Study of Belzutifan (MK-6482) Plus Fulvestrant for ER+/HER2- Metastatic Breast Cancer (MK-6482-029/LITESPARK-029)

Inclusion Criteria:

* Has a diagnosis of estrogen receptor positive (ER+)/human epidermal growth factor receptor negative (HER2-) invasive breast carcinoma that is either locally advanced disease not amenable to resection or metastatic disease not treatable with curative intent
* Has documented radiographic confirmation of disease progression during or after the last administered endocrine therapy (ET)
* Provides additional tissue from the same sample used to determine ER and HER2 status locally
* Has received ET in the noncurative setting and has 1) Radiographic disease progression on 12 months or more of ET in combination with CDK4/6 inhibitor in the noncurative setting or 2) Received at least 2 lines of ET in the noncurative setting including CDK4/6 inhibitor where the CDK 4/6 inhibitor was discontinued due to intolerance
* Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 assessed within 7 days of randomization
* Participants who have AEs due to previous anticancer therapies must have recovered to ≤Grade 1 or baseline. Participants with endocrine-related AEs who are adequately treated with hormone replacement or participants who have ≤Grade 2 neuropathy are eligible
* Participants who are hepatitis B surface antigen (HBsAg) positive are eligible if they have received hepatitis B virus (HBV) antiviral therapy for at least 4 weeks prior to the first dose of study intervention and have undetectable HBV viral load prior to randomization

Exclusion Criteria:

* Has Breast cancer amenable to treatment with curative intent
* Is unable to receive any of the endocrine therapies (ETs) (ie, fulvestrant or exemestane)
* Has known difficulty in tolerating oral medications, unable to swallow orally administered medication, or conditions which would impair absorption of oral medications such as uncontrolled nausea or vomiting (ie, CTCAE =Grade 3 despite antiemetic therapy), ongoing gastrointestinal obstruction, motility disorder, malabsorption syndrome, or prior gastric bypass
* Has advanced/metastatic, symptomatic visceral spread at risk of rapidly evolving into life-threatening complications
* Has active, bleeding diathesis, or on oral anti-vitamin K medication
* Has history of noninfectious pneumonitis/interstitial lung disease including radiation pneumonitis that required steroids or has current pneumonitis/interstitial lung disease
* Has a known germline BRCA mutation (deleterious or suspected deleterious) and has received previous treatment with poly-ADP ribose polymerase (PARP) inhibition either in the adjuvant or metastatic setting
* Has received prior fulvestrant in the adjuvant, unresectable locally advanced, or metastatic setting
* Has received any line of cytotoxic chemotherapy or PARP inhibitor in the unresectable or noncurative advanced/metastatic setting
* Has received prior radiotherapy for non-central nervous system (CNS) disease or required corticosteroids for radiation-related toxicities including radiation pneumonitis, within 14 days of the first dose of study intervention
* Is currently receiving either a strong inhibitor or inducer of CYP3A4 that cannot be discontinued for the duration of the study
* Has received prior systemic anticancer therapy including investigational agents within 4 weeks before randomization
* Has received a live or live-attenuated vaccine within 30 days before the first dose of study intervention
* Has concurrent active Hepatitis B and Hepatitis C virus infection
* Has clinically significant cardiac disease, including unstable angina, acute myocardial infarction within 6 months from Day 1 of study medication administration, or New York Heart Association Class III or Class IV congestive heart failure
* Has not adequately recovered from major surgery or have ongoing surgical complications