TB-500

01 · What it is

TB-500 is a synthetic, N-terminally acetylated heptapeptide with the sequence Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln (Ac-LKKTETQ), corresponding to residues 17 to 23 of thymosin β4. Its molecular weight, calculated from that sequence, is approximately 889 Da. The identity was established not by a developer but by a doping-control laboratory: in 2012 a group at Ghent University detected and identified Ac-LKKTETQ in a product sold as TB-500 using high-resolution mass spectrometry, synthesised the peptide independently to confirm the assignment, and proposed an analytical strategy for detecting it in plasma and urine ⁶.

TB-500 is not thymosin β4. Thymosin β4 is a 43-residue endogenous human protein with a decades-long research literature and a clinical programme spanning cardiac, ophthalmic and dermatological indications ^1,8. TB-500 is a seven-residue fragment of it, supplied as a research chemical.

The distinction is not pedantry, and the most important sentence in this compound’s literature makes that clear. In 2024 an independent group at the Korea Institute of Science and Technology stated the position directly: the biological functions ascribed to this sequence — actin binding, dermal wound healing, angiogenesis, skin repair — belong to the unacetylated form, LKKTETQ, and “the biological effects of TB-500, however, have not been documented” ¹⁴. The same study went further. Testing the parent compound and each of its metabolites in fibroblast wound-healing assays, only the metabolite Ac-LKKTE showed significant activity relative to control, leading the authors to conclude that previously reported wound-healing activity of TB-500 may be attributable to that metabolite rather than to the administered peptide ¹⁴.

Almost the entire published literature carrying the name TB-500 is analytical chemistry written to detect it in sport. As of September 2026 it holds no marketing authorisation from any regulator.

02 · Evidence at a glance

Evidence grade

Preclinical

Sequence

Ac-LKKTETQ, residues 17–23 of thymosin β4 ⁶

Molecular target

None identified for the acetylated fragment; the parent protein binds G-actin ^2,3

Human subjects in published studies

None identified

Human pharmacokinetic data

None identified, by any route

Randomised controlled trials

None

In vivo animal efficacy studies of TB-500 itself

One identified, in rat tendon ¹⁷

In vitro wound-healing activity of the parent compound

Not significant; activity found only in the metabolite Ac-LKKTE ¹⁴

Toxicology

No acute, repeat-dose, genotoxicity, reproductive or carcinogenicity study identified

Cytotoxicity

None observed for parent or metabolites in fibroblasts ¹⁴

Animal pharmacokinetics

Horse (subcutaneous) and rat (intraperitoneal) only ^7,9,14

Approval status

None, any jurisdiction

03 · Mechanism of action

The mechanism is inherited, and the inheritance is disputed

TB-500 has no mechanism of its own in the published literature. What it has is a mechanism borrowed from thymosin β4, and a 2024 paper stating plainly that the borrowing has never been justified: the documented biological functions belong to the unacetylated heptapeptide LKKTETQ, and the effects of the acetylated form sold as TB-500 have not been documented ¹⁴. Everything below describes what the parent molecule and the unacetylated fragment do. Whether any of it transfers is the open question of this compound.

G-actin sequestration and the LKKTET motif

Thymosin β4 binds actin monomers stoichiometrically and holds the bulk of the cellular monomer pool, buffering the concentration available for filament elongation ². Binding occurs through the β-thymosin/WH2 module, a short conserved actin-binding motif whose central element is the LKKTET sequence ^3,4. Because TB-500 contains that motif, the inference has been that it should bind G-actin similarly. The inference is reasonable and remains an inference; no published binding study of the acetylated heptapeptide to actin was identified for this guide.

Why acetylation is not a formality

N-terminal acetylation removes the positive charge of the free α-amino group, alters local electrostatics, and changes susceptibility to aminopeptidases. In a family of proteins where a short charged motif does the binding, that is a change at the business end of the molecule. The 2024 metabolite study is the empirical expression of this concern: in the same assay system, the acetylated parent did not produce significant wound healing while a shorter acetylated metabolite did ¹⁴.

Metabolism, and the possibility that the metabolite is the active species

TB-500 is hydrolysed by sequential removal of amino acids from the C-terminus. In rats, Ac-LK was the primary metabolite by concentration in the 0–6 hour window, and Ac-LKK persisted as a long-term metabolite detectable to 72 hours ¹⁴. Parallel in vitro work in human serum, liver microsomes, S9 fraction and other enzyme systems identified the same C-terminal truncation series ^12,14. Of these species, only Ac-LKKTE showed significant wound-healing activity in fibroblasts ¹⁴. If that finding holds, the pharmacologically interesting molecule is a five-residue breakdown product, and TB-500 functions as a prodrug for it — a hypothesis nobody has tested.

