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