Microbubbles plus ultrasound punched temporary holes in cancer cells - and 50% self-destructed

Can you pop a bubble to kill a cancer cell? That is the basic idea behind SonoPIN - Sonoporation-assisted Precise Intracellular Nanodelivery - a platform demonstrated by engineers at Duke University and published March 13 in the Proceedings of the National Academy of Sciences.

The approach addresses a specific and stubborn problem in cancer drug delivery. A promising class of therapeutics called PROTACs (proteolysis-targeting chimeras) can hijack the body's own protein recycling system to destroy cancer-driving proteins. But PROTAC molecules are too large to cross cell membranes on their own. If they cannot get inside the cell, they cannot do their job.

How SonoPIN works

The platform starts with prefabricated microbubbles - the same kind already used as contrast agents in diagnostic ultrasound. These bubbles are equipped with synthetic nucleic acid strands designed to bind to specific receptors found on cancer cell surfaces but not on healthy cells.

Once the microbubbles attach to their targets, a focused ultrasound pulse causes them to collapse rapidly. The collapse generates microjets and shock waves directed toward the nearby cell membrane, creating nanoscopic, temporary pores - a phenomenon called sonoporation. These pores are large enough for PROTAC molecules to slip through before the membrane self-heals, typically within seconds to minutes.

Yuqi Wu, a doctoral student working with Tony Jun Huang, the William Bevan Distinguished Professor at Duke, described the process as less like an explosion and more like a controlled mechanical opening. Cell membranes are fluid and dynamic; they naturally close small pores without lasting damage.

Killing cancer, sparing everything else

The team tested the platform in benchtop experiments by attaching fluorescent markers to PROTACs and measuring uptake. After one minute of ultrasound exposure, cancer cells treated with SonoPIN glowed seven times brighter than those given PROTACs through conventional delivery, indicating dramatically higher intracellular concentrations.

The specific PROTAC used targets a protein called BRD4, which cancer cells rely on for rapid reproduction and survival. Once BRD4 is tagged by the PROTAC, the cell's own ubiquitin system marks it for destruction. Without BRD4, the cancer cell's growth machinery collapses and it self-destructs through apoptosis.

The results: 50% of targeted cancer cells self-destructed. Ninety-nine percent of non-targeted healthy cells remained viable. That selectivity is notable because BRD4 is also essential in healthy cells - meaning the targeting depends on precise delivery, not on the drug's inherent specificity.

Why size matters in drug delivery

Most conventional drugs are small molecules that can cross cell membranes through diffusion or active transport. PROTACs are substantially larger - too big for those standard entry routes. Other large-molecule therapies face the same barrier, including gene-editing complexes like CRISPR-Cas9 systems.

SonoPIN's mechanical delivery mechanism is size-agnostic. Because it creates physical pores rather than relying on biological uptake pathways, it could theoretically deliver therapeutics of almost any size. Huang expressed particular interest in testing the platform with gene-editing payloads, which are currently limited by delivery challenges.

From benchtop to mouse models

The current results are entirely in vitro - cells in dishes, not tumors in bodies. The team has applied for a patent and plans to test SonoPIN in mouse models, where the approach would involve injecting both PROTACs and cancer-seeking microbubbles intravenously, then focusing ultrasound on tumor locations.

The translation from dish to animal introduces significant unknowns. Microbubbles need to reach tumors through the bloodstream, attach to cancer cells in a complex tissue environment, and respond to ultrasound that must penetrate through overlying tissue. Bubble concentration, ultrasound focusing precision, and potential off-target effects in nearby healthy tissue all need evaluation.

Still, the core principle - using mechanical force from collapsing bubbles to create entry points for oversized drugs, targeted specifically to cancer cells - is physically sound and the benchtop numbers are compelling enough to justify the next step.