Overcoming oncolysis resistance by double infections with vaccine-derived state-of-the-art virotherapeutics
Introduction: Virotherapy is a promising new approach in cancer therapy. However, in prior work we have identified tumor cell lines originating from colon (HCT-15, KM12) or renal (ACHN) cancer exhibiting a primary resistance towards vaccine-derived virotherapeutics (vaccinia virus, VACV; measles vaccine virus, MeV); variants of both viruses are currently tested in clinical phase I/II trials (e.g., NCT01443260 & NCT00408590). To optimize the outcome of the virotherapeutic treatment we here investigated different schemes of combinatorial applications of recombinant oncolytic viruses of VACV and MeV origin.
Aim: Evaluation of combinatorial virotherapeutic treatment regimens by performing double infections with (i) VACV-derived vector GLV-1h254 (encoding TurboFP635) and (ii) MeV-derived reporter gene vector MeV-GFP (encoding GFP).
Methods: We first identified a critical MOI (multiplicity of infection) for each virus vector which induced a tumor cell mass reduction of less than 25%. Secondly, we tested different orders of double infection approaches (first VACV, second MeV; and vice versa), different time points for secondary virus infections (2 – 24 hrs) and different dosages of both viruses (MOIs 0.001 – 1). Infected cells were visualized via fluorescence microscopy; tumor cell mass reduction was measured by SRB and MTT viability assays.
Results: Both viruses were able to infect target tumor cells. However, when performing sequential infections we observed the phenomenon of “viral competition”: single tumor cells were mostly infected by a single type of virus and not by both viruses. Nevertheless, when applying both viruses at low MOIs sequentially, oncolytic destruction of tumor cells worked much better in comparison to single virus infections. This effect was pronounced when GLV-1h254 was applied first followed by secondary infection with MeV-GFP. Only slight differences were observed when time points of addition of the second virus were varied.
Conclusion: Primary resistance to oncolysis could possibly be overcome by double infections employing vaccine-derived state-of-the-art virotherapeutics exhibiting an excellent safety profile. Thus, our newly established combinatorial virotherapeutic treatment regimens should further be improved.