New Drugs, New Toxicities: Side Effects of New and Emerging Breast Cancer Therapies

With the rapid introduction of novel breast cancer therapies, recognizing and managing side effects is essential to maintain adherence and improve outcomes. As novel oral endocrine therapies and combination strategies including targeted agents have prolonged progression and in some cases disease-free survival, early recognition and appropriate management of these toxicities is critical to optimize quality of life. Dermatologic adverse events are frequently associated with novel breast cancer therapies including immune checkpoint inhibitors (ICIs), targeted therapies, and antibody-drug conjugates (ADCs). These include various rashes, stomatitis, and alopecia, necessitating multidisciplinary dermatologic intervention to allow for prompt management of cutaneous toxicities and continuation of oncologic therapy. Targeted breast cancer therapies, including ADCs, can also induce ocular adverse events (OAEs), such as corneal pseudomicrocysts which lead to blurry vision and eye pain. Current preventative therapies have had limited success for these OAEs, necessitating dose interruptions. Although anthracycline-based chemotherapy and human epidermal growth factor receptor 2 (HER2)-targeted therapy are associated with an increased risk of heart failure and left ventricular (LV) dysfunction, novel breast cancer therapies including ADCs, HER2 tyrosine kinase inhibitors, cyclin-dependent kinase 4 and 6 inhibitors, ICIs, and oral selective estrogen receptor degrader are also associated with an increased risk of LV dysfunction, heart failure, corrected QT prolongation, myocarditis, and bradycardia. This review provides a comprehensive overview of novel and emerging breast cancer therapy toxicities, with suggested management strategies to prevent and mitigate these adverse events. Through a multidisciplinary approach involving preventative strategies, monitoring, proactive interventions, providers can minimize symptom burden and improve patient adherence, ultimately improving breast cancer outcomes.