For patients with focal segmental glomerulosclerosis (FSGS), a leading cause of kidney failure, the race to slow disease progression has long been defined by a single, stubborn marker: proteinuria. The more protein lost in the urine, the faster the kidneys decline. Now, new evidence from the phase 3 DUPLEX trial confirms that sparsentan—a dual endothelin-angiotensin receptor antagonist—delivers more rapid and sustained reductions in proteinuria than the standard treatment, irbesartan, regardless of how severe the condition is at the start. The findings mark a potential turning point for a disease with few effective therapies, offering hope to the tens of thousands of Americans living with FSGS.
FSGS is a rare but aggressive form of kidney disease, characterized by scarring in the kidney’s filtering units. Proteinuria, the leakage of protein into the urine, is both a hallmark and a harbinger of worsening kidney function. Lowering proteinuria has long been a primary goal of treatment, as studies show that achieving and maintaining low levels—below 0.3 grams per gram of creatinine, for example—is associated with slower progression to kidney failure. Yet, until now, the tools available to nephrologists have been limited in their ability to consistently achieve these targets, especially for patients with high baseline proteinuria.
The DUPLEX trial, published in the New England Journal of Medicine, directly compared sparsentan at 800 mg per day with irbesartan at its maximum labeled dose of 300 mg per day in 431 patients with FSGS. Over 108 weeks, sparsentan demonstrated superior efficacy in reducing proteinuria, a benefit that held true even when patients were divided by the severity of their disease at the outset. A post hoc analysis led by Jai Radhakrishnan, MD, and colleagues further illuminated these results, showing that sparsentan helped more patients reach clinically meaningful proteinuria thresholds—below 0.3 g/g (complete remission), below 0.7 g/g, and below 1.5 g/g—compared to irbesartan, regardless of whether their baseline urine protein-to-creatinine ratio (UPCR) was below or above 3 g/g.
Sparsentan’s Edge: Faster, Deeper, and More Consistent
The DUPLEX trial’s design and results reflect a growing understanding that FSGS is not a one-size-fits-all condition. Patients enter treatment with widely varying levels of proteinuria, and their responses to therapy can differ just as dramatically. Historically, drugs like irbesartan have been the mainstay for reducing proteinuria, but their effects are often modest and slower to manifest. Sparsentan, by contrast, appears to act more swiftly and profoundly, a distinction that could be critical for patients at high risk of rapid decline.
In the trial, patients on sparsentan achieved a 40% or greater reduction in UPCR more quickly than those on irbesartan, and this advantage persisted over the two-year study period. The post hoc analysis underscored that these benefits were not limited to those with the highest proteinuria at the start. Even patients with lower baseline UPCR levels saw greater and more sustained reductions with sparsentan, suggesting that the drug’s mechanism—blocking both endothelin and angiotensin II pathways—may offer a broader therapeutic window than current options.
Why Baseline Proteinuria Matters
Proteinuria is more than just a symptom; it is a powerful predictor of kidney survival. Research published in the American Journal of Kidney Diseases confirms that lower UPCR thresholds are associated with better long-term outcomes, with each incremental reduction in proteinuria linked to a slower rate of kidney function decline. The DUPLEX trial’s findings align with this body of evidence, showing that sparsentan not only lowers proteinuria more effectively but also does so in a way that may translate into better preservation of kidney function over time.
For nephrologists, these results present a compelling case for reconsidering treatment strategies. The ability to achieve and maintain low proteinuria thresholds—especially in patients with high baseline levels—could mean the difference between stabilization and progression for many with FSGS. Sparsentan’s approval by the FDA in 2023 as the first and only treatment specifically indicated for FSGS has already expanded the therapeutic landscape, but the DUPLEX trial’s detailed outcomes now provide a clearer picture of how to use it to maximum effect.
Beyond Proteinuria: Safety and Tolerability
While the focus has been on proteinuria reduction, the trial also highlighted sparsentan’s safety profile. The drug was well tolerated across both baseline UPCR groups, with no new safety signals emerging over the course of the study. Here’s particularly important for patients with FSGS, who often face a delicate balance between managing proteinuria and avoiding treatment-related side effects that could further compromise kidney function.
However, the DUPLEX trial did not show a significant difference between sparsentan and irbesartan in the rate of decline in estimated glomerular filtration rate (eGFR), a key measure of kidney function. This discrepancy—rapid proteinuria reduction without a clear eGFR benefit—has sparked discussion in the nephrology community about whether proteinuria alone should remain the primary endpoint in FSGS trials, or if longer-term studies are needed to fully capture the drug’s potential nephroprotective effects.
What’s Next for Patients and Providers
For now, the results from DUPLEX offer a strong rationale for clinicians to consider sparsentan as a first-line or early-line therapy for FSGS, particularly for patients with high proteinuria or those who have not responded adequately to other treatments. The drug’s approval and the trial’s outcomes have already prompted updates to clinical guidelines, with organizations like the American Society of Nephrology emphasizing the importance of individualized treatment approaches.
Looking ahead, the next checkpoint will be the long-term real-world data, which will help clarify whether the proteinuria reductions seen in DUPLEX translate into meaningful improvements in kidney survival and quality of life. Travere Therapeutics, the company behind sparsentan, has also announced plans to explore its use in other forms of kidney disease, including immunoglobulin A nephropathy (IgAN), further broadening the potential impact of this novel therapy.
For patients living with FSGS, the message is clear: treatment options are evolving, and the goal of achieving and maintaining low proteinuria is more attainable than ever. As the nephrology community digests these findings, one thing is certain—sparsentan represents a significant step forward in the fight against a disease that has long left clinicians and patients searching for answers.
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