To compare the efficacy of abemaciclib plus endocrine therapy versus endocrine therapy alone on invasive disease-free survival (iDFS) in patients with invasive lobular carcinoma versus invasive ductal carcinoma

Breast cancer is a common disease that affects millions of women worldwide, but it is not a single illness. There are different subtypes based on how the cancer cells look and behave. The two most common types are invasive ductal carcinoma (IDC), and invasive lobular carcinoma (ILC). ILC is different from IDC in important ways. Its cells grow in thin, single-file patterns, making it harder to detect on mammograms or physical exams. ILC can also spread to unusual parts of the body, such as the stomach or intestines, and has been historically understudied even though it is the second most common form of breast cancer.

Most ILC tumors grow in response to hormones such as estrogen. This is why endocrine therapy—treatment that blocks or lowers hormones—is a key part of care. A newer treatment called abemaciclib is a type of therapy that blocks proteins known as cyclin-dependent kinase 4 and 6 (CDK4/6), which help cancer cells grow. A major clinical trial called monarchE showed that adding abemaciclib to standard endocrine therapy after surgery lowered the chance of breast cancer returning in patients at high risk. However, we still do not know whether this benefit is the same for people with ILC as it is for people with IDC.

In this project, we will conduct a secondary analysis, meaning we will study data that were already collected in the monarchE trial. We will examine whether abemaciclib works similarly in both types of breast cancer or whether the benefit differs for those with ILC. This question is important because ILC has unique biological features, and it has often been underrepresented in clinical trials. Understanding whether the treatment effect varies will help ensure that patients with ILC receive care that is supported by strong evidence.

We will compare outcomes such as how long people remain free of breast cancer returning (invasive disease-free survival, or iDFS), how long before cancer spreads to distant organs (distant relapse-free survival, or DRFS), and overall survival (OS), meaning whether patients live longer. We will also look at where the cancer returns and whether certain characteristics—such as tumor grade, lymph node involvement, or a marker called Ki-67—affect treatment results.

To conduct this research, we will analyze the trial data by grouping participants according to whether they had IDC or ILC. We will then compare how well abemaciclib plus endocrine therapy worked versus endocrine therapy alone in each subtype. By studying these differences, we hope to understand whether treatment should be tailored more specifically for people with ILC.

This work will help doctors and patients make more personalized treatment decisions and may highlight whether people with different types of breast cancer benefit equally from CDK4/6 inhibitors like abemaciclib.