TB-500 (Thymosin β4) Research, Specifications & Scientific Information

Introduction

Most pages in this library begin with what a compound is. This one has to begin with which compound is meant, because "TB-500" names two molecules that differ by a factor of five in mass. Thymosin β4 is a 43-residue intracellular protein of the β-thymosin family that binds monomeric actin and keeps it from polymerising; it has an International Nonproprietary Name, timbetasin, and a clinical development history running from rodent wound models in the 1990s to completed phase 3 ophthalmic trials. TB-500, as PubChem and the FDA/NCATS register define it, is a seven-residue acetylated peptide — Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln — corresponding to residues 17 to 23 of that protein. Suppliers apply the name to either.

This page keeps the two apart throughout, and says which one each piece of evidence was gathered on. It is a reference record: structure, mechanism, and the published preclinical and clinical literature, with every source resolved against PubMed, Crossref or ClinicalTrials.gov at the time the page was built. It describes research, not use, and it contains no guidance of any kind on handling the material.

What Is TB-500?

Thymosin β4 is not a hormone and not a secreted signalling peptide, despite a name that suggests a thymic hormone. It is one of the most abundant intracellular proteins in many mammalian cell types, and its characterised function is cytoskeletal: it holds a pool of monomeric actin in reserve by binding it one-to-one and preventing polymerisation into filaments ^[1]. The β-thymosin sequence motif is now classified as a WH2 (WASP-homology 2) actin-binding module, a fold found across a family of cytoskeletal regulators.

The clinical development programme has been run almost entirely by RegeneRx Biopharmaceuticals and its partners, under formulation codes rather than under the peptide's own name: RGN-259 for the ophthalmic solution, RGN-137 for a topical gel, RGN-352 for the intravenous form. A separate recombinant preparation, NL005, has been developed in China for acute myocardial infarction ^[12]. None of these has been approved by the U.S. Food and Drug Administration.

"TB-500" belongs to a different lineage. It appears in the chemical registers as a distinct substance — CAS 885340-08-9, PubChem CID 62707662, UNII QHK6Z47GTG — with the structure Ac-LKKTETQ-OH and a mass of 889.0 g/mol. The catalog record behind this page carries the 43-residue figures, not the fragment figures, and the specification table below reflects that. The Chemical & Molecular Characteristics section sets out how to tell which is which.

TB-500 Specifications

The name on this page covers two different molecules, and the difference is the single most important fact about the compound. (1) Thymosin beta-4 is a 43-residue, N-terminally acetylated intracellular protein of the beta-thymosin family. Its identifiers are CAS 77591-33-4, PubChem CID 45382195, UNII 549LM7U24W, formula C212H350N56O78S and average mass 4963.4 g/mol; its INN is timbetasin. The sequence above is that 43-residue backbone in single-letter code and does not show the N-terminal acetyl group. (2) 'TB-500' in the chemical registers is not that protein: PubChem CID 62707662 and UNII QHK6Z47GTG both record TB-500 as Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH, a seven-residue acetylated peptide corresponding to residues 17 to 23 of thymosin beta-4, with CAS 885340-08-9, formula C38H68N10O14 and average mass 889.0 g/mol. The two differ by roughly 4,074 daltons. The identifiers shown in the table above are those of the 43-residue protein, because that is what the catalog record for this laboratory reagent carries: the molecular formula and mass in the supplier catalog are the full-length values, not the fragment values. Material sold elsewhere under the label 'TB-500' is frequently the seven-residue fragment instead. Nothing on this page resolves which of the two a given lot contains; the certificate of analysis and the mass spectrometry result supplied with a laboratory order are the record for that, and on this compound in particular the mass figure is the thing to read.

Values that a public register does not carry are shown as not publicly characterised rather than estimated. Identifiers are reference values; the certificate of analysis supplied with a laboratory order is the record for a given lot.

How Does TB-500 Work?

