Addition of Abemaciclib to Aromatase Inhibitors Associated with Durable Tumor Responses


Researchers have confirmed that the addition of the CDK4/6 inhibitor abemaciclib to aromatase inhibitors is associated with durable tumor responses.

In an extended follow-up to the MONARCH 3 trial, researchers have further confirmed that the addition of abemaciclib to nonsteroidal aromatase inhibitors (AI) is associated with durable tumor responses. This includes patients with clinically poor prognostic characteristics.

The research was presented at the 2019 San Antonio Breast Cancer Symposium in San Antonio, Texas.

In an initial treatment for postmenopausal women with hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer in the MONARCH 3 trial, abemaciclib plus nonsteroidal aromatase inhibitors demonstrated efficacy with a tolerable safety profile.

In the intent-to-treat (ITT) population, the updated progression-free survival (PFS) with 12 months of additional follow-up was 28.2 versus 14.8 months in the abemaciclib plus AI versus placebo plus AI arms.

MONARCH 3 was a randomized, double-blind, phase 3 trial with 493 postmenopausal women with HR-positive, HER2-negative advanced breast cancer and no prior systemic therapy in the advanced setting. Women received an AI (anastrozole or letrozole) plus abemaciclib or a placebo. Secondary endpoints included objective response rate (ORR), complete response, partial response, time to response (TTR), and duration of response.

The results showed that patients with measurable disease the abemaciclib plus AI arm had a 62.5% ORR compared with 44.7% in the placebo plus AI arm. Additionally, all prognostic subgroups received some benefit from the addition of abemaciclib to AI, consistent with the ITT population as evidenced by the change in ORR.

The largest effects were observed in subgroups of patients with liver metastases, progesterone receptor-negative tumors, high grade tumors, or a treatment-free interval of 36 months.

Median TTR was similar between the 2 treatments and was generally consistent across subgroups, according to the study. Responses were more durable in the abemaciclib plus AI arm versus the placebo plus AI arm, including poor prognostic subgroups.


Updated subgroup tumor response of abemaciclib plus aromatase inhibitor for hormone receptor positive (HR+), HER2 negative advanced breast cancer (MONARCH 3). Accessed Dec 3, 2019.

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