Cancer chemotherapy can be good, bad, or ugly. The good is that the drugs often effectively kill cancer cells. The bad is that the drugs can also damage other quickly dividing cells, causing side effects.
The ugly occurs when the drug dose needed to kill a tumor is more than what a person’s body can handle.
This might happen if the tumor doesn’t have much of a blood supply and very little of the drug can get in. A dose high enough to infiltrate the tumor could be deadly to other cells in the body.
Some recently approved therapies, called Antibody–drug conjugates (ADC), a type of monoclonal anti-body, get around this problem using antibodies to deliver a drug directly to tumors.
Now scientists have built on this antibody approach using an engineered protein rather than an antibody to direct the drug to the tumor.
Although the two techniques are conceptually similar, the specialized protein could treat brain tumors because it would have a potential advantage of being able to pass through the blood-brain barrier. It is also smaller than the antibody and might be able to reach dense tumors that have little blood supply.
Molecular Delivery Vehicle
says Jennifer Cochran, associate professor of bioengineering at Stanford University.
“Antibodies can be limited for treating solid tumors because they are too big to penetrate well. The idea is that a smaller molecule could diffuse into the tumor better.”
The new idea stemmed from the knowledge that cancer cells, and the blood supply that feeds them, often produce particular molecules known as integrins on their surface. Researchers wanted to create an engineered protein that could latch tightly onto those integrins and be used as a drug delivery vehicle.
The team started with a protein called knottin, so named for its knot-like shape, and used directed evolution to engineer a protein variant that would bind strongly to integrins.
Two Attachment Approaches
Researchers then worked on two strategies for attaching chemotherapeutic drugs to the evolved knottin. One strategy used a portion of an antibody to connect the drug to the knottin, mimicking antibody therapies that are already on the market, perhaps speeding the time it would take to get this therapy approved for patients.
The team tested this approach in a lab dish and in mice with implanted human tumors and in each case the knottin successfully delivered the drug to the tumor and killed the cancer cells.
A second approach, developed in collaboration with postdoctoral fellow Nick Cox, used a small chemical link to attach a chemotherapeutic drug directly to the knottin.
The knottin-drug combination effectively killed breast, ovarian, and pancreatic cancer cells in a lab dish. The targeted drug delivery was highly effective against cancer cells, including those that had developed a resistance to the drug alone.
“We found that when the drug was delivered by the knottin, its potency was greatly enhanced in treating highly resistant tumor cells, like those found in pancreatic cancer,” Cox says.
Which Is Best?
In both examples, the knottin portion of these multipart compounds binds to integrins present at high levels on cancer cells, delivering the drug directly into the cancer cell and bypassing healthy cells. Once inside, the drug is released and kills the cell.
Because the drug is less active when connected to the knottin and cannot get inside cells of the body that do not express integrins, this approach could significantly reduce side effects on other tissues and organs.
This isn’t the first time Cochran has taken advantage of knottin’s affection for cancer cells.
In previous work her team had attached the engineered knottin to a fluorescent dye that was visible by molecular imaging techniques. This dye-labeled knottin could seek out and attach to cancer cells in the brain and elsewhere in the body and make them visible to doctors, who often have a hard time detecting whether drugs are shrinking tumors.
That previous work gave Cochran and her team some useful information.
First, they learned that the knottin can get past a barrier that protects the brain from many molecules. And second, they learned that the knottin does seek out cancer cells compared to healthy tissue. If it didn’t, the imaging signal would show a blur of light rather than visibly highlighting tumors.
“We’ve shown that these knottins can pass the blood-tumor barrier so the hope is that we can use this to deliver chemotherapy to brain tumors,” Cochran says.
These two knottin-based therapies are a first step.
“These studies showed that we could use these molecules to deliver drugs to tumors,” Cochran adds. “What we still need to understand is which cancers it works best on and what is the best chemotherapeutic drug to use.”