In 2020, the American Society of Clinical Oncology (ASCO) published a guideline update on the selection of optimal adjuvant chemotherapy and targeted therapy in early-stage breast cancer (BC). However, these guidelines did not address the use of cyclin-dependent kinase 4 and 6 (CDK 4/6) inhibitors in this population.

The CDK 4/6 inhibitors approved for use in BC in the United States include palbociclib, abemaciclib and ribociclib. Palbociclib is approved for patients with hormone receptor–positive (HR+), human epidermal growth factor receptor 2–negative (HER2-) advanced or metastatic BC in combination with an aromatase inhibitor (AI) as initial endocrine therapy (ET) or with fulvestrant in patients with disease progression following ET.

Abemaciclib is indicated in combination with ET (tamoxifen or an AI) for the adjuvant treatment of HR+, HER2-, node-positive, early-stage BC at high risk of recurrence; in combination with an AI  as initial ET for HR+, HER2- advanced, or metastatic BC; in combination with fulvestrant for the treatment of HR+, HER2-negative advanced, or metastatic BC with disease progression following ET; and as monotherapy in patients with HR-positive, HER2-negative advanced or metastatic breast cancer with disease progression following ET and prior chemotherapy in the metastatic setting.

Ribociclib is indicated for HR+, HER2- advanced or metastatic BC in combination with an AI as initial ET; with fulvestrant as initial ET-based therapy; or following disease progression on ET in postmenopausal women or in men.

Since the publication of ASCO’s 2020 guidance, additional studies have been published including monarchE, a global, phase III open-label trial that examined the use of abemaciclib with ET in the treatment of high-risk, early-stage BC. This prompted a 2022 ASCO update focusing on abemaciclib with ET in high-risk early-stage BC. Additional changes have taken place in FDA approved labeling since then, however, including the removal of the need for Ki-67 testing. Recently, data from a 5-year overall survival interim analysis was published involving the use of abemaciclib in HR+, HER2-, high-risk, early-stage BC.

The NATALEE trial, which was an open-label, multicenter, randomized, phase II study, shed additional light on the use of ribociclib with ET in the treatment of early-stage BC.

Upon review of this new evidence, ASCO released two updated recommendations on the use of CDK4/6 inhibitors in early-stage BC. Among the recommendations are that abemaciclib can be prescribed for 2 years along with ET administration for >5 years for BC patients with resected HR+, HER2-, node+, early-stage BC at high risk of recurrence, which is defined as having four or more positive axillary lymph nodes or having one to three positive axillary lymph nodes plus one of the following additional risk factors: grade 4 disease, tumor size >5 cm, or Ki-67 index >20%. These criteria are similar to the criteria for the intention-to-treat group in the monarchE study. The second recommendation is that adjuvant ribociclib (400 mg once daily; 3 weeks on followed by 1 week off) for a period of 3 years can be administered along with ET therapy for BC patients with anatomic stage II or III disease who have met criteria for study entry, which include HR+, HER2- BC and for those with stage IIA disease and at least one of the additional following risk factors: grade 2 disease plus Ki-6 >20% or high genomic risk or grade 3 disease and have a high risk of recurrence.

The authors further qualified the use of these two CKD4/6 inhibitors indicating that these drugs may not provide benefit to lower risk patients; however, at this point there is insufficient evidence to specify which subgroups of BC patients would benefit the most. The recommendations consider weighing risk versus benefit and engaging in shared decision-making with the patient. The guideline panel also favored abemaciclib over ribociclib since the former has longer follow-up data; has demonstrated increasing benefit over time; requires a shorter duration of therapy; and is FDA approved as adjuvant therapy. The group recommended that ribociclib be reserved for patients who have preexisting high-grade diarrhea or intolerance to abemaciclib. The guideline authors acknowledged that more studies are needed for clarify the place of these agents in therapy.

Pharmacists should be aware of these latest guidelines and should help guide the discussion as to which BC patients may benefit most from the use of these expensive agents. As the use of these CDK 4/6 inhibitors expands beyond their initial indications, pharmacists should also closely monitor patients for signs and symptoms of adverse effects.

The content contained in this article is for informational purposes only. The content is not intended to be a substitute for professional advice. Reliance on any information provided in this article is solely at your own risk.