Empaveli (pegcetacoplan) works by binding to a protein called C3 in the complement system, a part of the immune system that normally helps destroy foreign invaders but in certain diseases mistakenly attacks the body’s own red blood cells. By blocking C3, Empaveli prevents the chain reaction that leads to red blood cell destruction in paroxysmal nocturnal hemoglobinuria (PNH), a rare blood disorder. It was the first drug to target this early step in the complement cascade, and that distinction matters because it addresses a type of anemia that older treatments could not fully control.
The Complement System and PNH
Your immune system includes a group of proteins called the complement system. These proteins work in a chain reaction, or “cascade,” where one activates the next. In PNH, red blood cells are missing protective surface proteins that normally shield them from complement attack. Without that shield, the complement system treats those red blood cells as threats and destroys them. This destruction is called hemolysis, and it causes anemia, fatigue, blood clots, and dark-colored urine.
How C3 Inhibition Differs From Older Treatments
Before Empaveli, the main treatments for PNH were drugs that block a protein called C5, which sits near the end of the complement cascade. These C5 inhibitors stop the most dramatic form of red blood cell destruction, called intravascular hemolysis, where cells burst apart inside blood vessels. But blocking C5 leaves an earlier part of the cascade running. C3 fragments still coat PNH red blood cells, tagging them for removal by the spleen and liver. This slower process, called extravascular hemolysis, leaves a significant proportion of patients on C5 inhibitors still anemic.
Empaveli works further upstream by targeting C3 itself. Because C3 sits at a junction where multiple complement pathways converge, blocking it shuts down both the tagging process that drives extravascular hemolysis and the downstream cascade that causes intravascular hemolysis. The result is that PNH red blood cells survive at much higher rates, approaching levels above 90% in some patients.
Where Empaveli Binds on C3
The C3 protein has a complex structure built around a ring of six connected domains. Empaveli binds in a groove between two of these domains (known as MG4 and MG5), physically blocking C3 from being cleaved into its active fragments. It binds to both intact C3 and its breakdown product C3b, which means it can intercept the cascade at multiple points. The binding strength is in the nanomolar range, effective enough to suppress complement activity but potentially sensitive to rare structural variations in the C3 protein that some patients carry.
What Treatment Looks Like
Empaveli is given as a subcutaneous infusion, meaning a small needle delivers the drug just under the skin rather than into a vein. The standard dose is 1,080 mg twice per week. Each infusion takes about 30 minutes if you use two infusion sites simultaneously, or about 60 minutes with a single site. Most patients learn to do this at home after initial training.
The drug has a half-life of roughly 8 to 10 days, meaning it clears from the body relatively slowly. This supports the twice-weekly dosing schedule and helps maintain steady complement suppression between infusions.
Clinical Results
In the PEGASUS trial, patients who switched from a C5 inhibitor (eculizumab) to Empaveli saw meaningful improvements in hemoglobin, the protein in red blood cells that carries oxygen. At the start of the switch, average hemoglobin was 8.58 g/dL, well below the normal range. By 48 weeks on Empaveli, it had risen to 11.57 g/dL, bringing many patients close to normal levels. That increase reflects the drug’s ability to prevent both types of hemolysis simultaneously.
Infection Risk and Vaccination Requirements
Because the complement system is a key defense against certain bacteria, suppressing it with Empaveli increases the risk of serious infections. The most concerning are infections caused by encapsulated bacteria, organisms that have a protective coating the complement system normally helps neutralize. These include the bacteria responsible for meningococcal disease and pneumococcal pneumonia.
The FDA requires vaccination against these bacteria at least two weeks before starting Empaveli. Specifically, you need vaccines covering Neisseria meningitidis (serogroups A, C, W, Y, and B) and Streptococcus pneumoniae, following the schedule recommended for people on complement inhibitors, which differs from the standard adult schedule. If treatment is urgent and vaccines haven’t been completed, preventive antibiotics are given while the vaccination series catches up.
This infection risk is serious enough that Empaveli carries a boxed warning, the FDA’s strongest safety alert. Prescribers, pharmacies, and patients must all enroll in a Risk Evaluation and Mitigation Strategy (REMS) program before the drug can be prescribed or dispensed. Doctors complete a certification process confirming they understand the risks, and pharmacies designate a representative to oversee compliance.
Limitations of C3 Inhibition
While targeting C3 covers more ground than C5 inhibitors, it is not completely foolproof. In situations that strongly activate the classical complement pathway, such as infections or other inflammatory triggers, a bypass mechanism can occur. Surface-deposited fragments from the classical pathway can recruit and activate C5 directly, sidestepping the C3 blockade entirely. This means breakthrough hemolysis episodes are still possible during acute illnesses or other complement-amplifying conditions, and patients on Empaveli need monitoring for signs of worsening anemia during these events.