Immune enhancement of nitroreductase‐induced cytotoxicity: Studies using a bicistronic adenovirus vector

Abstract

The nitroreductase (NR)/CB1954 enzyme prodrug system has given promising results in preclinical studies and is currently being assessed in phase I clinical trials. It is well established that there is an immune component to the bystander effect observed with other systems such as thymidine kinase and cytosine deaminase; however, such an effect has not previously been described using NR. We have preliminary data suggesting an immune bystander effect with NR to further examine these effects and their potential enhancement by cytokines, an adenoviral vector containing CMV-NR, an internal ribosome entry site (IRES) and the gene for murine GM-CSF (mGM-CSF) was constructed. The NR-GM-CSF virus was validated in 2 experimental models and demonstrated increased therapeutic efficacy in the MC26 murine colorectal tumour model. These data illustrate that the combination of suicide gene therapy using NR and CB1954 with immune stimulation via GM-CSF gives an improved response compared to either modality alone and suggests that the immune component of this response may be beneficial in combating unresectable, metastatic disease and preventing tumour recurrence.

Mechanism of cell death induced by the novel enzyme-prodrug combination, nitroreductase/CB1954, and identification of synergism with 5-fluorouracil

“…This distinction is important as it is thought that apoptotic cell death may not be optimal in generating immune responses to tumour antigens, which might enhance tumour cell killing in vivo (Melcher et al, 1999;Steinman et al, 2000). So, immune-mediated bystander killing in vivo may be enhanced by combined NR/CB1954 gene therapy with immunostimulatory molecules such as GM-CSF (Green et al, 2003).…”

Enhanced efficacy of Escherichia coli nitroreductase/CB1954 prodrug activation gene therapy using an E1B-55K-deleted oncolytic adenovirus vector

“…Our previous work suggests that NTR/CB1954-induced cell death also contributes to an antitumour immune response and such an effect could be further enhanced in the context of an oncolytic virus, providing an additional immune bystander effect. 33 CB1954 exposure of cells infected with CRAd-NTR(PS1217H6) led to a reduction in the yield of virus progeny ( Figure 5). Similar effects have also been observed on the replication of CRAds expressing TK or CD, when treated with their respective prodrugs.…”

Testing double mutants of the enzyme nitroreductase for enhanced cell sensitisation to prodrugs: Effects of combining beneficial single mutations

“…We are therefore investigating a number of complementary strategies that could improve efficacy, including co-expression of the cytokine GM-CSF [29], use of conditionally replicating adenovirus vectors [30,31], and engineering the enzyme NTR for greater catalytic activity with CB1954 [32,33].…”