Made from amorphous silica, a form of silicon dioxide found naturally in foods and the fossilized remains of microscopic organisms, the engineered nanoparticles appear to attack prostate cancer in multiple ways at once.

Tiny nanoparticles with a dual cancer fighting strategy

The nanoparticles, known as ultrasmall fluorescent core shell silica nanoparticles or Cornell Prime dots (C' dots), were originally created to improve medical imaging. They have already advanced into late stage clinical trials for image guided surgery and other therapeutic uses.

More recently, researchers discovered that the particles themselves can selectively damage cancer cells while leaving healthy cells largely unharmed.

In the new study, published June 15 in Cancer Research, a journal of the American Association for Cancer Research, the team tested the nanoparticles in mice with aggressive prostate cancer. They found that the particles made tumor cells highly vulnerable to a form of self destruction while also transforming the tumor environment from an immune resistant "cold" state into an immune active "hot" state. This shift could significantly improve the effectiveness of existing immunotherapies.

"We're very encouraged by these results; a treatment that directly induces tumor-cell death while transforming the immune microenvironment, as this does, would represent a new clinical paradigm," said senior author Dr. Michelle Bradbury, the Endowed Professor of Imaging Research in Radiology and director of the Molecular Imaging Innovations Institute at Weill Cornell Medicine and a neuroradiologist at NewYork-Presbyterian/Weill Cornell Medical Center.

The work is part of a long running collaboration between Dr. Bradbury's laboratory and the laboratory of co corresponding author Dr. Ulrich Wiesner, the Spencer T. Olin Professor in the Department of Materials Science and Engineering and a professor in the Department of Design Tech in the College of Architecture, Art, and Planning. The research received support in part from the Parker Institute for Cancer Immunotherapy at Weill Cornell Medicine.

How the silica particles kill cancer cells

One of the most unusual findings involved a process called "ferroptosis," a specialized form of cell death driven by overwhelming oxidation inside cells. During ferroptosis, oxidation damages critical molecules, particularly the fatty molecules that make up cell membranes, eventually causing the cells to break down.

Scientists do not yet fully understand how the nanoparticles trigger this process. However, evidence suggests the particles, which were initially designed to carry imaging agents, can collect positively charged iron ions from the bloodstream and transport them into tumor cells. Once inside, those iron ions may fuel the intense oxidation that drives ferroptosis.

Reawakening the immune system

Beyond directly killing tumor cells, the nanoparticles also reshaped the immune environment surrounding the cancer.

The researchers observed that T cells, macrophages, and other immune cells near the tumors shifted from inactive or immune suppressing states into active cancer fighting cells. The nanoparticles also made tumors much more responsive to approved immunotherapy drugs. At the same time, they disrupted metabolic processes across several types of cells within the tumor microenvironment, further slowing tumor growth.

To ensure the treatment reached prostate cancer cells, the team attached a targeting molecule that recognizes PSMA, a protein found on the surface of prostate tumor cells. Although some particles briefly accumulated in other organs such as the spleen, the researchers found no signs of toxicity outside the tumors.

"It seems unreal -- how is it possible that rather than a single pathway we see all these effects happening simultaneously and only in tumors and not in healthy tissues?" Dr. Wiesner said. "I have to wonder whether ultrasmall silica's very early and ubiquitous presence in the environment and foods like leafy greens or cereal grains has given it a connection to biology that we're only beginning to glimpse."

Combination therapy produced the strongest results

The most dramatic findings came from survival studies involving mice with aggressive prostate cancer.

On their own, both the C' dots and immunotherapy modestly improved survival compared with no treatment. However, combining the nanoparticles with an immune checkpoint blockade therapy produced complete or nearly complete remissions and indefinite survival in four out of ten mice.

Adding a third treatment called CSF-1R blockade, which targets tumor associated macrophages, increased the number of complete remissions to five out of ten mice.

"We think there's nothing else out there that has such a strong and durable tumor growth suppressing effect," Dr. Bradbury said.

"One of the most intriguing aspects of this work is the convergence of direct tumor cell killing with broad immune remodeling," said study co author Dr. Jedd Wolchok, the Meyer Director of the Sandra and Edward Meyer Cancer Center, professor of medicine at Weill Cornell Medicine, director of the Parker Institute for Cancer Immunotherapy at Weill Cornell Medicine Meyer Cancer Center and an oncologist at NewYork-Presbyterian/Weill Cornell Medical Center. "By creating conditions that support a more effective antitumor immune response, these particles may help unlock the full potential of immunotherapy in prostate cancer, where durable responses have historically been difficult to achieve."

Next step is human clinical trials

Dr. Bradbury also recognized the work of the study's co first authors, Drs. Nabil Siddiqui, Li Zhang, and Gabriel DeLeon, who led many of the biological, mechanistic, and translational studies, along with graduate students Nada Naguib and Rachel Lee from Dr. Wiesner's laboratory, whose careful synthesis and characterization of the nanoparticles were essential to the project.

"This study reflects years of collaborative effort across multiple laboratories and would not have been possible without the dedication, creativity and perseverance of this tremendous research team that helped drive the science forward," she said.

The research team is continuing to investigate these ultrasmall core shell silica particles as a potential new class of cancer therapies capable of influencing inflammatory, immune, and metabolic pathways at the same time. Their long term goal is to evaluate the safety and effectiveness of the treatment in human clinical trials.

Drs. Michelle Bradbury and Ulrich Wiesner are inventors on patents related to the technology described in this study.

The study was funded by the Department of Defense (PC220534); the National Cancer Institute, part of the National Institutes of Health, through grant numbers R01CA253658, R01CA243085, U54CA199081, the Cancer Center Support Grant (P30 CA008748), and Cycle for Survival/Parker Institute funding.