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

This research study is studying a combination of targeted therapies as a possible treatment for estrogen-receptor positive (ER+) endometrial cancer. The drugs involved in this study are: * Abemaciclib (also known as Verzenio™) * Letrozole (also known as Femara®) * Metformin (also known as Glucophage®) * Gedatolisib (also known as PF-05212384)

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 \ 150 mmHg or diastolic blood pressure \> 90 mmHg. * For Cohorts 6 and 7: Inability to determine the corrected QT interval using Fridericia's formula (QTcF) on the ECG (i.e., unreadable or not interpretable) or QTcF\>480 msec (determined by mean of triplicate ECGs at screening). * For Cohorts 6 and 7, participants with type 1 diabetes or uncontrolled type 2 diabetes. * For Cohorts 6 and 7, participants who have previously received any PI3K pathway inhibitors are ineligible. Up to 10 patients in Cohort 6 and up to 10 patients in Cohort 7 may have received prior mTOR inhibitor therapy such as everolimus.