Blood platelets play a critical role in helping blood to clot and stop bleeding after injury. However, activated platelets can also contribute to the formation of unwanted, dangerous blood clots, and are associated with a range of disorders, including cancer. Although antiplatelet therapies are available to help address platelet-related disorders, treated patients are then at risk of uncontrolled bleeding if injured, and the effects of these drugs aren’t easily reversed.

An international research team has now developed a reversible antiplatelet treatment that harnesses deactivated human platelet decoys. The researchers, headed by a team at the Wyss Institute for Biologically Inspired Engineering at Harvard University, suggest that the new technology could feasibly be used to reduce the risk of blood clots in patients undergoing surgery, while in vivo preclinical tests also showed that the platelet decoys can help to inhibit tumor metastasis.

Platelet Decoys SEM Composite
When platelets are activated, they send out long tendrils and clump together, helping to form a blood clot (left). When the decoys were exposed to the same platelet-stimulating molecules, they failed to activate and stayed in their round “resting” shape (right). [Wyss Institute at Harvard University]
“The reversibility and immediate onset of action are major advantages of our platelet decoys, and we envision them to be useful in hospital-based situations such as preventing clotting in high-risk patients just before they undergo surgery, or when given alongside chemotherapy to prevent existing tumors from spreading,” said Anne-Laure Papa, PhD, previously a postdoctoral fellow who worked on the technology in the laboratory of the Wyss Institute’s founding director and study lead Donald Ingber, MD, PhD.

Papa, who is now an assistant professor at George Washington University, is lead author of the team’s published paper in Science Translational Medicine, which is titled, “Platelet decoys inhibit thrombosis and prevent metastatic tumor formation in preclinical models.” Research head Ingber is the Judah Folkman professor of vascular biology at Harvard Medical School and the vascular biology program at Boston Children’s Hospital, as well as professor of bioengineering at Harvard’s School of Engineering and Applied Sciences.

Platelets play a key role in regulating hemostasis, as well as protecting the body against bleeding and helping to maintain the vascular system. However, the authors explained, activated platelets can also contribute to disorders including ischemic heart disease, stroke, sepsis, and cancer, which the World Health Organization acknowledges are among the leading causes of death worldwide.

Different antiplatelet drugs have been developed to prevent unwanted thrombosis by inhibiting platelet binding and activation, but reversing the effects of these drugs requires the formation of new platelets, which takes at least a week to 10 days. As a result, the authors wrote, “the use of antiplatelet drugs is a major risk factor for patients experiencing life-threatening situations such as trauma or hemorrhage, where the need for immediate reversal of antiplatelet therapy is critical.”

Platelets and Decoys
Normal platelets in human blood samples flowing through a microfluidic chip attached in large numbers to a collagen surface (left), while the addition of decoys to the blood sample (right) greatly reduced the amount of platelet binding. [Wyss Institute at Harvard University]
Working with collaborators in the U.S., Australia, and Israel, the Wyss Institute-led team has now developed modified human platelets, or platelet decoys, which have lost the ability to aggregate and activate clotting, but which retain cell surface receptors that allow them to bind to other cells in the blood. Generating these platelet decoys—which are about a third of the size of normal platelets—involves centrifugation and the use of detergent to strip the cell fragments of their outer lipid membrane and contents.

Initial laboratory tests showed that platelet decoys didn’t display normal clotting behavior when perfused into a microfluidic channel that was designed to mimic a blood vessel. When the researchers then added decoys to normal human blood in the channel, in the ratio of one decoy to five normal platelets, the decoys were unable to bind to the channel wall, but they also prevented the normal platelets from aggregating and binding to the channel wall.

“The decoys, unlike normal intact platelets, are unable to bind to the vessel wall and likely hinder the normal platelets’ ability to bind as well,” Papa said. “A way to imagine this would be that the decoys are fast-moving skaters skating along the wall of an ice rink, and their high speed prevents other skaters from getting to the wall, thus limiting them from slowing down and grabbing onto it.”

