FLASH-Breast: Evaluating the Efficacy of Fezolinetant in Reducing Vasomotor Symptoms in Women With Breast Cancer on Endocrine Therapy

Inclusion Criteria: Women with diagnosed, histologically confirmed, clinical stage I-III, HR+ invasive breast cancer as defined by ASCO CAP guidelines for whom adjuvant endocrine therapy would be indicated, regardless of HER2 status. Age 18 years and older. Currently on endocrine therapy (tamoxifen or aromatase inhibitors) as part of standard of care treatment for HR+ breast cancer. Endocrine therapy may be administered with or without ovarian suppression and/or CDK4/6 inhibitors as permitted in Section 7.1.1. Participants with HER2-positive disease are eligible and may receive concurrent HER2 monoclonal antibody therapy (e.g., trastuzumab or pertuzumab), but not HER2 antibody-drug conjugates (e.g., ado-trastuzumab emtansine T-DM1, or trastuzumab deruxtecan T-DXd). Willing and able to provide written informed consent/assent for the trial. Postmenopausal as defined by spontaneous amenorrhea for at least 12 consecutive months, spontaneous amenorrhea for at least 6 months with biochemical criteria or menopause (FSH \> 40 IU/L), or bilateral oophorectomy for at least 6 weeks before the screening visit, or if premenopausal chemically suppressed by GnRH agonist therapy with ultrasensitive estradiol level \<10. On endocrine therapy for a minimum of 3 months and has planned duration of 12 weeks left in the treatment regimen. Participants receiving a CDK4/6 inhibitor must have been on treatment for at least 4 weeks prior to enrollment. Experiencing an average of seven or more moderate to severe hot flashes per day over a 7-day period as documented by Symptom Diary during the Screening Period and seeking treatment or relief for VMS. Able to swallow oral formulation of the study agent. Exclusion Criteria: Participants who have a diagnosis of stage IV metastatic disease. Receiving any other cancer treatment other than endocrine therapy. This includes chemotherapy, targeted therapies, and immunotherapy. Receiving cytochrome CYP1A2 inhibitors. Participants who have initiated, discontinued, or had a dose adjustment of treatment for vasomotor symptoms (prescription, over the counter, or herbal) within 28 days prior to screening. Pregnant or lactating patients. Known cirrhosis or active liver disease, jaundice, or elevated liver aminotransferases (ALT or AST) \>2x ULN, or elevated total bilirubin, OR elevated direct bilirubin, or elevated INR, or elevated alkaline phosphatase \>2x ULN. Creatinine \> 1.5 times upper limit of normal; or estimated GFR ≤ 30 mL/min per 1.73 m2 at screening. Judgement by the investigator that the participant should not participate in the study if the participant is unlikely to comply with study procedures, restrictions, and requirements.