The advent of pathotropic (disease-seeking) targeting technologies, combined with advanced gene delivery vectors, provides a unique opportunity for the systemic delivery of immunomodulatory cytokine genes to remote sites of cancer metastasis. When injected intravenously, such pathotropic nanoparticles seek out and accumulate selectively at sites of tumor invasion and neo-angiogenesis, resulting in enhanced gene delivery, and thus cytokine production, within the tumor nodules. Used in conjunction with a primary tumoricidal agent (e.g., Rexin-G) that exposes tumor neoantigens, the tumor-targeted immunotherapy vector is intended to promote the recruitment and activation of host immune cells into the metastastic site(s), thereby initiating cancer immunization in situ. In this study, we examine the feasibility of cytokine gene delivery to cancerous lesions in vivo using intravenously administered pathotropically targeted nanoparticles bearing the gene encoding granulocyte/macrophage colony-stimulating factor (GM-CSF; i.e., Reximmune-C). In vitro, transduction of target cancer cells with Reximmune-C resulted in the quantitative production of bioactive and immunoreactive GM-CSF protein. In tumor-bearing nude mice, intravenous infusions of Reximmune-C-induced GM-CSF production by transduced cancer cells and paracrine secretion of the cytokine within the tumor nodules, which promoted the recruitment of host mononuclear cells, including CD40+ B cells and CD86+ dendritic cells, into the tumors. With the first proofs of principle established in preclinical studies, we generated an optimized vector configuration for use in advanced clinical trial designs, and extended the feasibility studies to the clinic. Targeted delivery and localized expression of the GM-CSF transgene was confirmed in a patient with metastatic cancer, as was the recruitment of significant tumor-infiltrating lymphocytes (TILs). Taken together, these studies provide the first demonstrations of cytokine gene delivery to cancerous lesions following intravenous administration and extend the applications of cancer immunization in vivo.