What Is Filspari, and Why Does It Matter?
Filspari (sparsentan) is a once‑daily oral medicine approved to slow the decline of kidney function in adults with kidney diseases such as primary immunoglobulin A nephropathy (IgAN). The FDA has now approved it to reduce urine protein levels in adults and children ages 8 years or older with focal segmental glomerulosclerosis (FSGS) who do not have nephrotic syndrome.
FSGS is a rare kidney condition in which parts of the kidney's filters become scarred over time, which can lead to kidney failure. This damage causes protein to leak into the urine, which can further harm the kidneys. Some people with FSGS develop nephrotic syndrome, a more severe form marked by very high urine protein, swelling, and low protein levels in the blood, but many people have milder forms without these symptoms. In the U.S., more than 30,000 people are estimated to have FSGS without nephrotic syndrome.
Filspari blocks two hormone pathways that strain the kidneys by targeting endothelin A and angiotensin II type 1 receptors. Blocking both signals helps protect kidney filters and reduce urine protein. Reducing urine protein is one way to help protect kidney health over time.
This approval is important because it marks the first FDA-approved treatment specifically for FSGS. It provides a much needed, more focused treatment option and new hope for people living with FSGS. Filspari was first approved in 2023, for the treatment of IgAN in adults at risk of their disease getting worse.
Why Was It Approved?
The FDA's approval was based on clinical trial results showing Filspari reduces urine protein and is generally safe and well tolerated in people with FSGS.
The phase 3 study involved 371 patients with FSGS ages 8-75 years. It compared Filspari with irbesartan, a commonly used treatment. After about two years (108 weeks), patients taking Filspari had a 46% reduction in urine protein, compared to 30% for patients taking irbesartan. In patients without nephrotic syndrome, results were even better, with a reduction of 48% vs. 27%. Filspari also slowed kidney function decline slightly, and overall, it was well tolerated with a safety profile similar to that of irbesartan.
The most common side effects were similar across conditions. In patients with FSGS, common effects included swelling, low blood pressure (including on standing up), high potassium levels, dizziness, and anemia.
What Do I Need to Know?
Filspari comes as tablets that you need to swallow with water before your morning or evening meal. If you cannot swallow tablets, you may crush and mix them in water and take the dose right away. Do not save the mixture for later. If you miss a dose, take the next dose at the usual time. Do not take extra doses.
For FSGS in adults and children ages 8 years or older, dosing is once daily based on body weight, with a possible increase after about two weeks if you tolerate it. Before starting Filspari, you should stop taking certain blood pressure medicines or medicines for your kidneys that work in similar ways. These include medicines that affect the body's hormone system for controlling blood pressure (called the renin-angiotensin-aldosterone system) as well as specific types of medicines like angiotensin receptor blockers, endothelin receptor antagonists, and aliskiren. Taking these together with Filspari is not recommended because this may increase the risk of side effects.
Filspari has boxed warnings for liver injury and harm to a fetus. It is available only through the Filspari Risk Evaluation and Mitigation Strategies (REMS) program, which requires you, your health care provider, and your pharmacy to enroll.
Your health care provider will check liver enzyme and bilirubin levels before you start treatment and then every three months. Do not use Filspari during pregnancy. If you can become pregnant, get a pregnancy test before starting treatment, use effective birth control during treatment and for two weeks after the last dose, and stop the medicine right away if you get pregnant.
Low blood pressure (including on standing up), acute kidney injury, high potassium levels, and fluid retention can happen while taking Filspari. Your health care provider may adjust your blood pressure medicines, water pills, or your Filspari dose. They will also check your kidney function and potassium levels from time to time.
Be sure to tell your health care provider about all medicines, over-the-counter products, supplements, and herbal remedies you take before starting Filspari to avoid possible interactions. Avoid medicines that strongly affect how your body processes medicines (called CYP3A inhibitors or inducers).
Filspari may also affect how certain other medicines (such as P-gp substrates) are transported in your body. And be careful with anything that raises potassium levels, including potassium supplements and some salt substitutes. Ask your pharmacist to check if you are taking any of these medications.
Use caution with common pain relievers like nonsteroidal anti-inflammatory drugs (such as ibuprofen), as they can worsen kidney function, especially if you are dehydrated or already have kidney disease.