This project aims to optimize and validate a promising therapeutic tool for combined cancer therapy, 2shRNA, in an ex vivo model system. Combined therapies are of great significance nowadays, due to their key role in fighting, for instances, resistance processes during cancer treatment. Nonetheless, the drug combinations approved to date face several problems, such as cooperative toxicity effects, lack of efficiency and poor bioavailability. We have designed and synthesized 2shRNA, a new bifunctional RNA tool that can simultaneously attack two therapeutic targets involved in drug resistance pathways, and that can additionally bind other molecules such as peptide carriers or fluorophores, to improve delivery and efficacy. The 2shRNA nanostructure displayed low toxicity and excellent anti-proliferative properties in resistant HER2+ breast cancer cell lines. The present proposal is aimed at optimizing and validating this novel and promising RNA tool by combining state-of-the-art bioinformatics design and cycles of chemical refinement with biological evaluation in PDx-derived primary cell cultures and biodistribution studies in PDx mouse models. The proposed strategy presents a novel therapeutic approach with great potential to circumvent drug resistance in breast cancer, which is a major health challenge for our society. The ability of our biostable RNA tool to administer two drugs in a single dose could improve the quality of life of the patients, as fewer doses might be needed to stall disease progression.