percent with venetoclax in combination with rituximab. More than one-quarter (27.3%) of patients achieve minimal residual disease-negativity in the bone marrow, and the median duration of response is 33 months.
Initially, venetoclax was approved only for the treatment of patients with CLL whose tumors had a genetic alteration in which a piece of one chromosome was missing. In June 2018, the FDA expanded the drug's approval to include all patients with CLL whose disease progressed after at least one previous treatment, regardless of whether their cancer cells have this genetic alteration.
The newly discovered mutation in BCL2 significantly reduces the ability of venetoclax to latch on to and block the protein, said lead author Piers Blombery, MD, of the Peter MacCallum Cancer Centre and Royal Melbourne Hospital in Melbourne, Australia. "Although venetoclax is a highly effective drug for CLL, most patients eventually relapse," said Blombery. "The mutation we have found helps to explain why venetoclax stops working in some patients. Furthermore, we have shown that in some cases the mutation can be detected in patients' bone marrow years before clinical signs of relapse appear."
Blombery presented the study's results at a press conference at the 2018 ASH Annual Meeting (Abstract LBA-7).
Methodology, Results
CLL disease progression includes Richter's transformation and CLL-type progression. "The molecular basis of venetoclax resistance is largely unknown. Abnormalities are acquired in BTG1, TP53, CDKN2A/B, SF3B1, and BRAF1 genes. Acquired mutations in BCL2 induced in a murine cell line include BCL2 Phe104Cys and Phe104Leu2," said Blombery.
The researchers performed genomic sequencing on tumor samples from 67 patients with relapsed CLL who were treated with venetoclax on three early phase clinical trials. They performed focused genomic evaluation in those with CLL-type progressions as opposed to large cell Richter's transformation.
Next-generation sequencing of a panel of 33 genes recurrently mutated in lymphoid malignancy was performed where suitable pre- and post-progression samples were available.
Of 21 patients with CLL-type progressions, 15 patients had suitable samples for genomic analysis both before and after venetoclax treatment. A single heterozygous nucleotide variant was detected in BCL2 in progression samples in seven of the 15 patients.
"We also tested almost 400 patients with CLL and other blood cancers who had never been treated with venetoclax, and we did not find this mutation in any of those patients," said Blombery. "This is evidence that the mutation only develops during venetoclax treatment, so it is something that patients can be screened for. In patients who develop this mutation at a low level, it would be prudent for the clinician to begin to look for other therapies to use instead."
One recurrent mutation, BCL2 Gly101Val, was detected in four patients with CLL-type progression on venetoclax, but was absent in pre-venetoclax samples in all four patients. This mutation is detectable in patients before overt clinical relapse. "Cell lines overexpressing BCL2 Gly101Val are resistant to venetoclax, and BCL2 Gly101Val cells display a growth advantage over wild-type cells in the presence of venetoclax, which results in impaired venetoclax binding," said Blombery.
Identification of the BCL2 mutation helps to explain why CLL recurs in some patients treated with venetoclax, however, much still remains to be understood about the factors underlying CLL recurrence after venetoclax treatment. "We don't know what caused the cancer to relapse in the eight patients in whom we did not find the BCL2 mutation," said Blombery. "Moreover, in the seven patients who had the mutation, we found it in only some of their cancer cells-and yet the cells that did not carry the mutation were still contributing to the cancer's growth. So, clearly, even among patients who have the mutation, other factors are at play in causing resistance to venetoclax therapy."
In one patient, the investigators found a second genetic abnormality generating resistance to venetoclax treatment in cells that did not carry the BCL2 mutation, suggesting that alternative resistance mechanisms can co-exist with BCL2 Gly101Val. "This demonstrates the importance of considering multiple angles of attack with combination therapies-for example, venetoclax and rituximab-rather than relying on a single targeted agent in a disease that has multiple genomic tricks up its sleeve," said Blombery.
In summary, Blombery said: "BCL2 Gly101Val is the first acquired BCL2 mutation described in CLL patients on venetoclax. It is associated with a marked reduction in binding affinity of venetoclax to BCL2 and has been observed subclonally, implicating multiple mechanisms of venetoclax resistance in the same patient."
The researchers are planning to conduct a more extensive genomic analysis of the patients who did not develop the BCL2 mutation to identify other mechanisms of resistance to venetoclax, and also plan to gather more data on the time course of acquisition of the BCL2 mutation.
Mark L. Fuerst is a contributing writer.