MicroRNA-Directed Biomarkers and Breast Cancer Therapeutics—Potential to Advance Personalised Approaches in Clinical Trials

The advent of breast cancer molecular subtyping has transformed management, enabling treatment personalisation and de-escalation beyond traditional stage-based approaches. Established biomarkers, such as Ki-67 in luminal disease, HER2 amplification, and PD-L1 expression in triple-negative breast cancer, underpin seminal clinical trials yet remain imperfect predictors of response and long-term outcome. MicroRNAs have emerged as promising next-generation biomarkers and therapeutic tools. As master regulators of gene expression, both tumour-derived and circulating microRNAs can refine diagnosis and molecular subclassification, inform prognosis and therapeutic selection, act as treatment sensitisers, and potentially serve as direct therapeutic targets. Well-characterised miRNAs such as miR-221 have been implicated in endocrine resistance, while recent liquid-biopsy approaches have enabled the identification of circulating miR-145 and exosomal miR-155 as predictors of pathological complete response in HER2-positive disease. Their detectability in tissue, blood and other biofluids offers a minimally invasive means to dynamically monitor cancer behaviour and response, supporting more precise therapeutic decision-making. This review synthesises the current evidence for miRNA-based biomarkers across oestrogen-receptor positive, HER2-positive and triple-negative breast cancer and outlines their potential integration into biomarker-driven clinical trial designs and personalised treatment strategies.