Alternative Mechanism of Action Suggested for Vemurafenib in Hairy Cell Leukemia With BRAF V600E Mutation


Key Points

  • BRAF inhibition and cell death were observed in the absence of change in ERK or MEK phosphorylation.
  • The findings suggest an alternative mechanism of action for vemurafenib in this setting.

The finding that the BRAF V600E mutation is present in nearly all cases of hairy cell leukemia has resulted in the use of BRAF inhibitors to treat chemotherapy-resistant disease, with good responses to vemurafenib (Zelboraf) being observed. BRAF inhibition has been thought to result in leukemic cell death by inhibiting the phosphorylation of ERK and MEK resulting from BRAF activation. In a letter to the The New England Journal of Medicine, Samuel et al describe studies in a patient with hairy cell leukemia that suggest an alternative mechanism of vemurafenib activity.

The investigators evaluated a 72-year-old male patient with a 22-year history of hairy cell leukemia who had been free of disease manifestations for 14 years after initial splenectomy. Treatment with rituximab (Rituxan) and 2-chlorodeoxyadenosine for a third relapse a year before the report resulted in complete response, but relapse occurred within a year. DNA sequencing showed a biallelic BRAF V600E mutation, and treatment with vemurafenib at 240 mg twice daily was started.

Response Not Accompanied by Decreased ERK-MEK Phosphorylation

Since the patient had high levels of circulating hairy leukemia cells, the effect of vemurafenib on cell populations could be investigated directly. Assessment showed an initial increase followed by a rapid decrease in peripheral white blood cell counts and viability of CD103-positive cells after starting vemurafenib, with hematologic recovery beginning at 2 months into therapy.

Pull-down and kinase assays showed that BRAF activity was reduced by 75% at 1 day after and by 90% at 8 days after starting vemurafenib. However, Western Blot analysis showed no changes in levels of phosphorylated ERK or MEK and flow cytometry showed no reduction in ERK1 or ERK2 phosphorylation in CD103-positive cells in the setting of prolonged vemurafenib exposure.

Effect on Alternative Signaling Pathway?

The studies thus show an “uncoupling” between reduction in BRAF activity accompanied by increased cell death (shown by increased levels of propidium iodide staining) and MEK-ERK inhibition. The investigators noted that they could not rule out the possibility that BRAF inhibition resulted in inhibition of ERK activation in an anatomic compartment other than the blood before leukemic cell death. However, this scenario was considered unlikely since the in vivo data show BRAF inhibition without any change in ERK phosphorylation in leukemic cells while cells were dying. Further, the absence of effect of BRAF inhibitors on MEK and ERK phosphorylation had been previously shown during prolonged in vitro incubation of leukemic cells from the same patient, whereas the ERK phosphorylation was inhibited and cell death induced with use of the MEK1/2 inhibitor PD325901.

The investigators concluded, “An alternative signaling pathway, as yet uncharacterized, may therefore be affected by vemurafenib, either directly or through BRAF inhibition, and it may have a strong impact in hairy-cell leukemia cell survival in vivo. These data have implications for the design of possible combination therapies.”

Martin J.S. Dyer, DPhil, of University of Leicester, is the corresponding author for The New England Journal of Medicine letter.

Dr. Dyer has received grants and financial support from Roche Pharmaceuticals. Salvador Macip, MD, PhD, received grants from the Medical Research Council.

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