Evaluation of Estrogen Responsiveness and Sensitivity to Endocrine Therapy in ER-Positive Breast Cancer Organoids Using an Optimized Culture Medium

Breast cancer is not a single disease but a collection of different conditions, and one of the most common types is driven by a hormone called estrogen. In these cases, the cancer cells have receptors that act like antennas, catching estrogen signals from the body and using them to tell the cells to grow and divide. For decades, doctors have treated this specific type of cancer by blocking those signals, using drugs that either stop the body from making estrogen or prevent the cancer cells from receiving it. This approach, known as endocrine therapy, is the standard of care for most patients. However, a significant challenge remains: in about one out of every three or four patients, this treatment fails. The cancer either does not respond from the start or returns quickly after treatment begins. Currently, doctors rely on genetic tests that analyze the static blueprint of a tumor to guess how it might behave, but these tests cannot see how the living cells actually react to a drug in real time. There is a need for a way to watch the cancer cells themselves respond to treatment before a patient ever takes a pill.

To solve this, researchers at Kyoto University turned to a technology called organoids. Imagine taking a tiny piece of a patient's tumor and growing it in a lab dish, where the cells arrange themselves into tiny, three-dimensional clusters that mimic the structure and behavior of the original cancer. These living models are powerful because they keep the unique characteristics of the patient's disease. However, for years, scientists struggled to use these models to test hormone therapies. The standard liquid food used to keep these organoids alive was packed with powerful growth factors—chemicals that force cells to multiply rapidly. While this kept the cells alive, it was like shouting so loudly that the cells could no longer hear the quiet whisper of the estrogen hormone. The constant, loud signal from the growth factors drowned out the specific signal from estrogen, making it impossible to tell if the cancer cells were truly dependent on the hormone or if they were just growing because of the food. Consequently, when researchers tried to test hormone-blocking drugs on these organoids, the results were confusing and often useless.

The team set out to fix this by redesigning the food the organoids ate. They began by systematically removing the loud, overpowering growth factors from the standard mixture. They reduced the amount of one specific factor called heregulin to a tiny, precise amount and completely removed three others. This created a new, leaner environment where the cells could still survive but were no longer being forced to grow by external chemicals. In this quieted-down setting, the cancer cells finally began to listen to estrogen again. When the researchers added estrogen to this new medium, the organoids started to grow, just as they would in a human body. When they added a hormone-blocking drug called tamoxifen, the growth stopped. This proved that the new medium, which the researchers named BH-SAM, successfully restored the natural connection between the hormone and the cancer cells.

With this new tool in hand, the researchers tested it on organoids grown from nine different patients who had estrogen-positive breast cancer. The results were striking. In six of the nine cases, the organoids responded exactly as the researchers hoped: the hormone-blocking drug effectively stopped them from growing. In the other three cases, the drug had no effect, and the organoids continued to grow despite the treatment. This finding was not just a lab curiosity; it matched real-world history. One of the resistant organoids came from a patient who had clinically relapsed while on hormone therapy after surgery. The lab test had correctly predicted that the drug would not work for that specific patient. This suggests that the new system can identify which patients will benefit from standard hormone therapy and, just as importantly, which ones will not.

The study went a step further by looking at what could be done for the patients whose cancer was resistant to hormone therapy. The researchers tested the resistant organoids against other types of cancer drugs, including chemotherapy and targeted therapies. They found that even though the hormone drug failed, the resistant organoids reacted differently to the other medicines. Some were sensitive to certain chemotherapy drugs, while others responded to different targeted treatments. This indicates that the system does more than just confirm failure; it opens a door to finding a new, effective treatment for each individual. By watching how a patient's specific cancer cells react to a menu of different drugs, doctors could potentially skip the trial-and-error phase of treatment and move directly to the therapy that works.

This work represents a shift from guessing based on static genetic data to observing dynamic, living responses. The researchers did not claim to have cured breast cancer or to have a perfect solution for every patient. They acknowledged that their models were grown from cells that had been passed down in the lab for some time, and that the ideal scenario would be to test fresh cells directly from a patient. However, they established that by simply adjusting the chemical environment, they could unlock the ability to see how estrogen-dependent cancers truly behave. The new medium provides a reliable way to screen for resistance and explore personalized alternatives, offering a clearer path forward for treating a disease that has long been complicated by unpredictable responses to standard care.