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Individualizing CDK46 Inhibitors
FAQs: Individualizing CDK4/6 Inhibitor–Based Therapy in Breast Cancer

Released: July 17, 2026

William J. Gradishar
William J. Gradishar, MD
Heather McArthur
Heather McArthur, MD, MPH, FASCO
Joanne Mortimer
Joanne Mortimer, MD, FACP, FASCO
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Key Takeaways
  • Treatment choice among adjuvant CDK4/6 inhibitor–based regimens should be individualized and consider treatment duration, toxicity profiles, monitoring requirements, and patient preferences.
  • For treatment consideration, aggressive advanced HR-positive/HER2-negative disease and visceral crisis should not be viewed as the same clinical scenario.
  • Proactive toxicity management and ongoing reinforcement of patient education are important to maintain treatment adherence and avoid switching between CDK4/6 inhibitors.

In this commentary, William J. Gradishar, MD; Heather McArthur, MD, MPH, FASCO; and Joanne Mortimer, MD, FACP, FASCO, address key questions raised during a recent live symposium titled, “The Next Frontier for CDK4/6 Inhibitors in Breast Cancer: Expert Strategies for Applying the Latest Guidelines and Advances in Practice,” including how treatment duration, toxicity profiles, monitoring requirements, and patient preferences can influence treatment selection in the adjuvant disease settings. They also address clinical judgment and the choice between endocrine therapy (ET) and chemotherapy for patients with aggressive presentations of hormone receptor (HR)–positive/HER2-negative advanced disease, the evolving role of everolimus-based therapy, and supportive strategies for managing fatigue, venous and arterial thromboembolic events, and tolerability challenges.

In your practice, what do you discuss with your patients who are eligible for either abemaciclib or ribociclib in the adjuvant setting?
Joanne Mortimer, MD, FACP, FASCO:
For patients with high-risk HR-positive/HER2-negative early-stage breast cancer who may be eligible for either adjuvant abemaciclib or ribociclib, I discuss several practical considerations, including the difference in treatment duration (ie, 2 years with abemaciclib vs 3 years with ribociclib), as well as each agent’s toxicity profile and monitoring requirements. In practice, diarrhea can be one of the most challenging aspects associated with abemaciclib because it may substantially affect quality of life, even if it does not lead to treatment discontinuation. For some patients, particularly those who are concerned about diarrhea or its impact on daily activities, I may favor 3 years of ribociclib.

Heather McArthur, MD, MPH, FASCO:
Treatment duration is an important part of the discussion. Although I personally would prefer 2 years over 3 years of therapy, ultimately, the decision needs to account for toxicity, monitoring requirements, and patient preferences. In my experience, switching between CDK4/6 inhibitors because of tolerability issues is uncommon, particularly when patients receive thorough education and proactive supportive care. Severe fatigue or concerning laboratory changes can occur, but those situations have been relatively rare in my practice.

Considering the results from the RIGHT Choice trial, how should we distinguish aggressive disease from visceral crisis?
William J. Gradishar, MD:
The randomized phase II RIGHT Choice trial of first-line ribociclib plus ET vs combination chemotherapy in pre/perimenopausal women with advanced HR-positive/HER2-negative breast cancer reinforced the idea that aggressive disease and visceral crisis should not be viewed as the same clinical scenario. Aggressive disease may include bulky tumor burden, diffuse metastatic involvement, symptomatic disease, or rapid progression, but that alone does not necessarily constitute visceral crisis. Visceral crisis is more narrowly defined by rapidly progressive disease that is causing, or threatening to cause, clinically significant organ dysfunction. In my opinion, the RIGHT Choice trial gives healthcare professionals more confidence to consider ribociclib plus ET in selected patients with aggressive or even fairly diffuse HR-positive/HER2-negative advanced breast cancer. However, if a patient has clear evidence of impending or established organ failure, that is a different situation and may warrant a more urgent treatment approach using chemotherapy.

