AuthorisedTherapeutic use (Phase IV)Interventional

ENDO-67 project: Investigating ENDOcrine therapy-related questions using Ki67 changes in the window-of-opportunity before breast surgery

This study, called ENDO-67, is for people with breast cancer who are taking tamoxifen, a common hormone treatment. Researchers are investigating how well tamoxifen works by looking at changes in a marker in cancer cells called Ki67, both before and after a short period of treatment. They also want to understand the levels of a substance called endoxifen in the blood and within the tumour itself. By doing this, they hope to learn more about why tamoxifen works differently for some people and how to predict if a patient might be resistant to it. The aim is to improve breast cancer treatment in the future.

At a glance

Status

Authorised

Phase

Therapeutic use (Phase IV)

Sponsor

Erasmus Universitair Medisch Centrum Rotterdam (Erasmus MC)

Enrolment target

175

Start

21 Oct 2025

Breast cancer

What is this study about?

This study, called ENDO-67, is trying to find out more about how a breast cancer medicine called tamoxifen works. Tamoxifen is often used to treat certain types of breast cancer by blocking hormones that can help cancer grow. Before surgery, some patients take tamoxifen for a short time – this is often called a 'window of opportunity' because it allows doctors to see how the cancer reacts to the drug.

The researchers are particularly interested in two things: a substance in the body called endoxifen, and a marker in cancer cells called Ki67. When you take tamoxifen, your body changes it into endoxifen, which is the active form of the drug. The Ki67 level tells doctors how quickly cancer cells are growing. By measuring endoxifen levels and seeing how Ki67 levels change after taking tamoxifen, the study aims to understand why tamoxifen is very effective for some people but not as much for others.

Ultimately, this research could help doctors better understand who will benefit most from tamoxifen and identify early on if someone might need a different treatment. This could lead to more personalised and effective care for people with breast cancer.

Key takeaways

The study explores how tamoxifen, a breast cancer drug, works in your body.
Researchers are looking at endoxifen levels and cancer cell growth (Ki67).
It helps understand why tamoxifen works differently for different people.
Participation involves taking tamoxifen, blood tests, and a small tumour sample.
The study aims to improve future breast cancer treatments.
It lasts about three months and focuses on treatment before surgery.

Who may be eligible?

To take part in this study, you need to be an adult, aged 18 years or older. There is no upper age limit, meaning people of all adult ages can be considered. This study is open to both men and women.

The study is specifically for people who are receiving treatment for breast cancer. The researchers are looking at how a specific medication, tamoxifen, works before surgery. So, you would need to be in a situation where tamoxifen is part of your treatment plan before having surgery.

More detailed information about your specific type of breast cancer and other health conditions would be discussed with you by the study team to make sure this study is the right fit for you.

Could this study suit you?

Answer these quick questions to see if you may be eligible. This is a guide only — the research team makes the final call.

Are you 18 years old or older?
Are you being treated for breast cancer?
Is tamoxifen part of your treatment plan before surgery?
Are you willing to have blood tests and a small tumour sample taken?

Answer every question to see your result.

What does participation involve?

If you decide to take part, you would receive the standard dose of tamoxifen (20mg) continuously for three months, as part of your normal treatment. During this time, the study team would take some blood samples to check your endoxifen levels at different points. They would also take a small sample of your tumour (a biopsy) after three weeks of taking tamoxifen to look at the Ki67 levels and other measurements within the tumour.

The study also involves checking the levels of other hormones like oestradiol and progesterone in your blood after three weeks and three months. You would also fill out a questionnaire called FACT-ES, which asks about your quality of life, to see how you are feeling during the treatment. The total duration of active participation in the study for you would be around three months, aligning with your tamoxifen treatment before surgery.

Potential risks and benefits

Participating in this study might help us learn more about how tamoxifen works, which could benefit future patients with breast cancer. You'll also receive careful monitoring of your treatment. However, it's possible that taking part won't directly improve your own health. As with any medical treatment or procedure, there could be some risks, such as discomfort from blood tests or biopsies. You are free to withdraw from the study at any time, for any reason, without it affecting your medical care.

Locations (1)

Some site locations are approximate. We're improving this — please verify with the trial team before travelling.

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Unverified
Netherlands

Common questions

What is Ki67 and why is it important?

Ki67 is a marker that shows how quickly cancer cells are dividing and growing. It helps doctors understand how active the cancer is and how it might respond to treatment.

What is endoxifen?

When you take tamoxifen, your body changes it into an active substance called endoxifen. This is the part of the drug that helps to fight the cancer.

Will I get a new drug in this study?

No, this study is looking at how a standard and commonly used breast cancer drug, tamoxifen, works. You won't be given an experimental drug.

Will this study change my usual breast cancer treatment?

The study aims to understand your response to tamoxifen as part of your existing treatment plan. Any changes to your overall cancer care would be discussed by your medical team separate from the study.

What is a 'window-of-opportunity' study?

This means the study takes place in a short period, typically before surgery, to see how the cancer responds to a treatment like tamoxifen. It helps researchers gather information quickly.