[64/67Cu]-FAP theranostics in combination with immunotherapy in a preclinical model of TNBC

Breast cancer is not a single disease but a collection of different conditions, each with its own behavior and response to treatment. One particularly aggressive form, known as triple-negative breast cancer, lacks the specific receptors that many standard drugs target, leaving fewer options for patients. In recent years, scientists have turned their attention to the environment surrounding the tumor, rather than just the cancer cells themselves. This environment, called the tumor microenvironment, is a complex neighborhood filled with various support cells, blood vessels, and structural materials. Among these support cells are cancer-associated fibroblasts, which act like construction workers that can sometimes help the tumor grow and spread. These cells carry a specific protein on their surface called fibroblast activation protein, or FAP. Because this protein is abundant in the tumor's support system but rare in healthy tissue, it offers a unique address that doctors might use to deliver medicine directly to the disease while sparing the rest of the body.

Researchers at the University of Alabama at Birmingham set out to test a new way of using this address to treat triple-negative breast cancer in mice. They focused on a specific molecule, a small chain of amino acids known as FAP2287, which acts like a key designed to fit the FAP lock on the cancer-supporting cells. To make this key useful for both seeing the disease and treating it, they attached it to two different versions of the same metal element, copper. One version, copper-64, emits signals that can be detected by a PET scanner, allowing doctors to see exactly where the key is going inside the body. The other version, copper-67, releases radiation that can damage and kill the cells it attaches to. By using these two forms together, the team created a "theranostic" approach, where the same targeting molecule is used for both diagnosis and therapy. They also wanted to see if combining this targeted radiation with immunotherapy, a treatment that wakes up the body's own immune system, would work better than either treatment alone.

The team began by confirming that the mice they were using, which carried tumors similar to human triple-negative breast cancer, indeed had high levels of the FAP protein on their cancer-supporting cells. They injected the mice with the copper-64 version of the targeting molecule and took pictures over the course of a day. The scans showed that the molecule successfully found and stuck to the tumors, and the brightness of the signal on the scan matched the actual amount of the molecule found in the tissue when the tumors were examined later. This confirmed that the imaging technique could accurately predict where the treatment would go.

Next, the researchers divided the mice into several groups to test different treatment strategies. Some mice received no treatment, some received only immunotherapy, some received only the copper-67 radiation, and others received a combination of the radiation and immunotherapy at two different strength levels. The results showed that the radiation treatment alone was effective at slowing tumor growth and extending the lives of the mice compared to the untreated group. However, the most dramatic results came from the combination groups. The mice that received both the targeted radiation and the immunotherapy saw their tumors shrink significantly more than those receiving radiation alone. In fact, the group receiving the higher dose of radiation combined with immunotherapy had the best outcome, with a substantial reduction in tumor size and a much higher rate of survival.

The study also looked at whether the initial PET scan could predict which mice would respond best to the treatment. The researchers found a clear link: mice that showed a stronger signal on their initial scan, indicating more of the targeting molecule had accumulated in the tumor, tended to survive longer after treatment. This suggests that the diagnostic scan could serve as a guide, helping doctors identify which patients are most likely to benefit from this specific type of therapy before they even begin treatment. Furthermore, the mice that survived the longest and had their tumors completely disappear were tested again later by introducing new cancer cells. These survivors showed a remarkable resistance to the new tumors, suggesting that the combination treatment may have trained their immune systems to recognize and fight the cancer long-term.

While the study was conducted in mice, the findings offer a promising path forward for treating a difficult form of breast cancer. The research demonstrates that using a chemically matched pair of copper isotopes allows for precise targeting of the tumor's support system. It also provides evidence that combining this targeted radiation with immunotherapy creates a powerful synergy, where the two treatments work together to produce results greater than the sum of their parts. The ability to use a non-invasive scan to predict the success of the treatment adds another layer of precision, potentially allowing for personalized care plans that maximize effectiveness while minimizing unnecessary exposure to radiation. The work highlights a shift in strategy, moving from attacking the cancer cells directly to disrupting the environment that allows them to thrive, and doing so with a level of precision that was previously difficult to achieve.