Tumour vasculature: Friend or foe of oncolytic viruses?

In the past two decades there have been substantial advances in understanding the anti-cancer mechanisms of oncolytic viruses (OVs). OVs can mediate their effects directly, by preferentially infecting and killing tumour cells. Additionally, OVs can indirectly generate anti-tumour immune responses. These differing mechanisms have led to a paradoxical divergence in strategies employed to further increase the potency of oncolytic virotherapies. On one hand, the tumour neovasculature is seen as a vital lifeline to the survival of the tumour, leading some to use OVs to target the tumour vasculature in hopes to starve cancers. Therapeutics causing vascular collapse can potentiate tumour hypoxia, nutrient restriction and pro-inflammatory cytokine release, which has shown promise in oncological studies. On the other hand, the same vasculature plays an important role for the dissemination of OVs, trafficking of effector cells and other therapeutics, which has prompted researchers to find ways of normalizing the vasculature to enhance infiltration of leukocytes and delivery of therapeutic agents. This article describes the recent developments of therapies aimed to shut down versus normalize tumour vasculature in order to inform researchers striving to optimize OV-based therapies.