What are your thoughts on everolimus today? Does it still have a role in the current therapeutic landscape?
Heather McArthur, MD, MPH, FASCO:
Everolimus is not necessarily one of my preferred treatment options because of the risk of stomatitis and mucositis. However, I do believe it is making a comeback based on the phase III evERA study, which evaluated giredestrant, an oral SERD, in combination with everolimus vs standard-of-care ET in combination with everolimus in patients with estrogen receptor–positive/HER2-negative advanced breast cancer after progression on a CDK4/6 inhibitor plus ET. Initial results from evERA showed an improved median progression-free survival (PFS) in both the overall patient population (8.77 vs 5.49 months; hazard ratio: 0.56) and patients with ESR1-mutated disease (9.99 vs 5.45 months; hazard ratio: 0.38). Recent updates presented at ASCO 2026 showed an improvement in median PFS2 (19.0 vs 13.2 months; hazard ratio: 0.69), while the overall survival data were not yet mature. Although it remains to be seen how this regimen will ultimately fit into practice, these data suggest that everolimus may continue to play a role, particularly in selected patients, including those with ESR1-mutated disease, where the median PFS2 in evERA was 19.0 vs 12.9 months (hazard ratio: 0.61).

How do you manage fatigue from CDK4/6 inhibitors?
Joanne Mortimer, MD, FACP, FASCO:
Fatigue is a common side effect, and it can be more frequent when a CDK4/6 inhibitor is added to ET. I often recommend exercise because it is one of the better supported nonpharmacologic interventions for cancer-related fatigue, including in patients with breast cancer. I do consider dose reduction for patients who are experiencing significant fatigue with a CDK4/6 inhibitor, particularly when fatigue is affecting their ability or willingness to continue therapy. For example, in some older patients receiving ribociclib, reducing the dose from 600 mg daily to 400 mg daily can be a practical strategy to help manage fatigue and support treatment adherence. The ongoing FastER trial is evaluating whether prolonged overnight fasting, exercise, a combination of prolonged overnight fasting and exercise, or general health education sessions can reduce fatigue in women with advanced or metastatic breast cancer receiving first- or second-line ET with a CDK4/6 inhibitor with or without HER2-directed therapy, or in combination with both a CDK4/6 inhibitor and a PI3K inhibitor (NCT06123988), and may provide additional support for exercise.

Does smoking increase venous thromboembolism (VTE)/arterial thromboembolism (ATE) risk associated with CDK4/6 inhibitor treatment?
Joanne Mortimer, MD, FACP, FASCO:
There is not a clear answer to this question. Available data suggest that thromboembolic events, particularly VTE, can occur with CDK4/6 inhibitors, and some analyses have raised concern that patients with underlying cardiovascular risk factors or cardiovascular disease may be at higher risk. However, the evidence is not definitive, and smoking has not been clearly established as an independent risk factor for VTE or ATE in patients receiving CDK4/6 inhibitors. In practice, I would consider smoking as part of the patient’s overall cardiovascular and thrombotic risk profile, but I would not say that there are CDK4/6 inhibitor–specific data showing that smoking itself increases VTE or ATE risk.

William J. Gradishar, MD:
Although there are no clear CDK4/6 inhibitor–specific data showing that smoking independently increases the risk of VTE or ATE, smoking is an established cardiovascular risk factor and may contribute to a patient’s overall thrombotic risk profile. For that reason, I would strongly encourage smoking cessation as part of broader cardiovascular risk reduction and supportive care.

In your practice, how often are you switching from one CDK4/6 inhibitor to another because of adverse events?
Heather McArthur, MD, MPH, FASCO:
In my practice, switching between CDK4/6 inhibitors is very uncommon. I think much of that comes down to patient education and proactive toxicity management. In an academic setting, we may have more time and resources to support that education, including access to a patient-facing pharmacist who can reinforce expectations and management strategies. There are occasional exceptions. For example, I have had patients with significant laboratory changes, such as a marked creatinine increase on abemaciclib that required further evaluation with cystatin C, or profound fatigue that interfered with their ability to work. However, these are anecdotal examples that I do not consider the general rule.

Your Thoughts
What are your current questions related to CDK4/6 inhibitor therapy for patients with breast cancer? Do you have any interesting experiences related to the care of patients on CDK4/6 inhibitor therapy that you would care to share with the community?

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In your practice, how often are you using dose reductions of CDK4/6 inhibitors to help your patients with early-stage breast cancer remain adherent to their prescribed adjuvant therapy? 

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