Peptide Therapeutics for Pulmonary Fibrosis
Pulmonary Fibrosis Peptides
12 Peptides
At least twelve distinct peptides have demonstrated antifibrotic effects in preclinical lung fibrosis models, with two now in human clinical trials.
Li et al., Int J Mol Sci, 2023
Li et al., Int J Mol Sci, 2023
View as imageIdiopathic pulmonary fibrosis kills roughly half of those diagnosed within three to five years.[1] The two approved drugs, nintedanib and pirfenidone, slow the decline in lung function but do not stop or reverse the scarring. They also cause significant gastrointestinal side effects that lead many patients to discontinue treatment. That gap between what exists and what patients need has driven researchers toward peptide therapeutics for pulmonary fibrosis, a class of molecules with structural diversity, low toxicity profiles, and the ability to target multiple fibrotic pathways simultaneously.
This article maps the full landscape of peptide-based approaches to lung fibrosis, from endogenous fragments like Ac-SDKP to engineered clinical candidates like LTI-03. Each section covers a different peptide or peptide class, what it does at the molecular level, and how far the evidence extends. The angiotensin II pathway, which drives fibrosis through a peptide signaling cascade, gets its own dedicated deep dive as a companion to this overview.
Key Takeaways
- GHK peptide reduced bleomycin-induced lung fibrosis at all three tested doses by suppressing TGFβ1/Smad signaling in mice (Zhou et al., 2017)
- A caveolin-1 derived peptide called CSP7 (now LTI-03) reversed fibrotic markers in human IPF lung tissue ex vivo and entered a Phase 2 clinical trial (Marudamuthu et al., 2019)
- Ac-SDKP, a thymosin β4 fragment, showed both preventive and therapeutic antifibrotic effects in bleomycin mouse models (Conte et al., 2015)
- A long-acting D-amino acid isomer of Ac-SDKP attenuated pulmonary fibrosis through PI3K/AKT and Smad2 pathway inhibition, solving the short half-life problem (Qiu et al., 2020)
- XFB-19, a synthetic tetrapeptide targeting C/EBPβ in myofibroblasts, became the first antifibrotic peptide to enter a Phase 1 human trial
- All twelve peptides reviewed here remain in preclinical or early clinical stages. None has completed a Phase 3 trial for pulmonary fibrosis.
Why Peptides for Lung Fibrosis?
The current treatment landscape for IPF is limited. Nintedanib, a tyrosine kinase inhibitor, slows FVC decline by roughly 50% but causes diarrhea in over 60% of patients. Pirfenidone, which modulates TGF-β among other targets, produces similar efficacy but with nausea and photosensitivity as common side effects. Neither drug stops disease progression. Neither reverses existing fibrosis. And roughly 20-30% of patients discontinue treatment due to tolerability issues, leaving them with no pharmacological options at all.
Pulmonary fibrosis involves a cascade of overlapping pathological processes: epithelial cell death, fibroblast-to-myofibroblast differentiation, excessive collagen deposition, and chronic inflammation. Small molecule drugs tend to hit one or two of these nodes. Peptides, by contrast, can interact with multiple biological processes due to their structural diversity and specific targeting capabilities.[1]
The practical advantages are measurable. Peptides are characterized by low toxicity, high biological activity, easy absorption, and fewer off-target effects compared to small molecules.[2] Their primary limitation, short plasma half-life, is being addressed through chemical modifications like D-amino acid substitution and cyclization. The inhaled delivery route, which deposits drug directly at the site of fibrosis, sidesteps many systemic exposure concerns entirely.
What makes the current pipeline notable is its diversity. At least twelve peptides target lung fibrosis through distinct mechanisms: TGF-β/Smad signaling, epithelial-mesenchymal transition (EMT), oxidative stress, matrix metalloprotease regulation, myofibroblast differentiation, and alveolar epithelial cell survival. No single peptide covers all pathways. But collectively, they map the biology of fibrosis from multiple angles.
