Cancer-Killing Viruses

Viruses have been a problem for humans for a long long time. From the day we are born, some virus or another infects us. Some can cause life-threatening disease (AIDS, Ebola). Others can make us very ill but in time we will recover (Measles, Mumps, Varicella-Zoster (chickenpox, shingles virus)). While still others are an annoyance and make us ill but not ill enough to stay home from work (common cold viruses (rhinoviruses)). Most viruses are very picky about the cell types they infect. Some prefer the cells that line our nasal passages (rhinoviruses). Others only infect certain white blood cells (AIDS virus). Another virus only infects cells that have an activated Ras pathway (Reovirus).

Cancer has also been a very common problem for us as well. Cancer is the second most common cause of death for people living in the United States. Unfortunately, many of the treatments for cancer are also quite toxic and make a person very ill. If only there was a way to selectively kill the cancer cells without harming the normal human cells. Certain viruses may be an answer to this problem.

About 30 percent of all human tumors have activated the Ras pathway. Scientists reported in Science (Nov. 13, 1998, Vol. 282, pg 1332-1334) that a mouse virus (Reovirus) can only infect and kill cells that have their Ras pathway activated. These researchers lead by Dr. Patrick W. K. Lee demonstrated that this reovirus could kill certain tumor cells in test tubes as well as in mice.

Special mice lacking an immune system were used in some of their experiments. These mice will not reject the human cancer cells and they develop tumors quite rapidly. Once the human tumors had developed to a certain size (0.31cm²) they injected the tumors with reovirus. Of the 8 mice injected, 6 mice showed an 80 percent repression in tumor growth. When they looked at the normal mouse cells around the tumors, they were unable to detect infection of the normal mouse cells. This is a great finding. A virus that kills tumor cells but not normal tissue. They then went on to show that mice with tumors and a normal immune system could be injected with the reovirus and the virus could eliminate the tumor cells.

This reovirus does not cause disease in humans. It will only infect the tumor cells and kill them. This may be a more selective way of killing tumor cells without destroying normal cells. There are still problems with this method of killing cancer cells. The virus must be directly injected in the tumors. It won't be much use for cancers that are not injectable.