RESOLVE: Abemaciclib + Letrozole +/- Metformin or Gedatolisib in Endometrial Cancer

Inclusion Criteria: * Participants must have cytologically or histologically confirmed endometrial cancer that is recurrent or metastatic and/or resistant to standard therapies, or for which no standard therapy is available. Participants enrolled in the second stage of Cohort 1A, or into Cohort 3, 6 and 7, must have histologically confirmed either i) endometrioid endometrial cancer or ii) endometrial carcinosarcoma with endometrioid epithelial component * Participants must have ER-positive disease, defined as ≥ 1 percent of tumor cell nuclei being immunoreactive by immunohistochemistry (IHC). If multiple analyses have been performed, judgment should be based on the most recent biopsy or pathology specimen analyzed in a CLIA-certified laboratory. * Participants must have measurable disease, defined as at least one lesion that can be accurately measured in at least one dimension (longest diameter to be recorded for non-nodal lesions and short axis for nodal lesions) as ≥20 mm with conventional techniques or as ≥10 mm with spiral CT scan, MRI, or calipers by clinical exam. * Age ≥ 18 years * ECOG performance status of 0 or 1 * Participants must have normal organ and bone marrow function as defined below: * Absolute neutrophil count ≥ 1,500/mcL * Platelets ≥ 100,000/mcL * Total bilirubin ≤ 1.5 × institutional upper limit of normal (ULN). Patients with Gilbert's syndrome with a total bilirubin \</= 2.0 times ULN and direct bilirubin within normal limits are permitted. * …