The mechanism of thymosin β4 is better characterised than that of most compounds in this library, because its binding partner is known and the interaction has been mapped residue by residue.

Actin exists in cells in two forms: free monomers, G-actin, and polymerised filaments, F-actin. The balance between them drives cell shape, motility and division. β-thymosins bind G-actin and hold it out of the filament pool, which makes them buffers rather than enzymes: they do not catalyse anything, and their effect scales with how much of them is present.

Two consequences follow, and both matter for reading the rest of this page. First, the site of action is inside the cell, which raises a question the extracellular literature does not fully answer — how a 43-residue peptide applied outside a cell exerts effects attributed to an intracellular binding partner. Second, activity is a property of a specific structural arrangement rather than of the sequence as a whole, so a fragment is not automatically a miniature of the parent.

A separate mechanism has been reported that does not involve actin sequestration at all: complex formation with PINCH and integrin-linked kinase, leading to activation of Akt ^[5]. That pathway is the one invoked in the cardiac literature.

TB-500 Mechanism of Action

In vitro research

Actin sequestration, and which parts of the sequence are required for it. Using the DNase I inhibition assay — which reports G-actin but not F-actin — thymosin β4 was shown to sequester G-actin at a one-to-one ratio and to block polymerisation in high-salt solution. Oxidation of the single methionine at position 6 did not abolish the property. Two C-terminal fragments did: neither thymosin β4 13-43 nor thymosin β4 24-43 inhibited polymerisation of G-actin at all, and the authors concluded that structural features before position 13 are obligatory for the function ^[1].

The binding site mapped. Chemical synthesis of full-length variants, cross-linking, native-gel complex formation and circular dichroism placed the actin-binding site in two separate structural entities: an N-terminal part, residues 1 to 16, which must adopt an α-helix and contacts actin through a hydrophobic patch, and a hexapeptide motif at residues 17 to 22. Electrostatic contacts involving lysine 18 in the motif and lysine 14 in the helix were identified as important. The critical residues are conserved across the β-thymosin family ^[2].

Put together, these two papers are the reason this page separates the fragment from the protein rather than treating them as interchangeable. Residues 17 to 22 are genuinely part of the actin-binding site — but they are one of two parts, and removing the other part abolished sequestration in the assay that measures it. The seven-residue peptide sold as TB-500 begins at residue 17.

Endothelial cell behaviour. Thymosin β4 stimulates directional migration of human umbilical vein endothelial cells, which is the in vitro observation underlying the angiogenic claims made for it ^{[3, 10]}.

These are observations in cell-free assays and in cultured cells. They describe biochemistry and establish nothing about animals or people.

What Is TB-500 Being Researched For?

Registered and published research on thymosin β4 clusters into four areas, in rough order of how far each has progressed:

• Ophthalmic surface disease. Dry eye disease through phase 2, phase 2/3 and two phase 3 trials ^{[8, 9, 17, 19, 20]}, and neurotrophic keratopathy through two phase 3 trials ^[18].
• Dermatological repair. Epidermolysis bullosa ^[14], venous stasis ulcers ^[16] and pressure ulcers ^[15], all at phase 2.
• Cardiac research. Preclinical work on coronary artery ligation ^[5], a phase 1 intravenous programme ^[7], a recombinant first-in-human study ^[12] and a phase 2 trial in acute myocardial infarction ^[21].
• Cytoskeletal biochemistry. The actin work described above, which is basic science rather than a development programme ^{[1, 2]}.

Every one of those studied thymosin β4. The seven-residue TB-500 fragment has one registered human trial, opened in 2026, with no results ^[22].

Human Research on Thymosin β4

Human clinical research

Results from pharmaceutical clinical trials describe the investigational material and populations used in those studies and should not be interpreted as establishing the effects of research-grade materials offered for laboratory use.

