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

Inclusion Criteria:

- Women (regardless of menopausal status) or men ≥18 years of age (or per local
regulations).

- The participant has confirmed HR+, HER2-, early stage resected invasive breast cancer
without evidence of distant metastases.

- The participant must have undergone definitive surgery of the primary breast tumor.

- 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:

- 4 or more positive axillary lymph nodes

- Tumor size of at least 5 centimeters

- Grade 3 defined as at least 8 points on the Bloom Richardson grading system

- Ki-67 index by central analysis of ≥20% on untreated breast tissue

- The participant must be randomized within 16 months from the time of definitive breast
cancer surgery.

- 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.

- Participants must have recovered (grade ≤1) from the acute effects of chemotherapy and
radiotherapy and from surgical side effects following definitive breast surgery.

- Women of reproductive potential must have a negative blood pregnancy test and agree to
use highly effective contraceptive methods.

- The participant has a Eastern Cooperative Oncology Group (ECOG) performance status ≤1.

- The participant has adequate organ function.

- The participant is able to swallow oral medications.

Exclusion Criteria:

- Metastatic disease (including contralateral axillary lymph nodes) or node-negative
disease.

- Participants with inflammatory breast cancer.

- 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.

- Females who are pregnant or lactating.

- The participant has previously received treatment with any CDK4 and CDK6 inhibitor.

- The participant is receiving concurrent exogenous reproductive hormone therapy (for
example, birth control pills, hormone replacement therapy, or megestrol acetate).

- The participant has previously received endocrine therapy for breast cancer prevention
(tamoxifen or aromatase inhibitors) or raloxifene.

- The participant has serious preexisting medical condition(s) that, in the judgment of
the investigator, would preclude participation in this study.

- 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).

- The participant has active systemic infections or viral load.

- The participant has received an experimental treatment in a clinical trial within the
last 30 days or 5 half-lives, whichever is longer.