What is absent from the mechanistic account

No receptor. No demonstrated cellular uptake. No account of how a charged heptapeptide administered subcutaneously reaches the cytoplasmic actin pool it is supposed to act on. These gaps exist for thymosin β4 as well, but the parent at least has structural and biophysical work behind its binding claim ^2,4,5. TB-500 has the sequence and the assumption.

04 · Key research findings

Identity and analytical chemistry — the bulk of the literature. Ac-LKKTETQ was identified in a product sold as TB-500 and independently synthesised for confirmation ⁶. Subsequent work developed detection methods in equine urine and plasma ⁷, multi-analyte methods covering seven bioactive peptides in horse plasma ⁹, in vitro metabolism models for sport drug testing ¹², and reviews of analytical approaches for emerging peptidic drugs ^10,11. One study addressed a purely practical problem: TB-500 adsorbs to laboratory glassware and plasticware to a degree that affects recovery, which has implications for every quantitative result reported for it ¹³.

This is a well-executed analytical literature written to catch a compound in sport, and it is the only mature body of work the name TB-500 has.

Wound healing — tested once, in vitro, and negative for the parent. The 2024 Korean study measured cytotoxicity and wound-healing activity of TB-500 and each metabolite in fibroblasts. No cytotoxicity was found for the parent or any metabolite. Only Ac-LKKTE produced significant wound healing relative to control, and the authors concluded that wound-healing activity previously reported in the literature may be due to that metabolite rather than the parent ¹⁴.

This is the only direct test of the compound’s headline claim, it is independent, it declares no competing interests, and its result is that the compound itself did not work in the assay.

Tendon — the single in vivo efficacy study. In a 2026 rat Achilles transection-and-repair model, 32 animals were randomised to control, BPC-157, TB-500 at 60 µg/kg/day intraperitoneally, or the combination, for four weeks. The TB-500 group showed significantly higher maximum load to failure than controls (p < 0.05), the lowest total Bonar score (p = 0.016), a significantly lower Movin score (p = 0.017), and increased type I collagen organisation on Sirius red birefringence ¹⁷.

This is the strongest positive result TB-500 has: one exploratory study, eight animals per arm, one dose level, four weeks — and the paper describes its test article as “synthetic thymosin beta-4 (TB-500)” without reporting independent characterisation of what was administered, which is precisely the conflation the identity literature exists to prevent.

Pharmacokinetics — two species, neither human. After 10 mg subcutaneously in thoroughbred geldings, the parent peptide and its metabolites were detectable in plasma and urine, with plasma concentrations peaking in the low picogram-per-millilitre range within two hours and falling below quantification within hours ^7,9. In rats given TB-500 intraperitoneally, urinary metabolites followed the same C-terminal truncation pattern ¹⁴. No intravenous arm was run in either species, so absolute bioavailability by the subcutaneous route is unestablished.

Everything known about the disposition of this molecule was learned in the course of trying to detect it, not to develop it.

Cardiac, ophthalmic, dermatological — none of it belongs here. The infarct, dry eye, neurotrophic keratopathy, venous ulcer and stroke findings routinely attributed to TB-500 were generated with full-length thymosin β4 ⁸. Recent reviews written for orthopaedic and sports medicine audiences list TB-500 among compounds whose reported musculoskeletal benefits are largely unvalidated in human trials ^{13,15,16,18,19}.

The evidence base people cite for this compound is a different molecule’s.

05 · Evidence overview

06 · Safety profile

Animal data. No acute toxicity, repeat-dose toxicity, genotoxicity, reproductive or developmental toxicity, or carcinogenicity study of TB-500 was identified in the indexed literature during preparation of this guide. The 2026 rat tendon study did not report systemic toxicity outcomes for any arm ¹⁷. The pharmacokinetic work in horses and rats was designed to characterise excretion for detection purposes and did not include toxicological endpoints ^7,9,14.

In vitro data. The 2024 metabolite study assessed cytotoxicity of the parent compound and each metabolite in fibroblasts and found none ¹⁴. That is a genuine, if narrow, safety observation: one cell type, one assay, acute exposure.

Human data. None exists. No clinical study, no case report, and no pharmacokinetic or pharmacodynamic data in humans by any route were identified.

