High-Risk Node-Positive Hormone Receptor-Positive/HER2-Low Breast Cancer Relapse on Adjuvant Abemaciclib Treatment with ER Loss at Metastatic Recurrence: A Case Report and Literature Review

<b>Background and Clinical Significance</b>: Up to 30% of patients with HR+/HER2-early breast cancer (eBC) may experience distant recurrence, and patients with high-risk clinical features have a higher likelihood of recurrence. For these patients, the addition of a CDK4/6 inhibitor to standard adjuvant endocrine therapy (ET) has demonstrated a significant reduction in the risk of invasive and distant recurrence. Despite that, a small subset of patients still experience distant relapse. To date, the characteristics of patients who relapse on adjuvant CDK 4/6i are not well defined. <b>Case Presentation</b>: Here we report the case of a patient with early-stage HR+/HER2- breast cancer at high risk of recurrence, who experienced distant metastatic relapse during adjuvant therapy with abemaciclib plus ET, with a shift in the tumor subtype to triple-negative. Genomic alterations associated with ET and cyclin-dependent kinase 4 and 6 inhibitor resistance were analyzed with next-generation sequencing (NGS) carried out at recurrence. Results showed P53 and PI3KCA pathway alterations, but no ESR1 mutation or RB1 mutations. The duration was 12 months for adjuvant abemaciclib, and the first-line metastatic treatment lasted less than 3 months. Conversely, T-DXd administered due to the presence of HER2-low showed excellent effectiveness. <b>Conclusions</b>: The management of breast cancer relapse occurring during adjuvant therapy with CDK4/6 inhibitors is challenging. Performing a re-biopsy is crucial due to the possible loss of estrogen receptors, which would require a change in therapeutic strategy no longer based on endocrine therapy. In cases that remain luminal, knowledge of the mutational profile may help to offer patients novel targeted treatments.