Thymosin β4

Thymosin β4 is a naturally occurring peptide that helps cells manage actin, a building block of their internal framework. Its connection with cell movement led researchers to investigate wound repair, the eye's surface and injured organs. Full-length formulations have reached human trials. Some measurements improved, but several planned main comparisons did not establish a benefit. Understanding these results requires the exact preparation, disease and physical outcome measured in each experiment.

Overview

Molecular profile

Parent identity: Mature full-length thymosin β4 Source
Mature chain length: 43 amino acids Source
Amino-terminal modification: Acetylated; established in the original thymosin β4 chemical characterization Source
Actin interaction: One thymosin β4 molecule per actin monomer in the biochemical assay Source
PK-study preparation: NL005, a 44-residue recombinant human thymosin β4 product Source

Forms and derivatives

RGN-259

Investigational ophthalmic thymosin β4 formulation tested in dry eye and persistent epithelial defects. Its route and clinical condition determine which outcomes the trials address.

NL005

A 44-residue recombinant preparation studied intravenously in healthy volunteers. Its numerical PK values are identified as NL005 data.

Thymosin β4 sulfoxide

Oxidation of the parent's methionine modifies the full chain. The 1993 purified assay found that this modification retained actin-sequestering ability.

Ac-SDKP

Four-residue amino-terminal product released through successive enzymatic cleavage of thymosin β4; studied in renal and fibrosis-related biology.

LKKTETQ and Ac-LKKTETQ/TB-500

Internal seven-residue sequences distinguished by terminal acetylation. Unacetylated LKKTETQ was studied in aged-mouse wounds; Ac-LKKTETQ is the fragment characterized in TB-500 analytical papers.

Mature thymosin β4 contains 43 amino acids and has an acetylated amino terminus. Early chemical work established this identity, while later biochemical experiments demonstrated actin binding. Synthetic versions of the mature peptide and recombinant products have both been developed. One recombinant trial product, NL005, was explicitly described as containing 44 residues, making its preparation relevant when interpreting its pharmacokinetics. Low, Hu and Goldstein, Wang et al.

Several related forms belong within this family. Oxidized thymosin β4, called thymosin β4 sulfoxide, retains the chain but modifies its methionine. Ac-SDKP is a four-residue product derived from the parent's amino terminus. LKKTETQ is an internal seven-residue segment; its acetylated version is the preparation identified as TB-500 in analytical papers. These molecules have their own experimental properties, while RGN-259 names an ophthalmic formulation studied clinically. Hannappel and Wartenberg, Kumar et al., FDA identity assessment

Uses and research areas

Tissue-repair research asks whether thymosin β4 can influence cell migration, matrix formation and the response to injury. Rodent dermal experiments provided early evidence of faster repair in selected impaired-healing models. Human development has concentrated on defined local conditions, including venous ulcers, dry eye and persistent corneal epithelial defects. Healthy-volunteer intravenous studies have examined tolerability and exposure as preparation for systemic development.

These conditions involve different obstacles to repair. A venous ulcer develops in tissue affected by impaired venous return. Dry eye combines tear-film instability with surface injury and symptoms. Neurotrophic keratopathy involves impaired corneal sensation and support, which can allow an epithelial defect to persist. The trials therefore require different endpoints: wound closure, corneal staining, discomfort or complete epithelial healing. Their findings become clearer when grouped by those clinical questions. Guarnera et al., Sosne and Ousler, Sosne et al.

Mechanism of action

Managing the actin pool

Actin exists as individual units and as assembled filaments. Cells continually reorganize these filaments to change shape, move and divide. Thymosin β4 binds individual actin units, a process called sequestration, and influences the pool available for assembly. Hannappel and Wartenberg measured a one-to-one interaction with actin and inhibition of polymerization in a purified biochemical system. Oxidation of methionine at position 6 did not abolish this function, while fragments spanning residues 13–43 or 24–43 lacked it. Hannappel and Wartenberg

This intracellular function gives a biological connection to repair, where cells must move and reorganize. Clinical formulations introduce peptide from outside the cells and tissues, however, so the biochemical result alone does not identify every step responsible for an observed clinical response. Research must also establish local access and the processes changed in the treated tissue.