Rapidly reversing the platelet-inhibiting effects of the decoys involved simply adding platelets. “The addition of fresh platelets significantly reversed the antiplatelet effect of the decoys …” the researchers stated. This means that in a prospective clinical setting, a patient being treated using platelet decoys could have the antiplatelet effects rapidly reversed—for example, if they became a bleeding risk or needed to regain the ability for form clots—by administering normal platelets intravenously. Infusing platelets is a procedure that is already carried out routinely in hospitals. It may also be feasible to produce decoys for a patient using their own platelets, minimizing the risks of immune reactions.

Although the technology isn’t yet ready for use in humans, tests using the decoys in rabbits showed that the same 1:5 blood ratio of decoy (D):normal platelets (P) effectively prevented blood clots developing after blood vessel injury. Rabbits are a particularly useful model of thromboembolism because they have similar circulating platelet counts and cell size distribution to humans, the authors noted.

Platelets also play a role in cancer metastasis by binding to circulating tumor cells. In a subsequent set of experiments, the researchers showed that the platelet decoys retained their ability to bind to cancer cells. Whereas human breast cancer cells were able to stick to and start to penetrate the walls of cell-lined microfluidic channels in the presence of normal platelets, adding decoys to the system prevented the normal platelets from aiding the cancer cell’s invasive behavior. “Coadministration of the decoys with intact platelets (1:5 ratio) completely inhibited the ability of the normal platelets to promote tumor cell arrest and extravasation,” the authors wrote.

Encouragingly, mice receiving human breast cancer cell transplants after treatment with both platelets and decoys developed fewer, and smaller metastatic tumors than cancer cell-bearing animals that had been retreated using normal platelets but no decoys. “… when platelet decoys were preincubated with both intact platelets and cancer cells at a P:D ratio of 5:1 or 5:2 before being injected into the mice, we observed dose-dependent inhibition of the metastatic tumor load by the addition of the decoys, with the higher decoy:platelet ratio producing significant reduction of metastatic tumor mass relative to the platelet group,” the researchers noted. These in vivo results suggest that it may one day be feasible to give human cancer patients platelet decoys alongside their chemotherapy or at the time of cancer surgery, to help prevent tumor spread.

“In this study, we report the development of human platelet decoys as a reversible, drug-free, cell-based, antiplatelet therapy that might be useful for treating patients with thrombotic disorders,” the authors concluded. “Moreover, our results in the metastatic tumor model in mice raise the possibility that the platelet decoys might also reduce metastasis formation … Unlike most of the current clinically approved antiplatelet drugs, a major potential benefit of this cellular therapeutic approach is the immediate onset of action when administered as an intravenous infusion and the ability to rapidly reverse this inhibition simply by transfusing functional platelets.”

“In this study, we were able to create what is effectively a dominant-negative cellular therapy to prevent platelet activation-induced clotting and metastatic cascades,” said Ingber. “It’s another example of how seemingly unrelated diseases often have common contributing factors, such as inflammation, stress, or in this case, activated platelets, and that we can develop new therapies for multiple disorders by targeting one of those key factors.”

Work in Papa’s lab is currently ongoing to help boost the longevity of decoys in the bloodstream, and to investigate whether they can also be loaded with drugs that could directly target blood clots or tumors, or potentially kill circulating tumor cells. “Platelet decoys also have the potential to be used as drug carriers to specifically target chemical or molecular therapies to native platelets, thrombotic sites, or circulating tumor cells, which could represent the first step toward targeted metastatic prevention therapy,” the researchers noted.

The team acknowledged that further safety and dose-escalation studies will be required, but suggest that any future clinical studies could potentially start with very low doses of patient-derived decoys used in the same patient. “Human platelets could be collected, modified, and transfused in the same patient to limit any potential toxicity, and thus, the best assessment of toxicity might be to use patient-derived platelet decoys in the same patient and start with very low doses if this modality moves to clinical studies in the future.”

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