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When 2 years of adjuvant abemaciclib was approved for use in late 2021 along with endocrine therapy in our highest-risk, node-positive, hormone receptor-positive breast cancer patients, it quickly became a new standard of care in this population. However, abemaciclib has potentially serious side effects (such as diarrhea and neutropenia) and also necessitates more intensive clinical and laboratory monitoring over a 2-year time frame than this group of patients previously required when on endocrine therapy alone. As indications for adjuvant abemaciclib further broadened to include all patients who would have been eligible for the monarchE trial, knowing the best ways to optimize care for patients on adjuvant abemaciclib has been challenging and broadly discussed.

As of Sept. 2024, a second adjuvant CDK 4/6 inhibitor, ribociclib, was approved, based on data from the NATALEE trial, which created an opportunity for choice but also some confusion among clinicians as they try to apply the data from both monarchE and NATALEE to the patient sitting in front of us.

The recent offers an excellent summary of both trials, including the patient populations that were studied and how the results apply to patients in specific stage categories.

The authors conclude that patients at highest risk, who would have been eligible for both NATALEE and monarchE, should probably be offered abemaciclib unless there is a contraindication, given the longer duration of follow-up on this study and the 2-year (rather than 3-year) duration of treatment; but that if these patients do not tolerate abemaciclib (specifically because of diarrhea, a side effect that is specific to that agent), a switch to ribociclib would be appropriate.

For patients at intermediate risk, who would have qualified for NATALEE but not monarchE, adjuvant ribociclib can be considered, carefully taking the risks and benefits into consideration and using shared decision-making with the patient. For patients at lowest risk of recurrence -- for example, most T2N0 patients -- the risks of ribociclib may outweigh the benefits, and patients and providers should take this into account when considering offering therapy.

The optimal management of early-stage breast cancer is rapidly evolving, and given that we do not have a biomarker to indicate which patients are likely to benefit (or not) from adjuvant CDK 4/6 inhibition, shared decision-making among patients and their informed providers will be paramount.

Equally important is optimal support of our patients while on treatment, to make sure that side effects are optimally managed and that quality of life is maintained as much as possible while doing all that we reasonably can to reduce breast cancer recurrence risk.

Jane L. Meisel, MD, is a professor in the Departments of Hematology and Medical Oncology and Gynecology and Obstetrics and co-director of Breast Medical Oncology at Emory University School of Medicine and the Glenn Family Breast Center at Winship Cancer Institute in Atlanta.

Read the companion article to the guideline update here and an interview about it here.