An antibody originally designed to treat hemophilia A replaces a missing clotting protein, helping patients prevent bleeding with infrequent injections instead of frequent intravenous infusions. The innovation has been recognized with the 2026 Lasker Award.

Antibodies are best known for helping the immune system recognize and fight infections. They are also used to treat cancer and other diseases by binding to specific proteins in the body. But one antibody has been engineered to do something quite different: help blood clot.

Emicizumab, developed by scientists at Japan’s Chugai Pharmaceutical, is a bispecific antibody that restores a critical step in blood clotting. Rather than replacing a missing protein directly, it brings two other clotting proteins together so they can work as if the missing protein were present.

The approach has transformed treatment for people with hemophilia A, an inherited bleeding disorder. Its developers, Kunihiro Hattori, Tomoyuki Igawa and Takehisa Kitazawa, received the 2026 Lasker-DeBakey Clinical Medical Research Award for this innovation.

When a Missing Protein Disrupts Clotting

Blood clotting is a carefully coordinated process involving a series of proteins known as clotting factors. When a blood vessel is damaged, these proteins activate one another in a cascade of reactions that ultimately produces a stable blood clot.

One of these proteins, clotting factor VIII, plays a crucial role in amplifying the process. It helps two other clotting factors, activated factor IX and factor X, come together so factor IX can activate factor X. This triggers the production of thrombin, an enzyme that helps form the clot.

In people with hemophilia A, factor VIII is missing or does not function properly. Without it, the clotting cascade cannot generate enough thrombin to form stable clots, leaving patients vulnerable to prolonged bleeding and repeated internal bleeding, particularly in joints.

For decades, treatment relied on replacing the missing factor VIII with infusions. Although effective, the protein is cleared from the bloodstream relatively quickly, requiring frequent intravenous doses. Some patients also develop antibodies that block the replacement protein, making treatment less effective.

Building a Bridge Between Two Clotting Proteins

Conventional antibodies typically bind to a particular target. A bispecific antibody, however, is designed to recognize two different targets at once. This allows it to bring molecules together and influence how they interact. If an antibody could position factor IX near factor X close enough to activate it, the antibody could effectively mimic the missing factor VIII.

The idea was simple in principle but difficult to achieve. Factor VIII is a large, complex protein whose activity depends on precisely positioning other molecules. A bispecific antibody would need to reproduce that function despite having a completely different structure.

The team screened and refined approximately 40,000 different bispecific antibodies to find one that could do the job. They measured how effectively each candidate promoted factor X activation and improved its stability, solubility and other properties.

An important discovery was that stronger binding was not necessarily better. The antibody needed to bring the clotting factors together temporarily, allowing them to interact and then separate normally. The resulting antibody, emicizumab, acts as a molecular bridge between activated factor IX and factor X. By bringing these proteins together, it restores the missing function of factor VIII and helps restart the clotting cascade.

From Frequent Infusions to Infrequent Injections

Because emicizumab is structurally different from factor VIII, the antibodies that some patients develop against factor VIII do not recognize it. This allows emicizumab to work even in patients whose immune systems have made factor VIII replacement ineffective.

Its antibody structure also gives it a longer lifespan in the bloodstream. Instead of requiring frequent intravenous infusions, emicizumab can be administered as an injection under the skin every week, every two weeks or every four weeks.

Clinical trials confirmed that the treatment substantially reduces bleeding in people with hemophilia A, including those with factor VIII inhibitors. It has since become an established preventive treatment for the disorder. The change is more than a different way to administer a drug. Emicizumab replaces a missing biological function without replacing the missing protein itself.

A New Way to Engineer Medicines

The development of emicizumab demonstrates how antibodies can be used to do more than recognize disease-related targets. By bringing two proteins together, they can recreate functions normally performed by complex biological molecules.

The principle could have applications beyond blood clotting. Many biological processes depend on proteins interacting in precise ways. Engineering antibodies to control these interactions offers another way to restore or redirect biological activity.

Emicizumab does have important limitations. It does not eliminate the need for careful monitoring, and certain combinations with other clotting treatments can increase the risk of dangerous clots. But its success has established a new approach to treating hemophilia A.

The 2026 Lasker Award recognizes not only a treatment that has changed care for people with hemophilia, but also a different way of thinking about drug design. Rather than simply replacing what the body lacks, a medicine can be engineered to bring existing components together and make them work again.