TB-500 and thymosin beta-4: what the research literature shows

What it is

Three things behind one name

Review Thymosin beta-4 is the natural molecule, first isolated from calf thymus and at first thought to be a thymic hormone.⁵ It was later found in large amounts in many cell types, and a review of the family concludes that it is not really a thymic hormone.¹⁴ It is a 43-residue polypeptide of about 5 kDa.⁴

Cell study Its actin-binding region includes a short stretch in the middle of the chain, mapped to residues 17 to 22; later papers call the seven residues LKKTETQ, positions 17 to 23, the actin-binding domain.¹⁹¹⁵

TB-500 is a commercial name. A racing laboratory that analyzed a veterinary preparation sold under it reported that the key ingredient was LKKTETQ with an acetyl group added to the N-terminus.¹ A later metabolism study and PubChem define TB-500 as Ac-LKKTETQ, and the FDA gives TB-500 as another name for the LKKTETQ fragment.³¹³¹⁶

Values are from the two PubChem records.¹⁶¹⁷

How it is studied

Three kinds of experiment recur, on different rungs of the evidence ladder.

• Cell study Bench and cell work: actin polymerization assays, endothelial cell migration and vessel-sprouting assays, and mass spectrometry of the peptide and its breakdown products.³⁴⁵
• Animal study Injury models: skin wounds in diabetic and aged mice, coronary artery ligation in mice, cardiopulmonary bypass in pigs, alkali burns of the mouse cornea and tendon repair in rats.²⁷⁸⁹¹⁸
• Human study Clinical trials of full-length thymosin beta-4 in healthy volunteers, venous ulcers, dry eye and neurotrophic keratopathy, a disease of the corneal surface.^6111920

What the literature reports

Actin sequestration (thymosin beta-4)

Cell study A peptide purified from human platelets as an actin-sequestering factor turned out to have the same sequence as thymosin beta-4, which formed a 1:1 complex with actin monomers and inhibited their polymerization into filaments.⁵

Review A later review describes beta-thymosins as the main peptides holding actin monomers in reserve in most vertebrate cells, binding loosely enough to release actin quickly, and notes that very little is known about how the effects attributed to them outside the cell are produced.¹⁴

Cell study A mapping study with chemically synthesized full-length variants found two parts of the chain holding actin: an N-terminal segment (residues 1 to 16), which had to form a helix, and a six-residue motif at residues 17 to 22.¹⁵ It used full-length variants, so it does not test the seven-residue fragment on its own.¹⁵

Cell migration and vessel sprouting (both molecules)

Cell study In endothelial cell migration assays and chick aortic arch sprouting assays, thymosin beta-4 and a seven-residue peptide carrying the actin-binding motif showed near-identical activity, while peptides lacking any part of the motif were inactive.⁴

Cell study In a fibroblast wound-closure assay of the acetylated form sold as TB-500, the parent peptide Ac-LKKTETQ did not differ significantly from control; only one of its breakdown products, Ac-LKKTE, did.³ The authors suggest that activity reported for TB-500 may come from that metabolite, not from the parent peptide.³

Skin wound models (mostly thymosin beta-4)

Animal study In full-thickness skin wounds, thymosin beta-4 was followed by more wound contracture and collagen deposition than in controls in both diabetic and aged mice, with more migration of keratinocytes (skin cells) in the aged mice only.⁹ The same paper holds the only wound experiment we found on the short sequence: synthetic LKKTETQ gave repair in aged mice comparable to the parent molecule.⁹ A later paper assigns that work to the unacetylated sequence, not to the acetylated TB-500.³

Cardiac models (thymosin beta-4)

Animal study In mice, thymosin beta-4 given after coronary artery ligation was followed by higher activity of two signalling enzymes, integrin-linked kinase and Akt, in the heart and greater early survival of heart muscle cells, and the authors reported better cardiac function.⁷

Animal study A later paper recalls an earlier report in which cells of the epicardium, the heart’s outer layer, became heart muscle cells in mice pretreated with thymosin beta-4 before an infarction.²¹ Its own experiment gave the peptide after the infarction and traced the cells genetically: the epicardium thickened and capillary density rose, but its cells did not become heart muscle.²¹

Animal study In pigs undergoing cardiopulmonary bypass with aortic cross-clamping, cell death, cardiac function and myocardial blood flow did not differ between animals given thymosin beta-4 and those given vehicle.¹⁸

Corneal models

Animal study In mouse corneas injured with alkali, eyes treated with thymosin beta-4 regrew their surface cell layer (re-epithelialized) faster than saline-treated controls, with fewer infiltrating white blood cells (neutrophils) at seven days and lower transcript levels of several inflammatory mediators.⁸