Phase 1, intravenous, healthy volunteers

Population. Four cohorts of ten healthy subjects each, 40 in total, given placebo or synthetic thymosin β4 intravenously ^[7].

Endpoint and design. Safety evaluations, incidence of treatment-emergent adverse events and pharmacokinetic parameters, first as a single administration of 42, 140, 420 or 1260 mg, then — after safety review — as the same amount daily for 14 days ^[7].

Result. Adverse events were infrequent and mild or moderate in intensity. There were no limiting toxicities and no serious adverse events. The single-administration pharmacokinetic profile was proportional to the amount given, with half-life increasing as the amount increased ^[7].

Limitations. Forty subjects, safety and pharmacokinetics only. No efficacy endpoint was assessed.

Phase 1, recombinant preparation, healthy volunteers

Population. 54 healthy subjects in seven single-administration cohorts and 30 in three multiple-administration cohorts, in China ^[12].

Endpoint and design. Safety, tolerability, pharmacokinetics and anti-drug antibodies for recombinant human thymosin β4 (NL005), intravenously, at 0.05 to 25.0 µg/kg as a single administration and at 0.5, 2.0 or 5.0 µg/kg once daily for ten days, with 28 days of observation ^[12].

Result. Adverse events were mild to moderate; there were no limiting toxicities and no serious adverse events. Plasma concentration, Cmax and AUC rose with the amount administered, terminal clearance was consistent across groups, and no obvious accumulation was seen after continuous administration ^[12].

Limitations. A phase 1 in healthy volunteers, with amounts roughly four orders of magnitude below those in the earlier intravenous study — a difference that reflects a different preparation and a different development route, and that makes the two studies hard to read against each other.

Phase 2, severe dry eye disease

Population. Nine patients with severe dry eye at two United States sites ^[8].

Endpoint and design. A 56-day multicentre, randomised, double-masked, placebo-controlled phase 2 trial with 28 days of follow-up; the ophthalmic solution at 0.1% or vehicle control six times daily for 28 days ^[8].

Result. At day 56 the treated group, 12 eyes, showed a 35.1% reduction in ocular discomfort against vehicle control, six eyes (p = 0.0141), and a 59.1% reduction in total corneal fluorescein staining against vehicle control (p = 0.0108) ^[8].

Limitations. Nine patients. The authors describe it as a small trial, and it is.

Phase 2, controlled adverse environment model

Population. 72 subjects with moderate to severe dry eye at a single centre, randomised 1:1 ^[9].

Endpoint and design. Primary endpoints ocular discomfort score and inferior corneal staining at day 29, using a controlled adverse environment challenge; 28 days of treatment ^[9].

Result. Neither primary endpoint reached significance. Several secondary endpoints did: discomfort scores in the challenge at day 28 were reduced by 27% against placebo (p = 0.0244), with significant improvements in central and superior corneal staining (p = 0.0075 and p = 0.0210). No adverse events were observed ^[9].

Limitations. A trial that misses both primary endpoints and reports secondary ones is a trial that did not demonstrate what it set out to demonstrate, whatever the secondary findings show. Reporting the secondary results without that sentence would misrepresent it.

The phase 3 programme

Three larger ophthalmic trials have completed: ARISE-1, a phase 2/3 trial with 317 participants ^[17]; ARISE-2, phase 3 with 601 ^[19]; and ARISE-3, phase 3 with 700 ^[20]. A phase 3 trial in neurotrophic keratopathy, SEER-1, was terminated with 18 participants ^[18].

Limitations, and an important one. At the time this page was built, no primary journal publication reporting the results of ARISE-1, ARISE-2 or ARISE-3 could be located in PubMed. Summary results are posted to the ClinicalTrials.gov records for ARISE-2 and ARISE-3. A completed phase 3 trial whose findings have not been published in the peer-reviewed literature supports far less than a published one, and no efficacy figure from those trials is quoted on this page for that reason.

