Antifibrotic Drugs: What They Are and How They Work

Antifibrotic drugs are medications designed to slow or stop the buildup of scar tissue inside organs. The two that have reached widespread clinical use, pirfenidone and nintedanib, are approved primarily for idiopathic pulmonary fibrosis (IPF), a progressive lung disease where normal tissue is gradually replaced by stiff, fibrous scarring. Both drugs work by interrupting the chemical signals that push cells to overproduce collagen and other structural proteins, though they do so through different molecular routes. The story of antifibrotics is still being written, with dozens of experimental compounds in development targeting fibrosis in the liver, kidneys, heart, and skin.

Why Organs Scar in the First Place

Fibrosis is wound healing that never turns off. When you cut your skin, your body sends repair cells to the site, lays down new collagen, and once the wound closes, the process winds down. In fibrotic diseases, the “wind down” signal either never arrives or gets overridden. The cells responsible for this runaway repair are called myofibroblasts, specialized cells that churn out extracellular matrix, the scaffolding material that gives tissues their structure. In healthy tissue, that scaffolding stays in balance. In fibrotic tissue, it piles up unchecked.¹

A protein called transforming growth factor beta (TGF-β) sits at the center of nearly all fibrotic diseases. When TGF-β levels spike and stay elevated, it drives normal fibroblasts to transform into those aggressive myofibroblasts, ramps up collagen production, and suppresses the enzymes that would normally break old matrix down.² Under abnormal conditions, excessive TGF-β also triggers a process in which surface-lining cells lose their identity and start behaving like scar-producing cells, further accelerating tissue remodeling.³ This makes TGF-β the single most attractive drug target in fibrosis research, but it is also one of the hardest to tackle safely, because TGF-β does useful work elsewhere in the body, from regulating immune responses to suppressing tumor growth.

Pirfenidone and How It Works

Pirfenidone was originally developed as an anti-inflammatory compound, and it retains that dual personality. On the fibrosis side, it dials down TGF-β1 and other growth factors, which in turn reduces fibroblast proliferation, slows the transformation of fibroblasts into myofibroblasts, and cuts the production of collagen and fibronectin.⁴ On the inflammation side, it tamps down the secretion of pro-inflammatory cytokines and reduces inflammatory cell accumulation.⁵ This two-pronged action matters because inflammation and fibrosis feed each other: inflamed tissue signals for more repair, and the excess scarring perpetuates inflammation.

In IPF specifically, pirfenidone slows the rate at which the lungs lose function rather than reversing damage already done.⁶ That distinction is important for patients to understand: the goal of treatment is to flatten the downward curve of lung capacity, not to climb back to where things were before diagnosis.

Nintedanib and How It Differs

Where pirfenidone acts broadly on inflammatory and fibrotic signaling, nintedanib takes a more targeted approach. It is a small-molecule inhibitor that blocks three families of receptor tyrosine kinases: platelet-derived growth factor (PDGF) receptors, fibroblast growth factor (FGF) receptors, and vascular endothelial growth factor (VEGF) receptors.⁷ Each of those receptor families plays a role in fibroblast growth, migration, or the formation of new blood vessels that support fibrotic tissue. By plugging those receptors simultaneously, nintedanib starves the fibrotic process of several of its key growth signals at once.⁸

In practice, the two drugs produce similar outcomes for lung function decline. A single-center study comparing pirfenidone and nintedanib head to head found that the average annual drop in forced vital capacity (a key measure of how much air the lungs can hold) was roughly 1.7% with pirfenidone and roughly 2.4% with nintedanib, with no statistically meaningful difference between the two.⁹ Real-world registry data from a Czech cohort showed that after two years on pirfenidone, fewer than a quarter of patients had experienced a major decline in lung function.¹⁰ These drugs do not cure IPF, but they meaningfully slow its march.

Side Effects and Sticking with Treatment

A drug that works brilliantly on paper is only useful if patients can tolerate taking it. That is a real challenge with antifibrotics. In a large multicenter study, about seven in ten patients on either drug reported at least one side effect.¹¹ The side-effect profiles differ in ways that sometimes guide the choice between the two drugs:

Nintedanib: Diarrhea is the dominant complaint, reported by about 63% of patients in one Spanish cohort. Weight loss is also more common with nintedanib.
Pirfenidone: Gastrointestinal symptoms still occur but are less frequent. Pirfenidone’s signature side effects are skin rash and photosensitivity (heightened sunburn risk), reported by roughly 23% of patients.

