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

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Open to people ages 18 years and up

• Have confirmed HR+, HER2+ in initial diagnostic tissue, 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 therapy
• Have completed approximately nine months to one year of standard HER2-targeted therapy without evidence of disease recurrence (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 (chemotherapy administered with

HER2-targeted therapy): Single agent adjuvant ado-trastuzumab emtansine or

• For participants not treated with neoadjuvant therapy: Adjuvant pertuzumab with trastuzumab
• Have high risk disease, defined by one of the following:
• For participants treated with neoadjuvant therapy (as defined above): Pathologically detected axillary nodal disease in the surgical specimen
• For participants not treated with neoadjuvant therapy: Axillary node positive disease meeting one of the following criteria:
• Pathological tumor involvement in greater than or equal to (≥) four ipsilateral axillary lymph nodes OR
• Pathological tumor involvement in one to three ipsilateral axillary lymph node(s) and at least 1 of the following criteria:
• Histological Grade 3
• Primary invasive tumor size ≥5 centimeters determined pathologically

• Have breast cancer with any of the following features:
• Disease recurrence or distant metastatic disease (including contralateral axillary lymph nodes)
• Lymph node-negative status
• Pathological complete response from any prior systemic treatments for early breast cancer
• 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, investigational HER2 directed therapy, or trastuzumab deruxtecan 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 therapy for their breast cancer at study enrollment