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 localregulations).

• The participant has confirmed HR+, HER2-, early stage resected invasive breastcancer 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 forexploratory biomarker analysis available prior to randomization.

• Pathologic lymph node involvement and at least one of the following indicating ahigher 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 definitivebreast cancer surgery.

• The participant may receive up to 12 weeks of endocrine therapy until randomizationfollowing the last non-endocrine therapy (surgery, chemotherapy, or radiation)whichever is last.

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

• Women of reproductive potential must have a negative blood pregnancy test and agreeto 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-negativedisease.

• Participants with inflammatory breast cancer.

• Participants with a history of previous breast cancer, with the exception ofipsilateral ductal carcinoma in situ (DCIS) treated by locoregional therapy alone ≥5years ago. Participants with a history of contralateral DCIS treated by localregional therapy at any time may be eligible. Participants with a history of anyother 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 dateof 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 (forexample, birth control pills, hormone replacement therapy, or megestrol acetate).

• The participant has previously received endocrine therapy for breast cancerprevention (tamoxifen or aromatase inhibitors) or raloxifene.

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

• The participant has a personal history of any of the following conditions: syncopeof cardiovascular etiology, ventricular arrhythmia of pathological origin or suddencardiac 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 withinthe last 30 days or 5 half-lives, whichever is longer.