These side effects drive real-world discontinuation. In the Spanish study, adverse effects accounted for about 42% of nintedanib discontinuations and 23% of pirfenidone discontinuations, and time to withdrawal was longer in the pirfenidone group.¹² Women were more likely to stop treatment because of gastrointestinal problems in that study. A nationwide French claims analysis found that the 12-month persistence rate with antifibrotic therapy overall was about 71%, and that regular follow-up visits and early supportive care interventions, such as temporary dose interruptions, were strongly linked to patients staying on treatment.¹³

The practical takeaway is that managing side effects, rather than simply prescribing the drug, is a big part of successful antifibrotic therapy. Dose reductions, anti-diarrheal medications, sun-protection counseling, and taking pirfenidone with food all help patients stay on treatment long enough to benefit. About 19% of patients in the multicenter study had their dose reduced, and roughly 31% used medical treatment specifically to manage side effects, allowing them to continue therapy.¹¹ Switching from one antifibrotic to the other is also common: in the French study, about 17% of patients switched, mostly during the first year.¹³

Fibrosis Beyond the Lungs

IPF gets most of the attention because it was the proving ground for both approved antifibrotics, but fibrosis affects virtually every organ, and researchers are exploring whether the same drugs, or their relatives, can help elsewhere.

In the liver, activated stellate cells play the same role that myofibroblasts play in the lung, churning out collagen in response to chronic injury from hepatitis, alcohol, or metabolic disease. Decades of preclinical work have identified multiple mechanisms behind liver fibrosis and highlighted potential drug targets.¹⁴ Experimental nanoplatforms are now being designed to deliver antifibrotic agents directly to activated stellate cells, using vitamin A as a targeting molecule because those cells naturally take it up. One such platform carrying lanifibranor reduced collagen accumulation in animal models by both blocking TGF-β signaling and inducing cell death in the fibrosis-producing cells.¹⁵

In the kidneys, fibrosis is a hallmark of chronic kidney disease (CKD) progression. The same TGF-β pathway is central, alongside others. Both nintedanib and pirfenidone have shown the ability to reduce fibrosis in preclinical kidney models, and researchers are investigating drugs that target connective tissue growth factor (CTGF) and a protein called galectin-3 for kidney-specific applications.¹⁶ One area of active interest involves a protein called BPIFA2, whose levels rise in damaged kidney tubules and correlate with the severity of scarring. In lab models, blocking BPIFA2 partially reversed the fibrotic changes, suggesting it could become a therapeutic target.¹⁷

In the heart, fibrosis after a heart attack contributes to stiffening and eventual heart failure. Animal studies have identified molecules that can dampen cardiac fibroblast activation and reduce scar formation after injury, some working through the same TGF-β and related signaling routes used in the lung.¹⁸ Natural compounds are also under investigation: arbutin, a molecule found in certain plants, improved cardiac function and reduced fibrosis in post-heart-attack animal models, partly by regulating a different signaling arm.¹⁹

Systemic sclerosis (scleroderma) is another fibrotic disease that causes skin thickening and internal organ scarring. Nintedanib has already demonstrated benefit in scleroderma-related lung disease, and researchers are exploring early combinations of pirfenidone with immunosuppressive agents to address the disease’s dual nature of immune dysfunction and fibrosis.²⁰²¹

Experimental Targets in the Pipeline

The fact that pirfenidone and nintedanib slow but do not stop fibrosis has pushed researchers to look for additional or complementary drug targets. Several stand out.

Galectin-3 is a protein that activates both fibroblasts and immune cells called macrophages in chronically inflamed tissues. Elevated galectin-3 drives myofibroblast activity and scar formation across multiple organs.²² In rats with advanced liver fibrosis amounting to cirrhosis, galectin inhibitors given weekly over several weeks led to marked reduction in fibrosis, reduced the number of galectin-3-positive macrophages, and lowered portal pressure, with treated animals showing resolved or resolving cirrhosis compared to vehicle-treated controls that remained cirrhotic.²³ That animal result is striking because it suggests the possibility of reversing established scarring, not just slowing new scar formation.

