New hope in the fight against melanoma

WASHINGTON - Scientists have offered new hope in the fight against melanoma, most aggressive form of skin cancer.

This past summer saw a revolution in melanoma therapy. Patients whose melanoma lesions contain a mutation in the BRAF gene were successfully treated with a BRAF-specific inhibitor, PLX4032. Reports of the drug trial described shrinking tumors and improved health. Yet seven months after therapy began the tumors returned and resumed growing.

Now, scientists at The Wistar Institute explain why: the tumor learns to signal around the blocked gene by adjusting its molecular wiring. They also show how to overcome resistance by simultaneously targeting multiple signaling pathways.

The researchers see this as further evidence that some cancers must be treated with multiple targeted drugs at the outset of treatment.

“The evidence suggests that targeting mutant BRAF can kill cancer cells, but it is not enough by itself to finish off melanoma,” said Meenhard Herlyn, director of The Wistar Institute Melanoma Research Center.

“The good news is that drugs are being developed to work in combination with BRAF inhibitors, which our data clearly shows is our best option if we intend to beat advanced melanoma,” Herlyn said.

Melanoma is the deadliest, most aggressive form of skin cancer. While surgical treatment of early melanoma leads to 90 percent cure rates, advanced melanoma is notoriously resistant to chemotherapy and has a tendency to metastasize, or spread, throughout the body.

To study how melanoma responds to BRAF inhibitors, the Herlyn lab took melanoma cells with the BRAF mutation and tested them against a variety of anti-mutant BRAF drugs.

When exposed to the drugs, the cells died off dramatically only to grow back again. In fact, cells that became resistant to one type of BRAF drug became resistant to all of them, which suggests that the cells were biochemically “rewired” in such a way that they no longer needed BRAF to form tumors.

First, they found that resistant cells used a protein similar to BRAF to carry the signal down the chain. Second, they found these cells received an additional boost from the IGF-1 receptor, a protein that sits on the surface of cells and sends signals that prevent cells from being killed.

The resistant cells re-route the signal around BRAF by switching to an alternate protein (CRAF or ARAF), which promotes tumor cell growth, while IGF-1R signaling promotes survival of the resistant cells.

Fortunately, there are a number compounds in clinical development that could block signals along both these pathways. So-called MEK inhibitors target a protein along the same pathway as BRAF, and IGF-1 receptor inhibitors (and inhibitors of P13K, a protein that can be activated by the IGF-1 receptor pathway) block the cancer-enabling survival signal.

As predicted, a combination of these two inhibitors killed BRAF-resistant melanoma cells in the Wistar 3-D model.

Their findings are published in the December 14 issue of the journal Cancer Cell. (ANI)