C3 Inhibitors: New Hope for Rare Kidney Diseases (C3G & IC-MPGN)
For people diagnosed with rare kidney diseases C3 glomerulopathy (C3G) and immune complex membranoproliferative glomerulonephritis (IC-MPGN), the path to kidney failure and the need for dialysis has historically been a daunting one. About half of those affected reach end-stage renal disease within ten years of diagnosis, and even kidney transplantation doesn’t guarantee long-term success, as the disease can recur. But a new class of drugs, complement inhibitors, is offering a potential turning point, targeting the underlying immune system dysfunction that drives these conditions. Two drugs, iptacopan (Fabhalta) and pegcetacoplan (Empaveli), have recently received FDA approval, and others are under investigation.
Understanding C3G and IC-MPGN: A Problem with the Immune System
Both C3G and IC-MPGN involve overactivity of the complement system, a crucial part of the immune system normally responsible for defending against infections. This system consists of a cascade of proteins that, when activated by a pathogen, help eliminate it. However, in these kidney diseases, the complement system becomes dysfunctional, leading to inflammation and damage within the kidneys’ filtering units, called glomeruli.
In C3G, the complement proteins, including one called C3, break down and accumulate in the kidneys, causing inflammation. IC-MPGN, involves the buildup of C3 deposits alongside other protein fragments and antibodies, forming “immune complexes” that damage the glomeruli. The precise cause of this malfunction remains unclear. In C3G, genetic factors or autoantibodies – antibodies that mistakenly attack the body’s own tissues – are suspected. IC-MPGN is often linked to autoimmune diseases, certain cancers, or chronic infections. Learn more about the complement system from the National Institutes of Health.
How Complement Inhibitors Work: Targeting the Root Cause
Traditional treatments for C3G and IC-MPGN have focused on managing symptoms and slowing disease progression. These include medications to control high blood pressure, steroids to reduce inflammation, and immunosuppressants like mycophenolate mofetil and rituximab. However, complement inhibitors represent a fundamentally different approach. They directly target the overactive complement system, aiming to reduce the damage caused by C3 deposits in the kidneys.
Iptacopan, approved in March 2025, is an oral medication that blocks a specific signaling pathway within the complement system, reducing the breakdown of C3 and disrupting the cascade. Clinical trials showed a 35-37% reduction in proteinuria – the presence of protein in the urine, a key indicator of kidney damage – at both 6 and 12 months, and helped stabilize the decline in estimated glomerular filtration rate (eGFR), a measure of kidney function. Pegcetacoplan, approved in July 2025, is administered via twice-weekly injection and blocks all three signaling pathways in the complement system, further preventing C3 breakdown. Trials demonstrated a 68% reduction in proteinuria at 26 weeks compared to a placebo. Apellis Pharmaceuticals provides detailed information on their C3 targeting therapies.
Proteinuria and Kidney Health: A Critical Connection
The presence of protein in the urine is a significant sign of kidney dysfunction. Healthy kidneys filter waste products from the blood although retaining essential substances like protein. Elevated protein levels in the urine indicate that the kidneys’ filtering system is compromised. The urine protein creatinine ratio (uPCR) test is used to assess the amount of protein in the urine, providing a valuable diagnostic tool for C3G and IC-MPGN.
“Protein in the urine actively drives progressive kidney injury through multiple pathways that cause direct injury to the kidney tubules, kidney inflammation and fibrosis, and irreversible kidney scarring,” explains Emilia Maria C. Cadiz, DO, a pediatric nephrologist at Phoenix Children’s Hospital. Reducing protein levels in the urine is therefore a key goal of treatment, as it helps protect the kidneys from further damage.
What Does This Mean for Long-Term Prognosis?
While complement inhibitors offer a promising new avenue for treatment, it’s still too early to definitively say whether they represent a cure. Marc Richards, MD, a nephrologist and director of the Florida Kidney Physicians Glomerulonephritis Center of Excellence, emphasizes that while pegcetacoplan has been approved to reduce proteinuria, “true data on preservation of GFR will be known after two years of data is accrued.” However, initial findings from the VALIANT trial showed a reduction in C3 deposits in the kidneys of patients treated with pegcetacoplan, suggesting potential long-term benefits.
Rossana Malatesta Muncher, MD, a pediatric nephrologist at Baylor College of Medicine, notes that reducing proteinuria is consistently linked to slowing kidney failure progression, regardless of the underlying cause. “Pegcetacoplan has been shown to significantly decrease this proteinuria,” she says. “Otherwise, I suppose it’s too soon to tell if this therapy will be curative or a long-term stabilization strategy.” The Mayo Clinic provides a comprehensive overview of C3 glomerulopathy.
Navigating Treatment and Monitoring Your Health
If you’ve been diagnosed with C3G or IC-MPGN, it’s crucial to have an open conversation with your nephrologist about whether complement inhibitor therapy is right for you. Key questions to ask include: Are you a candidate for this treatment? How does it compare to your current regimen? Will your insurance cover the cost? Are there any clinical trials you could participate in? And what are the potential risks associated with these therapies?
Proactive monitoring of your health is as well essential. This includes regularly tracking your blood pressure, weight, and urine output, as well as monitoring the results of your lab tests, particularly eGFR and uPCR. Pay attention to any new symptoms and discuss them with your doctor.
Questions to Ask Your Nephrologist
- Am I a candidate for complement inhibitor therapy?
- How do they compare to the treatment I’m currently using?
- Will my insurance cover the cost?
- Are there any clinical trials of complement inhibitors that I could join?
- What risks are associated with these therapies?
Complement-targeting therapies represent a significant step forward in the treatment of C3G and IC-MPGN. While not a guaranteed cure, they offer the potential to modify the course of these diseases and potentially prevent or delay the need for dialysis and kidney transplantation. Continued research and long-term follow-up will be essential to fully understand the benefits and risks of these new treatments.
Looking Ahead: Researchers are continuing to investigate complement inhibitors and explore other potential therapies for C3G and IC-MPGN. Ongoing clinical trials will provide further insights into the long-term efficacy and safety of these treatments, and may lead to the development of even more targeted and effective therapies in the future. Regularly check with your healthcare provider and reputable sources like the National Kidney Foundation for updates on the latest research and treatment options. The National Kidney Foundation offers resources and information on C3 glomerulopathy.