Therapeutic Targeting of Melanoma Cells Using Neural Stem Cells Expressing Carboxylesterase, a CPT-11 Activating Enzyme

Neural stem cells (NSCs) have been investigated in preclinical models as delivery vehicles for therapeutic genes for treatment of tumors in the central nervous system and other organs. Melanoma at early stages is effectively treated with surgery and radiotherapy, however metastatic disease is almost universally fatal, thus novel therapeutic approaches are needed. We studied the use of HB1.F3.CD NSCs, a well-characterized clonal cell line derived from human fetal telencephalon, for their potential to secrete a prodrug-activating enzyme. HB1.F3.CD cells were transduced by adenovirus encoding rabbit carboxylesterase (rCE), which converts CPT-11 (irinotecan) into SN-38, a potent topoisomerase 1 inhibitor and anti-cancer agent. In vitro cell migration assays revealed robust migration of NSCs to conditioned media from human melanoma cells. Cytokine profiles showed that IL-6, IL-8, MCP-1 and TIMP-2, known chemoattractants for stem cells, were highly expressed by melanoma cells. Exposure of melanoma cells to conditioned media from the HB1.F3.CD.rCE cells in the presence of CPT-11 increased the tumor cell-killing effect by approximately 100-fold when compared to CPT-11 alone. Our data demonstrate the rationale for a NSC-based enzyme/prodrug therapeutic approach to target metastatic melanoma. Future experiments will evaluate the therapeutic efficacy of NSC-mediated melanoma therapy in animal models, which will provide the basis for targeted therapy in patients with advanced melanoma.