- Canadian scientists have discovered a
virus that infects and kills cancer cells without affecting normal cells.
- The virus, called a reovirus, has been
used to kill a human cancer tumour transplanted into a mouse.
- About six different viruses that kill
cancer cells without affecting normal cells have now been discovered.
- Although work on them is still at quite
an early stage, they are regarded as a promising approach to therapy because,
unlike existing cancer therapy using chemicals or radiation, they appear
to have no side effects on normal tissue.
- Because the viruses spread naturally
from cancer cell to cancer cell, there is also the hope that they might
be capable of completely eliminating tumours, whereas some cancer cells
often survive conventional cancer treatments and grow to form new tumours.
- Normal cells
- Reoviruses, the type being used by the
team at the University of Calgary in Alberta, Canada, are completely harmless
to normal, healthy cells.
- "We found that the receptor [for
the virus] is very ubiquitous, so the virus attaches itself to all types
of cells," virologist Dr Patrick Lee said.
- "But we found that not all the cells
it binds to are infected by this virus. We found clues that oncogenes,
genes that cause cancer, could be important in allowing the virus to cause
- "So we introduced oncogenes into
cells that are normally resistant to reovirus infection, and lo and behold
we found that once the oncogenes had made the cells cancerous, they became
susceptible to reovirus infection."
- Normally, cells are naturally protected
against reovirus. But a reovirus kills cancer cells because an oncogene
known as the Ras gene makes a protein that destroys that protection.
- Brain tumour
- Next, Dr Lee tested the reovirus against
laboratory cultures of cancer cells from human breast, prostate and pancreatic
cancers, with good results. Then he tested it against a human brain tumour
transplanted into mice.
- "We injected the tumours directly
with the virus," he said. "We were able to see tumour regression
within three to four weeks. The regression appears to be complete and the
mice are still living after five to six months."
- The tumour tissue seems to have been
completely eliminated. The next step is tests in human patients.
- Dr Lee has applied to the Canadian regulatory
authorities for permission to carry out human trials, and hopes it will
be possible to start the first tests, designed to see if the treatment
is safe to use and how best to use it, within a year to 18 months.
- If the treatment does prove valuable
then it will only be for some cancers - only some have the Ras oncogene
that makes them vulnerable to the virus. Some other cancers may also be
too inaccessible for the virus to be used.