ARTICLE ABSTRACTPurpose: Exosomes deliver signals to target cells and could thus be exploited as an innovative therapeutic tool. We investigated the ability of membrane TRAIL-armed exosomes to deliver proapoptotic signals to cancer cells and mediate growth inhibition in different tumor models.Experimental Methods and Results: K562 cells, transduced with lentiviral human membrane TRAIL, were used for the production of TRAIL+ exosomes, which were studied by nanoparticle tracking analysis, cytofluorimetry, immunoelectronmicroscopy, Western blot, and ELISA. In vitro, TRAIL+ exosomes induced more pronounced apoptosis (detected by Annexin V/propidium iodide and activated caspase-3) in TRAIL-death receptor (DR)5+ cells (SUDHL4 lymphoma and INT12 melanoma), with respect to the DR5−DR4+KMS11 multiple myeloma. Intratumor injection of TRAIL+ exosomes, but not mock exosomes, induced growth inhibition of SUDHL4 (68%) and INT12 (51%), and necrosis in KMS11 tumors. After rapid blood clearance, systemically administered TRAIL+ exosomes accumulated in the liver, lungs, and spleen and homed to the tumor site, leading to a significant reduction of tumor growth (58%) in SUDHL4-bearing mice. The treatment of INT12-bearing animals promoted tumor necrosis and a not statistically significant tumor volume reduction. In KMS11-bearing mice, despite massive perivascular necrosis, no significant tumor growth inhibition was detected.Conclusions: TRAIL-armed exosomes can induce apoptosis in cancer cells and control tumor progression in vivo. Therapeutic efficacy was particularly evident in intratumor setting, while depended on tumor model upon systemic administration. Thanks to their ability to deliver multiple signals, exosomes thus represent a promising therapeutic tool in cancer. Clin Cancer Res; 22(14); 3499–512. ©2016 AACR.