The TGF-β Problem
Nearly every peptide in this pipeline converges on transforming growth factor beta (TGF-β), the central orchestrator of fibrotic signaling. TGF-β1 drives fibroblast proliferation, stimulates collagen production, and induces epithelial cells to transform into mesenchymal cells through EMT.[3]
The challenge is that TGF-β also regulates immune function, wound healing, and tumor suppression. Blocking it completely creates unacceptable risks. The peptide approach offers a potential solution: rather than globally suppressing TGF-β, individual peptides modulate specific downstream nodes in the signaling cascade. GHK targets TGFβ1/Smad2/3 and IGF-1. DR8 targets TGF-β/MAPK. M10 targets Smad2 phosphorylation specifically. Ac-SDKP works upstream through the renin-angiotensin system. Each peptide addresses the TGF-β problem from a different angle, which is why the pipeline contains so many candidates rather than a single lead compound.
GHK and GHK-Cu: The Copper Peptide in Lung Tissue
GHK (glycyl-L-histidyl-L-lysine) is a naturally occurring tripeptide found in human plasma, saliva, and urine. Plasma levels decline from roughly 200 ng/mL at age 20 to 80 ng/mL by age 60, a decline that correlates with reduced tissue repair capacity. The peptide is best known for its role in skin remodeling and wound healing, but its ability to suppress TGF-β1 secretion made it a logical candidate for fibrosis research. Its relevance to pulmonary fibrosis was established in a 2017 study by Zhou and colleagues, who administered GHK intraperitoneally to mice with bleomycin-induced lung fibrosis.[3]
GHK at all three tested doses (2.6, 26, and 260 μg/ml/day) reduced inflammatory cell infiltration and interstitial thickness. The peptide reversed bleomycin-induced increases in TGF-β1, phosphorylated Smad2, phosphorylated Smad3, and insulin-like growth factor 1 (IGF-1) expression. The mechanism mapped to inhibition of epithelial-to-mesenchymal transition through the TGF-β1/Smad2/3 and IGF-1 pathway.
The copper-complexed form, GHK-Cu, was tested separately by Ma et al. in 2020. Their study demonstrated that GHK-Cu protected against bleomycin-induced pulmonary fibrosis through anti-oxidative stress and anti-inflammatory pathways, adding a complementary mechanism to the Smad-targeting effects of the uncomplexed peptide.[4]
Both studies used the bleomycin mouse model, which reproduces some but not all features of human IPF. Neither GHK nor GHK-Cu has been tested in human pulmonary fibrosis. The broader profile of GHK-Cu's gene-modulating effects extends well beyond lung tissue, which raises questions about systemic effects that would need addressing before clinical development.
Ac-SDKP: The Thymosin β4 Fragment
N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is an endogenous tetrapeptide generated by prolyl oligopeptidase from thymosin β4. It circulates naturally in human plasma and is degraded primarily by angiotensin-converting enzyme (ACE), which is why ACE inhibitor use correlates with elevated Ac-SDKP levels. The peptide's connection to the angiotensin II signaling axis makes it a natural counterweight to the profibrotic effects of the renin-angiotensin system in lung tissue.
Conte et al. (2015) demonstrated both preventive and therapeutic effects of Ac-SDKP in the bleomycin mouse model of pulmonary fibrosis. When administered alongside bleomycin (preventive protocol), Ac-SDKP reduced fibrotic markers. When administered after fibrosis was already established (therapeutic protocol), it still produced measurable antifibrotic effects. The peptide inhibited TGF-β-induced myofibroblast differentiation and reduced extracellular matrix deposition in human lung fibroblasts.[5]
The primary limitation of native Ac-SDKP is its extremely short half-life in vivo, measured in minutes due to rapid ACE-mediated degradation. Qiu et al. (2020) addressed this by synthesizing a D-amino acid isomer of Ac-SDKP that resists enzymatic breakdown. The long-acting isomer attenuated pulmonary fibrosis through SRPK1-mediated PI3K/AKT and Smad2 pathway inhibition, maintaining antifibrotic activity over a sustained period that native Ac-SDKP could not achieve.[6]
This stabilization strategy, replacing L-amino acids with their D-isomer counterparts, represents a general solution applicable to other short-lived peptide candidates in the fibrosis pipeline.
