Phase II study of pembrolizumab plus nab-paclitaxel in Hormone Receptor–positive, HER2- negative metastatic breast cancer: clinical and molecular insights into response

Breast cancer is not a single disease but a collection of different conditions, each behaving in its own way. One of the most common types is driven by hormones, specifically estrogen or progesterone, which act like fuel for the cancer cells to grow. Doctors have long known how to cut off this fuel supply using hormone-blocking drugs, and for many patients, this approach works well for years. However, when the cancer returns and spreads to other parts of the body, it often learns to ignore these drugs. In recent years, scientists have turned to a different strategy called immunotherapy, which trains the body's own immune system to recognize and attack cancer cells. This approach has been a major success for a different type of breast cancer that lacks hormone receptors, but it has struggled to work in the hormone-driven kind. The reason is that these hormone-positive tumors are often quiet places for the immune system; they do not attract many immune cells, making it difficult for the body to see the cancer as a threat.

A team of researchers at New York University set out to see if they could wake up the immune system in these quiet tumors by combining a standard chemotherapy drug with an immunotherapy agent. They chose a specific chemotherapy called nab-paclitaxel, which is known to be effective and does not require heavy steroid premedication that might suppress the immune system. They paired it with pembrolizumab, a drug that removes the brakes on the immune system, allowing it to attack more freely. The study involved twenty patients with metastatic hormone-positive breast cancer who had already tried other treatments. The researchers did not just give the drugs and wait to see if the tumors shrank; they also took small tissue samples from the tumors at different times during treatment. This allowed them to watch the microscopic world inside the cancer change in real time, looking for clues about why some patients responded while others did not.

The results showed that the combination was safe and manageable, with no unexpected severe side effects, but the overall success rate was modest. Out of the nineteen patients who could be evaluated, five saw their tumors shrink significantly, and seven saw their disease remain stable for a period of time. The remaining patients saw their cancer continue to grow despite the treatment. On average, the time before the cancer started to grow again was about four months, and the average time patients lived after starting the treatment was about fifteen and a half months. While these numbers are not a cure, they are comparable to what other studies have found when using chemotherapy alone or with other drugs for this specific type of cancer. The researchers noted that the treatment worked particularly well for a small group of patients who had previously taken a class of drugs called CDK4/6 inhibitors, suggesting that the history of a patient's treatment might matter more than the tumor's appearance at the start.

To understand why the treatment worked for some and not others, the scientists looked deep inside the tumor samples. They examined the immune cells living within the cancer tissue and the genetic messages the tumor cells were sending. They found that simply having immune cells present at the start was not enough to predict who would get better. Instead, the key seemed to be how the tumor reacted to the treatment. In the patients who responded well, the treatment appeared to change the environment inside the tumor, making it more welcoming to the immune system. The cancer cells in these patients seemed to have a genetic profile that allowed them to adapt to the immune attack. In contrast, the tumors that continued to grow were often dominated by signals that told the cells to divide rapidly and resist death, essentially overpowering the immune system's attempts to fight back.

One of the most interesting discoveries came from looking at the diversity of the immune cells. The researchers found that patients who lived longer had a higher fraction of T-cells, a type of immune soldier, in their tumors at the very beginning of the study. This suggests that the baseline state of the immune system inside the tumor is a crucial factor. However, the study also showed that the immune system alone was not enough. Even when the treatment successfully activated the immune cells, if the tumor cells were too aggressive or had strong internal programs for survival, the attack failed. It was like trying to stop a runaway train with a small push; the push might be strong, but if the train is too heavy and fast, it will not stop. The researchers concluded that for immunotherapy to work in hormone-positive breast cancer, it likely needs to be combined with other strategies that slow down the tumor's rapid growth and weaken its defenses, rather than relying on the immune system alone.

The study did not find a single simple test that could predict who would benefit from this specific combination of drugs. While some patients had high levels of a protein called PD-L1, which is often used as a marker for immunotherapy success, many of the patients who responded well actually had low levels of this protein at the start. This indicates that the usual markers used for other types of cancer do not work well here. The researchers emphasized that their findings are exploratory, meaning they offer new ideas and directions for future research rather than definitive answers. They suggest that future trials should focus on selecting patients based on a more complete picture of their tumor's biology, looking at both the immune environment and the tumor's own genetic drive to grow. Until such methods are developed, using immunotherapy broadly for all patients with this type of breast cancer is not supported by the evidence, but the door remains open for a carefully selected group of patients who might still benefit.