Breaking Barriers: Immune Checkpoint Inhibitors in Breast Cancer

Breast cancer remains the most commonly diagnosed malignancy among women worldwide and continues to pose significant therapeutic challenges, particularly in advanced and refractory disease. Although traditionally considered less immunogenic compared with other solid tumours, growing evidence demonstrates that subsets of breast cancer, particularly triple-negative and HER2-positive subtypes, exhibit immune-responsive features. This recognition has spurred the development and clinical evaluation of immunotherapeutic strategies, with immune checkpoint inhibitors (ICIs) emerging as the most prominent approach. This new class of drugs targeting the programmed death-1 (PD-1)/programmed death-ligand 1 (PD-L1) axis has demonstrated meaningful clinical activity in select patient populations, leading to regulatory approvals in combination with chemotherapy for advanced triple-negative breast cancer. Despite these advances, response rates remain modest, and the benefits are largely restricted to patients with PD-L1-positive tumours. Ongoing studies are evaluating predictive biomarkers, optimal treatment combinations, and mechanisms of resistance to expand the efficacy of ICIs across broader breast cancer subtypes. Furthermore, novel checkpoint targets such as cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), lymphocyte-activation gene 3 (LAG-3), and T cell immunoreceptor with immunoglobulin and immunoreceptor tyrosine-based inhibitory motif domains (TIGIT) are under investigation, with the potential to enhance or complement PD-1/PD-L1 blockade. This review summarises the current state of knowledge on breast cancer immunotherapy with an emphasis on ICIs, highlighting key clinical trial findings, as well as emerging biomarkers of response, and strategies to overcome therapeutic resistance, if cancer cells eventually develop resistance. By integrating preclinical insights with clinical progress, we aim to provide a comprehensive overview of the evolving role of checkpoint blockade in breast cancer and outline future directions to optimise patient outcomes.