Breakdown products and tissue context

Thymosin β4 can generate Ac-SDKP through successive enzyme actions. Kumar and colleagues found that meprin-α and prolyl oligopeptidase together released this tetrapeptide in laboratory systems; either enzyme alone was insufficient. Experiments with kidney homogenates and genetically altered mice supported the same pathway. This provides a concrete reason to consider both intact peptide and metabolites when investigating repair-related effects. Kumar et al.

The consequences of thymosin β4 also depend on which cells are active. In a 2023 mouse study, Kim and colleagues selectively deleted endogenous thymosin β4 in activated hepatic stellate cells, the cells that produce much of the scar tissue in liver fibrosis. Deletion reduced fibrosis in chemical-injury and bile-duct-ligation models. This genetic experiment concerns the role of endogenous peptide in those cells, rather than the clinical effects of administering a formulation. It shows that a repair-associated process can contribute to scarring in another setting. Kim et al.

Pharmacokinetics

Wang and colleagues measured plasma Tβ4 after intravenous NL005, the 44-residue recombinant preparation, in healthy Chinese volunteers. In the selected eight-person single-dose cohort studied at 5.0 μg/kg, Table 4 reports a terminal half-life of 1.383 hours. The table gives neither an averaging convention nor a standard deviation, standard error or confidence interval for that entry. The reported LC-MS/MS assay followed plasma Tβ4; the accessible methods did not provide enough assay detail to independently determine how the measurement distinguished intact peptide from related molecules. This estimate does not describe the distinct seven-residue TB-500 fragment or exposure in the eye.

Wang X, Liu L, Qi L, Lei C, Li P, Wang Y, Liu C, Bai H, Han C, Sun Y, Liu J. A first-in-human, randomized, double-blind, single- and multiple-dose, phase I study of recombinant human thymosin β4 in healthy Chinese volunteers. Journal of Cellular and Molecular Medicine. 2021;25(17):8222–8228. DOI: 10.1111/jcmm.16693

Wang and colleagues measured plasma Tβ4 by a validated LC-MS/MS method after intravenous NL005 in healthy Chinese volunteers. Their single-dose study enrolled 54 people across active and placebo cohorts; the multiple-dose study enrolled another 30. Reported cohort half-lives varied from 0.5 to 2.084 hours in the single-dose table. One eight-person active cohort had a reported terminal half-life of 1.383 hours. Wang et al., Table 4 and Methods 2.4

NL005 is the 44-residue recombinant preparation described in that publication. These intravenous plasma measurements do not determine the exposure produced by an eye drop, a skin formulation or the TB-500 fragment. The paper provides no half-life uncertainty for the selected table entry, and its LC-MS/MS description does not supply enough detail to independently compare analyte specificity across other preparations. A concentration decline also cannot determine how long a tissue-repair response lasts.

Research findings

Dermal repair in impaired-healing models

Philp and colleagues studied full-thickness wounds in diabetic and aged mice. Full-length thymosin β4 increased wound contraction and collagen deposition in diabetic animals. Keratinocyte migration did not differ, with nearly complete surface coverage observed across groups at the reported assessment. Aged mice showed improvements in migration as well as contraction and collagen deposition. Unacetylated LKKTETQ also promoted repair in the aged-mouse experiment. Philp et al.

The variation across measurements matters: surface closure, tissue contraction and collagen deposition are related parts of repair, each with a different biological meaning. These experiments established plausible targets for clinical testing without resolving which chronic human wounds would respond.

A 2026 study cut and surgically repaired rat Achilles tendons, then compared a product called synthetic thymosin β4/TB-500 with vehicle, the preparation without peptide. Repaired tendons from the product group withstood more pulling force before breaking than control tendons. The paper does not report the vehicle's composition or independently establish the product's peptide sequence or chemical form. Its result therefore cannot confidently be assigned to mature thymosin β4 or Ac-LKKTETQ. The TB-500 profile describes the full experiment and the number of tendons tested for each outcome. Biçer et al.

Venous ulcers

Guarnera and colleagues randomized 73 patients at eight European sites in a double-blind, placebo-controlled phase 2 trial of topical synthetic thymosin β4. The publication described an exploratory healing signal in one formulation group, especially among smaller or less severe wounds. Its abstract did not provide a precise placebo-adjusted effect with uncertainty. Safety was described as comparable with placebo. Guarnera et al.

This study supports further examination of local chronic-wound treatment. The subgroup emphasis and small formulation groups limit how confidently the result can predict benefit for a new patient or another injury type.

