Bursting cancer-seeking microbubbles to deliver deadly drugs

(Press-News.org) Engineers at Duke University have demonstrated a technique that uses microbubbles and ultrasound to help relatively large cancer drugs enter tumor cells and cause them to self-destruct.

Dubbed “Sonoporation-assisted Precise Intracellular Nanodelivery”—or SonoPIN for short—the technology caused 50% of targeted cancer cells in a benchtop experiment to self-destruct, while leaving 99% of non-targeted cells healthy. The results show promise for precisely delivering a wide variety of large-molecule therapeutics to cells with few off-target effects.

The research appears online March 13 in the journal Proceedings of the National Academy of Sciences.

A class of up-and-coming therapeutics called “proteolysis-targeting chimeras,” or PROTACs for short, have shown great promise for degrading “undruggable” proteins and overcoming drug resistance in cancer therapy. PROTACs work by binding to a specific target protein and recruiting an enzyme called an E3 ubiquitin ligase, which attaches ubiquitin to the target protein. As the name suggests, ubiquitin is ubiquitous in all complex cellular life, and it marks proteins for destruction by the body’s natural garbage collection system.

In cancer cells, PROTACs have been used to target and degrade a protein called BRD4. Once destroyed, this breaks the cancer cells’ ability to rapidly reproduce and survive, effectively forcing them to self-destruct.

While PROTACs might seem like a miracle drug, there are, of course, a few catches. First, BRD4 is also essential to healthy cells, so off-target effects are harmful. Then there’s the size issue.

“PROTAC molecules are too big to get into cells in the first place,” said Yuqi Wu, a doctoral student working in the laboratory of Tony Jun Huang, the William Bevan Distinguished Professor of Mechanical Engineering and Materials Science at Duke. “But with our SonoPIN platform, the PROTACs can enter into targeted cancer cells while almost completely ignoring non-targeted cells.”

SonoPIN relies on prefabricated microbubbles that are commonly used to increase ultrasound contrast. When attached to cells and probed very gently with ultrasound waves, they create echoes that enhance the resulting images.

When probed vigorously, however, the bubbles collapse rapidly causing a phenomenon in nearby cells called sonoporation. While the full mechanisms are not well understood, the current hypothesis is that the bubbles’ collapse forms high velocity microjets and emit shock waves directed toward nearby cells. These forces create nanoscopic, temporary pores in the cell membrane large enough for PROTACs to enter.  

“This process is less like an explosion and more like a temporary, controlled mechanical opening,” explained Huang. “While it involves physical force, because cell membranes are fluid and dynamic, they naturally self-heal and close these pores within minutes if not seconds.”

In the study, Huang, Wu and their colleagues equipped these microbubbles with synthetic nucleic acid strands designed to bind with specific biochemical receptors that appear on the cell membranes of cancer cells but not healthy cells. They then tried several combinations of ultrasound frequencies and intensities to find the perfect pairing for opening pores in the cell membranes to allow the PROTACs to enter.

Once the optimal settings were identified, the researchers validated the platform by attaching fluorescent molecules to the PROTACs. They conducted separate experiments on cancer cells and healthy cells to compare the delivery efficiency. After a minute of ultrasound exposure, the cells treated with SonoPIN glowed seven times brighter than those treated with traditional PROTAC delivery methods, indicating that they were taking in many PROTACs. This resulted in half of the cancer cells self-destructing, while 99% of the healthy cells remained viable.

Moving forward, the researchers plan to test this approach in mouse models and have already applied for a patent covering the work. By injecting the PROTACs and cancer-seeking microbubbles into their veins and focusing the ultrasound waves on tumor locations, they believe SonoPIN could form a highly potent cancer-killing technology with few side effects.

“And because SonoPIN relies on a mechanical delivery approach rather than biological engulfment, it could theoretically deliver therapeutics of almost any size,” Huang said. “We would also be excited to see how it performs with therapeutics such as large gene-editing complexes.”

This work was supported by the National Institutes of Health (R01AG084098, R01CA282939, R01GM141055, R44GM154514, and R44GM154515) and the National Science Foundation (CMMI-2104295).

CITATION: “SonoPIN Enables Precise, Noninvasive, and Efficient Intracellular Delivery of PROTACs.” Yuqi Wu, Mingyuan Liu, Ke Li, Shanglin Li, Lai Yee Phoon, John Mai, Ying Chen, Wei Yan, Shu Nakajima Lan, Joseph Rufo, Graham Milford, Ye He, Qian Wu, Shujie Yang, Li Lan, Stephen J. Benkovic, and Tony Jun Huang. PNAS, 2026. DOI: 10.1073/pnas.2534439123

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