CSP7/LTI-03: The Caveolin-1 Peptide in Clinical Trials
The most clinically advanced peptide therapeutic for pulmonary fibrosis is LTI-03, a seven-amino-acid peptide (sequence: FTTFTVT) derived from the caveolin-1 scaffolding domain. Caveolin-1 is a structural protein in cell membrane invaginations called caveolae. In healthy lung tissue, caveolin-1 acts as a brake on fibrotic signaling by inhibiting TGF-β receptor activation and suppressing matrix metalloproteinase expression. In IPF, caveolin-1 expression is markedly reduced in both epithelial cells and fibroblasts, effectively releasing the brake and allowing fibrotic signaling to proceed unchecked. The peptide fragment CSP7 was designed to restore this protective signaling without requiring the full protein.
Marudamuthu et al. published the foundational preclinical data in Science Translational Medicine in 2019. When delivered to three distinct mouse models of lung fibrosis, CSP7 reduced extracellular matrix deposition, promoted alveolar epithelial cell survival, and improved lung function. The peptide also demonstrated efficacy when applied ex vivo to lung tissue from patients with end-stage IPF, reducing profibrotic markers in human tissue.[7]
The dual mechanism is what distinguishes LTI-03 from other candidates. Most antifibrotic peptides either inhibit profibrotic signaling OR promote epithelial cell survival. CSP7 does both: it suppresses TGF-β-driven fibroblast activation while simultaneously protecting alveolar epithelial cells from apoptosis.[8]
Rein Therapeutics (formerly Lung Therapeutics) formulated LTI-03 as a dry powder for inhalation, delivering the peptide directly to the lungs. The Phase 1 trial in healthy volunteers (NCT04233814) demonstrated tolerability. The company received FDA clearance in late 2025 to resume the Phase 2 "RENEW" trial (NCT05954988), which is enrolling patients with IPF across approximately 50 sites in the U.S., U.K., Germany, Poland, and Australia. Topline data is expected in Q3 2026.
The inhaled delivery route is central to LTI-03's design. By depositing the peptide directly at the site of fibrosis, systemic exposure is minimized and local concentrations can reach therapeutic levels that would be impractical with intravenous administration.
DR8 and M10: Synthetic TGF-β Pathway Inhibitors
Two synthetic peptides, DR8 and M10, have been tested specifically in bleomycin and silica models of pulmonary fibrosis, each targeting distinct nodes in TGF-β signaling.
Wang et al. (2019) demonstrated that peptide DR8 protected against bleomycin-induced pulmonary fibrosis by regulating the TGF-β/MAPK signaling pathway and reducing oxidative stress. The peptide decreased inflammatory infiltration, collagen deposition, and expression of fibrotic markers including α-smooth muscle actin.[9]
A follow-up study by Wang et al. (2021) tested structural analogs of DR8, designated DR8-3D and DR8-8A, which showed improved efficacy. These analogs alleviated pulmonary fibrosis by suppressing TGF-β1-mediated epithelial-mesenchymal transition and ERK1/2 signaling both in vivo and in vitro. The analog approach allowed researchers to optimize potency while retaining the peptide's favorable safety profile.[10]
M10, tested by Li et al. (2019), takes a more targeted approach: it attenuates silica-induced pulmonary fibrosis specifically by inhibiting Smad2 phosphorylation. Rather than broadly modulating TGF-β, M10 blocks a single phosphorylation event that is required for profibrotic signal transduction. In the silica model, M10 treatment reduced lung collagen content and preserved alveolar architecture.[11]
Neither DR8 nor M10 has advanced beyond animal models. Their value at this stage is primarily as proof-of-concept: synthetic peptides can be designed to hit specific signaling nodes in the fibrotic cascade with precision that small molecules have difficulty matching.
The DR8 story also illustrates a general principle in peptide drug development. The original DR8 peptide showed activity but was not optimized. By systematically modifying the peptide's structure to create DR8-3D and DR8-8A, researchers improved both potency and pharmacological properties. This iterative analog design is a standard strategy in peptide therapeutics, and the same approach could accelerate development for other preclinical candidates in the fibrosis pipeline.