Preclinical Research on Thymosin β4

Animal research

Full-thickness dermal wounds in rats. Applied topically or intraperitoneally, thymosin β4 increased re-epithelialisation by 42% over saline controls at four days and by as much as 61% at seven days, and treated wounds contracted at least 11% more than controls by day seven. Increased collagen deposition and angiogenesis were observed in the treated wounds ^[3].

The fragment, in vivo. In db/db diabetic mice and in 26-month-old aged mice, thymosin β4 in saline or in a hydrogel increased wound contracture and collagen deposition. In the aged animals it also increased keratinocyte migration. Critically for the question this page keeps returning to, the authors also tested the actin-binding domain as a seven-amino-acid synthetic peptide, LKKTETQ, and reported that it promoted repair in the aged animals comparably to the parent molecule ^[4].

That is the single most relevant preclinical result for material sold as TB-500, and it should be read for exactly what it says: one endpoint, one model, aged mice, a comparison of the fragment against the full protein in a dermal wound. It does not say the fragment sequesters actin — the biochemical work says it should not ^[1] — and it does not extend to any other tissue or species.

Cardiac. After coronary artery ligation in mice, thymosin β4 administration resulted in up-regulation of integrin-linked kinase and Akt activity in the heart, enhanced early myocyte survival and improved cardiac function. In the same work the peptide promoted myocardial and endothelial cell migration in the embryonic heart and retained that property in postnatal cardiomyocytes ^[5].

Tendon. A 2026 rat Achilles study assessed TB-500 alongside BPC-157 histopathologically and biomechanically, and is the only published head-to-head rodent comparison of the two ^[13].

Findings described in this section were observed in animals, and nothing in them establishes anything about humans.

Other Areas of Thymosin β4 Research

Human clinical research

Results from pharmaceutical clinical trials describe the investigational material and populations used in those studies and should not be interpreted as establishing the effects of research-grade materials offered for laboratory use.

Dermatological programme. Three phase 2 trials were registered by RegeneRx in wound indications: epidermolysis bullosa, which was terminated with 30 participants ^[14], venous stasis ulcers with 72 ^[16], and pressure ulcers with 72 ^[15]. Summary results are posted to all three registry records. No primary journal publication of any of the three could be located in PubMed at the time this page was built, and the epidermolysis bullosa work is represented in the literature only by a narrative article describing the rationale rather than the outcome ^[6]. This is a common pattern for the compound and it is a real limit on what can be said about it.

Cardiac programme. A phase 2 trial of thymosin β4 in acute myocardial infarction, with 62 participants, is recorded as completed ^[21]. A separate randomised pilot took a different route entirely: autologous endothelial progenitor cells were pre-treated with thymosin β4 ex vivo and then transplanted into patients with acute ST-segment elevation myocardial infarction ^[11]. That study tested a cell product prepared with the peptide, not administration of the peptide, and it should not be counted as evidence about the peptide itself.

The fragment. One registered trial of TB-500 as such exists: a phase 1/2 study described in its registry record as the thymosin β4 17-23 fragment, recruiting from 2026 with a planned enrolment of 80 and cardiovascular biomarker endpoints ^[22]. It has no results.

Current Research Status

Status as of September 22, 2026. This block is rendered from maintained fields, not from prose, so it cannot go stale in one place and stay current in another.

TB-500 vs Related Compounds

Composition and mechanism only; no comparison of effect is made between these compounds.

Molecular class

Thymosin β4 — a 43-residue intracellular protein with a WH2 actin-binding module. TB-500 — a seven-residue acetylated fragment of it. BPC-157 — a fifteen-residue synthetic peptide from a gastric juice fraction. Thymosin α1 — a 28-residue acetylated peptide from prothymosin α, unrelated to the β-thymosins despite the shared family name.

The thymosin name

"Thymosin" is a historical label from fractions of calf thymus, not a structural or functional family. Thymosin α1 and thymosin β4 are not related by sequence, fold, target or pathway; they were named for where they were first found.

