Breaching Barriers
A look at the promise of focused ultrasound to outsmart the brain’s natural defenses and smuggle therapies into deep-brain territory to treat intractable disorders of the central nervous system
Illustration: Jim Frazier/Illustration Source
Researchers know more about the biology of neurological disorders than ever before. They have identified genetic mutations and defects in cell signaling pathways linked to disease that may serve as therapeutic targets. Moreover, pharmaceutical companies have spent billions developing drugs for brain disorders—ones that might promote growth of brain cells, repair damage or slow progress of tumours, something not possible now. Obstacles to effective therapy delivery remain, however, most notably, the blood-brain barrier (BBB).
“The blood-brain barrier is the interface between the blood vessels and the brain,” says Dr. Isabelle Aubert, a senior scientist in brain biology at Sunnybrook Research Institute (SRI) and a professor at the University of Toronto. “The barrier is lined with brain endothelial cells sealed together by tight junctions and maintained through cellular and molecular interactions.”
The BBB protects brain chemistry by maintaining a constant environment. Unlike elsewhere in the body where gaps between endothelial cells allow fluid to pass through vessel walls, endothelial cells lining the brain’s blood vessels are tightly knit; this cohesiveness excludes large molecules, like antibodies, and 98% of small molecules, including most drugs, from the brain. There are exceptions; for example, proteins like insulin, needed to regulate eating, are ushered through cell membranes to neurons via transporter proteins. Caffeine and active components of sleeping pills are other examples.
Dr. Isabelle Aubert is working with Dr. Kullervo Hynynen on evaluating the use of imaging-guided focused ultrasound to sneak therapeutics past the blood-brain barrier.
Photo: Doug Nicholson
While protecting the brain from toxins, the BBB also prohibits agents that could help heal it—a not unsizeable problem that needs to be solved before biologists and drug companies can capitalize fully on the advances they are making.
Dr. Kullervo Hynynen, director of Physical Sciences at SRI, first used ultrasound to disrupt the BBB in the 1980s. He developed technology showing he could pass ultrasound beams through the skull safely, something thought impossible. Over the next two decades he worked with industry to refine and commercialize the technology.
The device works by emitting therapeutic ultrasound at high and low frequencies. Application of acoustic power changes properties of biological tissue: high-intensity focused ultrasound generates heat to destroy tissue, such as a lesion, either in the body or deeper in the brain, away from bone, which can overheat.
In 2001, Hynynen was the first to show that low-power ultrasound could open the BBB focally and reversibly without damaging surrounding brain tissue. His lab has worked since then on advancing the technology, taking it from theory to where it is now: showing strong preclinical results and the potential to transform clinical care.
Dr. Todd Mainprize (left) operates on a patient. He is leading the world's first clinical trial assessing magnetic resonance imaging-guided focused ultrasound for targeted delivery of anti-cancer drugs to the brain.
Photo: Doug Nicholson
The technique combines ultrasound with tiny gas bubbles that are injected into the bloodstream. Guided by magnetic resonance imaging (MRI), low-frequency ultrasound is applied when bubbles reach the target area. As the bubbles expand and shrink, the tight junctions between endothelial cells loosen, which allows molecules to cross over into brain tissue temporarily. “We don’t disrupt the blood-brain barrier permanently. We just increase permeability for about six hours. It reseals by itself after,” says Aubert.
Hynynen says the biggest challenge was controlling acoustic power such that the barrier is opened enough to let therapeutics through. His group developed controllers with microphones that “listen” to how the bubbles behave. “By listening at different locations we can pinpoint where the bubbles are active. We can see how they are expanding and contracting, and use that information to control the power at that location,” says Hynynen, who is also a professor at U of T.
Hynynen and Aubert’s preclinical studies have led to pioneering publications detailing how MRI-guided low-intensity focused ultrasound can be used to deliver immunotherapy, and gene and cell therapies across the BBB.
In 2010, they published the first experiment using the technique to deliver immunotherapy to mouse models of Alzheimer’s disease (AD), for which there is neither a cure nor a treatment that will halt disease progression. Their results showed antibodies cleared amyloid-beta, a protein found in the plaques that are a hallmark of AD, in four days—much faster than injecting antibodies into the bloodstream, which can take up to 18 months to get rid of the protein. “The big breakthrough here was that we were able to reduce the pathology much faster in a localized manner,” says Aubert.
Compared with other options, delivering immunotherapy using focused ultrasound is noninvasive, more precise and safer. For example, administering antibodies intravenously works by pulling amyloid-beta from the brain into the circulation; much higher doses are needed compared with direct delivery to the brain; moreover, drawing amyloid-beta into the bloodstream can harm the circulatory system and sometimes causes brain swelling. Another method, convection-enhanced delivery, is a newer, aggressive treatment for brain tumours that have not responded to other treatments. Doctors drill a hole in the skull and insert a tiny catheter into the brain to dispense chemotherapeutic drugs. In addition to being invasive, it lacks precision; the medication can follow uncontrolled pathways.
