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Study of LY3484356 (imlunestrant) with Tamoxifen Citrate and Goserelin Acetate in Premenopausal Women with ER‑Positive, HER2‑Negative Early Breast Cancer

Fast replyRegistered drugNo placebo
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What is this trial about?

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

The trial focuses on early Breast Cancer that is Estrogen Receptor-Positive and HER2-negative in premenopausal women. The investigational oral tablet imlunestrant (LY3484356) is taken daily and is compared with the standard oral pill tamoxifen and with a combination of goserelin given as a subcutaneous injection together with imlunestrant. The study aims to evaluate how these treatments influence tumor cell growth and the occurrence of ovarian cysts.

Participants receive the assigned medication for several months and attend regular clinic visits where blood samples and imaging are performed to monitor the cancer and any side effects. A laboratory test called Ki-67 is used to measure how quickly cancer cells are dividing; a higher level means faster growth. The term ovarian cysts refers to fluid‑filled sacs that can develop on the ovaries and sometimes cause pain or other symptoms. Throughout the study, safety checks and symptom assessments are conducted to ensure participant well‑being.

The research process

The trial runs in 5 steps – from screening to follow-up. Each step says what happens and what the team monitors.

  1. Step 1

    Baseline assessment

    After joining the study, you will undergo a series of baseline examinations. these include a physical exam, blood tests, imaging of the breast, and a biopsy to measure the level of ki-67, a marker of tumor cell growth.

    The results of these tests are used to compare changes that occur during the study.

  2. Step 2

    Treatment assignment

    Based on the study design, you will be placed into one of three groups: a group receiving imlunestrant alone, a group receiving tamoxifen, or a group receiving imlunestrant together with goserelin.

    The specific group is determined by the study randomization process.

  3. Step 3

    Start of medication

    If assigned to the imlunestrant group, you will take one 400 mg tablet by mouth each day.

    If assigned to the tamoxifen group, you will take one 20 mg tablet by mouth each day.

    If assigned to the combination group, you will take the same imlunestrant tablet daily and also receive a 3.6 mg subcutaneous injection of goserelin according to the study schedule.

  4. Step 4

    Regular follow‑up visits

    You will return to the clinic at regular intervals (typically every four weeks) for safety checks, blood tests, and assessment of any side effects.

    During these visits, doctors will perform ultrasound examinations to look for symptomatic ovarian cysts and may repeat the ki-67 measurement to evaluate how the tumor is responding.

    Any adjustments to medication timing or dosage will be made only by the study team according to the protocol.

  5. Step 5

    Completion of treatment and final evaluation

    After the prescribed treatment period ends, you will have a final set of assessments similar to the baseline tests, including imaging, blood work, and a repeat biopsy for ki-67.

    The study will then compile all data to determine the effect of the different treatments on tumor growth and the occurrence of ovarian cysts.

Who can join the trial?

14 criteria

  • Have a breast cancer diagnosis that has been histologically confirmed – a lab test looked at your tissue under a microscope to be sure.
  • The cancer must be Stage I to III, meaning it is still localized and has not spread far beyond the breast.
  • The tumor must be Estrogen Receptor positive (ER+) (grows with estrogen) and HER2‑negative (does not have excess HER2 protein).
  • The cancer must be invasive, which means it has grown out of the original breast ducts or lobules.
  • The tumor must have a Ki‑67 level of at least 10%, a lab measure that shows how quickly the cancer cells are dividing.
  • You must have had the tumor removed (resected) and completed definitive local treatment such as surgery or radiation, with no signs that the cancer has spread to distant parts of the body.
  • You must have already taken at least 4.5 years of any hormone (endocrine) therapy, or at least 2 years of hormone therapy without planning additional ovarian suppression.
  • Be willing and able to provide tumor tissue samples before starting treatment and during the study.
  • Have an ECOG performance status of 0 or 1, which means you are fully active or able to do light work.
  • Be able to swallow capsules or tablets.
  • Be a premenopausal woman (still having menstrual cycles).
  • If you could become pregnant, you must use one highly effective non‑hormonal contraception method while receiving the study medication and for the time required after treatment.
  • Have adequate organ function, meaning your blood, liver, and kidneys are working well enough for the study.
  • Must be female.

Who cannot join the trial?