XFB-19: First Antifibrotic Peptide in Human Trials
XFB-19 is a synthetic tetrapeptide (Acetyl-Lys-D-Ala-D-Val-Asp-NH2) developed by Xfibra, Inc. It inhibits activation of the transcription factor C/EBPβ (CCAAT-enhancer binding protein beta), which mediates myofibroblast differentiation in the lung. This target is distinct from TGF-β, making XFB-19 mechanistically complementary to most other candidates in the pipeline.
The Phase 1 trial (NCT05361733) is a randomized, double-blind, placebo-controlled study of single and multiple ascending doses in healthy volunteers, evaluating safety, tolerability, and pharmacokinetics. XFB-19 incorporates D-amino acids (D-Ala and D-Val) into its sequence, a deliberate design choice to extend half-life by resisting enzymatic degradation, the same stabilization principle demonstrated with the long-acting Ac-SDKP isomer.
What makes XFB-19 noteworthy is its specificity. The peptide targets only the C/EBPβ activation step in myofibroblast differentiation, which could theoretically allow recovery from fibrosis rather than merely slowing progression. If validated, this mechanism would represent a reversal strategy rather than a stabilization strategy, a distinction that matters to patients already living with significant scarring.
No efficacy data in IPF patients is yet available. The Phase 1 trial is focused solely on safety and pharmacokinetics in healthy volunteers. If XFB-19 clears this hurdle, a Phase 2 trial in IPF patients would be the next step, but that timeline depends on the safety data and the company's ability to secure funding for a larger study. Xfibra is a small biotech based in Del Mar, California, and the progression from Phase 1 to Phase 2 for rare disease indications can take two or more years.
The Broader Pipeline
Beyond the individually characterized peptides above, several additional candidates have shown antifibrotic effects in lung models.
Yu et al. (2020) demonstrated that a peptide called TAT-Y127WT, which blocks protein phosphatase 2A (PP2A), protected mice against bleomycin-induced pulmonary fibrosis. PP2A appears to promote endothelial-mesenchymal transition, and its peptide-based inhibition prevented the transformation of endothelial cells into fibrotic myofibroblasts.[12]
Simon et al. (2023) reviewed the full preclinical landscape, cataloging scorpion-derived immunomodulatory peptides ToAP3 and ToAP4, which controlled experimental IPF by regulating fibrosis-associated cytokine production while maintaining normal tissue architecture.[2]
The E4 peptide, developed at the Medical University of South Carolina, takes a fundamentally different approach from TGF-β-focused candidates. E4 activates an antifibrotic pathway through the urokinase plasminogen activator system, which ultimately activates enzymes that break down excess collagen. This pathway operates across organ systems, and E4 reversed established fibrosis in lung, liver, and kidney tissue in preclinical models. The cross-organ activity is conceptually appealing because fibrotic diseases share common downstream mechanisms even when their initiating triggers differ. If E4's mechanism translates to humans, a single peptide could potentially address fibrosis in multiple organs.
C-type natriuretic peptide (CNP) analogs have also demonstrated anti-inflammatory and antifibrotic effects in lung fibroblasts. A long-acting CNP analog abrogated experimental lung inflammation and fibrosis by signaling through the NPR-B receptor to generate cyclic GMP, which counteracts profibrotic pathways. CNP is technically a 22-amino-acid peptide hormone, placing it at the upper boundary of what is typically classified as a peptide rather than a small protein.
Li et al. (2023) provided the most comprehensive review of the field, documenting how peptides including PAP, CGEN25009, liraglutide, ANP, CNP, M10, Ac-SDKP, DR8, and E4 collectively target the NF-κB signaling pathway, TGF-β signaling, oxidative stress, and extracellular matrix remodeling in pulmonary fibrosis.[1]
Delivery Challenges: Getting Peptides to Fibrotic Lungs
The central obstacle for peptide therapeutics in lung fibrosis is pharmacokinetics. Native peptides are degraded within minutes by circulating proteases. Oral bioavailability is near zero for most candidates because stomach acid and digestive enzymes destroy peptide bonds before absorption occurs. Intravenous administration achieves systemic levels but requires frequent dosing, exposes non-target organs, and in the case of Ac-SDKP, the peptide is cleared from circulation in under five minutes by ACE-mediated hydrolysis.
