A Study of Abemaciclib (LY2835219) Plus Hormone Therapy in Participants With Early Breast Cancer

Inclusion Criteria: * Have confirmed HR+, HER2+ early invasive breast cancer without evidence of disease recurrence or distant metastases * Have undergone definitive surgery of the primary breast tumor(s) * Have tumor tissue from breast (preferred) or lymph node * Have received a minimum of four cycles of chemotherapy in either the neoadjuvant or adjuvant setting per standard of care * Have completed approximately nine to 20 months of standard HER2-targeted therapy (neoadjuvant/adjuvant combined duration) * Have received one of the following eligible HER2-targeted adjuvant regimens AND be randomized within 12 weeks of completing the regimen: * For participants treated with neoadjuvant therapy and HER2-targeted therapy: A minimum of 4 cycles of T-DM1 in the adjuvant setting. NOTE: Participants may have received up to approximately 6 cycles of adjuvant trastuzumab prior to initiation of T-DM1. Additionally, participants may have switched to trastuzumab-based therapy (monotherapy or in combination with other HER2-targeted therapies) after 4 cycles of T-DM1 * For participants who had definitive surgery prior to systemic therapy, a minimum of 4 cycles of adjuvant pertuzumab with trastuzumab. * Have high risk disease, defined by one of the following criteria: * Those who received neoadjuvant chemotherapy along with HER2-targeted treatment must have: * residual disease in at least one axillary lymph node, or * a residual tumor ≥ 5 cm, or * a residual tumor of any size that has direct extension to the chest wall and/or skin (ulceration or skin nodules). * Those who had definitive surgery prior to systemic therapy and completed adjuvant chemotherapy along with HER2-targeted therapies (trastuzumab and pertuzumab) must have * tumor involvement in ≥4 ipsilateral axillary lymph nodes, or * tumor involvement in 1 to 3 ipsilateral axillary lymph node(s) and histological Grade 3, or * primary invasive tumor size of ≥ 5 cm on pathological evaluation. Exclusion Criteria: * Have breast cancer with any of the following features: * Disease recurrence or distant metastatic disease (including contralateral axillary lymph nodes) * Pathological complete response from any prior early breast cancer treatments. Participants are required to have residual primary tumor and/or lymph node disease at the time of definitive surgery as indicated in inclusion criteria. * Inflammatory breast cancer * Have other medical conditions including: * Previous breast cancer (Exceptions: Ipsilateral ductal carcinoma in situ \[DCIS\] treated by locoregional therapy alone ≥5 years ago; contralateral DCIS treated by locoregional therapy at any time) * Other cancer being treated and/or not in complete remission within the last 5 years (Exceptions: Appropriately treated non-melanomatous skin cancer or carcinoma in situ of cervix, bladder, or colon) * Females who are pregnant or lactating * History of venous thromboembolism * Other serious medical conditions * Have previously received treatment with: * Any cyclin-dependent kinase (CDK)4 and CDK6 inhibitor * Prior adjuvant treatment with immunotherapy, tucatinib, neratinib, any investigational HER2 directed therapy, or T-DXd (DS8201) for treatment of breast cancer * Endocrine therapy (ET) (i.e., tamoxifen, raloxifene or aromatase inhibitor) for breast cancer prevention (without diagnosis of breast cancer) * Additional chemotherapy, anti-cancer ET, or HER2-targeted therapy beyond standard of care at study enrollment