A novel suicide gene-expressing oncolytic measles vaccine virus for the treatment of hepatocellular carcinoma

Introduction: Despite tremendous efforts in the search for new therapies, a truly efficient cure for hepatocellular carcinoma (HCC) is not available. Oncolytic viruses are promising agents under investigation as new cancer therapeutics. These replication competent viruses selectively infect and kill cancer cells. To enhance cell killing, oncolytic viruses can be engineered to express suicide genes which convert non-toxic substances (prodrugs) into toxic metabolites.

Aims: Development of new anti-HCC regimens based on oncolytic measles vaccine virus (MeV) expressing functionally optimized suicide genes.

Methods: By means of reverse genetics, we have cloned and rescued (i) MeV vectors expressing the suicide gene SCD (MeV-SCD), a fusion protein of the yeast cytosine deaminase and yeast uracil phosphoribosyltransferase. Additionally, we have produced a (ii) MeV vector expressing VP22SCD (MeV-VP22SCD) which is a fusion of SCD to the intercellular transport protein VP22. Different hepatoma cell lines (HepG2, Hep3B, PLC/PRF/5) were used to test the susceptibility of HCC to MeV oncolysis. To enhance cell killing, infected cells were incubated with the prodrug 5-fluorocytosine (5-FC). Conversion of 5-FC to 5-FU (5-fluorouracil) was demonstrated by HPLC. Cell death pathways of the infected cells were examined by caspase assay, cell death ELISA, and Nicoletti staining. In addition to these in vitro experiments, we also tested the effect of MeV-SCD in vivo.

Results: All cell lines tested were found to be susceptible to MeV mediated oncolysis, albeit to different extends. Cell killing was shown to be enhanced by treatment of the infected cells with the prodrug 5-FC. Comparison of the different MeV vectors suggests that VP22 does not enhance the bystander effect significantly in the context of an oncolytic virus with fusogenic glycoproteins. Of note, survival of tumor bearing mice was significantly prolonged by combinatorial treatment with MeV-SCD + 5-FC.

Conclusions: Oncolytic virotherapy represents a promising addition for multimodal oncological concepts. By employing the SCD/5-FC system in combination with cytolytic MeV vectors, partial resistances of HCC to oncolysis can be overcome and the efficiency of MeV-mediated oncolysis can be enhanced significantly.