This pharmacokinetic barrier has historically been the single biggest reason peptide drug candidates fail to reach the clinic. A peptide that works in a cell culture dish or even in a mouse may be completely impractical in humans if it cannot survive long enough to reach its target in sufficient concentrations.
Three strategies have emerged in this pipeline:
Inhaled delivery deposits the peptide directly at the fibrotic site. LTI-03 uses a dry powder inhaler formulation that achieves therapeutic lung concentrations without significant systemic exposure. This approach is particularly well-suited to lung fibrosis because the target tissue is directly accessible through the airway. Inhaled peptide drug delivery is advancing rapidly across multiple therapeutic areas, and the fibrosis pipeline benefits from that broader development.
D-amino acid substitution replaces protease-susceptible L-amino acids with their mirror-image D-isomers. Both the long-acting Ac-SDKP isomer and XFB-19 use this approach, extending half-life from minutes to hours or days without altering the peptide's binding properties.
Analog optimization iterates on a lead peptide's structure to improve potency, selectivity, or stability. The DR8-3D and DR8-8A analogs exemplify this approach, achieving better efficacy than the parent compound through targeted structural modifications.
None of these strategies is mutually exclusive. A future clinical candidate could combine D-amino acid stabilization with inhaled delivery, for example, to maximize both duration and local concentration.
Where the Evidence Stands
Honesty about the state of this field requires stating clearly what has not happened yet. No peptide has completed a Phase 3 trial for pulmonary fibrosis. No peptide has demonstrated efficacy in a randomized controlled trial in IPF patients. The two clinical candidates, LTI-03 and XFB-19, are in Phase 2 and Phase 1 respectively, meaning years of development remain before any regulatory approval is possible.
The preclinical evidence is extensive but relies heavily on the bleomycin mouse model, which reproduces acute fibrosis but does not perfectly replicate the chronic, progressive nature of human IPF. The silica model used for M10 captures occupational fibrosis mechanisms but has different pathobiology from idiopathic disease. Translation from mouse to human has failed for many promising fibrosis candidates in the past.
What the evidence does support is the biological plausibility of peptide-based approaches. The twelve peptides reviewed here target at least six distinct pathways in fibrotic signaling. Several show effects in multiple independent models. The caveolin-1 peptide (LTI-03) demonstrated activity in human IPF tissue ex vivo, which is a stronger translational signal than mouse data alone. And the pipeline's diversity means that failure of any single candidate does not invalidate the approach.
The timeline for this field is measured in years, not months. LTI-03's Phase 2 topline data, expected in Q3 2026, will be the first real test of whether peptide-based antifibrotic activity translates to measurable clinical benefit in IPF patients. If the RENEW trial shows a signal on FVC decline, it would validate not just LTI-03 but the broader concept of peptide therapeutics for fibrotic lung disease. If it does not, the preclinical candidates behind it will face even greater scrutiny from investors and regulators.
The lung's airway defense system, including defensin peptides that protect against infection, operates in the same tissue where fibrosis develops. Understanding how endogenous peptide defenses interact with fibrotic processes remains an open question that could inform future therapeutic design.
For collagen-related peptide markers used in fibrosis diagnosis, the evidence is substantially more mature than for therapeutic peptides. The diagnostic and therapeutic sides of fibrosis research inform each other, but they are at very different stages of clinical validation.
The Bottom Line
The peptide therapeutic pipeline for pulmonary fibrosis contains at least twelve candidates targeting distinct molecular pathways, with LTI-03 (Phase 2) and XFB-19 (Phase 1) as the only two in human clinical trials. The preclinical evidence supports biological plausibility across multiple mechanisms, but no peptide has yet demonstrated efficacy in a controlled human trial for IPF. The field's strength is its mechanistic diversity; its limitation is the distance between mouse models and human disease.