The autotaxin-lysophosphatidic acid (LPA) axis is another route under investigation, particularly for lung fibrosis. Autotaxin is an enzyme that produces LPA, a lipid signaling molecule that promotes fibroblast migration and survival. Inhibitors of this axis have shown promise in preclinical studies and have moved into clinical trials for fibrotic lung disease.²⁴

Integrins, particularly one called αvβ6, offer yet another angle. This integrin sits on the surface of epithelial cells and activates latent TGF-β right where it is needed. Mice that lack αvβ6 develop exaggerated inflammation but are protected from pulmonary fibrosis, a finding that flagged the integrin as a potential drug target.²⁵ Antibodies and small molecules targeting this integrin have moved into clinical testing, with the goal of blocking TGF-β activation locally in the lung without suppressing TGF-β everywhere else in the body.²⁶

A newer compound called BI 1015550 (nerandomilast), a phosphodiesterase 4B inhibitor, has shown promising results in preventing lung function decline and has been flagged in recent systematic reviews as a potential addition to the antifibrotic toolkit.²⁷

The Stiffness Feedback Loop

One reason fibrosis is so hard to reverse is mechanical. As scar tissue accumulates, the organ gets stiffer. That stiffness itself acts as a signal. Cells sense the rigidity of their surroundings through surface receptors called integrins and through mechanosensitive channels, and when they detect a stiff environment, they respond by producing even more matrix, making the tissue stiffer still. This creates a self-reinforcing cycle. Research into this mechanobiological feedback loop has identified pathways like Hippo-YAP/TAZ signaling as potential intervention points, raising the prospect of drugs that could interrupt the stiffness signal and break the cycle.²⁸ This is still largely experimental, but it represents a fundamentally different approach from simply blocking growth-factor signals.

Getting Drugs Where They Need to Go

Both pirfenidone and nintedanib are taken orally, meaning they circulate through the entire body to reach the lungs. That systemic exposure contributes to gastrointestinal side effects and limits how much drug actually reaches the fibrotic tissue. Researchers are working to change that with inhaled formulations that deliver antifibrotics directly to the lungs.

Inhaled nanocarriers can improve the solubility of poorly soluble drugs, penetrate biological barriers within the lungs, and concentrate treatment at the fibrotic lesion itself, reducing the dose needed and lowering systemic toxicity.²⁹ Several nanoparticle platforms are under investigation, including liposomes and polymer-based carriers.³⁰

One recent example used albumin-based nanoparticles to co-deliver nintedanib along with a second compound, dihydroartemisinin, directly into the lungs. In animal testing, about 23% of the nanoparticle-delivered drug remained in the lungs after 48 hours, compared to free drug that cleared within 12 hours. The combination suppressed TGF-β signaling and improved lung retention substantially.³¹ None of these inhaled systems have reached routine clinical use yet, but if they pan out, they could reshape how antifibrotic therapy is delivered by making treatment more tolerable and more potent at the site of disease.

Why Fibrosis Exists at All

It is worth stepping back to ask a basic question: if fibrosis is so destructive, why did evolution keep it around? The answer is that scarring is an emergency survival strategy. In multicellular organisms, clotting, inflammation, and fibrosis were all positively selected because they are overwhelming advantages in the short term. If you are attacked by a predator and your liver is lacerated, rapid scar formation keeps you alive. The problem arises when these danger-response programs are triggered by chronic, low-grade injury, such as decades of viral hepatitis, autoimmune attack, or inhaled toxins, and they never shut off.³² Antifibrotic drugs, in that light, are an attempt to rein in a system that evolution designed for sprints, not marathons. The challenge for drug developers is suppressing the chronic overshoot without undermining the acute repair the body still needs.

Tracking Response with Biomarkers

One frustration in treating fibrosis is that the standard way to track lung disease progression, repeated lung function tests every few months, measures decline only after it has already happened. Researchers want predictive biomarkers that can flag who is getting worse and who is responding to treatment before the lung function numbers shift. A serum marker called KL-6, a glycoprotein shed by damaged lung cells, has shown potential as a predictor of response to nintedanib.³³ The French claims analysis reinforced a simpler practical point: patients who had fewer than two pulmonary function tests during follow-up were three times more likely to stop treatment, suggesting that the act of monitoring itself helps keep patients engaged with therapy.¹³ The search for better blood-based or imaging-based biomarkers remains one of the more active and practically important fronts in fibrosis research.

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