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what is the difference

TB-500 and thymosin beta-4

One name covers two molecules of very different size, and the clinical record quoted for the short one was produced by the long one, in a tissue nobody buys it for.

Last Reviewed Editorial policy Methodology

TB-500 is a short synthetic peptide. A doping-control laboratory that analysed the product sold under that name identified Ac-LKKTETQ, the acetylated 17 to 23 fragment of thymosin beta-4, seven amino acids long [1]. Thymosin beta-4 is the whole protein that fragment sits inside: 43 amino acids, made throughout the body, and studied for decades because it binds actin, the structural protein cells use to hold their shape and move.

Sellers use the two names as if they were one product, and the trial counts have followed the names rather than the molecules. A ClinicalTrials.gov term search for thymosin beta-4 returned 18 registrations on 2 August 2026, and an intervention search for TB-500 returned one [8]. A single phase 1/2 study appears in both sets, because its registered title carries both names. Everything else in that group of 18 registered the parent protein, in eyes, skin wounds and the heart.

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The short answer

TB-500 is a seven amino acid piece of thymosin beta-4, sold for tendon and muscle recovery. Thymosin beta-4 is the 43 amino acid protein, and it is the molecule that has been through registered clinical trials: phase 2 work in dry eye and a phase 3 in neurotrophic keratopathy, all of it given as eye drops [2] [3] [4]. Not one registration in that set studied tendon or muscle, and not one studied the fragment. The single registration naming TB-500 is a phase 1/2 record of 80 people, first posted 23 March 2026, and its own registered brief summary states that it is a fictional study and an example of a ClinicalTrials.gov-style record [8].

Side by side

TB-500 and thymosin beta-4, compared on what each molecule is and what the registry and the literature hold for each, checked 2 August 2026
PropertyTB-500Thymosin beta-4
What it isA synthetic peptide seven amino acids long, characterised in marketed product as Ac-LKKTETQ, the acetylated 17 to 23 fragment [1]A 43 amino acid protein made throughout the body, which binds actin, the structural protein cells use to hold their shape and move
Registered human trials1, and it declares itself an example. A phase 1/2 record in cardiovascular disease, 80 participants, first posted 23 March 2026, recruiting, no result posted, whose registered brief summary opens by stating that it is a fictional study and an example of a ClinicalTrials.gov-style record [8]18 records on a term search, of which 17 registered the protein. The eighteenth is the TB-500 record above, matched because its title names both molecules [8]
What those registrations studiedCardiovascular biomarkersDry eye, neurotrophic keratopathy, skin ulcers, epidermolysis bullosa, myocardial infarction, and two phase 1 studies in healthy volunteers [8]
Registered trials in tendon or muscle00
Published human resultsNonePhase 2 in severe dry eye, 9 patients [2]. Phase 2 in moderate to severe dry eye, 72 subjects, neither primary endpoint separating from placebo [3]. Phase 3 in neurotrophic keratopathy, defects closing in 6 of 10 treated and 1 of 8 on placebo, short of the conventional significance threshold [4]. Three further phase 3 dry-eye trials totalling 1,618 participants have completed and none is published; the one that posted registry results moved in the worse direction on both primary endpoints. The TB-500 review carries that record in full
Published work on the molecule itselfA rat Achilles tendon model of 32 animals [7], a 2024 study tracking it and its metabolites in rats [6], equine pharmacokinetics, and two further animal studies in which it was given inside an engineered gel or alongside a second peptide. The hub sets all of them outA large ophthalmic and preclinical literature, plus a 2018 piece indexed as a review and a personal narrative rather than a study [5]
Evidence grade on this siteGrade D, derived from the claim rows on its own hub. No completed human study of the fragment exists for any useNot graded here either. The individual trials are cited by number instead
Regulatory positionNamed on the face of class S2.3 by its own name, as the example in the entry reading thymosin beta-4 and its derivatives; prohibited at all times and non-Specified [9]. In FDA 503A category 2 for immunogenicity and aggregation risk. The use FDA evaluated is wound healing, taken up by the compounding committee on 23 July 2026 [10]Named in class S2.3 [9]. Not before the same committee under its own name [10]
What is missingAny registration of it that is not a self-declared example, and any human study of any design, in any tissueA registered trial in tendon or muscle, and a study of the fragment rather than the protein

Why the two get mixed up

The confusion is built into the naming. TB stands for thymosin beta, so a product labelled TB-500 reads as a quantity of thymosin beta-4 rather than as a different molecule, and vendors list the two as synonyms. Once the names are interchangeable, so are the trial counts, and 18 registrations attach themselves to a compound that has one.

