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Purpose: The integrin-targeted conditionally replicating adenovirus Ad5-delta 24RGD has been shown to possess strong oncolytic activity in experimental tumors and is currently being developed toward phase I clinical evaluation for ovarian cancer and malignant glioma. Previously, we reported that combination therapy of Ad5-delta 24RGD with irradiation led to synergistic antitumor activity in s.c. glioma xenografts. In the current study, the underlying mechanism of action to this synergy was studied and the effects of combined therapy were assessed in an orthotopic glioma model., Experimental Design and Results: Sequencing studies in U-87 monolayers showed that delivery of irradiation before Ad5-delta 24RGD infection led to a greater oncolytic effect than simultaneous delivery or infection before irradiation. This effect was not due to enhanced virus production or release. Experiments using a luciferase-encoding vector revealed a small increase in transgene expression in irradiated cells. In tumor spheroids, combination therapy was more effective than Ad5-delta 24RGD or irradiation alone. Staining of spheroid sections showed improved penetration of virus to the core of irradiated spheroids. Mice bearing intracranial tumors received a combination of Ad5-delta 24RGD with 1 x 5 Gy total body irradiation or with 2 x 6 Gy whole brain irradiation. In contrast to the in vitro data and reported results in s.c. tumors, addition of radiotherapy did not significantly enhance the antitumor effect of Ad5-delta 24RGD., Conclusions: Combined treatment with Ad5-delta 24RGD and irradiation shows enhanced antitumor activity in vitro and in s.c. tumors, but not in an orthotopic glioma model. These differential results underscore the significance of the selected tumor model in assessing the effects of combination therapies with oncolytic adenoviruses.

Background: The conditionally replicating adenovirus (CRAd) AdDelta24-p53 replicates selectively in Rb mutant cells, and encodes the p53 suppressor protein. It has shown improved oncolytic potency compared to the parental control AdDelta24. As exogenous p53 has been shown to enhance radiosensitivity, the combination of AdDelta24-p53 and AdDelta24 with radiotherapy was assessed in vitro and in vivo against the therapy resistant gliomas., Methods: In glioma cells, multicellular spheroids and animal xenografts the efficacy of combination therapy was assessed. P53 phosphorylation, induction of apoptosis and viral replication were determined following single or combination therapies., Results: In vitro, AdDelta24-p53 was more effective against glioma cells than the control AdDelta24. Addition of irradiation equally increased the efficacy of both AdDelta24-p53 and AdDelta24 resulting in improved oncolysis compared to single agent treatment. Radiotherapy did not significantly change the replication kinetics of AdDelta24-p53 or AdDelta24. No detectable increase in p53 phosphorylation was observed but combination of radiotherapy and AdDelta24-p53 caused an increase in the percentage of apoptotic cells. In vivo, combination therapy with either AdDelta24 or AdDelta24-p53 significantly increased the number of mice demonstrating tumor regression (100%) as well as long-term survival (50%). No differences between viruses were noted., Conclusions: Exogenous p53 expression does not appear to increase the synergistic interaction of CRAds combined with radiotherapy. These results however do indicate that radiotherapy provides the time frame in which AdDelta24 and AdDelta24-p53 can eradicate established tumors that would otherwise escape treatment, and establishes the need to combine these modalities to form an effective anti-cancer treatment.