Identification and evaluation of new oncolytic adenoviruses with enhanced therapeutic efficacy

Virotherapy with oncolytic adenoviruses has proved to be safe in clinical trials but has not fulfill the expectations regarding efficacy. After systemic delivery, preexisting immunity and sequestration by cellular and non-cellular blood components limit the number of particles that can target the tumor. Infectivity is further reduced by the lack or low expression of viral receptors on cancer cells, resulting in low efficiency of transduction. In addition, the ability of oncolytic viruses to induce a tumor-directed immune system activation is limited by the immunosuppressive status of the tumor microenvironment. Cancer cells use different immune evasion strategies. One of these is the accumulation of high levels of adenosine in the extracellular environment, which has an overall inhibitory effect on different immune cell types and promotes cancer cells growth. The present thesis addresses two independent but interconnected projects. Firstly, we reported the successful purification of twelve human adenovirus types at high titer and we analyzed the ability of the viruses to target and lyse different cancer cells after transduction in vitro. Moreover, preliminary information about the seroprevalence and the interaction with blood cells have been presented. Secondly, we evaluate an oncolytic adenovirus armed with the enzyme adenosine deaminase (ADA). The goal of this second part was to produce an ADA-encoding conditionally replicating adenovirus type 5 to induce the breakdown of the immunoinhibitory adenosine in the tumor bed. High levels of ADA were detected both in cells infected with ADA-expressing CRAd5 and in cell culture supernatants after virus-induced cell lysis, which were also proven to be enzymatically functional.