A Study of Giredestrant in Participants With Grade 1 Endometrial Cancer

Inclusion Criteria: * Confirmed Grade 1 endometrial cancer (EC) of endometrioid histology for which participants are willing to receive 6-cycles of study therapy. An endometrial biopsy (EMB) or dilation and curettage (D\&C) fresh collected within the screening period or archival sample collected within 3 months prior to screening must be provided to a central laboratory for histologic confirmation to determine eligibility. * Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1 * Magnetic resonance imaging (MRI)-confirmation of non-deeply invasive tumor (\<50% myometrial invasion) * MRI or computed tomography (CT)-confirmation of no extrauterine disease * Willing to undergo a minimum of 6 continuous cycles of therapy before decision on surgery * No prior treatment for endometrial cancer * Able and willing to take oral medications * Willingness and ability to comply with scheduled visits, treatment plans, laboratory tests, and other study procedures * Adequate hematologic and end-organ function, as defined in the protocol * Negative HIV test at screening, with the following exception: Patients with a positive HIV test at screening are eligible provided they are stable on anti-retroviral therapy, have a CD4 count ≥200/μL, and have an undetectable viral load. * For female participants of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraception, and agree to refrain from donating eggs, during the treatmen…