The chemistry keeps it going, because the 17 to 23 stretch is the actin-binding region and a fragment built from it gets described as the active part of the protein. That is a statement about a binding site, not about what happens in a body: a seven amino acid peptide and a 43 amino acid protein differ in stability and distribution, and the 2024 rat work found that a metabolite of the fragment, rather than the fragment, separated from control in a cell assay [6]. The last piece is where the parent's trials were run. A reader looking for tendon evidence finds a phase 3 trial with the right molecule name attached, in an eye [4], and stops reading.

What the published record covers for each

The parent protein carries the human record, and what those trials measured matters more than that they exist. Two randomized, placebo-controlled phase 2 trials tested thymosin beta-4 eye drops in dry eye. The first covered nine patients with severe dry eye and reported separation from vehicle control on discomfort and on corneal staining at day 56 [Human RCT] [2]. The second randomized 72 subjects using a controlled adverse environment model, and neither of its primary endpoints, ocular discomfort and inferior corneal staining, separated from placebo at the primary visit [Human RCT] [3]. The largest published result is a randomized, double-masked phase 3 in neurotrophic keratopathy, where persistent epithelial defects closed at four weeks in 6 of 10 treated subjects against 1 of 8 on placebo, short of the conventional significance threshold [Human RCT] [4]. The 2018 piece cited alongside these is indexed as a review and a personal narrative, so it summarises that programme rather than adding to it [5].

The fragment has its own literature and it is in rodents. In 32 male Sprague-Dawley rats with a transected and repaired Achilles tendon, split into four groups of eight, the TB-500 group reached a higher maximum load to failure than controls at four weeks and scored lower on two histological degeneration scales; combining it with BPC-157 added nothing over either alone [Animal] [7]. A 2024 analytical study quantified the compound and its breakdown products in rats and screened them in a fibroblast wound-closure assay, where TB-500 itself did not separate from control at the concentration tested and the metabolite Ac-LKKTE did [In-vitro] [6]. What this evidence can and cannot show: these results come from rats with a surgically created tendon injury and from cells in a dish. The injury is created on purpose, the animal is young and healthy, and the concentrations that produce an effect in a dish are often far above anything a body reaches. Mechanism in an animal does not establish an effect in a person.

The arithmetic is the part worth carrying away from this page. Seventeen registrations and a phase 3 endpoint belong to a 43 amino acid protein studied in the eye. One registration, which states in its own registered text that it is a fictional example, and a short animal literature belong to the seven amino acid fragment sold for tendons. The fragment now has a full evidence review on this site, graded from its own claim rows; the protein has none, because it is a molecule this site reports on rather than a compound in its catalogue.

Our takeEvery trial count published for TB-500 is really a count for a molecule six times its length, tested in an eye. That substitution is the single most useful thing to know about the name, and the one record that does name TB-500 says of itself that it is an example.

Frequently asked questions

Is TB-500 the same thing as thymosin beta-4?

No. Thymosin beta-4 is a 43 amino acid protein. Product sold as TB-500 was analysed by a doping-control laboratory and characterised as Ac-LKKTETQ, the acetylated 17 to 23 fragment, seven amino acids long [1]. They share a stretch of sequence, which is why the names travel together, and they are different molecules with different published records.

How many human trials has TB-500 been in?

None. One registry record names it, and that record states in its own registered brief summary that it is a fictional study and an example of a ClinicalTrials.gov-style record: a phase 1/2 in cardiovascular disease with 80 participants, first posted 23 March 2026, listed as recruiting, with no results posted [8]. The 18 registrations usually counted come from a term search on thymosin beta-4; 17 of them registered the protein and the eighteenth is that same record, matched because its registered title names both molecules. The TB-500 review sets the registry position out in full.

What did the thymosin beta-4 trials actually test?

