The study will however continue as planned on the recommendation of an independent data monitoring committee (DMC).
The MONARCH 2, Phase 3 trial is comparing a combination of abemaciclib plus fulvestrant (AstraZeneca's Faslodex) versus placebo with fulvestrant in women with hormone-receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) locally advanced or metastatic breast cancer.
Lilly Oncology senior vice president of product development and medical affairs Richard Gaynor said: "We had stringent criteria set for this interim analysis and we look forward to receiving the final MONARCH 2 results in the first half of 2017.
"We remain optimistic that treatment with abemaciclib, in combination with fulvestrant could offer improved outcomes for patients."
The primary endpoint of the trial is progression-free survival, but the company will also analyze overall survival and safety data.
Lilly is currently evaluating its drug as a single agent in breast cancer patients with refractory metastatic breast cancer, whose disease had progressed following multiple prior treatments, including chemotherapy.
Three other trials are also investigating abemaciclib in combination with other therapies.
Abemaciclib is an oral cell cycle inhibitor, designed to block the growth of cancer cells by specifically inhibiting cyclin-dependent kinases, CDK 4 and CDK 6.