Animal study A 2025 materials study built the LKKTETQ sequence into a self-assembling hydrogel conjugate, not the free peptide, and described the sequence as not previously explored for corneal disease.¹⁰ It was tested on human corneal cells in culture and in an alkali burn model, where the authors reported faster regrowth of the surface layer and less inflammation.¹⁰

Tendon repair (material labelled TB-500)

Animal study In a rat Achilles tendon transection-and-repair model, the group given TB-500, which the abstract calls synthetic thymosin beta-4 without giving a sequence, had a higher load to failure and lower histological degeneration scores than controls at four weeks.² Combining it with BPC-157 added nothing over either peptide alone, and the authors call the findings exploratory.²

Tumour models (thymosin beta-4 expression)

Animal study Mouse melanoma cells engineered to overexpress the thymosin beta-4 gene formed larger tumours and more lung metastases in mice than control cells, with more cell migration and more blood vessels in the tumours; no peptide was added from outside.²²

Human evidence

We found no human evidence for the seven-residue fragment: no published trial, and the FDA states that it has not identified human exposure data on drug products containing it.¹³ Every trial below used full-length thymosin beta-4.

• Human study Phase 1, healthy volunteers. Four cohorts of ten healthy volunteers were given synthetic thymosin beta-4 or placebo once and then repeatedly, in amounts rising from cohort to cohort; adverse events were infrequent, mild or moderate, and none was serious or stopped the escalation.¹⁹
• Human study Venous ulcers. A double-blind, placebo-controlled phase 2 trial randomized 73 patients; the authors judged safety comparable to placebo and called the efficacy results suggestive at one tested concentration, with no statistical comparison in the abstract.²⁰
• Human study Dry eye. In a phase 2 trial of an ophthalmic solution in 72 subjects, neither primary endpoint differed significantly from placebo, although some secondary measures did.¹¹ A phase 3 trial, ARISE-3, enrolled 700 participants; the registry shows nearly identical mean changes in both primary measures in the two arms and posts no statistical analysis.²³
• Human study Neurotrophic keratopathy. In a phase 3 trial that enrolled 18 patients, complete closure of the corneal defect at four weeks was recorded in 6 of 10 treated and 1 of 8 placebo patients, which was not statistically significant (p = 0.0656).⁶ Three of the six authors were employees of the sponsor or its parent company.⁶ The registry lists that trial as terminated for a business decision, and a second trial, SEER-2, as recruiting in a record last updated in December 2025.¹²²⁴
• Human study Acute myocardial infarction. A recombinant form has reached phase 2 in China; the registry lists a 90-participant trial as completed, with no results posted.²⁵

Regulatory status

• Canada. A search of Health Canada’s Drug Product Database on 8 October 2026 for thymosin and timbetasin as active ingredients returned no products.²⁶
• United States. On the same date, the same search of the FDA’s Drugs@FDA data returned no matches, and the September 2026 data file of the Purple Book, the FDA’s list of licensed biological products, had no entry for either name.²⁷²⁸ An FDA compounding page, current as of 22 April 2026, lists the fragment LKKTETQ, with TB-500 as its other name, among bulk substances once placed in category 2 of its interim policies (substances that may present significant safety risks) and later withdrawn by their nominators; it cites a possible unwanted immune response (immunogenicity) from aggregation and peptide-related impurities.¹³
• Sport. As displayed on 8 October 2026, the World Anti-Doping Agency’s Prohibited List names thymosin beta-4 and its derivatives, with TB-500 as the example, among growth factors prohibited at all times under section S2.3.²⁹

Material sold on this site is for laboratory research use only.

In the laboratory

TB-500 is supplied here as a lyophilized (freeze-dried) powder in sealed vials, alone and in a two-peptide vial with BPC-157. A pre-mixed liquid format is also listed.

Because the name covers two molecules, identity comes first. The two differ more than fivefold in mass, so a mass spectrum, where a batch document gives one, shows which molecule a lot contains.¹⁶¹⁷ Our guide to reading a certificate of analysis shows where to look, and batch documents are posted on the Lab Results page when they exist.

The two sequences also differ in the residues usually watched for oxidation: thymosin beta-4 has one methionine, at position 6, and a review mentions an oxidized form of the protein, while the fragment has no methionine, cysteine or tryptophan.¹⁴¹⁶¹⁷ Oxidation and the other routes are explained in peptide stability and degradation.

Cell study A doping-control study followed the breakdown of Ac-LKKTETQ in human serum, enzyme preparations and urine from rats given the peptide, and reports shorter acetylated pieces that include Ac-LK, Ac-LKK and Ac-LKKTE.³

Temperature, light and moisture are covered in the peptide storage guide.