Identified binding partner

Thymosin β4 — monomeric actin, with the site mapped to an N-terminal helix plus the 17-22 hexapeptide ^[2]. TB-500 — carries the hexapeptide but not the helix; the assay that measures actin sequestration finds C-terminal fragments inactive ^[1]. BPC-157 — none identified. Thymosin α1 — no single receptor; activity reported through Toll-like receptor signalling in dendritic cells.

Clinical position

Thymosin β4 has completed phase 3 trials in an ophthalmic indication without approval. Thymosin α1 is an approved pharmaceutical product in a number of countries under the name thymalfasin. BPC-157 and the TB-500 fragment have no completed and published controlled trial between them.

Chemical & Molecular Characteristics

Two molecules, one name. This section exists to make the difference checkable rather than arguable.

Thymosin β4. A 43-residue peptide, N-terminally acetylated, sequence SDKPDMAEIEKFDKSKLKKTETQEKNPLPSKETIEQEKQAGES. The registers carry CAS 77591-33-4, PubChem CID 45382195 and UNII 549LM7U24W; the INN is timbetasin. Molecular formula C212H350N56O78S, average mass 4963.4 g/mol. The single-letter string above is the backbone and does not show the N-terminal acetyl group, which is part of the molecule.

TB-500. A seven-residue peptide, Ac-LKKTETQ-OH, corresponding to residues 17 to 23 of the sequence above. The registers carry CAS 885340-08-9, PubChem CID 62707662 and UNII QHK6Z47GTG. Molecular formula C38H68N10O14, average mass 889.0 g/mol.

How to tell them apart on paper. The mass difference is about 4,074 daltons — roughly 5.6-fold — so any mass spectrometry result distinguishes them immediately, as does the presence or absence of sulfur in the molecular formula: thymosin β4 contains one methionine at position 6 and therefore one sulfur atom, and the fragment contains none. A catalog listing quoting 4963.4 g/mol with formula C212H350N56O78S is describing the protein. One quoting 889.0 g/mol with C38H68N10O14 is describing the fragment. The catalog record behind this page carries the former.

Why the difference is not cosmetic. The two register entries are not two names for one substance; they are two substances with separate CAS numbers and separate UNIIs, and the published evidence does not transfer freely between them. Actin sequestration is demonstrated for the 43-residue protein and was specifically absent from C-terminal fragments in the assay that measures it ^[1]. Every completed human trial listed on this page used the protein. The fragment's in vivo record consists of one endpoint in one rodent dermal model ^[4] and one registered human trial with no results ^[22].

A note on the methionine. Position 6 is the only sulfur-containing residue in thymosin β4, and it oxidises. Oxidation to the sulfoxide did not abolish actin sequestration in the DNase I assay ^[1], but it changes the mass by 16 daltons and is visible in mass spectrometry, which is one reason a lot-specific analytical record matters for this compound more than for most.

Analytical Specifications

Analytical figures are lot-specific. Fields the catalog does not carry for the current lot are omitted rather than filled with a typical value. The certificate of analysis and the safety data sheet for the exact lot supplied are provided with a laboratory order; no purity figure on this page is a substitute for that document.

