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Study of tamoxifen loading dose and Ki67 response in breast cancer patients before surgery (ENDO-67)

Verified siteRegistered drugNo placebo
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What is this trial about?

A plain-language summary of the goals, design and what participants do

This study focuses on patients with breast cancer that is hormone receptor-positive and HER2-negative. The research examines how the medication tamoxifen, which is used to treat breast cancer, works in patients before they undergo surgery. Tamoxifen is taken as a tablet by mouth and belongs to a group of medications called oncolytic agents that help fight cancer.

The main purpose of this research is to understand how quickly tamoxifen becomes effective in the body and how it affects certain markers in the tumor tissue. The study will investigate whether a shorter treatment period of three weeks can be as effective as the standard twelve-week treatment period. During the study, participants will receive tamoxifen tablets daily, with a maximum daily dose of 40 mg.

The research involves taking small samples of tumor tissue to measure a protein called Ki67, which helps determine how fast cancer cells are growing. The study will also measure levels of endoxifen (a substance that tamoxifen is converted into in the body) and monitor how the treatment affects hormone levels in the blood. Participants will be asked to complete questionnaires about their symptoms and overall well-being during the treatment period.

The research process

The trial runs in 6 steps – from screening to follow-up. Each step says what happens and what the team monitors.

  1. Step 1

    Initial treatment phase

    You will begin taking tamoxifen medication orally for three weeks before your scheduled breast surgery

    The medication will be administered according to a specific dosing schedule to achieve optimal blood levels

    The standard dose is 20mg of tamoxifen taken by mouth daily

  2. Step 2

    Tumor assessment - first stage

    Multiple samples (biopsies) of your breast tumor will be taken to measure a protein called Ki67

    The Ki67 measurement helps determine how your tumor is responding to the treatment

  3. Step 3

    Blood tests - first stage

    Blood samples will be collected to measure the levels of endoxifen (a substance that tamoxifen converts to in your body)

    Hormone levels (estradiol and progesterone) in your blood will also be measured

  4. Step 4

    Quality of life assessment

    You will complete a questionnaire called FACT-ES about your experience with the treatment

  5. Step 5

    Continued treatment phase

    After surgery, you will continue taking tamoxifen for three months

    The standard dose of 20mg daily will be maintained during this period

  6. Step 6

    Final assessments

    After three months, final blood tests will be performed to measure endoxifen levels

    Hormone levels will be measured again

    You will complete another FACT-ES questionnaire

Who can join the trial?

10 criteria

  • Patient must have newly diagnosed breast cancer that is:
    • At least 1.5 cm in size (T1c or larger)
    • Positive for estrogen receptors (ER+)
    • Negative for HER2 protein (HER2-)
    • Has a Ki-67 level of 10% or higher (Ki-67 is a protein that indicates how fast cancer cells are growing)
    • The tumor must still be in the breast (not yet surgically removed)
    • Must be 18 years or older
    • Must be planned to receive hormone therapy after surgery
    • Must have good overall health status (WHO performance status of 0-2, meaning able to carry out most daily activities)
    • The breast tumor must be suitable for taking multiple tissue samples
    • Must be suitable for treatment with tamoxifen (a type of hormone therapy medication)

Who cannot join the trial?

13 criteria

  • Patients with known metastatic disease (cancer that has spread to other parts of the body)
  • Previous chemotherapy or hormonal therapy within the last 6 months
  • Currently taking medications that interact with tamoxifen (including certain antidepressants)
  • History of blood clotting disorders or deep vein thrombosis
  • Severe liver disease or abnormal liver function
  • Pregnancy or breastfeeding
  • Known hypersensitivity to tamoxifen or its components
  • Participation in another clinical trial within the last 30 days
  • Psychiatric conditions that could interfere with study compliance
  • Uncontrolled medical conditions (such as diabetes or high blood pressure)
  • History of other malignancies within the past 5 years (except adequately treated non-melanoma skin cancer)
  • Unable to provide informed consent
  • Unable to follow study procedures or attend scheduled visits
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Investigated drugs

  • Tamoxifen

    is a hormone therapy medication used in breast cancer treatment. It works by blocking the effects of estrogen in breast tissue. This medication helps prevent breast cancer cells from growing and dividing. It belongs to a class of drugs called selective estrogen receptor modulators (SERMs).

  • Endoxifen

    is an active metabolite that forms in the body after taking tamoxifen. It is considered one of the most important compounds that makes tamoxifen effective in treating breast cancer. The body creates endoxifen when it processes tamoxifen, and the levels of endoxifen in the body are important for the medication's effectiveness.

What is already known about the treatment

  • Tamoxifen

    An oral medication used primarily in the treatment and prevention of breast cancer, particularly in estrogen receptor-positive cases. This selective estrogen receptor modulator (SERM) works by blocking estrogen receptors in breast tissue, effectively preventing estrogen from stimulating the growth of hormone-dependent breast cancer cells. Tamoxifen is typically administered as a daily tablet, and while it can take several weeks to reach optimal therapeutic levels in the body, research is exploring ways to achieve effective concentrations more quickly through modified dosing schedules. The medication has been a cornerstone of breast cancer treatment for decades and continues to be extensively studied in clinical trials to optimize its therapeutic benefits.

  • Endoxifen

    A key active metabolite of tamoxifen that is formed in the body after tamoxifen administration and is considered to be primarily responsible for the drug's anti-cancer effects. Endoxifen works by binding to estrogen receptors more potently than its parent compound tamoxifen, making it particularly effective at blocking estrogen's ability to stimulate breast cancer cell growth. Current research is focusing on measuring intratumoral endoxifen levels and their correlation with treatment response, as indicated by changes in cellular markers like Ki67, to better understand and predict treatment effectiveness in individual patients.

Investigated diseases

Breast cancer - A disease that occurs when cells in the breast tissue begin to grow abnormally and divide more rapidly than healthy cells. It typically starts in the cells of the lobules (milk-producing glands) or the ducts (passages that drain milk from the lobules to the nipple). As the cancer grows, cells can invade nearby healthy breast tissue and can travel through the lymphatic system or bloodstream to other areas of the body. The disease can develop in both women and men, though it's much more common in women. Breast cancer may present as a painless lump, changes in breast size or shape, or changes in the skin over the breast.
Trial detailsLast updated 2 Oct 2026
Age18+ yearsPhasePhase IIITrial ID2025-521385-10-00Estimated enrolment175 patientsSponsorErasmus Universitair Medisch Centrum Rotterdam (Erasmus MC)

sourced from the EU Clinical Trials Register and site verification

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