Assessment of virotherapy resistance phenomena with subsequent evaluation of means helping to overcome such resistancies

Introduction: Measles vaccine virus (MeV) has shown to possess profound oncolytic capability. However, tumor resistances to MeV might endanger broad clinical success, a hypothesis which is underlined by our analysis of the NCI-60 tumor cell panel being infected with a suicide gene-armed vector (MeV ld-SCD); this SCD suicide gene locally converts the non-toxic prodrug 5-fluorocytosine (5-FC) into the cytotoxic chemotherapeutic agent 5-fluorouracil (5-FU), which is able to kill also neighbouring cells by diffusion („bystander effect”).

Aim: We aimed at identifying mechanisms of resistance against the oncolytic vector MeV ld-SCD and searched for simple ways to overcome resistances.

Methods: Experiments were performed based on the well-characterized NCI-60 tumor cell panel comprising 60 tumors of gastrointestinal and other origin. 96 hours post infection (hpi) at a multiplicity of infection (MOI) of 1 the remaining tumor cell mass was determined using a standard viability assay. Expression of MeV vaccine strain CD46 receptor was proven on all cell lines by FACS analysis. Primary infection rates of resistant cell lines were determined by infection with a MeV strain encoding for green fluorescent protein (MeV ld-GFP) and subsequent FACS analysis. Virus growth curves were generated in order to examine replication of MeV. MeV protein expression and interferon response of infected cells was determined by Western Blot.

Results: 50% of the tested cell lines proved to be susceptible to MeV, exhibiting a loss of cell mass >50%. Nearly 40% of the cell lines were categorized as partially resistant (50–75% remaining cell mass). Six high-grade resistant tumor cell lines showed a remaining cell mass >75%. Interestingly, these high-grade resistant tumor cell lines showed a high variability in (i) primary infection rates by MeV, in (ii) inhibition of virus replication, and in (iii) interferon response. Irrespective of this, all of these resistance mechanisms were shown to be overcome by increase of MOI combined with addition of the prodrug 5-FC.

Conclusion: All resistance phenomena could be easily overcome by usage of the suicide gene function of MeV ld-SCD, thus enabling a much more efficient treatment of patients exhibiting gastrointestinal or other tumors.