Neuroblastoma is the most common extra-cranial solid cancer of childhood; typically effecting young children aged 1-5. High-risk patients have a 5-year survival rate below 50%. Treatment for these patients involves high-dose multi-layered chemotherapy, which is often not curative and causes severe health impacts later in life. There is therefore an urgent need to find alternative drugs to either replace or significantly improve upon conventional therapy. MicroRNAs (miRNA) are potent regulators of gene expression and are emerging as an exciting new class of therapeutic. However, our understanding of miRNA function in neuroblastoma is fragmented, restricting our ability to capitalise on their therapeutic potential. We have performed a functional genomic screen of >1200 miRNA mimics in the Kelly cell line, a model of poor-prognosis disease. This was performed in combination with low doses (IC30) of doxorubicin and vincristine. This uncovered 22 synthetic lethal miRNAs that synergised with chemotherapy to kill multiple cell line models of MYCN-amplified high-risk neuroblastoma, but not normal cells. Three miRNAs, miR-99b-5p, miR-380-3p and miR-485-3p, had potent synthetic lethal interaction with doxorubicin, acting to prevent DNA damage repair and cause apoptosis. These miRNAs are potentially novel tumour suppressors as they undergo recurrent copy number loss in neuroblastoma, and low expression predicts poor outcome. Pre-clinical modelling is currently being conducted in the COG-519 patient-derived xenograft model, originating from a patient with high-risk disease after several lines of treatment. We are using Star-POEGMA nanoparticles (1) to deliver candidate miRNAs in combination with chemotherapy. Ongoing work includes short-term mechanistic experiments to assess miRNA uptake and knockdown of predicted mRNA targets, and long-term therapeutic experiments to monitor tumour size and survival. We hypothesise that as observed in vitro, the overexpressed miRNAs will sensitise neuroblastoma tumours to chemotherapy by reducing proliferation, increasing apoptosis, and lowering overall tumour burden. We predict that the addition of miRNA treatment to chemotherapy will increase therapeutic outcome, while reducing the toxicity associated with conventional high-dose chemotherapy.