E4-6/7 protein forms a complex with two E2F transcription factors and induces the cooperative and stable binding of this complex to the palindromic E2F-binding site structure present in theE2promoter20and in theE1Apromoter of ICOVIR-15, activating a positive feedback that should culminate in higher production of virus progeny and lysis of the tumor cell. the endogenousE1Apromoter. The insertion of these sites controlling E1A-24 results in a low systemic toxicity profile in mice. Importantly, the E2F-binding sites also increased the cytotoxicity and the systemic antitumor activity relative to wild-type adenovirus in all cancer models tested. The low toxicity and the increased potency results in improved antitumor efficacy after systemic injection and increased survival of mice carrying tumors. Furthermore, the constrained genome size of this backbone allows an efficient and potent expression of transgenes, indicating that this computer virus holds promise for overcoming the limitations of oncolytic adenoviral therapy. == Introduction == Despite great advances in the treatment of cancer, it remains one of the leading causes of mortality worldwide. Therefore, research on novel cancer therapies with a high therapeutic index limited to malignant tissues is crucial. Among new treatments proposed to target cancer, oncolytic adenoviruses are a promising and appealing therapy due to their ability to self-amplify selectively at the tumor site.1Several oncolytic adenoviruses have already been tested in clinical trials involving a variety of tumors and routes of administration. Clinical data revealed a good toxicological and safety profile, but some potentially concerning adverse effects were observed after administration at high doses.2With regard to efficacy, most responses detected were transient and the treatment was not able to alter significantly the GCN5L course of the disease. Overall, this data indicate out a critical need for improved oncolytic potency to result in sustained therapeutic responses in humans. For an efficient treatment of tumors at an advanced stage, systemic delivery is preferred.3However, the computer virus encounters some limitations when injected systemically. First of all, it is quickly eliminated from the bloodstream by the liver or inactivated by binding to blood cells, neutralizing antibodies, or complement,4and only a minimal proportion of the Sec-O-Glucosylhamaudol injected dose reaches the tumor. Once in the tumor, the stroma and the antiviral immune response limit the spread of the computer virus throughout the tumor.5The expression of a therapeutic transgene from the adenoviral backbone is a rational and efficient approach to circumvent these limitations. Armed replicating adenoviruses are a combination of virotherapy and gene therapy strategies in which the input of transgene dose is amplified by replication of the computer virus and, above all, gene transfer can amplify the antitumor activity of virotherapy. In this regard, several transgenes have been inserted into Sec-O-Glucosylhamaudol oncolytic adenoviruses in order to increase cytotoxicity, to stimulate immune responses, or to digest the connective tissue to facilitate intratumoral spread.6However, encapsidation size of the adenovirus type 5 is limited to 105% of the wild-type genome, and larger genomes result in genetic instability and packaging problems.7E3genes have been deleted to create space for transgenes, but E3 has important immune inhibitory functions that may facilitate computer virus spread in immunocompetent hosts.8Thus, further research is needed to optimize the transgene expression machinery and the adenoviral backbone in which the transgene is inserted in order to Sec-O-Glucosylhamaudol minimize genome size and make transgene expression compatible with both selective and potent replication. Taking into consideration the concerning adverse effects observed in clinical trials after systemic injection of oncolytic adenoviruses,2it is important to restrict computer virus gene expression to tumor cells to ensure computer virus safety. In mice,E1Aexpression in hepatocytes is enough to cause transaminitis and severe liver Sec-O-Glucosylhamaudol injury,9and this toxicity is not prevented by deletions in other viral genes that confer selectivity. Modification ofE1Atranscriptional control is a useful approach to avoid this toxicity. Tissue-specific promoters have been tested in this regard to treat certain types of cancer, such as thePSApromoter to target prostate cancer10or theuPARpromoter for pancreatic tumors.11However, promoters with a broader tropism are more appealing due to their applicability to different tumor types. Those active in tumors and repressed in normal tissues, such asE2F-1(ref.12) orhTERT,13are an ideal option. Nevertheless, some losses of potency with respect to wild-type transcription control were reported when these promoters were placed to controlE1A, especially when tested in a wide range of cancer models. In a recent work,14we.