Frequently Asked Questions

Scientific References

1. 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. PMID 8471179 doi:10.1515/bchm3.1993.374.1-6.117
2. Van Troys M, Dewitte D, Goethals M, et al.. The actin binding site of thymosin beta 4 mapped by mutational analysis The EMBO journal; 1996. PMID 8617195
3. Malinda KM, Sidhu GS, Mani H, et al.. Thymosin beta4 accelerates wound healing The Journal of investigative dermatology; 1999. PMID 10469335 doi:10.1046/j.1523-1747.1999.00708.x
4. Philp D, Badamchian M, Scheremeta B, et al.. Thymosin beta 4 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 : official publication of the Wound Healing Society [and] the European Tissue Repair Society; 2003. PMID 12581423 doi:10.1046/j.1524-475x.2003.11105.x
5. Bock-Marquette I, Saxena A, White MD, et al.. Thymosin beta4 activates integrin-linked kinase and promotes cardiac cell migration, survival and cardiac repair Nature; 2004. PMID 15565145 doi:10.1038/nature03000
6. Fine JD. Epidermolysis bullosa: a genetic disease of altered cell adhesion and wound healing, and the possible clinical utility of topically applied thymosin beta4 Annals of the New York Academy of Sciences; 2007. PMID 17468231
7. Ruff D, Crockford D, Girardi G, et al.. A randomized, placebo-controlled, single and multiple dose study of intravenous thymosin beta4 in healthy volunteers Annals of the New York Academy of Sciences; 2010. PMID 20536472 doi:10.1111/j.1749-6632.2010.05474.x
8. 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. PMID 25826322 doi:10.1097/ICO.0000000000000379
9. 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. PMID 26056426 doi:10.2147/OPTH.S80954
10. Goldstein AL, Kleinman HK. Advances in the basic and clinical applications of thymosin β4 Expert opinion on biological therapy; 2015. PMID 26096726 doi:10.1517/14712598.2015.1011617
11. Zhu J, Song J, Yu L, et al.. Safety and efficacy of autologous thymosin β4 pre-treated endothelial progenitor cell transplantation in patients with acute ST segment elevation myocardial infarction: A pilot study Cytotherapy; 2016. PMID 27288307 doi:10.1016/j.jcyt.2016.05.006
12. Wang X, Liu L, Qi L, et al.. 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. PMID 34346165 doi:10.1111/jcmm.16693
13. Biçer O, Adanir O, Güleryüz Y, et al.. Effects of BPC-157 and TB-500 on Achilles tendon healing in rats: A histopathological and biomechanical study Joint diseases and related surgery; 2026. PMID 42542926 doi:10.52312/jdrs.2026.2951
14. A Phase 2 Study on Effect of Thymosin Beta 4 on Wound Healing in Patients With Epidermolysis Bullosa 2006. NCT00311766
15. Study of Thymosin Beta 4 in Patients With Pressure Ulcers 2006. NCT00382174
16. Study of Thymosin Beta 4 in Patients With Venous Stasis Ulcers 2006. NCT00832091
17. Assessment of the Safety and Efficacy of RGN-259 Ophthalmic Solutions for Dry Eye Syndrome: ARISE-1 2015. NCT02597803
18. Assessment of the Safety and Efficacy Study of RGN-259 Ophthalmic Solutions for Neurotrophic Keratopathy : SEER-1 2015. NCT02600429
19. Assessment of the Safety and Efficacy of RGN-259 Ophthalmic Solutions for Dry Eye Syndrome : ARISE-2 2016. NCT02974907
20. Assessment of the Safety and Efficacy of RGN-259 Ophthalmic Solutions for Dry Eye Syndrome: ARISE-3 2019. NCT03937882
21. Safety and Efficacy Study of Thymosin Beta 4 in Patients With Acute Myocardial Infarction.Infarction 2020. NCT05485818
22. TB-500 (Thymosin Beta 4 17-23 Fragment) for Cardiovascular Biomarkers in Stable ASCVD 2026. NCT07487363

Every identifier above is resolved against PubMed, Crossref or ClinicalTrials.gov at build time, and the title returned by the register is compared with the title stored here. A page does not publish if a reference fails to resolve.

Research-Use Information

For in vitro research use only. This material is a laboratory reagent. It is not a drug, food, dietary supplement, or cosmetic and is not for human or veterinary use, including ingestion, injection, or any other administration. No information on this page describes or implies any effect in humans or animals. Sold only to researchers under our Terms of Sale.

Related laboratory reagent: TB-500 (Thymosin β4) specifications and lot documentation