For the roughly 40,000 Americans living with Focal Segmental Glomerulosclerosis, April 13, 2026 is the day the math changed. The FDA approved Filspari (sparsentan) to reduce proteinuria in adults and pediatric patients ages 8 and older with FSGS without nephrotic syndrome (Travere Therapeutics, 2026). It is the first therapy ever specifically approved for the disease. Before today, every FSGS treatment in a nephrologist's toolkit was borrowed from somewhere else.
Patients and families know what that has meant in practice. Off-label ACE inhibitors. High-dose steroids with predictable side effects. Calcineurin inhibitors that can damage the kidney while they try to protect it. A slow march toward dialysis or transplant for a large share of people diagnosed.
Getting here took 3 hard years
The path to today's approval was not clean. Travere Therapeutics had hoped to seek FSGS approval for sparsentan after the Phase 3 DUPLEX trial, but did not file until 2025. DUPLEX was a 371-patient, 108-week study comparing sparsentan to irbesartan, a standard-of-care blood pressure pill widely used off-label in FSGS (Rheault et al., 2024).
The trial did two things at once. It missed its primary goal of slowing estimated glomerular filtration rate (eGFR) decline over 108 weeks, with a p-value that was not close to statistical significance. It also showed a large and durable reduction in proteinuria, with 42% of sparsentan patients reaching partial remission of proteinuria at week 36 versus 26% on irbesartan (Rheault et al., 2023).
After DUPLEX missed its main kidney-function goal in 2023, Travere held off on filing for FSGS. It submitted only after an independent workgroup (PARASOL) backed proteinuria as a meaningful measure in FSGS and after a meeting with the FDA in early 2025, and the FDA accepted its FSGS application in May 2025. That delay was the first setback. A second came in January 2026, when the agency extended its review by 3 months after Travere submitted requested information on the drug's clinical benefit. For FSGS families watching the calendar, each delay was another year on a disease that does not wait.
What the drug actually does, in plain English
Filspari is a once-daily pill that blocks two receptors at the same time. The first is the endothelin ETA receptor, a signal that tightens blood vessels inside the kidney and triggers scarring. The second is the angiotensin AT1 receptor, the target of widely used blood pressure drugs like losartan and irbesartan.
In FSGS, the tiny filters of the kidney called glomeruli are under too much pressure. Protein leaks through them into urine. That leaking protein is itself toxic to the filter, which then scars, which leaks more protein, which causes more scarring. Filspari interrupts that loop at two different points instead of one, which is the clinical case for using a dual-action pill rather than stacking two separate medicines.
Filspari is dispensed only through the FILSPARI REMS, a restricted distribution program required by the FDA. That is because of the risk of liver injury; the drug also has a boxed warning about harm to an unborn baby. Patients get liver tests before starting and every 3 months after, and people who can become pregnant need a pregnancy test before starting and must use effective birth control. The REMS paperwork is a hassle, though the same paperwork has been running smoothly for IgA nephropathy patients on Filspari since 2023.
When can patients actually start taking it
This is the question every family is asking right now. The honest answer has 3 parts.
Day 1 to week 2
Filspari is already in pharmacies for IgA nephropathy, so the supply chain exists today. A nephrologist can write a prescription for an adult FSGS patient starting this week. The central REMS pharmacy ships the medicine, and liver testing kicks off immediately.
Week 3 to month 3
Commercial insurance and Medicare Part D plans will take several weeks to update their formulary rules for the new FSGS indication. Prior authorization is the usual bottleneck. Expect requirements around prior use of standard ACE or ARB therapy, baseline liver function, and documentation of biopsy-confirmed FSGS. Travere TotalCare offers a $0 copay program for eligible commercially insured patients and a nurse educator who can help with coverage, which is worth asking about on the first nephrology visit.
3 to 6 months
Pediatric access will lag adult access by a few weeks while pharmacy systems adjust the age-eligibility rules. Medicaid coverage varies by state, and some states add an extra layer of prior authorization for any drug with a REMS. Patients in rural areas may need to coordinate more carefully with the central pharmacy for home liver draws, which Travere covers at no cost.
“The drug has been commercial since 2023. The infrastructure is there. What we are really waiting on is payers updating their rules for a new indication.”
nephrology access coordinator, speaking to Trial Friend, April 2026
Credit where credit is due
Travere Therapeutics is a small San Diego biotech that most people have never heard of. The company was originally Retrophin, founded in 2011, and rebranded as Travere in 2020 after a change in leadership. CEO Eric Dube and Chief Medical Officer Jula Inrig, a practicing nephrologist, ran the FSGS program through the 2023 DUPLEX miss, the 2025 filing, and the January 2026 extension. Running a Phase 3 kidney trial for a disease with no approved drug, no validated surrogate endpoint, and no commercial precedent is a hard ask at any size company. Doing it at a mid-cap biotech without a larger partner shouldering the risk is harder.
Real credit also belongs to the FSGS patients who enrolled in DUPLEX, to the nephrologists who referred them, and to advocacy groups like NephCure and the FSGS Foundation who kept pressure on regulators and payers for more than a decade. In rare disease work, the drug is only one part of the story.
The part most press releases are going to skip
Here is the angle worth understanding. Filspari was approved for FSGS even though its pivotal trial did not hit its own primary goal. That is unusual. It happened because the FDA, the kidney research community, and patient advocates spent years working on a different question. Can proteinuria reduction count as a meaningful clinical outcome for rare progressive kidney diseases, instead of requiring trials to run long enough to see kidneys actually fail?
NephCure led a collaborative project called PARASOL, short for Proteinuria and GFR as Clinical Trial Endpoints in Focal Segmental Glomerulosclerosis, working directly with the FDA's Division of Cardiology and Nephrology to validate proteinuria data across dozens of historical studies (NephCure, 2024). The case was that long-term proteinuria reduction reliably predicts long-term kidney survival in FSGS, which means you can approve a drug on the proteinuria data without waiting 10 years for dialysis rates.
That groundwork is what let FDA accept the DUPLEX result. It is also why, unlike sparsentan's original accelerated IgA nephropathy approval in 2023, the FSGS approval does not require Travere to run a separate post-marketing confirmatory trial to convert to full approval. The approval landed as full approval on day one. For a disease with fewer than 50,000 U.S. patients and no other approved drug, that matters. A required confirmatory trial in a rare disease often takes 5 to 8 years and can delay next-generation competitors from entering the same space.
What to ask your nephrologist this week
A short, practical list for patients and families heading into the next appointment.
Ask whether Filspari is appropriate for your specific FSGS subtype, since the approval covers patients without nephrotic syndrome. Ask about the baseline liver function test and whether to stop or taper an existing ACE or ARB before switching. Ask about the REMS enrollment steps so you know what to expect before the first dose. Ask whether your insurance plan has posted a Filspari FSGS coverage policy yet, and if not, ask your clinic how they plan to handle the prior authorization. Ask about Travere TotalCare, which is the manufacturer's patient support program that handles copay assistance and delay-period bridge supply.
For pediatric patients ages 8 and older, ask specifically about weight-based dosing and how pediatric REMS monitoring works at your center. Pediatric nephrology programs are usually excellent at this, though paperwork volume tends to be higher.
Filspari is not a cure for FSGS. It is the first therapy specifically designed to slow the disease, and it arrives after a long and uneven road. The work of actually getting treatment to every patient who qualifies starts this week.