Hynynen and Aubert have also shown they can direct gene therapy to specific brain regions, including the hippocampus, which is important for memory and learning. Gene therapy, which holds out hope of being able to treat disease without drugs or surgery, involves delivery of a vector containing genetic coding to make proteins that may be restorative. “By modifying brain cells with therapeutic genes, we allow them to use their own cell machinery to produce the proteins required to rescue neuronal function, in effect creating a ‘mini pump’ of therapeutic factors produced by brain cells,” says Aubert.
Dr. Kullervo Hynynen is working with Dr. Isabelle Aubert on evaluating the use of this technology to deliver therapies into the brain.
Photo: Doug Nicholson
The next challenge was transporting stem cells, which are even bigger than vectors, across the barrier. They administered stem cells into the blood and delivered them to certain areas of the brain in healthy preclinical models. They confirmed the cells’ entry by labeling them with markers. The stem cells not only survived after transplantation, but they were also able to differentiate into neurons, a feat that gives rise to the possibility that a damaged brain could be supplemented by healthy new cells, including neurons.
They’ve also demonstrated that areas of the brain treated with only ultrasound reduced plaque in models of AD—another first. While the mechanism is unknown, they surmise the effect might be owing to entry of natural antibodies in the circulation into the brain, or activation of glial cells, which are supportive cells that sweep away debris, including amyloid-beta. In recent studies, they found ultrasound alone rescued memory, thereby restoring function, and stimulated production and development of new brain cells.
The pair is pushing the boundary of what is known about the technology’s potential. They are not the only ones working in this field (most others are Hynynen’s former trainees), but they are at its forefront. “I think we’ll probably be the first ones to do clinical work. Once we do the first phase, everybody will be doing it,” says Hynynen.
As a start, neurosurgeons at Sunnybrook recently launched the world’s first clinical trial evaluating focused ultrasound for targeted delivery of chemotherapeutics to the brain. The outlook for patients with glioblastoma, the most common malignant brain tumour, is grim: average survival is 14 months from diagnosis. There is, however, cautious cause for hope.
Hynynen and Dr. Todd Mainprize, a clinician-scientist who heads neurosurgery at Sunnybrook, are leading the Phase 1 trial to see whether low-frequency ultrasound safely increases delivery of doxycycline, a drug with known anti-tumour effects. Subsequent trials will evaluate efficacy. They will use the technique to open the BBB only where they want the drug to go, making it safer than osmotic disruption, which opens the barrier everywhere.
“There’s going to be very little drug delivered to healthy tissues. The drug concentrations will only be high where we are delivering the ultrasound treatment. It should theoretically be extremely safe and very effective,” says Mainprize.
Health Canada approved use of Hynynen’s focused ultrasound brain device for the trial, which involves six to eight of Mainprize’s patients. (They are also using the device to conduct a Phase 1 clinical trial to ablate tumours nested deeply in the brain, which permits use of high-frequency ultrasound to destroy lesions.)
Delivering therapies across the blood-brain barrier: focused ultrasound is applied through the skull to the brain region of interest. Microbubbles are injected into the bloodstream. When the bubbles reach the ultrasound field, they vibrate. This changes the property of the blood vessel wall, making it more permeable. This temporary effect allows therapeutic molecules like antibodies to move from the blood to the brain. Here, the aim is clearance of amyloid plaques, the telltale signs of Alzheimer’s disease.
Illustration: Emmanuel Thévenot / lab of Dr. Isabelle Aubert
The trial is a gateway to treatment of other brain disorders, says Mainprize. “Once we show we can do it, there’ll be opportunities to try and treat various conditions [including] Parkinson’s disease and psychiatric illnesses—to open up selective areas of the brain where we think the problems are, and get drugs we think will be helpful into the areas where they can help.”
The establishment of the Slaight Centre for Image-Guided Brain Therapy and Repair will also accelerate translation. At its core will be a unique molecular imaging system that will fuse positron emission tomography, MRI and Hynynen’s focused ultrasound device. This will enable them to map brain anatomy and pathological changes simultaneously during therapy, thus tracking delivery and outcome in real time. The technology, combined with Aubert’s expertise in brain biology and the leadership of clinical expert Dr. Sandra Black, head of the Brain Sciences Research Program at SRI and an authority in neurodegenerative disorders, will give rise to trials in patients with Alzheimer’s disease and stroke.
Translation is still some years away. While his goal is for the technology to be used routinely to treat patients, Hynynen says getting there will take time. “Clinical trials are a significant amount of work. That’s where you really learn how to do things. The idea is that we do multiple Phase 1 trials and see what can be done with the device.”
As one case in point, before moving forward to human trials they have to ensure the treatment doesn’t just get in, but also improves function, as Aubert emphasizes: “We need to show much greater improvement in cognitive function, learning, memory, mood-related behaviour—everything that AD can present with, and that we can reproduce in preclinical models. We need to be more successful at curing all of this compared with other treatments before we can move forward.”
This research is supported by the Alzheimer Society of Canada, Canada Foundation for Innovation, Canadian Institutes of Health Research, National Institutes of Health, Ontario Mental Health Foundation, Ontario Ministry of Research and Innovation, Slaight Family Foundation and W. Garfield Weston Foundation. Hynynen holds the Canada Research Chair in Imaging Systems and Image-Guided Therapy.
