Ultrasound for Alzheimer’s?

by Jim Schnabel

March 24, 2015

Researchers in Australia have tested what must be one of the oddest Alzheimer’s therapeutic strategies yet devised: They used an ultrasound beam to vibrate open the blood-brain barrier in transgenic Alzheimer’s mice. They didn’t add any drug, but something about the treatment stimulated brain-resident immune cells, microglia, to increase their usual consumption of amyloid beta aggregates.

The result, reported in Science Translational Medicine on March 11, 2015, was a strong clearance of these Alzheimer’s proteins from the mouse brains—as strong as expensive monoclonal antibody treatments have achieved. And although the mice had showed significant memory impairments beforehand, the treatment restored their cognitive performance to normal.

The study points to a possible drug-free strategy for preventing Alzheimer’s or even treating its earliest stages. Moreover, although it remains unclear why the ultrasound stimulated microglial activity, it may ultimately prove possible to duplicate this effect by even easier methods.

“It was kind of a crazy idea,” says Jürgen Götz, the senior investigator for the study, who researches Alzheimer’s at the University of Queensland. “We were surprised that it worked.”

Medical ultrasound is nowadays best known as a non-invasive, sonar-like imaging technology, typically used for detecting disease in internal organs and visualizing fetuses in the womb. But ultrasound began to be used as a therapeutic tool many decades ago, and the Götz lab study is one example of that concept’s resurgence.

“Back in the 1940s and ’50s, even before its use in imaging, people were using high intensity focused ultrasound for thermal ablation therapies,” says Elisa Konofagou, an ultrasound researcher at Columbia University.

The tissue-burning technique eventually fell out of use, she adds, because there was no way to monitor the targeted burning zone inside the body during treatment. But the technique re-emerged with the advent of magnetic resonance imaging (MRI) guidance systems. MRI-guided high-intensity focused ultrasound is currently FDA-approved for burning away uterine fibroids as well as pain-sensing nerves on bone metastases.

At intermediate energies—less intense than those used to burn tissue, but more intense than those used for imaging—ultrasound merely vibrates cells. This vibrational effect can be amplified by injecting the gas microbubbles that are often used to increase the contrast of ultrasound imaging.“Those bubbles are engineered to enter resonance at the frequencies of the ultrasound beam,” explains Konofagou.

In recent years, Konofagou and others have been finding that that ability to deliver vibrational force non-invasively to a precise spot within an organ shows a lot of promise for treating various ailments.

Good vibrations

One set of potential applications involves vibrating precise volumes of tissue to enhance their uptake of drug molecules. Experimental targets have included specific organs and even tumors that normally resist drug uptake.

The brain too has become a target. Konofagou and others have reported using focused ultrasound to vibrate open the blood-brain barrier—the tightly packed endothelial lining of cerebral capillaries that normally blocks many molecules, especially large ones, from leaving the bloodstream and crossing into the neuronal space.

“We’ve worked with several different therapeutic molecules, including neurotrophic factors and even the adenoviral gene-therapy vector AAV, and we’ve enabled them to cross the barrier despite being, in some cases, several orders of magnitude larger than the normal limit,” says Konofagou.

In such cases, the ultrasound beam can be focused in a way that opens the barrier in selected tiny volumes of the brain, thus in principle enabling enhanced drug delivery to a precise brain region, to help minimize a drug’s unwanted side-effects.

The technique has obvious potential applications for Alzheimer’s disease, since several proposed Alzheimer’s treatments/preventives, particularly antibodies targeting the Alzheimer’s-linked amyloid beta protein, involve large molecules that normally don’t diffuse efficiently across the blood-brain barrier.

Perhaps the most remarkable finding to emerge from such studies is that the ultrasound-induced opening of the blood-brain barrier may be therapeutic on its own. In a study reported late last year, researchers in Kullervo Hynynen’s laboratory at the University of Toronto used MRI-guided focused ultrasound to open the barrier in the memory-related hippocampus on both sides of the brains of transgenic “Alzheimer’s mice.” Just four weekly sessions of this treatment were apparently enough to reduce amyloid plaque loads and restore performance on memory tests to normal, compared with untreated mice.

The Götz lab study was a more comprehensive approach in a different mouse model of Alzheimer’s, but yielded similar results: A series of weekly ultrasound treatments greatly reduced insoluble amyloid beta deposits (plaques) in the mouse brains, as well as smaller, soluble aggregates of amyloid beta that have been more closely linked to the Alzheimer’s disease process.

What explains these striking therapeutic effects of opening the blood-brain barrier?

In the Hynynen study, there were hints that the affected brain regions might have benefited from a greater flow of growth factors across the barrier into the brain, and thus a greater rate of “neurogenesis.” The researchers noted that treated animals’ hippocampi had many more newborn neurons, and that these neurons also had longer and more numerous input stalks (dendrites).

Götz, by contrast, had thought that the blood-brain-barrier-loosening technique might work by enhancing the usual flow of brain-originating amyloid beta into the bloodstream—a clearance process that apparently helps keep brain levels of amyloid beta in a safe range.

“We hoped we would be able to detect [increased] amyloid beta in the bloodstream, but we did ELISA tests [using antibodies against amyloid beta] and we didn’t see anything,” he says.

Ultimately he and his postdoctoral researcher Gerhard Leinenga found one big difference to explain the amyloid beta reduction and memory benefits: microglia in treated mouse brains showed signs of being much more active. These immune cells normally help clear away excess amyloid beta, essentially by gobbling up the proteins and digesting them. Tests showed that microglia in the treated animals were filled with amyloid beta to a far greater extent than in control animals, and in many cases appeared to have consumed all the amyloid beta aggregates that would normally have been observed in inspected brain regions. (Yet in treated mice these brain regions didn’t show evidence of harmful inflammation.)

Leinenga and Götz suspect that a factor or factors in the bloodstream crossed the ultrasound-loosened blood-brain barrier and stimulated the microglia to greater amyloid-eating activity. In a preliminary, lab-dish test, they showed that albumin, a protein found abundantly in blood, can have this effect on microglia-like test cells. They also found evidence that albumin had readily crossed into the mouse brains after their ultrasound treatment. But the question of how ultrasound stimulates microglia remains open.

“It could be merely the mechanical force on the microglia from the ultrasound that stimulates them,” says Konofagou. “Or it could be that effect plus some factor crossing from the bloodstream. We don’t know yet, and it’s a difficult thing to study.”

Perhaps a more urgent question is whether the ultrasound blood-brain-barrier-opening technique can be scaled up from tiny mouse brains to much larger human brains, for clinical testing. Götz and Leinenga are collaborating with other researchers to test the technique in sheep brains. Konofagou and colleagues already have taken a big step towards human studies, by showing that the ultrasound barrier-opening method works and seems safe in monkeys.

“We have to reduce the frequency to get through the thicker skull and brain, so the focus spot size is larger,” she says. “But that’s what you want for a bigger brain—so the physics is on our side when we scale it up.”


Ultrasound for Alzheimer’s?

Aleta Payne

10/18/2016 5:02:31 PM

When are the clinical trials for humans to begin?