Food and Drug Administration (SFDA) for the treatment of head and neck cancer. Sunway's H101 and the very similar Onyx-15 (dl1520) have been engineered to remove a viral defense mechanism that interacts with a normal human gene p53, which is very frequently dysregulated in cancer cells. Despite the promises of early in vivo lab work, these viruses do not specifically infect cancer cells, but they still kill cancer cells preferentially. While overall survival rates are not known, short-term response rates are approximately doubled for H101 plus chemotherapy when compared to chemotherapy alone. It appears to work best when injected directly into a tumour, and when any resulting fever is not suppressed. Systemic therapy (such as through infusion through an intravenous line) is desirable for treating metastatic disease. It is now marketed under the brand name Oncorine. Mechanisms of action Immunotherapy With advances in cancer immunotherapy such as immune checkpoint inhibitors, increased attention has been given to using oncolytic viruses to increase antitumor immunity. There are two main considerations of the interaction between oncolytic viruses and the immune system. Immunity as an obstacle A major obstacle to the success of oncolytic viruses is the patient immune system which naturally attempts to deactivate any virus. This can be a particular problem for intravenous injection, where the virus must first survive interactions with the blood complement and neutralising antibodies. It has been shown that immunosuppression by chemotherapy and inhibition of the complement system can enhance oncolytic virus therapy. Pre-existing immunity can be partly avoided by using viruses that are not common human pathogens. However, this does not avoid subsequent antibody generation. Yet, some studies have shown that pre-immunity to oncolytic viruses doesn't cause a significant reduction in efficacy. Alternatively, the viral vector can be coated with a polymer such as polyethylene glycol, shielding it from antibodies, but this also prevents viral coat proteins adhering to host cells. Another way to help oncolytic viruses reach cancer growths after intravenous injection, is to hide them inside macrophages (a type of white blood cell). Macrophages automatically migrate to areas of tissue destruction, especially where oxygen levels are low, characteristic of cancer growths, and have been used successfully to deliver oncolytic viruses to prostate cancer in animals. Immunity as an ally Although it poses a hurdle by inactivating viruses, the patient's immune system can also act as an ally against tumors; infection attracts the attention of the