The Basics about Oncolytic Viruses
A press release issued by the Ottawa Hospital Research Institute in Ottawa, Ontario, Canada, on May 20, 2010, reported that Phase 1 clinical trials conducted conjunctively in Ottawa and three American cities has resulted in a huge step forward for oncolytic therapy in fighting cancer.
The term "oncolytic" actually means cancer killing (onco - cancer; lytic - killing). Oncolytic viruses are microorganisms that have been selected or engineered to grow inside tumor cells. As the tumor evolves, mutations in multiple genes contribute to the growth of the tumor. Oncolytic viruses target cancer cells and exploit the cells' ability to grow and spread.
Researchers have been able to genetically alter these viruses to do one of three things:
1) to take advantage of tumor-specific mutations in the cancer cells' antivirus defenses;
2) to feed off a tumor's signals or programs that would normally allow the cancer cells to grow or spread;
3) to restrict the spread of cancer cells into healthy cells based on antibody generators that are specialized or overexpressed on the surface of the tumor cells (www.medscape.com).
In other words, "viruses have been engineered to take advantage of specific tumor associated molecular changes" (http://oncolyticvirus.wordpress.com). These viruses are usually non-human in origin since the human body will not have built up immunity to these viruses. Clinical studies have shown that these cancer-killing viruses have a higher ratio of toxic dose to therapeutic dose - that means that more of the virus can be tolerated by a patient before the virus actually becomes toxic. Most oncolytic viruses have an index of 100,000:1. By contrast, chemotherapy is much more toxic at 6:1.
Initial human trials using oncolytic viruses (also known as virotherapy) generated a lot of excitement but yielded tumor regression that was followed by tumor progression in the later stages of the trials. As research into these viruses and cancer cells have continued, researchers and scientists have learned how to further manipulate the viruses to enhance results.