Delivering the promise of miRNA cancer therapeutics

► Delivery strategies for miRNA modulators (miRNA mimics/inhibitors) in cancer. ► Local versus systemic delivery in vivo. ► Modified miRNA modulators. ► Carriers for miRNA modulators delivery in vivo (viral, non-viral, and targeted strategies). MicroRNAs (miRNAs) are pivotal post-transcriptional gene expression regulators. These endogenous small non-coding RNAs aberrantly expressed in cancer have significant roles in tumorigenesis and progression. Currently, miRNAs are being pursued as diagnostic and prognostic biomarkers, and as therapeutic tools in cancer. miRNA modulation provides the unique ability to fine-tune multiple genes simultaneously, thereby regulating relevant signaling pathways involved in cell differentiation, proliferation and survival. This unique miRNA feature shifts the traditional one drug one target paradigm to a novel one drug multiple targets paradigm. We herein review in vivo strategies of miRNA modulator (mimic and/or inhibitor) delivery in cancer models, a subject that remains the key challenge to the establishment of this novel class of RNA therapeutics.