Impact of Omitting Chemo Based on Patient's Selection for ER-Positive, HER2-Negative Breast Cancer With Ribociclib and Endocrine Therapy

400 mg (2 x 200 mg tablets by mouth) once daily on days 1 to 21 of a 28-day cycle, followed by seven days off ribociclib (Days 22 to 28).

Letrozole will be administered as an endocrine therapy. The regimen will differ depending on the demographic of the patient. For postmenopausal women: Letrozole 2.5 mg by mouth daily continuously. For men or premenopausal women: Letrozole 2.5 mg by mouth daily continuously.

For postmenopausal women: Letrozole 2.5 mg by mouth daily continuously or anastrozole 1 mg by mouth daily continuously. For men or premenopausal women: Letrozole 2.5 mg by mouth daily continuously or anastrozole 1 mg by mouth daily continuously.

For men or premenopausal women: Letrozole 2.5 mg by mouth daily continuously or anastrozole 1 mg by mouth daily continuously, concurrently with goserelin 3.6 mg monthly injection.

Adjuvant chemotherapy dose modifications in Arm 2 will be per institutional guidelines and investigator discretion.