14 criteria

  • Have bilateral invasive, metastatic, occult primary, or inflammatory breast cancer – meaning cancer that is present in both breasts, has spread to other parts of the body, is hidden, or causes painful swelling of the breast.
  • Have had a prior bilateral oophorectomy or ovarian ablation – meaning both ovaries have been surgically removed or permanently shut down.
  • Have a serious medical condition – a health problem that is severe enough to affect overall safety.
  • Had major surgery within 28 days before randomization – any large operation performed less than four weeks ago.
  • Have a history of another cancer (except non‑melanoma skin cancer, Stage I uterine cancer, or carcinoma in situ of the cervix or other in‑situ cancer) unless it has been completely gone and no treatment has been needed for at least one year.
  • Plan to receive other cancer treatments at the same time that are not part of the study protocol – meaning you would be taking additional anticancer drugs or therapies.
  • Have had any previous treatment for an invasive or non‑invasive breast cancer – meaning you have already received therapy for breast cancer, whether it spread or not.
  • Have had prior radiation therapy to the same‑side chest wall for any cancer – meaning the chest area on the side of the breast has been treated with radiation before.
  • Have received prior anti‑estrogen therapy, including medicines used for osteoporosis or to prevent breast cancer – drugs that block estrogen effects.
  • Have had prior treatment with any GnRH agonist within 12 months before randomization – medications that stop the ovaries from working, given within the last year.
  • Are currently taking exogenous reproductive hormone therapy – hormone pills, patches, or injections that are not naturally produced by your body.
  • Have ovarian cyst(s) larger than 1 centimeter at screening – a fluid‑filled sac on the ovary bigger than a small pea.
  • Completed or stopped prior adjuvant ET (hormone therapy given after initial treatment) more than 6 months before screening.
  • Have received prior therapy with any selective estrogen receptor degrader (SERD) – a drug that removes estrogen receptors from cancer cells.
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Investigated drugs

  • LY3484356

    is an oral tablet taken by mouth. In this study it is being tested as a new medicine that works by blocking the action of estrogen on breast cancer cells. By reducing estrogen signaling, the drug aims to slow down the growth of tumors that need estrogen to grow. Participants will take this medication daily to see how well it can control the cancer compared with other treatments.

  • tamoxifen

    is a well‑known oral medicine that has been used for many years to treat breast cancer. It works by attaching to estrogen receptors on cancer cells and preventing estrogen from stimulating the cells. In this trial, tamoxifen is used as a standard comparison to see if the new drug can do a better job at stopping tumor cells from multiplying.

  • goserelin

    is given as a small injection under the skin. It belongs to a group of medicines that lower the amount of estrogen the body makes by temporarily stopping the ovaries from working. In the study, goserelin is used together with the new drug to see if lowering estrogen production plus blocking its action can more effectively reduce tumor growth in premenopausal women.

What is already known about the treatment

  • LY3484356

    This investigational drug is taken as a 400 mg oral tablet. It is currently being studied in clinical trials and is not yet approved for routine medical use. The medication is designed to treat estrogen‑receptor‑positive breast cancer by binding to the estrogen receptor and causing it to be broken down, which reduces the signal that drives tumor growth. It belongs to the class of selective estrogen receptor degraders (SERDs).

  • Tamoxifen

    Tamoxifen citrate is administered as a 20 mg oral tablet taken by mouth. It is an approved medicine that has been used for many years to treat estrogen‑receptor‑positive breast cancer. The drug works by attaching to estrogen receptors in breast tissue and blocking estrogen from activating them, while acting like estrogen in some other parts of the body. It is classified as a selective estrogen receptor modulator (SERM).

  • Goserelin

    Goserelin acetate is given as a 3.6 mg subcutaneous injection that forms a depot under the skin. It is an approved hormone therapy used to suppress ovarian function in premenopausal breast cancer patients and other hormone‑sensitive cancers. The medication mimics the natural hormone GnRH, initially stimulating and then shutting down the pituitary gland’s release of hormones that lead to estrogen production, thereby lowering estrogen levels. It belongs to the class of GnRH agonists.

Investigated diseases

Breast neoplasm - A breast neoplasm is an abnormal growth of cells in the breast tissue. It usually starts as a small cluster of cells that multiply faster than normal cells. Over time the cluster can become larger and may involve surrounding breast structures. The growth may spread within the breast and can sometimes move into nearby lymph nodes. The condition develops gradually, often without clear symptoms early on.
Trial detailsLast updated 7 Oct 2026
Age18+ yearsPhasePhase IITrial ID2025-523258-15-00Protocol codeJ2J-MC-JZLLEstimated enrolment600 patientsSponsorEli Lilly & Co.

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