The following article features coverage from the 2020 San Antonio Breast Cancer Symposium. Click here to read more of Cancer Therapy Advisor‘s conference coverage.

The addition of abemaciclib to standard adjuvant endocrine therapy continued to delay invasive disease for patients with node-positive, hormone receptor–positive, HER2-negative, high-risk early breast cancer, according to updated results from the phase 3 monarchE trial ( Identifier: NCT03155997).

The results of the planned interim analysis were presented at the 2020 Virtual San Antonio Breast Cancer Symposium (SABCS) in San Antonio, Texas.

The trial enrolled 5637 patients who were randomly assigned to receive adjuvant treatment with standard endocrine therapy or endocrine therapy plus abemaciclib for 2 years. To date, a quarter of patients have finished 2 years of treatment.

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Previously, the trial reported a median of 15.5 months of follow-up and showed superior invasive disease-free survival (IDFS) for patients on the abemaciclib arm compared with the endocrine therapy–alone arm.

At a median follow-up of approximately 19 months, the preplanned primary outcome analysis showed a 29% reduced risk of having invasive disease for patients treated with abemaciclib compared with endocrine therapy alone (P =.0009; hazard ratio [HR], 0.713; 95% CI, 0.583-0.871).

At 2 years, the IDFS rate was 92.3% for abemaciclib arm and 89.3% for the endocrine therapy–alone arm, and this benefit was seen for all subgroups.

Even among a subgroup of patients with high Ki-67, a marker for more aggressive disease, the risk of invasive disease declined by 31% for patients treated administered abemaciclib compared with endocrine therapy alone (P =.0111; HR, 0.691; 95% CI, 0.519-0.920).

The most common treatment-emergent adverse events on the abemaciclib arm were diarrhea (83%), neutropenia (45%), and fatigue (39%), while arthralgia (33%), hot flush (22%), and fatigue (17%) were most common on the endocrine therapy–alone arm.

The treatment discontinuation rate due to adverse events was considerably higher for the abemaciclib arm compared with the endocrine therapy alone arm (17% vs 1%, respectively). Also, most patients treated with abemaciclib (66%) had at least 1 dose hold, and dose reductions were also common with abemaciclib, affecting 43% of patients.

Overall survival data remain immature and the study is ongoing.

In light of the results of the phase 3 PENELOPE-B trial ( Identifier: NCT01864746), which showed adding palbociclib to endocrine therapy had no impact on IDFS with longer follow-up, study discussant Ruth O’Regan, MD, University of Wisconsin, cautioned that longer follow-up is “crucial” to make sure that the positive results seen in the monarchE trial remain positive.

Editor’s note: This article was updated on 12/9/20.

Read more of Cancer Therapy Advisor‘s coverage of the 2020 SABCS meeting by visiting the conference page.


O’Shaughnessy JA, Johnston S, Harbeck N, et al. Primary outcome analysis of invasive disease-free survival for monarchE: abemaciclib combined with adjuvant endocrine therapy for high risk early breast cancer. Presented at: 2020 Virtual San Antonio Breast Cancer Symposium (SABCS); December 8-11, 2020. Abstract GS1-01.