An oncolytic viral agent containing the oncolytic, live-attenuated, replication-competent strain of the avian paramyxovirus Newcastle disease virus (NDV) that has been engineered to include a transgene encoding granulocyte-macrophage colony-stimulating factor (GM-CSF), with potential antineoplastic and immunostimulating activities. Upon administration, MEDI5395 specifically infects and replicates in cancer cells. This may result in a direct cytotoxic effect involving the lysis of tumor cells via apoptotic mechanisms and may eventually lead to an inhibition of cancer cell proliferation through the increased production and secretion of pro-inflammatory cytokines and chemokines which are able to recruit mediators of both the innate and adaptive immune responses. Additionally, the inclusion of the GM-CSF transgene in the viral construct leads to expression of GM-CSF, which may potentiate and strengthen the anti-tumor immune response.