One of the standard treatments for women with breast cancer is hormone therapy, but this treatment can cause hot flashes. Hormone replacement therapy, or HRT, is most often prescribed for hot flashes for women in menopause but cannot be given to women on hormone therapy for breast cancer. Fezolinetant, an alternative to HRT, treats hot flashes for women in menopause. As hot flashes happen in the same way for women on hormone therapy for breast cancer, fezolinetant could help these women. In this study, women on hormone therapy for breast cancer who have moderate to severe hot flashes will take part. They will either take fezolinetant or a placebo to treat their hot flashes. The placebo looks like fezolinetant but doesn't have any medicine in it.

The main aim of this study is to confirm if women who take fezolinetant have fewer hot flashes that are less severe compared to women who take the placebo.

Women 18 years or older seeking treatment for hot flashes. They can take part in the study if they have an average of 7 or more moderate to severe hot flashes each day. They are having hormone therapy for breast cancer from stage 0 (cancer cells that have not spread to nearby tissue) up to stage 3+ (the cancer has spread from the breast to the lymph nodes near the breast or the chest wall).

The women will be assigned 1 of 2 study treatments (fezolinetant or placebo) by chance alone. Treatment will be double-blinded. That means that the women in the study and the study doctors will not know who takes which of the study treatments (fezolinetant or placebo). Women who take part in the study will take 1 tablet every day for 52 weeks (1 year). Each woman will be given an electronic handheld device with an app to track their hot flashes on a daily basis. Some women may be able to use the app on their own smartphone. They will also use another device to answer questions about how hot flashes affect their daily life. During the study, the women will visit their study clinic about every 4 weeks for a health check. The last clinic visit will be 3 weeks after the women take their last tablet of study treatment (fezolinetant or placebo). After this visit the women will be called twice to check their health. The women will be in the study for about 2 years.

Inclusion Criteria:

• Participant has a personal history of stage 0-3 hormone receptor positive (HR+), either human epidermal growth factor receptor (HER)-2+ or HER-2- breast cancer; appropriate documentation includes a written or electronic report.

• Participant must be receiving stable maintenance adjuvant endocrine therapy (e.g., tamoxifen or aromatase inhibitors, such as anastrozole, letrozole and exemestane) with or without gonadotropin-releasing hormone (GnRH) agonists/antagonists for a minimum of 4 months prior to randomization and be planning to continue on adjuvant endocrine therapy for the duration of the trial without change to therapy, brand or dose. If the participant is taking GnRH agonists/antagonists, therapy must also be stable for a minimum of 4 months prior to randomization. Add-on therapies for breast cancer adjuvant treatment (e.g., cyclin dependent kinase-4 (CDK4) inhibitors) are allowed.

• Participant has a minimum average of 7 moderate to severe hot flashes (HFs) (vasomotor symptoms (VMS)) per day as recorded in the electronic daily diary (data must be available for at least 7 of the last 10 days prior to randomization).

• Has an European Cooperative Oncology Group (ECOG) score 0 or 1.

• Has at least 12-month life expectation.

• Participant is born female.

• Female participant: Is not pregnant and at least 1 of the following conditions apply:

  • Not a woman of childbearing potential (WOCBP)
  • WOCBP who has a negative urine or serum pregnancy test at screening and day 1 and agrees to follow the contraceptive guidance from the time of informed consent through at least 30 days after final investigational study intervention administration.

• Female participant: Must not be breastfeeding or lactating starting at screening and while the participant is taking investigational study intervention and for 30 days after final investigational study intervention administration.

• Female participant: Must not donate ova starting at first administration of study intervention and while the participant is taking investigational study intervention and for 30 days after final investigational study intervention administration.

• Participant agrees not to participate in another interventional study while participating in the present study until the end of the 1-year extension follow-up period.

• Participant's condition is stable as determined on the basis of medical history and general physical examination, hematology and biochemistry parameters, pulse rate and/or blood pressure and electrocardiogram (ECG) (or showing no clinically relevant deviations obtained within the last 3 months or at screening).

• Participant has no new clinically significant findings on breast examination or from imaging (mammogram, breast ultrasound or equivalent). Results indicate that the participant is a good candidate for the study. Appropriate documentation includes a written or electronic report. In case of double mastectomy, imaging is not needed.

• Participant has a negative serology panel (including hepatitis B surface antigen (HBsAg), hepatitis C virus (HCV) antibody and human immunodeficiency virus (HIV) antibody screens).

Exclusion Criteria:

• Participant has diagnosis of metastatic breast cancer (stage 4).
• Participant has current or history (except complete remission for 5 years or more prior to signing informed consent) of any malignancy except for HR+ breast cancer (stage 0 to 3) or basal cell carcinoma.
• Participant has had surgery or non-surgical (chemotherapy or radiotherapy) treatment for breast cancer within the last 3 months prior to signing informed consent.
• Participant has active liver disease, jaundice, or elevated liver aminotransferases (alanine aminotransferase (ALT) or aspartate aminotransferase (AST)), elevated total bilirubin (TBL) or direct bilirubin (DBL), or elevated alkaline phosphatase (ALP) at screening. A participant with mildly elevated ALT or AST up to < 2 × upper limit of normal (ULN) can be enrolled if TBL and DBL are normal. Participant with mildly elevated ALP (up to < 1.5 × ULN) can be enrolled if cholestatic liver disease is excluded and no cause other than fatty liver is diagnosed. Participant with Gilbert's syndrome with elevated TBL may be enrolled as long as DBL, hemoglobin and reticulocytes are normal.
• Participant has creatinine > 1.5 x ULN; or estimated glomerular filtration rate (eGFR) using the Modification of Diet in Renal Disease formula < 30 mL/min/1.73 m2 at the screening visit.
• Participant has a history of endometrial hyperplasia (participant can be enrolled if she has undergone a hysterectomy) or uterine/endometrial cancer.
• Participant has a medical condition or chronic disease (including history of neurological [including cognitive], hepatic, renal, cardiovascular, gastrointestinal, pulmonary [e.g., moderate asthma], endocrine, or gynecological disease) or malignancy that could confound interpretation of the study outcome.
• Participant uses a prohibited therapy (menopause hormone therapy (MHT), estradiol-containing hormonal contraceptive progestin and progesterone-only medicines, any treatment for VMS [prescription medications, over-the-counter, or herbal] or CYP1A2 (cytochrome P450) inhibitors) or is not willing to wash out such drugs; in addition, medications that are contraindicated due to underlying breast cancer diagnosis and the adjuvant endocrine therapy.
• Participant has a known substance abuse or alcohol addiction within 6 months of screening.
• Participant has received any investigational therapy within 90 days or 5 half-lives, whichever is longer, prior to screening.
• Participant has any condition, which makes the participant unsuitable for study participation.
• Participant has a known or suspected hypersensitivity to fezolinetant, the adjuvant endocrine therapy being used, or any components of the formulations used.

Access to anonymized individual participant level data collected during the study, in addition to study-related supporting documentation, is planned for studies conducted with approved product indications and formulations, as well as products terminated during development. Studies conducted with product indications or formulations that remain active in development are assessed after study completion to determine if Individual Participant Data can be shared. Further details on Astellas' data sharing policy can be found at https://www.clinicaltrials.astellas.com/transparency/.

Access to participant level data is offered to researchers after publication of the primary manuscript (if applicable) and is available as long as Astellas has legal authority to provide the data.

Researchers must submit a proposal to conduct a scientifically relevant analysis of the study data. The research proposal is reviewed by an Independent Research Panel. If the proposal is approved, access to the study data is provided in a secure data sharing environment after receipt of a signed Data Sharing Agreement.