A Study of Elacestrant Alone or in Combination With Abemaciclib in People With Endometrial Cancer

The researchers are doing this study to find out whether elacestrant is an effective and safe treatment alone or in combination with abemaciclib for people with advanced or recurrent ER+ endometrial cancer.

Inclusion Criteria: * Age ≥18 years at the time of informed consent. * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 * Patients must have received previous platinum-based chemotherapy and treatment with a PD-1 inhibitor, together or separately, prior to enrolling on this trial. * Patients may have received no more than 2 prior lines of therapy for management of endometrial carcinoma. This includes platinum-based chemotherapy alone or combined with a PD-1 inhibitor, small molecule agents, and chemotherapy in combination with radiation therapy. A washout period of 14 days is required for chemotherapy °Adjuvant chemotherapy completed ≥ 12 months prior will not be counted toward prior therapy * Patients may have received ≤ one prior line of endocrine therapy for management of endometrial carcinoma. * No prior treatment with a CDK4/6 inhibitor * Measurable disease per RECIST v 1.1 criteria °Tumors within a previously irradiated field will be designated as "non-target" lesions unless progression is documented, or a biopsy is obtained to confirm persistence of tumor ≥ 90 days following completion of radiation therapy. * Advanced or recurrent endometrial carcinoma that is refractory to curative therapy. * Patients with the following histologic epithelial cell types are eligible: endometrioid adenocarcinoma, serous adenocarcinoma, undifferentiated carcinoma, de-differentiated, clear cell adenocarcinoma, mixed epithelial carcinoma, adenocarcinoma not otherwise specified (N.O.S.), mucinous adenocarcinoma, squamous cell carcinoma, and transitional cell carcinoma. * Patient must have ER-positive tumor status either from the most recent sample of advanced/recurrent disease or from an archival tissue. °Documentation of ER-positive tumor with ≥ 1% staining by IHC as defined in the 2010 or 2020 American Society for Clinical Oncology recommendations for ER testing (Hammond 2010, Allison 2020) * p53 wt by IHC or TP53 wt by next generation sequencing platform either from the most recent sample of advanced/recurrent disease or from an archival tissue. * No known dMMR or POLE mutation * If MSK IMPACT mutational profiling or mutational profiling performed in a CLIA laboratory is not already performed, must have tissue available for MSK IMPACT molecular profiling to be performed clinically * Female patients may be either postmenopausal or premenopausal or perimenopausal. Postmenopausal status is defined by: * Age ≥ 60 years * Age \< 60 years and amenorrhea for ≥12 months (in the absence of chemotherapy, tamoxifen, toremifene, or ovarian suppression) or a follicle-stimulating hormone value \>40 mIU/mL and an estradiol value \