Endocrine Therapy With or Without Abemaciclib (LY2835219) Following Surgery in Participants With Breast Cancer

Inclusion Criteria:

1. Women (regardless of menopausal status) or men ≥18 years of age (or per local regulations).
2. The participant has confirmed HR+, HER2-, early stage resected invasive breast cancer without evidence of distant metastases.
3. The participant must have undergone definitive surgery of the primary breast tumor.
4. The participant must have tumor tissue from breast (preferred) or lymph node for exploratory biomarker analysis available prior to randomization.

Pathologic lymph node involvement and at least one of the following indicating a higher risk of recurrence:

1. 4 or more positive axillary lymph nodes
2. Tumor size of at least 5 centimeters
3. Grade 3 defined as at least 8 points on the Bloom Richardson grading system
4. Ki-67 index by central analysis of ≥20% on untreated breast tissue
5. The participant must be randomized within 16 months from the time of definitive breast cancer surgery.
6. The participant may receive up to 12 weeks of endocrine therapy until randomization following the last non-endocrine therapy (surgery, chemotherapy, or radiation) whichever is last.
7. Participants must have recovered (grade ≤1) from the acute effects of chemotherapy and radiotherapy and from surgical side effects following definitive breast surgery.
8. Women of reproductive potential must have a negative blood pregnancy test and agree to use highly effective contraceptive methods.
9. The participant has a Eastern Cooperative Oncology Group (ECOG) performance status ≤1.
10. The participant has adequate organ function.
11. The participant is able to swallow oral medications.

Exclusion Criteria:

1. Metastatic disease (including contralateral axillary lymph nodes) or node-negative disease.
2. Participants with inflammatory breast cancer.
3. Participants with a history of previous breast cancer, with the exception of ipsilateral ductal carcinoma in situ (DCIS) treated by locoregional therapy alone ≥5 years ago. Participants with a history of contralateral DCIS treated by local regional therapy at any time may be eligible. Participants with a history of any other cancer (except non-melanoma skin cancer or carcinoma in situ of the cervix), unless in complete remission with no therapy for a minimum of 5 years from the date of randomization are excluded.
4. Females who are pregnant or lactating.
5. The participant has previously received treatment with any CDK4 and CDK6 inhibitor.
6. The participant is receiving concurrent exogenous reproductive hormone therapy (for example, birth control pills, hormone replacement therapy, or megestrol acetate).
7. The participant has previously received endocrine therapy for breast cancer prevention (tamoxifen or aromatase inhibitors) or raloxifene.
8. The participant has serious preexisting medical condition(s) that, in the judgment of the investigator, would preclude participation in this study.
9. The participant has a personal history of any of the following conditions: syncope of cardiovascular etiology, ventricular arrhythmia of pathological origin or sudden cardiac arrest. Any participant with a history of venous thromboembolism (VTE).
10. The participant has active systemic infections or viral load.
11. The participant has received an experimental treatment in a clinical trial within the last 30 days or 5 half-lives, whichever is longer.