Abemaciclib and Letrozole in Treating Patients With Endometrial Cancer

PRIMARY OBJECTIVES: I. To determine whether there are changes in Ki-67 expression from the pretreatment specimen (e.g. biopsy or dilation and curettage \[D\&C\]) to the post-treatment hysterectomy specimen following treatment with letrozole and abemaciclib. SECONDARY OBJECTIVES: I. To determine the proportion of tumors with complete cell cycle arrest (CCCA) response as measured by Ki-67 expression between the pre-treatment tumor and the posttreatment tumor. II. To identify biological characteristics of tumors (e.g. mismatch repair \[MMR\] status, PTEN mutational status, etc.) correlating with decreased Ki-67 expression induced by the letrozole and abemaciclib combination. III. To determine the frequency of adverse events associated with use of abemaciclib and letrozole. EXPLORATORY OBJECTIVES: I. To evaluate the correlation of the expression of Ki-67 with that of cyclin D1, p16, pRB, and PTEN as well as with MMR deficiency. OUTLINE: Patients receive letrozole orally (PO) once daily (QD) and abemaciclib PO twice daily (BID) on days 1-14. Patients then undergo standard of care hysterectomy on day 15. After completion of study treatment, patients are followed up at 30 days and at 2 and 6 weeks after surgery.