Since the body's immune system attacks the virus, the agent is injected directly into the tumor or, in the case of metastatic liver cancer, into the hepatic artery. The virus infects and multiplies in cancer cells but does not attack normal cells. In some instances it has made large solid tumors melt to nothing. One limit on its complete success, besides immune attack, is that tumors often contain layers of normal tissue that impede the virus from gaining access to cancer cells. Both agents are defective adenoviruses, one of the causes of the common cold.
Adenoviruses get themselves replicated by forcing an infected cell to divide.
But to do so they must overcome the cell's normal constraints on cell division, just as would-be cancer cells must. The virus' arsenal includes a gene called E1B, which inactivates the cell's p53 protein, and an E1A gene, which knocks out Rb. In Onyx-015, the virus' E1B gene has been inactivated. Hence it cannot grow in normal human cells whose p53 is active. But it can grow in the many kinds of cancer cell that have knocked out their own p53 gene.