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Brain Biopsy Method Uses Sound, Microbubbles to Boost ctDNA Mutation Detection

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NEW YORK – Researchers at Washington University in St. Louis improved the ability to detect specific brain cancer-related mutations in circulating tumor DNA by using focused ultrasound and microbubbles to briefly permeabilize the blood-brain barrier near glioblastomas, facilitating the passage of ctDNA into the bloodstream.

Although the work was conducted in mouse and pig models, the team is now exploring ways to commercialize their patented "sonobiopsy" and is beginning to apply for human trial approval.

The technique consists of locating a tumor via MRI and disrupting tumor tissue with focused ultrasound, or FUS, thereby encouraging it to shed DNA. Next, microbubbles — in this case Definity lipid-coated microbubbles from Lantheus — are delivered via infusion. These pulsate upon passing the FUS-targeted tissue.

"The interaction of focused ultrasound wave with the microbubbles leads to expansion and contraction of the microbubbles," Hong Chen, associate professor of biomedical engineering and the study's principal investigator, explained via email. The oscillating microbubbles, he continued, push and pull on the blood-brain barrier, causing it to grow more permeable.

The released ctDNA can then be assayed through standard liquid biopsy techniques, avoiding the risks involved in surgically acquiring a tissue biopsy.

Chen and his colleagues tested this technique in mouse and pig glioblastoma models. Each animal contained cells from the human U87 glioblastoma cell line, which overexpresses EGFRvIII and carries the TERT C228T mutation.

In both cases, sonobiopsy significantly enriched the amount of glioblastoma ctDNA in the bloodstream. The researchers recorded 920- and 270-fold increases in EGFRvIII ctDNA for mice and pigs, respectively, and 10- and ninefold increases in TERT C228T ctDNA, assayed by Bio-Rad Droplet Digital PCR.

Importantly, neither model showed significant signs of tissue damage. The investigators found evidence of microhemorrhage and apoptosis in the FUS-targeted tissues but this did not significantly differ from the microhemorrhage and apoptosis seen in non-FUS-targeted areas of the tumor.

In addition, the researchers noted that the observed tissue damage was consistent with the reversible damage observed in clinical trials of FUS-induced blood-brain barrier disruption for brain drug delivery, adding that they did not expect the observed tissue disruption to contribute to metastasis.

Although overexpression of EGFRvIII in U87 cells likely contributes to the magnitude of its increased ctDNA, the researchers argue that the strong rise in both biomarkers provides "convincing evidence" supporting the clinical translation of sonobiopsy.

Chen and WUSTL have patented the sonobiopsy technique and are in the early stages of seeking to commercialize it.

"We're in the process of looking for opportunities for commercialization by either starting our own company or finding commercialization partners," Chen said.

In support of that goal, the team is also applying for clearance to conduct clinical sonobiopsy trials in humans. They anticipate being able to begin a trial in 2022.

The transition into humans will provide numerous opportunities to better understand and to refine the technique. Chen anticipates that factors such as skull thickness, tumor size and location, and variation between individuals will provide the main challenges in adapting the technique. They will also evaluate the long-term safety of sonobiopsy.

Nonetheless, he remarked that "technology wise, we're ready to go for the human trials."

Although boosting the ability to obtain brain tumor ctDNA from liquid biopsies is currently the main goal of this research, it may yet find other applications.

In addition to increasing biomarker concentration, the release of tumor nucleic acid and other molecules could enable sensitive molecular cancer characterization for longitudinal monitoring, as well as information complementary to neuroimaging, where the latter alone remains challenging, such as in differentiating treatment-induced pseudoprogression from true relapse.

By enabling minimally invasive access to the brain, Chen pointed out that sonobiopsy may also prove useful in investigating diseases beyond cancer.

"We expect the application of sonobiopsy could be expanded to other brain diseases," he said, "such as Alzheimer's disease and Parkinson's disease."

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