**6. Conclusions**

With the translation of oncolytic virotherapy to clinical trials for GBM patients, impressive responses have been documented in small subgroups of patients. To increase these response rates, better understanding of factors affecting viral replication, oncolysis and subsequent immune activation is required for each of the OVs under development. Studies in clinically-relevant in vitro and in vivo models as well as trial-associated immune and tumor monitoring studies are crucial for defining these factors and are expected to offer leads for stratification of patients in future OV trials.

Based on current literature reviews, we have identified factors related to the sensitivity of GBM tumors for specific OVs including the expression of the viral entry molecules, activation state of (cell cycle) signaling or autophagy pathways and the induction of specific antiviral signaling pathways that clear the virus infection (Table 2). These individual markers of sensitivity or resistance may together yield predictive profiles. However, further investigations are needed to shed light on the interplay between oncolytic activity on the tumor cells and the immune system. Ultimately, the complexity of these interactions in a background of a heterogeneous tumor and interpatient immune status variations, may require development of personalized ex vivo models to aid in identifying the most promising OV for a specific patient. The convergence of these developments toward applicable tools will enable classification of each GBM patient as sensitive or resistant to specific OVs. Ideally, future clinical trial design will incorporate more than one OV in parallel arms, such that patients can be stratified to the OV that best matches their tumor properties and/or immune status (Figure 2). Such a selection and stratification approach is expected to significantly improve response rates in OV trials for GBM patients.


**Table 2.** Potential predictive markers for sensitivity or resistance to OVs. **Table 2.** Potential predictive markers for sensitivity or resistance to OVs.

**Figure 2.** Personalized oncolytic virotherapy. In an envisioned personalized OV trial, resected tumor material from each patient would be used for diagnostic tests to identify the most potent OV, in terms of oncolysis for the particular tumor. Additional tests to determine local and/or systemic immune status after OV infection could further aid in stratifying the patients based on their unique tumor microenvironment and immune status. Created with BioRender.com. **Figure 2.** Personalized oncolytic virotherapy. In an envisioned personalized OV trial, resected tumor material from each patient would be used for diagnostic tests to identify the most potent OV, in terms of oncolysis for the particular tumor. Additional tests to determine local and/or systemic immune status after OV infection could further aid in stratifying the patients based on their unique tumor microenvironment and immune status. Created with BioRender.com.

**Funding:** The authors gratefully acknowledge the support from Foundation Overleven met Al-

**Conflicts of Interest:** No conflict of interests to disclose.

**Funding:** The authors gratefully acknowledge the support from Foundation Overleven met Alvleesklierkanker and Foundation Support Casper.

**Conflicts of Interest:** No conflict of interest to disclose.
