WASHINGTON - Blood-thinning drugs such as aspirin may help fight cancer by denying shelter to wandering tumor cells, U.S. researchers reported on Friday.

Experiments in mice showed that combining aspirin with an experimental anti-clotting drug slowed the growth and spread of breast and melanoma tumors.

Blood cells called platelets shelter and feed tumor cells in the bloodstream, making it easier for cancer to spread, or metastasize, the team at Washington University in St. Louis said.

Writing in the Journal of Cellular Biochemistry, they said inactivating platelets may help slow or prevent this spread.

The study could help support other findings that show people who take aspirin or similar drugs that affect a gene and protein called COX-2, including aspirin, ibuprofen and the COX-2 inhibitor Celebrex, have a lower risk of some cancers.

There is also some suggestion that taking aspirin or ibuprofen along with chemotherapy may make the chemo more effective.

"Past research has shown that tumor cells activate platelets and that mice with defective platelets have significantly fewer metastases," Dr. Katherine Weilbaecher, who helped lead the study, said in a statement.

"We also know that platelets have several traits that can aid tumor cells, and we are working to break up that potentially lethal partnership."

The researchers used ordinary aspirin combined with an experimental antiplatelet drug called APT102. Made by St. Louis-based APT Therapeutics, the drug interferes with clotting.

When they injected mice with breast cancer and melanoma cells, the tumors quickly spread to the bone.

But when the mice got aspirin and APT102, the tumors that grew and spread were much smaller. Neither drug had an effect on its own, perhaps because platelet-making processes must be attacked from several angles, Weilbaecher said.

"Aspirin prevents platelets from making thromboxane, a substance that facilitates clotting," Weilbaecher said.

"APT102 is an especially interesting drug because it gets rid of a compound called ADP, which tumor cells release and which stimulates platelets to clump. So APT102 prevents platelet activation in response to tumor cells."

The company provided the drug but did not pay for the study.

"Anti-platelet drugs such as (aspirin) plus APT102 could be valuable experimental tools for studying the role of platelet activation in metastasis as well as a therapeutic option for the prevention of bone metastases," the researchers wrote.

Weilbaecher and colleagues are testing their theory in women with advanced breast cancer to test aspirin and the anti-clotting drug Plavix, another antiplatelet drug, to see if the combination reduces the number of tumor cells in the blood.

Plavix, one of the world's best-selling drugs, is sold by Bristol-Myers Squibb Co and Sanofi-Aventis.