A new study has uncovered a previously underappreciated mechanism that may help explain why estrogen receptor-positive (ER+) breast cancer can return years — or even decades — after patients are declared cancer-free.
Researchers found that some cancer cells do not fully shut down under hormone therapy, nor do they behave like rapidly dividing tumor cells. Instead, they enter a slow-growing, therapy-tolerant state that allows them to persist in the body, quietly forming microscopic tumors that retain metastatic potential.
Published in Nature Communications, the work from the Garvan Institute of Medical Research in Sydney, Australia, suggests these cells may represent an alternative route to late recurrence, challenging the prevailing view that relapse is primarily driven by fully dormant cancer cells that later “wake up.”
A long standing clinical blind spot
ER+ breast cancer accounts for around three-quarters of all breast cancers and is typically treated with endocrine therapies such as tamoxifen, aromatase inhibitors, or fulvestrant for five to ten years after surgery.
“We have become very good at treating primary breast cancer, but late relapses remain a major challenge,” Liz Caldon, cancer researcher and senior author of the study, said in the press release.
Up to 30 percent of ER+ breast cancer patients experience relapse, often many years after treatment has ended. Unlike more aggressive subtypes of breast cancer, where recurrence tends to occur within the first two to three years, ER+ disease shows a prolonged risk trajectory, with more than half of recurrences occurring after five years — and some emerging decades later.
This long latency has been attributed to so-called tumor dormancy, where disseminated cancer cells either enter a quiescent state or balance slow proliferation with cell death. But the biology of this process has remained poorly defined, in large part because dormant or slow-cycling cells are difficult to detect and existing experimental models fail to fully replicate long-term disease progression.
Models that miss the biology of late relapse
A major challenge in studying ER+ recurrence is that most preclinical models are optimized for rapid tumor growth rather than long-term persistence. Commonly used breast cancer cell lines are derived from advanced disease and tend to be highly proliferative, making them useful for studying signaling pathways and drug resistance — but less suited to modeling decades-long relapse dynamics.
Mouse xenograft systems also have limitations. ER+ tumors often require estrogen supplementation to grow and rarely produce spontaneous metastases within the animal’s lifespan. More aggressive experimental approaches, such as intracardiac injection, bypass early metastatic steps entirely. Even genetically engineered mouse models, while more physiologically relevant, typically do not recapitulate the bone-dominant, late-emerging metastatic pattern characteristic of ER+ disease.
As a result, the biology of slow-growing disseminated cells has remained largely inaccessible.
A slow growing but highly adaptable cancer state
To address this gap, the researchers isolated and tracked a population of ER+ breast cancer cells that survived endocrine therapy by dividing extremely slowly rather than stopping altogether.
Using a combination of hormone-deprived culture systems and long-term selection, the team generated two distinct therapy-resistant populations: one fast-growing and one slow-growing. Despite their differences in proliferation, both were capable of metastasizing in preclinical models.
“It took years to isolate these specific cells because they were dividing so slowly, almost in defiance of how we typically expect cancer to behave. But once we observed them in action, we realized that a slow clock doesn’t mean a stopped clock,” Kristine Fernandez, senior research assistant in the Caldon Lab and first author of the study, added in the press release.
When implanted into mammary tissue of immunocompromised mice, the slow-growing cells formed smaller primary tumors than their fast-growing counterparts. These tumors showed features often associated with cellular stress or senescence, including enlarged nuclei and increased collagen deposition. Critically, these cells still disseminated to distant organs, including the lung and bone, at similar frequencies to faster-growing cancer cells.
In other words, reduced proliferation did not equate to reduced metastatic potential.
A transcriptional switch linked to therapy resistance
Single-cell RNA sequencing of these cells revealed that endocrine therapy triggers a profound reprogramming of cancer cell states. Rather than arising from a distinct genetic clone, slow-growing cells appeared to emerge from within the broader tumor population in response to treatment.
These cells clustered with a hormone-treated subpopulation showing senescence-like features, but unlike fully senescent cells, they retained limited proliferative capacity. They also displayed altered metabolic and cell-cycle signatures consistent with prolonged survival under stress rather than full arrest.
This suggests that endocrine-tolerant cells may arise from a small subpopulation of cancer cells that enter a senescence-like state during hormone therapy — allowing them to survive treatment while retaining the ability to later drive disease relapse and spread.
A key finding was the activation of Rac1 signaling, a pathway involved in cell motility, survival, and cytoskeletal organization. One of its upstream regulators, P-Rex1 (Phosphatidylinositol 3,4,5-trisphosphate-dependent Rac exchanger 1), was significantly upregulated in slow-growing cells and in clinical datasets of ER+ breast cancer, including late-stage metastatic disease.
Importantly, P-Rex1 expression was associated with poorer outcomes specifically in the late recurrence window, rather than in early relapse, suggesting it may be linked to long-term disease persistence.
“For a long time, the idea that extremely slow-growing cells could drive relapse was just a theory,” said Caldon. “By identifying the pathways that are important in these slow-growing cells we have a new lever to potentially prevent these deadly outcomes.”
Rac1 as a driver of metastatic persistence
Imaging studies confirmed that Rac1 signaling is dynamically activated in endocrine-treated cells. This was observed across complementary systems, including in vivo xenograft models, genetically engineered mouse models, and patient-derived metastatic samples. Despite their differences in origin and complexity, all models showed that Rac1 activity was heterogeneous, with distinct subpopulations showing elevated signaling under therapeutic pressure.
Functional experiments showed that inhibiting the Rac1 pathway reduced cell survival, motility, and tumor burden across preclinical models, particularly when combined with endocrine therapy.
In patient-derived xenografts resistant to multiple lines of endocrine treatment, combining tamoxifen with Rac1 inhibitors led to tumor regression and improved survival compared with either treatment alone.
These findings suggest that Rac1 signaling may represent a key vulnerability in therapy-tolerant ER+ breast cancer cells.
Implications for late recurrence
Clinically, this research suggests that therapies targeting proliferating cells alone may be insufficient to prevent late recurrence, as slow-cycling populations can survive long-term endocrine therapy and later drive metastatic disease.
While Rac1 inhibitors used in this study are not yet clinically established in oncology, they point to a pathway that is already indirectly touched in clinical practice. Retrospective analyses suggest that peri-operative exposure to ketorolac — a commonly used analgesic drug with off-target Rac1 inhibitory activity — may be associated with reduced breast cancer recurrence in some patient cohorts. This is consistent with earlier studies showing that it can slow mammary tumor growth in mouse models and reduce tumor spread and burden in ovarian cancer xenograft models. Overall, this suggests that ketorolac, and other Rac1 inhibitors, could be a potential therapy for limiting metastatic spread and disease burden in ER+ breast cancer.
As researchers refine models that better capture this biology, the work opens new questions about how to detect, monitor, and potentially eliminate these long-lived cancer cell populations before they resurface as metastatic disease.
