cytokines requires transfection of host cells, which is highly variable from patient to patient. there has also been significant off-target transgene expression seen in clinical trials. small volumes of intratumoral injection of adenoviruses were shown to cause significant transgene expression in the liver, intestine, spleen, kidney, and brain. activity-on-target cytokines activity-on-target cytokines, known as actakines, are mutated cytokines that have reduced binding affinity for their native receptor complex and enhanced binding affinity for a specific tumor cell receptor. this causes the cytokine to be inactive in circulation, limiting systemic toxicity. the cytokine is activated upon binding to its tumor-specific antigen, allowing for local delivery. the cytokine tumor necrosis factor (tnf) causes rapid hemorrhagic tumor necrosis in both animal models and patients but is associated with high systemic toxicities. tnf is known to exert its antitumor effect through stromal cells in the tumor microenvironment. a tnf actakine was created to improve localized delivery of tnf and decrease systemic toxicity by changing its antitumor pathway to target endothelial cells. this mutated tnf cytokine was shown to only target endothelial cells of the tumor vasculature, allowing for a safe and effective delivery system. this tnf-based actakine resulted in a 100-fold increase in targeting efficiency, and when the actakine was targeted to cd13 expressed on endothelial cells of the tumor vasculature, it demonstrated selective activation of tumor neovasculature without any detectable toxicity in vivo. when administered with car-t cells, this therapy was shown to enhance t cell infiltration to control solid tumors, while combination with a cd8-targeted type ii interferon actakine led to eradication of solid tumors cytokine factories cytokine factories are cell-generated cytokines that can locally deliver a cytokine of interest, offering spatial and temporal control of dosing. the homing capacity and tumor tropism capabilities of mesenchymal stem/stromal cells (mscs) make them ideal drug delivery vehicles. mscs also have reduced immunogenicity due to their limited expression of costimulatory molecules. using mscs to express cytokines provides greater cytokine delivery to the tumor tissue, which increases therapeutic efficacy of the treatment. il-2 gene engineered mscs (msc-il-2) have been studied as a potential antitumor therapy since il-2 is an immunogenic cytokine. intratumoral injection of msc-il-2 was shown to significantly regress glioma tumor growth and improve the overall survival of rats with glioma. similarly, subcutaneous injections of bone marrow msc-ifn-α affected tumor growth in vivo and increased overall survival in a multiple myeloma mouse model. antitumor effects were attributed to increased apoptosis of tumor cells, decreased microvessel density, and ischemic necrosis. trail is an immunogenic cytokine that selectively targets tumor cells for apoptosis, reducing systemic toxicity. mscs engineered to overexpress trail have shown promising antitumor effects in xenograft models through apoptotic pathways. numerous studies of msc-trail systems are ongoing, including treatments for neuroblastoma, non-small-cell lung carcinoma, breast cancer, pancreatic cancer, glioblastoma, and multiple myeloma, among others human retinal pigmented epithelial (rpe) cells can also be engineered to express a cytokine of interest. rpe cells are ideal cytokine delivery systems because they are nontumorigenic, display contact inhibition, are amenable to genetic modification, have bene previously used in human trials for therapeutic delivery systems, and are safe to use. rpe cells engineered to produce different cytokines were encapsulated in alginate-based microparticles. the encapsulated cells will still viable after encapsulation, did not divide within the capsules, produced the cytokine of interest, and persisted longer in vivo than unencapsulated cells. the il-2 producing rpe cells eradicated peritoneal tumors in ovarian and colorectal mouse models, and computational modeling of pharmacokinetics, predicts clinical translation to humans, indicating potential success in future human clinical trials