Armed replicating adenoviruses for cancer virotherapy

Abstract

Conditionally replicating adenoviruses (CRAds) have many advantages as agents for cancer virotherapy and have been safely used in human clinical trials. However, replicating adenoviruses have been limited in their ability to eliminate tumors by oncolysis. Thus, the efficacy of these agents must be improved. To this end, CRAds have been engineered to express therapeutic transgenes that exert antitumor effects independent of direct viral oncolysis. These transgenes can be expressed under native gene control elements, in which case placement within the genome determines the expression profile, or they can be controlled by exogenous promoters. The therapeutic transgenes used to arm replicating adenoviruses can be broadly classified into three groups. There are those that mediate killing of the infected cell, those that modulate the tumor microenvironment and those with immunomodulatory functions. Overall, the studies to date in animal models have shown that arming a CRAd with a rationally chosen therapeutic transgene can improve its antitumor efficacy over that of an unarmed CRAd. However, a number of obstacles must be overcome before the full potential of armed CRAds can be realized in the human clinical context. Hence, strategies are being developed to permit intravenous delivery to disseminated cancer cells, overcome the immune response and enable in vivo monitoring of the biodistribution and activity of armed CRAds.

Figure 1

Schematic diagram of the Ad5 genome. The Ad5 genome is approximately 36 kb long, divided into 100 map units. The direction of transcription is indicated by arrows. Open arrows represent early transcripts; diagonally striped arrows represent late transcripts.

Figure 2

Schematic diagram of the Ad5 E3 region showing transgene insertion sites. Open boxes represent gene products encoded by the non-essential E3 transcription unit. Solid boxes represent therapeutic transgenes inserted into the E3 transcription unit in place of native genes. Transgenes are under control of endogenous E3 gene expression elements.

Figure 3

Schematic diagram of the Ad5 genome showing insertion sites of transgenes separated by native viral genes by IRES sequences. The direction of transcription is indicated by arrows. Open arrows represent early transcripts; diagonally striped arrows represent late transcripts; solid boxes represent therapeutic transgenes; horizontally striped boxes represent IRES sequences. Transgenes are under control of endogenous viral gene expression elements.

Figure 4

Schematic diagram of E3-deleted Ad5 genome showing exogenous promoter driving the expression of a therapeutic transgene linked by an IRES to the viral E1A gene. The direction of transcription is indicated by arrows. Vertically striped arrows represent exogenous promoters; open boxes represent E1A gene; solid boxes represent therapeutic transgenes; horizontally striped boxes represent IRES sequences. Open arrows represent early transcripts; diagonally striped arrows represent late transcripts.

Figure 5

Schematic diagram of the Ad5 genome showing insertion sites of transgenes driven by exogenous promoters. The direction of transcription is indicated by arrows. Open arrows represent early transcripts; diagonally striped arrows represent late transcripts; vertically striped arrows represent exogenous promoters; solid boxes represent therapeutic transgenes.