Purpose Deregulated expression of microRNAs (miRNAs) has been demonstrated in multiple myeloma (MM). A promising strategy to achieve a therapeutic effect by targeting the miRNA regulatory network is to enforce the expression of miRNAs that act as tumor suppressor genes, such as miR-34a. Experimental Design Here, we investigated the therapeutic potential of synthetic miR-34a against human MM cells in vitro and in vivo. Results Either transient expression of miR-34a synthetic mimics or lentivirus-based miR-34a-stable enforced expression triggered growth inhibition and apoptosis in MM cells in vitro. Synthetic miR-34a downregulated canonic targets BCL2, CDK6 and NOTCH1 at both the mRNA and protein level. Lentiviral vector-transduced MM xenografts with constitutive miR-34a expression showed high growth inhibition in SCID mice. The anti-MM activity of lipidic-formulated miR-34a was further demonstrated in vivo in two different experimental settings: i) SCID mice bearing non-transduced MM xenografts; and ii) SCID-synth-hu mice implanted with synthetic 3D scaffolds reconstituted with human bone marrow stromal cells and then engrafted with human MM cells. Relevant tumor growth inhibition and survival improvement were observed in mice bearing TP53-mutated MM xenografts treated with miR-34a mimics in the absence of systemic toxicity. Conclusions Our findings provide a proof-of-principle that formulated synthetic miR-34a has therapeutic activity in preclinical models and support a framework for development of miR-34a-based treatment strategies in MM patients.

Purpose Deregulated expression of microRNAs (miRNAs) plays a role in the pathogenesis and progression of multiple myeloma (MM). Among upregulated miRNAs, miR-21 has oncogenic potential and therefore represents an attractive target for the treatment of MM. Experimental design Here, we investigated the in vitro and in vivo anti-MM activity of miR-21 inhibitors. Results Either transient enforced expression or lentivirus-based constitutive expression of miR-21 inhibitors triggered significant growth inhibition of primary patient MM cells or IL-6-dependent/independent MM cell lines and overcame the protective activity of human bone marrow stromal cells. Conversely, transfection of miR-21 mimics significantly increased proliferation of MM cells, demonstrating its tumor promoting potential in MM. Importantly, upregulation of miR-21 canonical validated targets (PTEN, Rho-B and BTG2), together with functional impairment of both AKT and ERK signaling, were achieved by transfection of miR-21 inhibitors into MM cells. In vivo delivery of miR-21 inhibitors in SCID mice bearing human MM xenografts expressing miR-21 induced significant anti-tumor activity. Upregulation of PTEN and downregulation of p-AKT were observed in retrieved xenografts following treatment with miR-21 inhibitors. Conclusions Our findings show the first evidence that in vivo antagonism of miR-21 exerts anti-MM activity, providing the rationale for clinical development of miR-21 inhibitors in this still incurable disease.