Dry eye: positive observations and missed primary endpoints

Sosne, Dunn and Kim studied nine patients with severe dry eye. They compared RGN-259 eye drops with vehicle drops, the formulation without the study peptide, during 28 days of treatment and another 28 days of observation. At day 56, treated eyes had less discomfort and less corneal staining, in which a dye reveals damage to the eye's surface. Results were reported for 12 treated and six control eyes: 18 eyes from nine people. The small patient count limits precision, and the two eyes of one person can have related responses. Sosne, Dunn and Kim

A separate 72-person phase 2 trial exposed participants to an environment designed to provoke dry-eye signs and symptoms. They received RGN-259 eye drops or vehicle drops without peptide for 28 days. On day 29, eye-discomfort and lower-cornea staining scores were slightly higher with RGN-259 than with vehicle, an unfavorable numerical direction, but neither planned main comparison established a clear difference. Corneal staining uses dye to reveal surface damage. Some secondary measurements favored RGN-259, including discomfort during the environmental challenge and staining of the central or upper cornea. Those findings suggest responses worth testing further, while the trial did not establish benefit on its planned main outcomes. Sosne and Ousler

Persistent corneal defects

SEER-1 compared RGN-259 eye drops with placebo in people with stage 2 or 3 neurotrophic keratopathy, a condition in which impaired corneal nerves allow a surface wound to persist. Slow recruitment led to early termination after 18 participants completed the study, compared with 46 planned. At the main day-29 assessment, the corneal surface wound had closed completely in six of ten RGN-259 recipients and one of eight placebo recipients. Although this favored treatment, the comparison was too uncertain to establish the planned benefit (prespecified Fisher exact test, P=0.0656). On day 43, five of ten versus none of eight had complete closure (P=0.0359). The small sample and changing healing counts limit conclusions about both precision and durable healing. Sosne et al., Table 2

Subsequent development also contains a negative finding. In a June 2025 shareholder letter, RegeneRx reported that the European SEER-3 trial did not achieve statistical significance for its prespecified four-week complete-healing endpoint. This is a sponsor topline account without the complete data needed for independent analysis. It nevertheless belongs beside the earlier trial because it prevents the small published signal from standing as the only visible clinical result. RegeneRx shareholder letter

Safety and tolerability

The local trials provide short-term tolerability information for their formulations. In the 72-person dry-eye paper, the abstract states that no adverse events occurred, but the detailed Results report ocular treatment-emergent events in two of 36 active participants and five of 36 placebo participants. Only two events were suspected to be treatment-related, both in the placebo group. One participant withdrew with vomiting considered unrelated. The detailed accounting is the appropriate basis for interpreting the safety findings. Sosne and Ousler, Results

In the 18-person keratopathy trial, adverse events affected four of ten active and three of eight placebo participants; 16 events were reported altogether. One event was considered treatment-related, while one nonocular serious event was judged unrelated. No event caused withdrawal. The small trial provides little ability to detect uncommon harms. Sosne et al.

The NL005 healthy-volunteer study reported mild or moderate adverse events and no serious events or dose-limiting toxicity during its observation. During the multiple-dose study, one NL005 recipient and one placebo recipient withdrew because of abnormal electrocardiogram findings in the highest exposure cohort. These findings support the tolerability of that trial preparation over the studied exposure. They cannot establish long-term systemic safety or the safety of commercial products whose identity and quality differ. The preparations described remain investigational; a trial phase or a compounding discussion does not confer therapeutic approval. FDA's explicit warning about absent human exposure data for the LKKTETQ fragment concerns that fragment and should be read alongside its exact identity. Wang et al., FDA compounding context

What the research means

Thymosin β4 has a developed biological rationale and a real human development program. Its clinical evidence remains uneven: some local measures improved, two published trials missed their prespecified primary comparisons, and a later sponsor report describes another missed endpoint. This pattern supports condition-specific investigation rather than a broad promise of tissue recovery.

The preparation and the tissue are central to the next inference. An intravenous recombinant plasma half-life, a topical ulcer response and a corneal-healing endpoint each contribute useful information about a different setting. Future results need to establish reproducible benefit against an appropriate comparator, durable outcomes and sufficiently extensive safety observation. The endogenous liver-fibrosis experiment additionally argues for examining the effects in the particular cell population involved, because more activity in a repair pathway can have different consequences across tissues.

