Thrombotic nanoparticles

Researchers at Chapman University in California, working under the guidance of Associate Professor Thomas H. Barker and Dr. Andrew Lyon, have created tiny gel particles capable of taking on the role of platelets. In the future, they can be used to stop severe bleeding caused by injuries, as well as for the treatment of people with blood clotting disorders caused by genetic causes or taking certain medications.

Platelets play a key role in the body's ability to stop severe bleeding. When a blood vessel is damaged, inactive platelets circulating in the bloodstream are activated and form aggregates in the area of damage. After that, the protein fibrinogen comes into play, binding platelets to each other with the formation of a temporary platelet stopper that stops bleeding.

Fibrin molecules bind to each other, forming intertwining insoluble filaments, as a result of which the platelet plug turns into a stable thrombus.

An important ultimate task of platelets is thrombus retraction. When attached to the fibrin network, platelets spread out, changing their spheroidal shape to a stellate one. However, after about 7 hours, they begin to slowly shrink in size. Due to the simultaneous decrease in the volume of many platelets, the thrombus decreases, which ensures the squeezing of excess fluid from it and the tightening of the edges of the vessel damage. It is believed that this stage plays an important role in ensuring the stability of the thrombus and repairing damage.

Traditionally, patients in need of thrombosis stimulation are injected with a donor platelet preparation. However, donor platelets have a very short shelf life, which significantly limits their availability. In addition, transfusions of donor material can lead to undesirable side effects.

In order to overcome these problems, research groups have been working for a long time to create artificial platelet substitutes that would be biocompatible, stable and cheap to produce. However, earlier prototypes of such particles triggered the clotting process immediately after entering the bloodstream, which is associated with the risk of uncontrolled formation of blood clots that can block the blood vessels of the heart, lungs or brain. In addition, artificial platelet substitutes have not provided thrombus retraction until now. (This phrase is on the author's conscience: in an article by other American scientists published in the fall of 2014, the press release of which is almost verbatim repeated here, among other things it says: "blood clots formed with their [gel microparticles] participation shrink in size over time, although at a slower rate.")

During the formation of a blood clot, platelets bind to fibrin,
after that, they shrink in size, which leads to the retraction of the thrombus.

In contrast, the ultra-soft gel particles developed by the authors move directly into the thrombosis zone and, due to their unique ability to deform, are able to induce thrombus retraction. The surface of these particles is dotted with fragments of antibodies or nanoteles that selectively bind to fibrin, but do not bind to its precursor fibrinogen.

Since fibrin is formed exclusively after the start of the blood clotting mechanism, new particles are activated only in the area of active thrombosis, which minimizes the risk of the formation of life-threatening blood clots in other regions of the circulatory system. This provides the potential for their preventive use, for example, to prevent excessive blood loss in soldiers participating in military operations and patients with blood clotting disorders.

The developers tested new thrombocyte-like particles in microfluidic chambers simulating physiological conditions inside a blood vessel, as well as on an animal model of traumatic injury.

They also restored the reduced ability to thrombosis of platelet-depleted plasma of newborns who underwent surgical interventions for the correction of congenital heart defects.

Analysis of the blood clots formed in this case demonstrated a deep penetration of gel particles into the thickness of the fibrin matrix, comparable to the penetration of platelets. Additional experiments showed the ability of such blood clots to retraction, which corresponded to the degree of retraction of normal blood clots, proceeded for 24-48 hours – somewhat slower than in plasma with normal platelet concentrations. Finally, the administration of gel particles to rats five minutes before damage to a large blood vessel significantly reduced the bleeding time compared to the values obtained in the group of animals injected with platelets, the number of which was 100 times higher than the number of thrombocyte-like particles injected into the experimental group of rats.

The authors note that additional safety testing of experimental particles should be carried out before the start of clinical trials. It is also a prerequisite to find out their ultimate fate in the body, since, unlike traditional biocompatible materials, new particles are not biodegradable.

The developers are also interested in endowing their particles with the ability characteristic of platelets to secrete factors contributing to thrombosis. In a broader sense, they are inspired by the potential use of ultra-soft/deformable gels in the development of a new class of biomaterials whose components can directly interact with each other.

Article by Ashley C. Brown et al. Ultrasoft microgels displaying emergent platelet-like behaviours is published in the journal Nature Materials.