Eyes and skin, not tendon or muscle. The published set is two randomized placebo-controlled phase 2 trials of eye drops in dry eye, one of nine patients [2] and one of 72 subjects in which neither primary endpoint separated from placebo [3], and a phase 3 in neurotrophic keratopathy where defects closed in 6 of 10 treated subjects against 1 of 8 on placebo [4]. The unpublished registrations cover skin ulcers, epidermolysis bullosa and myocardial infarction [8].

Does the parent protein's evidence apply to the fragment?

This site says no, and the rule is written down: a peptide that is a fragment of a larger protein does not inherit that protein's trials. The parent's claims keep their design tier and are labelled as the parent's, and the fragment takes a tier only from work that tested the fragment. The 2024 rat study illustrates why, since what separated from control in its cell assay was a breakdown product of the fragment rather than the fragment [6].

Where do the two stand with regulators?

Both sit inside one anti-doping entry: class S2.3 of the 2026 WADA Prohibited List, growth factors and growth factor modulators, reading thymosin beta-4 and its derivatives e.g. TB-500, prohibited at all times, and every substance in class S2 is non-Specified [9]. TB-500 is on the face of that entry by its own name rather than reached by inference. On the compounding side, the use FDA evaluated for TB-500 was wound healing, the Pharmacy Compounding Advisory Committee took it up on the afternoon of 23 July 2026 on two voting questions [10], and FDA separately lists the fragment in 503A category 2 for immunogenicity and aggregation risk.

References

  1. Esposito S, Deventer K, Goeman J, Van der Eycken J, Van Eenoo P. Synthesis and characterization of the N-terminal acetylated 17-23 fragment of thymosin beta 4 identified in TB-500, a product suspected to possess doping potential. Drug Test Anal. 2012. PMID 22962027 DOI 10.1002/dta.1402
  2. 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
  3. 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. Clin Ophthalmol. 2015. PMID 26056426 DOI 10.2147/OPTH.S80954
  4. 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. Int J Mol Sci. 2022. PMID 36613994 DOI 10.3390/ijms24010554
  5. Sosne G. Thymosin beta 4 and the eye: the journey from bench to bedside. Expert Opin Biol Ther. 2018. PMID 30063853 DOI 10.1080/14712598.2018.1486818 Indexed by PubMed as a review and a personal narrative, not as a study
  6. Rahaman KA, Muresan AR, Min H, Son J, Han HS, Kang MJ, Kwon OS. Simultaneous quantification of TB-500 and its metabolites in in-vitro experiments and rats by UHPLC-Q-Exactive orbitrap MS/MS and their screening by wound healing activities in-vitro. J Chromatogr B Analyt Technol Biomed Life Sci. 2024. PMID 38382158 DOI 10.1016/j.jchromb.2024.124033
  7. 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. Jt Dis Relat Surg. 2026. PMID 42542926 DOI 10.52312/jdrs.2026.2951
  8. US National Library of Medicine, ClinicalTrials.gov. Registry searches run through API v2 on 2 August 2026. A term search for thymosin beta 4 returned 18 records, covering dry eye, neurotrophic keratopathy, venous stasis ulcers, pressure ulcers, epidermolysis bullosa, acute myocardial infarction and two phase 1 studies in healthy volunteers. An intervention search for TB-500 returned one, NCT07487363, a phase 1/2 study of 80 participants first posted 23 March 2026, which also appears in the 18 because its registered title names both molecules. ClinicalTrials.gov. 2026. Registry record
  9. World Anti-Doping Agency. The 2026 Prohibited List, international standard, effective 1 January 2026. Class S2.3, growth factors and growth factor modulators, carries the entry reading Thymosin-ß4 and its derivatives e.g. TB-500, so the fragment is named on the face of the list. Class S2 is prohibited at all times and its heading states that all prohibited substances in the class are non-Specified Substances. Read back from the final English document, 26 pages, retained off-site; the address below serves a zero-byte response to automated requests and is recorded as such in this repository's source ledger. WADA. 2026. Prohibited List
  10. US Food and Drug Administration. Questions for PCAC regarding whether FDA should include certain bulk drug substances on the 503A Bulks List, Pharmacy Compounding Advisory Committee, 23 and 24 July 2026. TB-500 was taken up in the afternoon session of 23 July on two voting questions, and the use FDA evaluated for it is recorded as wound healing in the Federal Register notice of 16 April 2026, document 2026-07361. FDA advisory committee materials. 2026. Voting questions