Bypassing Agents for Hemophilia Explained

Learn about bypassing agents as treatment for inhibitors and whether they might be right for you.

One of the most problematic issues that can occur with hemophilia are inhibitors—a complication that happens when the body’s immune system targets the factor replacement products meant to help the blood clot properly. When this occurs, the clotting factor becomes inactivated and can’t do its job to stop bleeding. And it happens relatively frequently: Roughly 20% to 30% of people with severe hemophilia A, 5% to 8% of people with mild or moderate hemophilia A, and 4% of people with hemophilia B develop inhibitors.

The reasons why this happens are not entirely clear, but someone is more likely to develop an inhibitor if they:

While inhibitors don’t affect the location, frequency, or severity of bleeds, they do make bleeds more difficult to control. In some cases, people with inhibitors are prescribed medications known as bypassing agents. Read on to learn about the types of bypassing agents for people with inhibitors and how they work.

Read More: Inhibitors 101: Frequently Asked Questions

How Do Bypassing Agents Work?

Instead of replacing the missing factor (factor VIII in hemophilia A or factor IX in hemophilia B), a bypassing agent contains special clotting factors that go around (or bypass) the inhibitor to help the body form a normal clot and stop bleeding.

Not all people with inhibitors need to take a bypassing agent. If someone’s inhibitor is mild, they can usually be treated with larger amounts of clotting factor because even though some factor binds to the inhibitor and becomes inactive, enough is left to form a clot.

Types of Bypassing Agents

Several bypassing agents are approved for use in the U.S. to treat inhibitors:

People taking a bypassing agent need to be monitored closely to make sure their blood is not clotting too much or clotting in the wrong place in the body.

Source: National Bleeding Disorders Foundation, HemAware, October 2023

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