Ultrasound-activated nanobubbles break down tumor barriers that block cancer drug delivery

Solid tumors are not just clusters of cancer cells. Over time they remodel the tissue around themselves, secreting enzymes that produce dense deposits of collagen - essentially scar tissue - that form a physical barrier around the tumor mass. This barrier is a major reason why many promising cancer therapies fail in solid tumors despite working well in liquid cancers or in laboratory conditions. Drugs, nanoparticle carriers, and immune cells that need to penetrate the tumor encounter a wall of compressed, stiff tissue that blocks or slows their entry.

The problem is particularly acute for newer immunotherapy approaches that deliver RNA molecules - including mRNA-based cancer vaccines and RNA that activates killer T cells - in lipid nanoparticle carriers. These carriers are large enough that the dense collagen matrix severely limits their distribution within tumor tissue. They tend to concentrate at the injection site rather than spreading throughout the tumor.

Researchers at Case Western Reserve University report a method that addresses this barrier directly, without destroying cells or requiring a new drug. Published in ACS Nano, the approach uses tiny gas-filled nanobubbles and focused ultrasound to physically remodel the tumor microenvironment.

How the nanobubble-ultrasound approach works

In a breast cancer mouse model, the researchers injected nanobubbles filled with perfluoropropane - an inert, biologically harmless gas - directly into tumors. They then directed ultrasound waves at the tumor to vibrate the bubbles at low intensity. The mechanical agitation from the oscillating bubbles gently disrupted the collagen network without destroying cells, reducing the stiffness and density of the tumor tissue.

"We developed a strategy that uses ultrasound-activated nanobubbles, which gently remodels the tumor microenvironment and effectively collapses the tumor walls, opening the door for drugs and immune cells," said Efstathios Karathanasis, vice chair and professor of biomedical engineering at Case Western Reserve and co-leader of the study.

The softening effect lasted for at least 5 days after treatment - a window substantially longer than the few hours typically associated with physical disruption approaches. Untreated tumors, by contrast, grew stiffer and denser over the same period. When the researchers subsequently injected lipid nanoparticles carrying RNA that enhances T cell activity, the nanoparticles distributed throughout the treated tumors rather than remaining at the injection site.

Immune activation that extended beyond the treated tumor

The most unexpected finding was what happened immunologically. The nanobubble treatment activated immune cells already present inside the tumor - without any additional immunotherapy drug being delivered. Those immune cells began secreting danger signals that recruited additional immune cells to the tumor site.

More striking: killer T cells activated at the treated tumor site subsequently targeted other tumors in the same animal that had not been directly treated. The immune response generalized beyond the site of physical intervention, a phenomenon sometimes called the abscopal effect when it occurs in response to radiation therapy.

"We drop the defenses of the cancer and give a fair chance for our therapies to actually win," said Agata Exner, Henry Willson Payne Professor of Radiology and director of the CWRU Center for Imaging Research, who co-led the study. "We did not invent a new drug, but it has the potential to make any existing or emerging therapy work much better."

Path toward clinical trials

The nanobubbles used in the study were developed in Exner laboratory and are currently being commercialized for detecting prostate cancer by Visano Theranostics, a company she co-founded. The ultrasound system is already FDA-approved and commercially available. These circumstances could accelerate the regulatory pathway: Exner said an Investigational New Drug application will be submitted to the FDA within the next 18 months, and the therapeutic application could piggyback on that submission, potentially enabling clinical trials within two years.

"Any tumor that you can biopsy can potentially have nanobubbles introduced," Exner said. "This is especially important for solid tumors that are difficult to treat, where ultrasound is already used, like liver, prostate and ovarian cancers."

Limitations and open questions

The research was conducted entirely in mouse breast cancer models. While mouse tumor models provide important mechanistic insights and have guided many cancer therapy developments, they differ from human tumors in important ways - particularly in tumor size, immune system composition, and the density and composition of the collagen matrix. The 5-day softening window and the abscopal immune response are promising findings, but whether they replicate in human patients at clinical scale requires testing in human trials.

The injection of nanobubbles directly into tumors requires that the tumor be accessible for injection - a constraint that limits applicability to tumors amenable to biopsy or direct imaging-guided access. Deep, poorly accessible tumors would require different delivery approaches. The intensity of ultrasound required to achieve therapeutic bubble oscillation without cell damage must also be precisely calibrated, and the optimal treatment parameters for different tumor types and depths remain to be established.

The research was funded by the Case Comprehensive Cancer Center and the National Institutes of Health.