Using sound waves to target tumours

Scientists looking to improve the next generation of cancer treatments have developed a way of using ultrasound to deliver high-potency cancer drugs directly to a tumour.

In the process, they have killed two birds with one stone: developing a method of tracking the drug’s progress through the body, and using ultrasound “tweezers” to guide it to where it’s needed, thereby minimising chemotherapy side effects.

The method, says Qifa Zhou, a biomedical engineer at the University of Southern California, in Los Angeles, hinges on the use of microbubbles, long used in imaging blood flow in coronary arteries. 

Because these bubbles reflect ultrasound beautifully, cardiologists are able to use them to produce ultrasound images sharp enough to reveal dangerous coronary blockages. 

If such bubbles are filled with chemotherapy drugs, they can be tracked the same way, Zhou says: a major improvement on other ways of tracking drug delivery, such as radioactive tracers, which can carry risk to the patient. 

“Ultrasound has been used for 40 years,” Zhou says. “It’s safe and easy.” 

And, once the bubbles have reached the right location, all that’s needed to get the drug out of them is to turn up the ultrasound intensity. That makes them pop, releasing their contents exactly when and where desired. 

But that’s just the beginning. 

In experiments reported in Applied Physics Letters, Zhou, Hanmin Peng, a visiting scholar from Nanjing University of Aeronautics & Astronautics, China, and colleagues demonstrated that they could use focused ultrasound to create “acoustic tweezers” that could steer the motion of microbubbles through an artificial blood vessel—basically a blood-vessel-sized silicone tube. (They placed the artificial blood vessel beneath a layer of animal tissue in order to make the experiment more realistic.)

This allowed them not only to control the movement of the microbubbles, but to trap them in whatever location they chose along the wall of the artificial blood vessel. There, they could simply turn up the power and pop them, just as oncologists of the future might do with chemotherapy agents.

“Currently, we are targeting relatively large arteries, about two to millimetres across,” Zhou says, but the same process should work in smaller ones or even capillaries.

So far, he adds, the study is merely a proof of concept. “I think in the future we will try an animal study and link a drug with a microbubble and see what happens,” he says.

His team is also consulting with oncologists to determine the type of cancers this approach might best work with. “We don’t have a specific [one],” he says.

Not that cancer is the only disease that could be addressed this way. 

It might also be possible, he agrees, to use the same approach to steer antibiotics directly to an infection, a possible way of attacking multi-drug resistant bacteria with super-high potency antibiotics too strong to be used by conventional methods.

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