By Rabiya S. Tuma, PHD
SAN FRANCISCO -- The vast majority of melanoma patients with BRAF-mutant tumors respond to BRAF inhibitors. The same is not true for colon cancer patients, though. One study reported that just 5% of colon cancer patients with BRAF-mutant tumors responded to the BRAF inhibitor vemurafenib. Now, data from a functional genomics screen may explain that paradox, researchers reported here at the AACR-NCI-EORTC Molecular Targets and Cancer Therapeutics Conference.
Remarkably, the data also suggest that combining an EGFR inhibitor with a BRAF inhibitor may dramatically improve responses in patients with BRAF-mutant colon cancer.
“Cells that don’t have EGFR expression don’t suffer from this feedback loop, and that is probably why BRAF inhibition has been so successful in melanoma,” said
René Bernards, PhD, Head of the Molecular Carcinogenesis Group at the Netherlands Cancer Institute in Amsterdam, who led the current study.
All of the drugs tested in the preclinical work are already approved, which could speed the translation of these findings into clinical care. “I think René’s findings should stimulate a clinical trial,” said William C. Hahn, MD, PhD, Associate Professor of Medicine at Harvard Medical School and a medical oncologist at Dana-Farber Cancer Institute, who was not involved in the study.
EGFR Protects Cells
Dr. Bernards said he was immediately struck by the incongruous response rates in melanoma and colorectal cancer. Because trial enrollment had been limited to patients with BRAF-mutant tumors in both diseases, the likely cause of the disparity, he reasoned, was a difference in the background genome or gene activity.
To find out what might be preventing responses in colon cancer patients, Dr. Bernards and colleagues performed a RNA interference screen in which they shut off each kinase gene in the human genome and examined the effect on cell growth in the presence and absence of vemurafenib. They found that knocking down expression of EGFR made BRAF-mutant colon cancer cells exquisitely sensitive to vemurafenib -- in other words, the BRAF-mutant cells could only grow in the presence of the drug if they had EGFR protein available.
“That to me was very surprising because we have learned that if you have a BRAF or K-Ras mutation in colorectal cancer, you will not respond to EGFR inhibition by cetuximab,” Dr. Bernards said during his presentation.
“Those patients are actually selected for no cetuximab treatment. But this experiment suggested that if you give these patients the BRAF inhibitor and co-inhibit EGFR, you would suddenly see cell death. And that indeed appears to be the case.”
When the team treated BRAF mutant colon cancer cells with vemurafenib and either cetuximab or gefitinib, the cells stopped growing and turned on the apoptosis pathway. As predicted, none of the agents worked as monotherapy, further supporting the notion that the simultaneous blockade of the two pathways is required.
The team found similar results in BRAF-mutant colon cancer xenograft models. Treating animals with a combination of vemurafenib and gefitinib or cetuximab induced apoptosis and tumor shrinkage, while single agents had little or no effect on tumor growth.
Dissecting a Feedback Loop
Dr. Bernards’ team performed a variety of cell-based experiments to characterize the crosstalk between the BRAF and EGFR signaling pathways in colon cancer cells. They found that vemurafenib triggered rapid phosphorylation of EGFR and its downstream signaling partner AKT in the colon cancer cells but not in melanoma cells.
Further experiments demonstrated that blocking BRAF signaling in BRAF-mutant colon cancer cells inhibited downstream signaling in the pathway, as expected, including shutting off MEK and ERK. The unexpected hitch in the process, though, is the finding that BRAF signaling to ERK is required to turn on the phosphorylase CDC25c that normally keeps EGFR switched off.
Without BRAF signaling, CDC25c is not activated and thus EGFR is allowed to run rampant in the cell.
Therefore, when BRAF-mutant colon cancer cells are exposed to vemurafenib, the EGFR pathway takes over as the driver of tumor growth.
So why would this pathway affect BRAF-mutant colorectal (and thyroid) cancer, but not melanoma? “The vast majority of colon cancer cells express very high levels of EGFR, and so do thyroid cancer cells,” Dr. Bernards said. “But melanomas, as they are derived from the neural crest lineage, tend to be have low levels of EGFR, if any. That might explain why this feedback loop is not operational in melanomas.”
Dr. Bernards’ presentation sparked an animated discussion, with numerous audience members asking for more details regarding the feedback loop, including the role of ligands and other proteins in the pathways. While Dr. Bernards seemed able to address all of the questions (and some audience members said they now needed to go back and reevaluate old experimental results), some people did not seem to be entirely convinced that Dr. Bernards’ team has nailed down the details of the pathway.
Many of those in the audience, however, did seem to share Dr. Hahn’s reaction that the results are strong enough to warrant a trial in these hard-to-treat colon cancer patients.
Dr. Bernards himself declined to answer specific questions in an email interview, because the work is unpublished. The group, however, does appear to be interested in moving forward with clinical trials if they can interest industry partners.