Cell-based carriers were recently exploited as a tumor-targeting tool to improve systemic delivery of oncolytic viruses for cancer therapy. However, the slow clearance of carrier cells from normal organs indicates the need for a controllable system which allows viral delivery only when the carrier cells reach the tumor site. In this study, we sought to develop a pharmaceutically inducible cell-based oncolytic adenovirus delivery strategy for effective targeting and treatment of renal cell carcinoma (RCC), which is one of the most malignant tumor types with an unfavorable prognosis. Herein, we demonstrated the intrinsic tumor homing property of human bone marrow-derived mesenchymal stem cells (hMSCs) to specifically localize primary and metastatic RCC tumors after systemic administration in a clinically relevant orthotopic animal model. The platelet derived growth factor AA (PDGF-AA) secreted from RCC was identified as a chemoattractant responsible for the recruitment of hMSCs. Like endogenous osteocalcin whose barely detectable level of expression was dramatically induced by vitamin D 3, the silenced replication of human osteocalcin promoter-directed Ad-hOC-E1 oncolytic adenoviruses loaded in hMSCs was rapidly activated, and the released oncolytic adenoviruses sequentially killed cocultured RCC cells upon vitamin D 3 exposure. Moreover, the systemic treatment of RCC tumor-bearing mice with hMSC cell carriers loaded with Ad-hOC-E1 had very limited effects on tumor growth, but the loaded hMSCs combined with vitamin D 3 treatment induced effective viral delivery to RCC tumors and significant tumor regression. Therapeutic effects of hMSC-based Ad-hOC-E1 delivery were confirmed to be significantly greater than those of injection of carrier-free Ad-hOC-E1. Our results presented the first preclinical demonstration of a novel controllable cell-based gene delivery strategy that combines the advantages of tumor tropism and vitamin D 3-regulatable human osteocalcin promoter-directed gene expression of hMSCs to improve oncolytic virotherapy for advanced RCC.
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