Data from the MONARCH 3 trial ( Identifier: NCT02246621) suggest that initial abemaciclib improves progression-free survival (PFS) among patients with hormone receptor–positive, HER2-negative advanced breast cancer. The findings were published in the Journal of Clinical Oncology.1

Previous data showed that monotherapy with abemaciclib, an orally administered CDK 4/6 inhibitor, improves outcomes among patients with endocrine therapy–refractory disease. For this phase 3 randomized trial, researchers evaluated the safety and efficacy of abemaciclib plus a non-steroidal aromatase inhibitor (AI) vs an AI alone among patients with hormone receptor–positive, HER2-negative disease. The primary endpoint was PFS.

Of 579 patients assessed for eligibility, 493 were randomly assigned 2:1 to receive abemaciclib plus AI (328 patients) or placebo plus AI (165 patients). Patient characteristics were similar between the groups.

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With a hazard ratio of 0.54, patients in the abemaciclib arm had a longer median PFS than those who received placebo (not reached vs 14.7 months, respectively; P = .000021). Patients with measurable disease also had a higher objective response with abemaciclib (59%) than with placebo (44%; P = .004).

Five complete responses were observed in the abemaciclib group vs 0 for placebo.

Serious adverse events (AEs) were observed in 27.5% of patients receiving abemaciclib vs 14.9% for placebo. Eight deaths in the abemaciclib arm and 2 in the placebo arm were related to AEs.

As previously reported, diarrhea was the AE most frequently observed with abemaciclib, occurring in 81.3% of patients, though it was usually mild.

Although overall survival data are not yet mature, the researchers concluded that “abemaciclib plus a nonsteroidal AI was an effective initial treatment with a tolerable safety profile for postmenopausal patients with [hormone receptor]-positive, HER2-negative advanced breast cancer.”


  1. Goetz MP, Toi M, Campone M, et al. MONARCH 3: abemaciclib as initial therapy for advanced breast cancer. J Clin Oncol. 2017 Oct 2. doi: 10.1200/JCO.2017.75.6155 [Epub ahead of print]