Study is terminated due to inability to enroll study participants

The main purpose of this study is to measure how well abemaciclib works in participants with early breast cancer who are taking hormone therapy after surgery. Participants must have breast cancer that is hormone receptor positive (HR+) and human epidermal receptor 2 positive (HER2+). Your participation could last up to 10 years depending on how you and your tumor respond.

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

Anonymized individual patient level data will be provided in a secure access environment upon approval of a research proposal and a signed data sharing agreement.

Data are available 6 months after the primary publication and approval of the indication studied in the US and EU, whichever is later. Data will be indefinitely available for requesting.

A research proposal must be approved by an independent review panel and researchers must sign a data sharing agreement.