Telomerase-selective oncolytic adenovirus OBP-405 induces autophagic cell death and enhances the antitumor effect of temozolomide and rapamycin on glioblastoma cells    in vitro and in vivo. Free

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Oncolytic adenoviruses are considered as a promising tool in cancer therapy. We have recently reported that hTERT-Ad, a conditionally replicating adenovirus regulated by the human telomerase reverse transcriptase promoter, has a great antitumor effect through induction of non-apoptotic autophagy (J. Natl. Cancer Inst., 98: 625-36, 2006). In the present study, we examined the in vitro and in vivo effects of hTERT-Ad with a tropism modification with RGD (OBP-405, Telomelysin-RGD) on the established cell lines (U87-MG, U251-MG, D54, and U373-MG) and primary cells (MDC-01) of the most malignant primary brain tumor, glioblastoma multiforme (GBM). OBP-405 exhibited a stronger antitumor effect regardless of the expression level of the coxsackievirus and adenovirus receptor in GBM cells than hTERT-Ad without RDG (OBP-301, Telomelysis). When OBP-405-induced autophagy was inhibited pharmacologically (by 3-methyladenine) or genetically (by small interfering RNA against autophagy-related gene, Atg5), the cytotoxicity of OBP-405 was suppressed. In addition, autophagy-deficient Atg5^-/- mouse embryonic fibroblast (MEFs) were less sensitive to OBP-405 than wild-type MEFs, indicating that OBP-405-induced autophagy was a cell killing effect. Because autophagy-inducing therapies might enhance the effect of OBP-405, we combined OBP-405 with a DNA alkylating agent temozolomide (TMZ) or rapamycin, an inhibitor of the mammalian target of rapamycin, which cause autophagy in GBM cells. Both treatments synergistically sensitized tumor cells to OBP-405 by stimulating the autophagic pathway. Interestingly, the quantitative real-time PCR analysis demonstrated that either TMZ or rapamycin did not affect viral replication of OBP-405. Moreover, nude mice carrying intracranial U87-MG tumors that were treated with three intratumoral injections of OBP-405 survived longer than after treatment with a single intratumoral injection of OBP-405 or non-replicating control adenovirus Ad-GFP. We are currently determining whether TMZ or rapamycin enhances the in vivo effect of OBP-405 using intracranial U87-MG tumor models. Our findings indicate that OBP-405 induces autophagic cell death in GBM cells and its efficacy is enhanced by RGD and that autophagy-inducers TMZ and rapamycin, which are clinically applicable, strengthen the antitumor effect of OBP-405. These data strongly suggest that OBP-405 is a novel approach for the treatment of patients with GBM.