Some targeted therapies used to treat breast cancer may raise the risk of thrombosis (blood clots) by as much as 20 percent, according to new research. Some medicines called PI3K and mTOR inhibitors had the highest risks. These drugs help to stop cancer by blocking the way it grows.

Blood clots are small clumps of blood that form blockages in blood vessels and can occur anywhere in the body. Certain targeted therapies may contribute to clot formation by affecting blood vessel health or increasing inflammation in the body. They can be serious, but many are treatable when found early.

In this large study, researchers analyzed real-world data from more than 13,000 adults with breast cancer who were taking targeted therapies, and compared the data with more than 776,000 people with breast cancer who were not on those treatments.

They found that people taking targeted therapies — including CDK4/6, PARP, mTOR, and PI3K inhibitors — had higher rates of both arterial clots (like heart attacks and strokes) and venous clots (like deep vein thrombosis or pulmonary embolism). In comparison, people with breast cancer who were not on these drugs had lower rates of clotting events.

Overall, the rate of blood clots was:

• 13.96 percent of people taking CDK4/6 inhibitors (ribociclib, palbociclib, abemaciclib)
• 19.5 percent of those on the mTOR inhibitor everolimus
• 20.7 percent of those on the PI3K inhibitor alpelisib
• 7.5 percent of those taking the PARP inhibitor olaparib

Those taking olaparib experienced an increase in risk for venous clots but not for arterial clots.

Researchers found the risks were higher for people with:

• A past history of blood clots
• Brain metastases (cancer that has spread to the brain)
• Nicotine dependence

Ways To Reduce Blood Clot Risks

The study’s findings suggest that the risk of thrombosis may be higher than previously reported in clinical trials — especially in real-world settings, where people often have other risk factors or complex medical histories.

Researchers suggest that healthcare providers may want to consider these risks when prescribing certain targeted therapies. People with higher risk factors for blood clots might benefit from closer monitoring or even preventive treatment with anticoagulants.

Symptoms of blood clots to watch for include swelling, cramping, or soreness in the leg, skin color changes, and a warm feeling in the leg that’s affected. More serious symptoms that warrant seeking emergency medical assistance include shortness of breath, chest pain, or feelings of dizziness.

If you’re currently on a targeted therapy for breast cancer or considering one, it may be helpful to talk with your doctor about your risk of blood clots. Let them know if you’ve had any prior clotting issues or other conditions that may raise your risk.

Learn more about targeted therapies for breast cancer, including how they work and what other side effects they may cause.

Find Your Team

On MyBCTeam, the social network for people living with breast cancer and their loved ones, members come together to ask questions, give advice, and share their stories with others who understand life with breast cancer.

Have you been treated with one of these targeted therapies? Did your doctor discuss blood clot risks with you? Share your experiences in a comment below, start a conversation on your Activities page, or connect with like-minded members in Groups.