What is genuinely unknown. Whether TB-500 is genotoxic. Whether it is carcinogenic. What repeated administration does over weeks or months in any species. What it does across a reproductive cycle. Whether an acetylated heptapeptide given parenterally provokes an antibody response — no immunogenicity data exist, and the parent protein’s phase 1 programme, which did include anti-drug antibody testing, tested a different molecule ⁸. Whether the thymosin β4 safety record transfers at all. And a question specific to the metabolite finding: if Ac-LKKTE is the active species ¹⁴, then the exposure that matters is to a molecule nobody is measuring and nobody has studied in vivo.

07 · US regulatory status

Current as of 6 September 2026. TB-500 is not approved as a drug in the United States or any other jurisdiction, and is not a controlled substance.

In September 2023 FDA placed “Thymosin beta-4, fragment (LKKTETQ), also known as TB-500” in category 2 of its interim bulk drug substances policy. As of FDA’s page dated 22 April 2026, that entry appears in the table of substances nominated but subsequently withdrawn by the nominators ²⁰. On 23 July 2026 FDA’s Pharmacy Compounding Advisory Committee reviewed TB-500 for possible inclusion on the 503A Bulks List, for the proposed use of wound healing ²¹. Advisory committee recommendations are non-binding; nothing has been added to that list as a result, and inclusion would require formal rulemaking. Full-length thymosin β4 was not on that agenda and is a separate regulatory question.

Under the World Anti-Doping Code, growth factors affecting muscle, tendon or ligament regeneration are prohibited at all times, and substances not approved by any governmental regulatory health authority for human therapeutic use fall within category S0. Detection methods for TB-500 in human and equine urine and plasma have been published since 2012 ^6,7,9,10,11.

08 · Limitations of the evidence

1. The compound’s own biological effects have not been documented. This is not a characterisation made by critics; it is the stated position of the 2024 primary study that examined the question most directly, which noted that the functions attributed to this sequence belong to its unacetylated form ¹⁴.
2. The only direct test of the headline claim was negative for the parent compound. In fibroblast wound-healing assays, TB-500 itself did not produce significant activity; only the metabolite Ac-LKKTE did ¹⁴. One in vitro assay is not definitive, and it is the only direct evidence there is.
3. The evidence people cite belongs to a different molecule. Thymosin β4’s clinical record — phase 1 pharmacokinetics, ophthalmic trials, cardiac trials — was generated with the full-length 43-residue protein ⁸. Attributing it to a seven-residue acetylated fragment is a category error, and it is the single most common error made about this compound.
4. Even the fragment literature is for the wrong fragment. The wound-healing and actin work most often invoked used unacetylated LKKTETQ ¹⁴. Acetylation changes charge and protease susceptibility at the peptide’s N-terminus, and no study has demonstrated equivalence.
5. There is one in vivo efficacy study, and it did not characterise its test article. The 2026 rat tendon study reported positive results ¹⁷ while describing its compound as “synthetic thymosin beta-4 (TB-500)” — two different substances named as one, with no reported verification of identity or purity. Given that TB-500 also adsorbs substantially to labware ¹³, quantitative confidence in any single study is limited.
6. No toxicology of any kind exists. No acute, repeat-dose, genotoxicity, reproductive or carcinogenicity data in any species. For most research compounds “understudied” means animal work has outrun human work; here neither has been done.
7. Pharmacokinetics are non-human and incomplete. Two species, two routes, no intravenous comparator, and therefore no bioavailability estimate ^7,9. The disposition data exist because regulators of horse racing wanted to detect the compound, not because anyone was developing it.

09 · References

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7. Ho EN, Kwok WH, Lau MY, et al. Doping control analysis of TB-500, a synthetic version of an active region of thymosin beta4, in equine urine and plasma by liquid chromatography-mass spectrometry. J Chromatogr A. 2012;1265:57–69.

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13. Judák P, Van Eenoo P, Deventer K. Adsorption effects of the doping relevant peptides Insulin Lispro, Synachten, TB-500 and GHRP 5. Anal Biochem. 2017;537:69–71.

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14. 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;1235:124033.

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17. 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. Jt Dis Relat Surg. 2026;37(3):822–837.

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20. US Food and Drug Administration. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks. Page current as of 22 April 2026. Regulatory document; no PMID.

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21. US Food and Drug Administration. July 23–24, 2026: Meeting of the Pharmacy Compounding Advisory Committee. Docket FDA-2025-N-6895. Regulatory document; no PMID.

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