FAQs

How is the mature peptide different from NL005?

Mature thymosin β4 has 43 residues and an acetylated amino terminus. Wang and colleagues described NL005 as a 44-residue recombinant product. The pharmacokinetic estimate therefore concerns NL005 given intravenously to healthy volunteers.

Why are some eye-study conclusions more cautious than the paper titles?

A trial sets out its main comparison before the results are known. In the 72-person dry-eye trial, RGN-259 did not establish an advantage over drops without peptide for either eye discomfort or lower-corneal surface damage on the planned assessment. In the 18-person keratopathy trial, more corneal wounds had closed with RGN-259 at day 29, but the comparison was too uncertain to establish the planned benefit (P=0.0656). Favorable secondary results remain useful reasons for further investigation.

Did the 18-person trial show durable corneal healing?

Complete-healing counts varied over follow-up. Six of ten active participants were healed at day 29 and five of ten at day 43; the corresponding placebo counts were one of eight and none of eight. The small sample and changing counts limit conclusions about durable healing.

Can more repair-pathway activity produce scarring?

The consequence depends on the cell and tissue. A mouse study found that deleting endogenous thymosin β4 in activated hepatic stellate cells reduced liver fibrosis. That genetic finding argues for evaluating the particular tissue response instead of assuming one direction of benefit across organs.

Do the full-length trials establish TB-500 efficacy?

The ocular and ulcer trials concern their full-length formulations. The acetylated seven-residue fragment has a separate experimental identity and metabolism. Its own measured outcomes are discussed on the TB-500 profile.

