Delta-24 increases the expression and activity of topoisomerase I and enhances the antiglioma effect of irinotecan

Purpose: In this study, we sought to determine whether Delta-24 could sensitize glioma cells to the topoisomerase I inhibitor irinotecan (CPT-11) and to identify the mechanisms underlying this enhanced anticancer effect. Experimental Design: We used human glioblastoma cell lines for the in vitro studies. The expression of topoisomerase I was determined in Western blot analyses, and topoisomerase I activity was determined by measuring the relaxation of a supercoiled DNA. The cell cycle distribution of cells was determined by flow cytometry analysis of the cellular DNA content. Cell viability was quantified by a 3-(4,5-dimethylthiazol-2-yl)-2,5- diphenyltetrazolium bromide assay. Tissue culture infection dose assays were used to quantitate adenovirus replication. For the in vivo studies, athymic mice received intracranial/intratumoral injections of Delta-24 in combination with CPT-11, after which animal survival was monitored. Results: Delta-24 infection caused human glioma cells to accumulate in the S phase and induced the expression and activity of topoisomerase I as shown by Western blot and in vitro enzymatic activity assays. Further, we showed that the sequential administration of Delta-24 and CPT-11 to human glioma cell cultures potentiated the CPT-11-mediated anticancer effect in vitro without modifying the replicative phenotype of the oncolytic adenovirus. In vivo experiments showed that the single intratumoral administration of Delta-24 to intracranially implanted human glioma xenografts followed by the systemic administration of CPT-11 resulted in significantly prolonged animal survival. Conclusions: The combination of Delta-24 treatment with CPT-11 showed an enhanced anticancer effect, which suggests that the interaction between adenoviral and human proteins can be exploited in rational anticancer therapies comprising replication-competent adenoviruses and conventional chemotherapeutic agents.