Based on earlier experiments, scientists had hoped the drug, ML464, would block the spread of a melanoma cell line into bones. They were pleasantly surprised to find that not only did the treatment block bone metastases, it also reduced the development of new tumors in organs like the liver, intestines and kidney.
"Bone metastases appear in 75 percent of all patients who develop metastatic breast and prostate cancer," says Katherine Weilbaecher, M.D., assistant professor of medicine and of pathology and immunology. "These metastatic tumors can be very painful and weaken the bone to the point of fracture."
Weilbaecher, the principal investigator in the new study, cautions that while it might be possible to use ML464 or other anti-platelet drugs to achieve the same effect in humans, such treatments have not been tested for their anti-metastatic effects yet and would leave patients at risk of bleeding. "This is a very exciting start, but it's just the beginning," says Weilbaecher. "The more we can understand this, the more narrowly we can target our therapy and explore the possibility that we might be able to block metastasis and only partially block clotting function."
The results are published today in the online early edition of the Proceedings of the National Academy of Sciences.
Weilbaecher's research group has been studying connections between bone metastases and osteoclasts, cells in bone marrow that normally break down the materials in bone for routine replacement. Scientists suspected that osteoclasts aid tumors' destruction of bone because they can make acid, an essential ingredient for breaking into bone.
Suzanne Bakewell, a Ph.D. graduate student in Weilbaecher's lab, led a series of experiments in mice that began with a test of the potential link between osteoclasts and bone metastases. After genetically disabling a protein important to osteoclasts, beta3 integrin, researchers injected the mice with melanoma tumor cells altered to produce a black pigment that makes them easy to spot.
"This is a very virulent cancer cell line," Weilbaecher says. "In 14 days, 75 to 80 percent of normal mice injected with these cells will have disseminated tumors throughout the body, including their bones and bone marrow, the spongy material inside bones that produces blood cells."
In contrast, the experimental mice lacking the beta3 integrin developed tumors in other parts of the body but had no tumor cells in their bones or bone marrow.
Because beta3 integrin is known to have a prominent role in other tissues of the body, the group then conducted an experiment involving bone marrow transplants from the genetically engineered mice into normal mice. The transplants protected normal mice from bone and bone marrow tumors, proving that the protective effects came from factors in the bone marrow, the place where osteoclasts are found.
However, the next experiment, conducted on mice genetically engineered with a defect very specific to osteoclasts, failed to produce equal levels of cancer protection. The tumors couldn't get into the bone itself, but they proliferated in the bone marrow.
"We were completely surprised by this," Weilbaecher says. "Blocking osteoclast function still seemed to be linked to less bone destruction by bone metastases, but that didn't tell us why these mice developed so many tumors in the bone marrow while mice with defective beta3 integrin didn't."
The group then turned to the next most likely cause of the protective effect: platelets, bits of membrane in the bloodstream that clump together to form blood clots. Like osteoclasts, they are produced in bone marrow, and a form of beta3 integrin plays a prominent role in their activity. Other researchers have linked platelets to the spread of lung tumors, and patients with metastatic cancer frequently have high platelet counts and excessive blood-clotting activity.
Weilbaecher's group treated experimental mice with high doses of ML464, which specifically blocks the form of beta3 integrin found on platelets. They dosed the mice every12 hours for the first two and a half days of the 14-day experiment.
"We gave the mice a dose of ML464 that would block all platelet aggregation," says Weilbaecher. "During this period, they were very susceptible to bleeding. No surgeon would have wanted to operate on them."
Injected cancer cells given to the experimental mice thirty minutes after the anti-clotting drug never made it into the bone or bone marrow, and were rarely able to find a foothold elsewhere and start building a tumor.
"The mice treated with the drug had much fewer metastases, and when they did get metastases they were smaller," Weilbaecher says. "There are other drugs that block platelet beta 3 integrin that are routinely used in patients who receive coronary artery stents, so this is definitely something that's worth exploring for potential clinical application."
Weilbaecher and others are working on several hypotheses for how platelets may help tumor cells metastasize. Most theories assume that platelets bind to tumor cells circulating in the bloodstream, and then begin to bind to other platelets, gathering tumor cells together. The platelets may hide tumor cells from the immune system, supply them with essential growth factors or just provide them with a ride.
"An aspirin a day is a very potent blocker of platelet function--it can impact survival in heart attack patients, because you get less clotting. And you don't need very much dosage to reduce cardiac risk," Weilbaecher notes. "Here, for metastasis prevention, I can't tell you if we need a lot of this anti-platelet effect or a little, or whether other drugs like aspirin or ticlopodine would be effective. That hasn't been explored yet in this model, but it will be."
Bakewell SJ, Nestor P, Prasad S, Dowland N, Mehrotra M, Scarborough R, Kanter J, Abe K, Phillips D, Weilbaecher K. Platelet and osteoclast B3 integrins are critical for bone metastasis. Proceedings of the National Academy Sciences, early online edition, November 3, 2003.
Funding from National Institutes of Health, National Institute on Aging, the Barnes Jewish Foundation Grant and the Edward G. Mallinckrodt, Jr foundation Grant.
The full-time and volunteer faculty of Washington University School of Medicine are the physicians and surgeons of Barnes-Jewish and St. Louis Children's hospitals. The School of Medicine is one of the leading medical research, teaching and patient-care institutions in the nation. Through its affiliations with Barnes-Jewish and St. Louis Children's hospitals, the School of Medicine is linked to BJC HealthCare.