“We haven’t had a drug that can tell the difference between a pathway signaling normally and one that is abnormally active,” Lin said. “We knew we needed a better strategy, a more rational way of treating cancer. But we’ve not had a way to do it until recently.”

Chung and her colleagues designed a synthetic protein consisting of two natural proteins fused together — one that binds to active ErbB receptors and another that cleaves a specific amino acid sequence. They then engineered a second protein that binds to the inner surface of the cell membrane and contains a customizable “cargo” sequence that can carry out specific actions in the cell. When the first protein binds to an active ErbB receptor, it cuts the second protein and releases the cargo into the interior of the cell.

“When the receptor protein is always on, as it is in cancer cells, the released cargo protein accumulates over time,” Chung said. “Eventually enough accumulates to have an effect on the cell. In this way, the system produces an effect only in cancer cells, and we can convert the always-on state of the receptor into different outcomes through the choice of cargo protein.”

After several rounds of tinkering, the team saw that their RASER system, which stands for “rewiring of aberrant signaling to effector release,” was highly specific for cancer cells dependent on ErbB receptor activity. For their first test they chose to use a protein involved in triggering cell death as the RASER cargo.

Killing only overactive cells

The team compared the RASER system to two therapies currently used for metastatic breast cancer — a chemotherapy regimen and a drug that blocks ErbB activity — on several types of cultured cells: breast and lung cancer cells in which the ErbB pathway was overly active; breast cancer cells in which ErbB activity was normal; and noncancerous breast and lung cell lines.

The researchers found that the traditional chemotherapy regimen of carboplatin and paclitaxel killed all the cells indiscriminately. The effect of the ErbB pathway inhibitor on the viability of the cells varied and did not reliably correlate with ErbB pathway activity levels. Only RASER specifically killed those cells in which the ErbB pathway was overly active while sparing those in which ErbB activity was normal.

We knew we needed a better strategy, a more rational way of treating cancer. But we’ve not had a way to do it until recently.

While much work remains to be done to learn whether RASER is effective in human tumors, the researchers are excited about the possibilities of re-engineering the system to recognize other receptors mutated in cancers and swapping the cargos to achieve different outcomes. Challenges include learning how best to deliver synthetic proteins into tumors and understanding how the immune system might react to RASER. But Lin is optimistic.

“We have so much more information now about cancer genomics, signaling and how cancer cells interact with the immune system,” Lin said. “It’s finally becoming practical to combine this knowledge with synthetic biology approaches to tackle some of these pressing human health problems. RASER is both customizable and generalizable, and it allows us for the first time to selectively target cancer cells while sparing normal signaling pathways.”

Other Stanford authors of the study are graduate student Xinzhi Zou; former undergraduate student Bryce Bajar; postdoctoral scholar Veronica Brand, PhD; research scientist Yunwen Huo, PhD; Javier Alcudia, PhD, director of Stanford’s Neuroscience Gene Vector and Virus Core; and James Ferrell, MD, PhD, professor of chemical and systems biology and of biochemistry.

Lin is a member of Stanford Bio-X and Stanford ChEM-H, as well as of the Stanford Maternal & Child Health Research Institute and the Wu Tsai Neurosciences Institute at Stanford.

The study was supported by the National Institutes of Health (grants P50GM107615 and 5R01GM098734), a Stanford graduate fellowship, the Ilju Foundation, the Burroughs Wellcome Foundation, a Damon Runyon-Rachleff Innovation award and an Alliance for Cancer Gene Therapy Young Investigator Award.

Stanford’s departments of Bioengineering and of Neurobiology also supported the work. The Department of Bioengineering is jointly managed by the School of Medicine and School of Engineering.