Biomedicine

A New Method of Getting Drugs into the Brain

Focused ultrasound waves can make a tiny, temporary hole in the barrier surrounding the brain.

One of the biggest challenges in treating neurological conditions such as Parkinson’s and Alzheimer’s disease is finding safe and non-invasive ways to enable drugs to penetrate the brain’s natural defense – the blood-brain barrier.

Light spot on the right of image shows the blood-brain barrier opening as focused ultrasound is targeted toward the mouse’s right hippocampus. (Courtesy of James Choi and Elisa Konofagou.)

Now scientists have developed a way to temporarily open a very small part of that barrier using focused ultrasound. They hope this precise targeting will allow drugs to enter specific parts of the brain – without exposing the rest of the brain and without damaging the barrier or surrounding neuronal tissue in the process.

[For images of this ultrasound barrier-piercing method, click here.]

In work presented at the annual meeting of the Acoustic Society of America in Providence, RI, earlier this month, researchers from Columbia University used magnetic resonance imaging to reveal how the hippocampus can be targeted with focused ultrasound, without affecting the rest of the brain, in mice genetically engineered to have Alzheimer’s disease. “The hippocampus is the region of the brain that controls the memory, among other things, and is the main region affected by Alzheimer’s and Parkinson’s,” says Elisa Konofagou, assistant professor in biomedical engineering at Columbia University, who carried out the work.

Using ultrasound in this way is a “huge deal,” says Al Kyle, president and CEO of Perfusion Technology, a startup medical research company in Boston that’s trying to develop similar technology. There are ways to open the blood-brain barrier using drugs, he says, “but it’s a really harsh treatment and requires several days in clinical care.” With more than 11 million people suffering from neurological diseases in the United States alone, says Kyle, a safer and less severe option is needed.

Research by Kullervo Hynynen at the University of Toronto, which first demonstrated the potential use of ultrasound to open the barrier in 2001, has suggested that using ultrasound to open the blood-brain barrier is safe. But Hynynen is still cautious about the applications for this use of ultrasound. “There could be significant clinical potential,” he says, but adds that it won’t be certain until someone does it in humans.

The blood-brain barrier protects the brain, which is why it can be difficult for drugs to penetrate it. The barrier consists of endothelial cells that line the small blood vessels in the brain. These cells are tightly packed to create a wall between most parts of the brain and the rest of the circulatory system, blocking bacteria and all but the smallest molecules.

Focused ultrasound works by directing sound waves toward a point in space. Individually, the waves are not powerful enough to affect the tissue, but when targeted, their collective intensity is much greater. High-intensity focused ultrasound (HIFU), which applies more intense sound waves, has been used to destroy tumors through heating, a process known as ablation.

When targeting the brain, though, Konofagou’s team used much lower-intensity levels, similar to those applied in diagnostic ultrasound, the technology used during a pregnancy sonogram. While researchers don’t know exactly how this technique is able to open the barrier, they say it’s not through heating.

Unlike tumor ablation –and this distinction is key – Konofagou’s technique appears to be reversible. Using an MRI contrast agent, she was able to show that the barrier closed up after about four hours. This is important, explains Pierre Mourad, principal physicist and research associate professor in the department of neurological surgery at the University of Washington, because “the longer the blood-brain barrier is open, the longer you let nasty stuff in the brain.”

“It’s an exciting and very viable field,” says Mourad. It is important to start applying this technique to animal models that simulate specific diseases, he says, just as Konofagou is doing, although he adds that the skulls of mice are extremely thin, unlike those of humans.

Konofagou says she’s now working on using higher-frequency ultrasound waves, which she believes will be able to penetrate human skulls.

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