A genetically modified herpes virus has been created that can block the spread of breast and ovarian cancer.
Scientists believe the virus, which has been reprogrammed so it no longer harms humans, could form the basis of a potent new cancer treatment.
The "oncolytic" herpes simplex virus (HSV) attacks especially aggressive tumours that have an over-active Her-2 gene.
Breast cancer drug Herceptin was designed to target the same cell sub-group.
When the modified virus was injected into mice growing human breast and ovarian tumours, it strongly inhibited the spread of cancer cells.
Cancer spread, or metastasis, is the main cause of death in cancer patients.
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Lead scientist Professor Gabriella Campadelli-Fiume, from the University of Bologna in Italy, said: "Numerous laboratories worldwide are using viruses as more specific weapons against cancer cells, called oncolytic viruses.
"Safety concerns prevailed so far, and all oncolytic herpes viruses now in clinical trials are debilitated viruses, effective only against a fraction of tumours.
"We were the first to obtain a herpes virus reprogrammed to enter Her-2-positive tumour cells, unable to infect any other cell, yet preserves the full-blown killing capacity of the wild-type HSV."
The research is reported in the online journal Public Library of Science Pathogens.
In mice with breast and ovarian cancer, the virus prevented tumour spread through the abdominal cavity.
Breast cancer spread to the ovaries and brain was greatly reduced, but the virus did not prevent lung metastasis.