Breaking tumor resistances by broad spectrum assessment of virotherapeutic resistance phenomena

Introduction: Measles vaccine virus (MeV) has proven its oncolytic potential in tumor models and early clinical trials, but primary and secondary resistances often emerge. In order to increase efficiency and to overcome resistance phenomena, a recombinant suicide gene armed MeV vaccine strain (MeV-SCD) was generated, converting the non-toxic prodrug 5-fluorocytosine (5-FC) into the cytotoxic compound 5-fluorouracil (5-FU). Because of its ability to diffuse freely, 5-FU can also kill neighbouring cells which are not infected primarily (“bystander effect”). Aim: We analyzed mechanisms of resistance to MeV-SCD and developed means to overcome them. Methods: All experiments were done based on the well-characterized NCI–60 cell panel which consists of 60 tumor cell lines, comprising tumors of both gastrointestinal and other origins. Tumor cells were infected with MeV-SCD using several multiplicities of infection (MOI) in the presence and in the absence of the prodrug. 96 hrs after infection the remaining cell mass was determined using a standard viability assay. Percentages of infected tumor cells were quantified by infection with marker gene encoding vector MeV-GFP and subsequent FACS analysis. To investigate replication of MeV vectors, viral growth curves were generated. Viral protein expression was confirmed by Western Blot. Expression of the MeV vaccine strain receptor was proven via FACS analysis. Results: Six cell lines proved to be highly resistant, i.e. loss of cell mass was less than 25% 96 hrs after infection with MeV-SCD (MOI 1, no prodrug added). We successfully overcame these resistance phenomena by increasing MOI and by addition of 5-FC. Conclusion: Resistance phenomena of many different tumor entities can be successfully overcome by usage of the suicide gene function of MeV-SCD. This probably will enable a much more efficient virotherapeutic treatment of patients exhibiting tumors of both gastrointestinal and non-gastrointestinal origin.