Armed oncolytic measles vaccine virus–a novel therapeutic agent for virotherapy of sarcomas of gastrointestinal and non-gastrointestinal origin

Introduction: The oncolytic potential of measles vaccine virus (MeV) has been demonstrated in several tumor models but it has also been shown that primary and secondary resistances exist or may arise. To improve the oncolytic potential of MeV and to overcome resistances, a suicide gene (SCD) was inserted (MeV-SCD) which allows the systemic application of the prodrug 5-fluorocytosine (5-FC) which is locally converted into the chemotherapeutic 5-fluorouracil (5-FU). 5-FU is diffusible and can kill primarily non-infected neighbouring cells. Aims: To investigate if MeV-SCD in combination with the prodrug 5-FC is a suitable tool to overcome primary resistances towards MeV mediated oncolysis. In addition, the molecular mechanisms of resistance should be elucidated. Results: MeV-SCD was used to infect eight sarcoma cell lines in the presence or absence of the prodrug 5-FC. Five cell lines were found to be susceptible to MeV-SCD mediated oncolysis whereas three showed a primary resistance which could be overcome by employment of an increased input of viral particles and application of the prodrug 5-FC. To elucidate the molecular mechanism of tumor cell resistance versus susceptibility we had a closer look at one MeV resistant (SRH) and one MeV susceptible (BRZ) sarcoma cell line. Viral growth curves revealed an inhibition of viral replication in the resistant sarcoma cell line. Real time PCR analyses showed a strong induction of TLR3, MDA5 as well as interferon beta mRNA in the resistant cell line whereas there was no induction of these transcripts in the susceptible cell line. Western Blot analyses showed constitutive phosphorylation of Stat1 in the resistant cell line which was increased after infection with MeV-SCD. No Stat1 phosphorylation/Stat1 activation could be detected in the susceptible cell line. Conclusion: Differences in interferon signaling were found to account for resistance or susceptibility of tumor cells towards MeV mediated oncolysis.