Tamoxifen Side Effects Linked to Uterine Tumors in Breast Cancer Patients: Study

Widely used breast cancer treatment has been found to activate certain forms of cancer in the uterus.

A new study suggests that side effects of the breast cancer drug tamoxifen may trigger a cellular process that increases the risk of uterine cancer in some users.

The findings were published in the journal Nature Genetics on August 22, following up on a well-established connection between use of tamoxifen and uterine cancer side effects.

Tamoxifen has been used in the U.S. since the late 1970s to block estrogen and slow or prevent the growth of breast cancer. It remains a standard treatment for breast cancer patients and is also prescribed to people at high risk of developing the disease.

However, over the last two decades, evidence has linked tamoxifen side effects to an increased risk of certain uterine cancers, which prompted the U.S. Food and Drug Administration to add a black box warning to the drug’s label in 2002, alerting patients about the potential for those risks.

In the new study, researchers studied molecular effects that may explain tamoxifen’s uterine cancer risk by analyzing 60 cases of tamoxifen-associated uterine cancer (TA-UC) from the Tamoxifen Associated Malignancies: Aspects of Risk (TAMARISK) study, and comparing them with a control group of uterine cancer patients with no tamoxifen use. 

The research team, led by Kirsten Kübler, focused primarily on the cancer-related PI3KCA gene, which can cause uterine cancer by activating what is known as the PI3K signalling pathway, a main driver in the development of these types of cancers.

The data showed that with tamoxifen, PI3KCA mutations are not required for cancerous cells to develop. Instead, the drug itself becomes a signal activator for the PI3K pathway.

Kübler’s team also examined the effects of tamoxifen on living mice, confirming the drug activated PI3K pathways in uterine tissue.

“While this PI3K mechanism is specific to tamoxifen-associated uterine cancer, the concept of treatment-induced signaling events may have broader applicability to other routes of tumorigenesis.”

– Kirsten Kübler, Tamoxifen induces PI3K activation in uterine cancer

The researchers cautioned that these findings do not mean patients should stop taking tamoxifen, only that physicians need to monitor the risks more closely.

Breast Cancer Lawsuits

While tamoxifen has raised concerns over potential side effects, other treatments used in breast cancer care have also come under scrutiny in recent years. Many women who underwent breast cancer surgery received a now recalled BioZorb tissue marker, which is an implantable device used to mark the site of tumor removal.

Over time, BioZorb markers are supposed to dissolve and resorb into the body, leaving only the clips to assist in long-term monitoring and future medical procedures. However, many breast cancer patients have found that their bodies have not reabsorbed the tissue marker, causing pain, infection, rash, erosion, seromas and other complications from the device potentially migrating, requiring the need for additional medical treatment and surgical removal.

Due to the severity and frequency of these complications, the FDA issued a Class I recall of BioZorb implantable breast tissue markers in October 2024, warning that continued use could cause serious injury or death.

Complications linked to tissue markers have led many breast cancer survivors to file BioZorb implant lawsuits against the manufacturer, Hologic, alleging the devices have dangerous design defects that were not properly disclosed to patients or the medical community.

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