Complete Cell Cycle Arrest of Hormone Receptor–Positive Bilateral Breast Cancer with Neoadjuvant Abemaciclib plus Letrozole: A Case Report and Review of Literature

Neoadjuvant therapy for early breast cancer (BC) is useful in evaluating biomarkers of responsiveness and anticipating therapy resistance. Hormone receptor (HR)–positive BC shows poor response to chemotherapy than other subtypes. A change in Ki-67 with 2 weeks of neoadjuvant endocrine therapy (NET) has been used as a biomarker that correlates with recurrence-free survival. Addition of CDK4/6 inhibitors (CDK4/6i) to ET has further improved the efficacy. However, bilateral BC patients have been excluded from such studies. Through this case, we will highlight the potential therapeutic benefit of neoadjuvant CDK4/6i + ET in attaining complete cell cycle arrest in a patient with bilateral HR-positive BC. This combination may be better than neoadjuvant chemotherapy in selected patients, sparing them from the toxicity of chemotherapy.