References

Hannappel E, Wartenberg F. Actin-sequestering ability of thymosin beta 4, thymosin beta 4 fragments, and thymosin beta 4-like peptides as assessed by the DNase I inhibition assay. Biological chemistry Hoppe-Seyler. 1993;374(2):117–122. DOI: 10.1515/bchm3.1993.374.1-6.117.
Purified biochemical assays measured one-to-one actin sequestration by full-length thymosin β4 and failure of two tested larger fragments to inhibit polymerization.
Guarnera G, DeRosa A, Camerini R, 8 European sites. The effect of thymosin treatment of venous ulcers. Annals of the New York Academy of Sciences. 2010;1194:207–212. DOI: 10.1111/j.1749-6632.2010.05490.x.
The 73-person topical venous-ulcer trial described exploratory healing observations and tolerability comparable with placebo.
Sosne G, Dunn SP, Kim C. Thymosin β4 significantly improves signs and symptoms of severe dry eye in a phase 2 randomized trial. Cornea. 2015;34(5):491–496. DOI: 10.1097/ICO.0000000000000379.
A nine-patient severe-dry-eye trial reported favorable day-56 discomfort and corneal-staining findings using eye-level outcomes.
Sosne G, Ousler GW. Thymosin beta 4 ophthalmic solution for dry eye: a randomized, placebo-controlled, Phase II clinical trial conducted using the controlled adverse environment (CAE™) model. Clinical ophthalmology (Auckland, N.Z.). 2015;9:877–884. DOI: 10.2147/OPTH.S80954.
In a dry-eye trial, RGN-259 did not establish an advantage over drops without peptide on the two planned main outcomes: eye discomfort and lower-corneal staining. Some secondary measurements favored RGN-259. Detailed Results record adverse events despite the abstract stating that none occurred.
Sosne G, Kleinman HK, Springs C, Gross RH, Sung J, Kang S. 0.1% RGN-259 (Thymosin ß4) Ophthalmic Solution Promotes Healing and Improves Comfort in Neurotrophic Keratopathy Patients in a Randomized, Placebo-Controlled, Double-Masked Phase III Clinical Trial. International journal of molecular sciences. 2022;24(1). DOI: 10.3390/ijms24010554.
The under-recruited keratopathy trial found 6/10 versus 1/8 complete healing at the primary visit, with P=0.0656; later results and harms are reported separately.
Low TL, Hu SK, Goldstein AL. Complete amino acid sequence of bovine thymosin beta 4: a thymic hormone that induces terminal deoxynucleotidyl transferase activity in thymocyte populations. Proceedings of the National Academy of Sciences of the United States of America. 1981;78(2):1162–1166. DOI: 10.1073/pnas.78.2.1162.
Early chemical characterization established a 43-residue, amino-terminally acetylated peptide isolated from calf thymus.
Philp D, Badamchian M, Scheremeta B, Nguyen M, Goldstein AL, Kleinman HK. Thymosin β4 and a synthetic peptide containing its actin-binding domain promote dermal wound repair in db/db diabetic mice and in aged mice. Wound Repair and Regeneration. 2003;11(1):19–24. DOI: 10.1046/j.1524-475X.2003.11105.x.
The rodent experiments tested full-length thymosin β4 and unacetylated LKKTETQ; migration, contraction and collagen results varied with the model.
Wang X, Liu L, Qi L, Lei C, Li P, Wang Y, Liu C, Bai H, Han C, Sun Y, Liu J. A first-in-human, randomized, double-blind, single- and multiple-dose, phase I study of recombinant human thymosin β4 in healthy Chinese volunteers. Journal of Cellular and Molecular Medicine. 2021;25(17):8222–8228. DOI: 10.1111/jcmm.16693.
Healthy-volunteer intravenous research on the explicitly 44-residue recombinant product NL005 supplies plasma measurements, tolerability observations and cohort half-lives. Abnormal ECG findings led to one active and one placebo withdrawal during the multiple-dose study.
Kumar N, Nakagawa P, Janic B, Romero CA, Worou ME, Monu SR, Peterson EL, Shaw J, Valeriote F, Ongeri EM, Niyitegeka JMV, Rhaleb NE, Carretero OA. The anti-inflammatory peptide Ac-SDKP is released from thymosin-β4 by renal meprin-α and prolyl oligopeptidase. American Journal of Physiology—Renal Physiology. 2016;310(10):F1026–F1034. DOI: 10.1152/ajprenal.00562.2015.
Combined enzyme and renal-model experiments establish successive cleavage of thymosin β4 to Ac-SDKP.
Kim J, Lee C, Han J, Jeong H, Wang S, Choi YH, Jung Y. Targeted Deletion of Thymosin Beta 4 in Hepatic Stellate Cells Ameliorates Liver Fibrosis in a Transgenic Mouse Model. Cells. 2023;12(12):1658. DOI: 10.3390/cells12121658.
Deleting endogenous thymosin β4 in activated hepatic stellate cells reduced fibrosis, showing a tissue- and cell-specific role in scarring.
Biçer O, Adanir O, Güleryüz Y, Balci EC, Dinçel YM, Yenigün MY, Aydin C, Bayrak BY. Effects of BPC-157 and TB-500 on Achilles tendon healing in rats: A histopathological and biomechanical study. Joint Diseases and Related Surgery. 2026;37(3):822–837. DOI: 10.52312/jdrs.2026.2951.
After Achilles-tendon cutting and surgical repair in rats, the investigators measured how much pulling force repaired tendons could withstand before breaking. The TB-500-labelled group withstood more force than the no-peptide vehicle group, but its sequence and chemical form were not independently established.
Finkelstein JJ, Goldstein AL, for RegeneRx. Letter to Stockholders. June 24, 2025. Sponsor communication reproducing ReGenTree/HLB Therapeutics' SEER-3 topline report.
The sponsor reported that SEER-3 missed the prespecified four-week complete-healing endpoint; full participant-level data were not supplied.
U.S. Food and Drug Administration. FDA Evaluation of TB-500-Related Bulk Drug Substances (TB-500 Free Base and TB-500 Acetate). Briefing document for the Pharmacy Compounding Advisory Committee, July 23–24, 2026; evaluation dated May 15, 2026.
The agency defines the acetylated fragment, differentiates its acetate salt and assesses the evidence and safety issues for compounding.
U.S. Food and Drug Administration. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks. Current agency web record, accessed October 13, 2026.
Agency safety context distinguishes injectable GHK-Cu and the thymosin β4 fragment from other preparations.

Glossary

Actin sequestration: Binding individual actin units so that their availability for filament assembly is regulated.
Keratinocyte: An epithelial cell that contributes to surface coverage and barrier repair.
Neurotrophic keratopathy: Corneal disease caused by impaired nerve support and sensation, which can lead to persistent surface defects.
Primary endpoint: The prespecified main outcome used to assess a trial's principal treatment question.
Hepatic stellate cell: A liver cell that can become activated and contribute to scar-matrix production.
LC-MS/MS: Liquid chromatography with tandem mass spectrometry, an analytical method used to quantify specified molecules.