It treats the cancer with existing HIV drugs. It operates not by killing cells or slowing their growth, but by blocking their journey to other parts of the body, where they become deadly. It works against a particularly aggressive category of breast cancers, known as triple negative, for which there are no targeted therapies.

"I wish we could have things like that we could give patients," said Richard Pestell, senior author of the paper published online in Cancer Research, which found the effects in mice.

The scientists had analyzed 2,254 human breast cancer specimens and noticed that certain subtypes produced more of a hormone (CCL5) that cells send as signals to other cells, and had more of the receptors (CCR5) to which it attaches.

The HIV virus invades immune cells by attaching to the same receptor. Two HIV medicines — maraviroc, which is used in current drug "cocktails," and vicriviroc, which passed safety trials but was abandoned by Merck & Co. Inc. because it wasn't effective enough — block that receptor.

When the Jefferson researchers introduced both drugs to breast cancer cells in the lab, and then injected maraviroc into mice with the cancer subtypes, they found that the cancers didn't travel.

"It was dramatic," said Pestell, recalling the moment he saw a 90 percent drop in metastases to mouse lungs. "It is the lung metastases that kills patients."

The research is still early. The next step is more mouse tests, said Pestell, director of the Kimmel Cancer Center. Since both drugs already passed safety trials for HIV, he said, the road to approval might be as short as five years.

An estimated 20 percent of breast cancers are classified as triple negative. They lack the receptors needed for targeted drugs like tamoxifen and Herceptin. They also are much more likely than others to metastasize.

Standard chemotherapy prevents that spread, but is so toxic that it cannot be tolerated for more than several months. The HIV drugs are relatively safe and are currently used as long-term treatment.

Contact Don Sapatkin at 215-854-2617 or dsapatkin@phillynews.com.