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TB-500 is not thymosin beta-4: fragment vs protein

Almost everything said about TB-500 comes from studies of full-length thymosin beta-4, a 43-amino-acid protein. TB-500 is a 7-amino-acid fragment. The FDA found no case in the medical literature of it being given to patients.

Published October 9, 2026 · 3 min read · 17 sources

Two molecules that get confused

Thymosin beta-4 is a natural 43-amino-acid protein, present in cells and body fluids, that has been studied in tissue repair3. Its full sequence was published in 1981, from calf thymus12, and it is the main actin-sequestering peptide in mammalian tissue14. TB-500 is something else: an N-acetylated synthetic fragment of 7 amino acids (LKKTETQ, residues 17–23), matching the region of thymosin beta-4 responsible for actin binding12. An anti-doping laboratory synthesized and characterized that acetylated fragment after identifying it in a product sold as TB-50013. In cell assays, that seven-amino-acid motif reproduced the angiogenic activity of the full protein14. TB-500 marketing often cites studies of the full protein as if they were its own, such as the animal work on wound healing15 and cardiac repair16.

What the FDA found

In its July 2026 briefing document the FDA wrote: "FDA did not find any information in the medical literature where TB-500 was administered to patients to treat any disease or condition". In an in-vitro fibroblast wound-healing assay, TB-500 at 50 µg/mL "did not induce wound healing", although one metabolite (Ac-LKKTE) produced a small but significant closure; and the FDA identified no acute, repeat-dose or genotoxicity studies2. A 2024 study quantified TB-500 and its metabolites in vitro and in rats, and screened their wound-healing activity in vitro17. Even so, the advisory committee voted 8–6, with one abstention, to add it to the compounding list5.

7 vs 43amino acids: TB-500 vs thymosin beta-41
0documented human uses of TB-500 according to the FDA2
P = 0.0656primary endpoint missed in the ocular thymosin beta-4 phase 33

The human data are for the full protein

Full-length thymosin beta-4 does have trials, but small ones in specific uses, mainly eye drops. In a phase 3 in neurotrophic keratopathy with 18 patients, complete healing at 4 weeks was 6/10 vs 1/8, P = 0.0656: it missed the primary endpoint; two weeks after treatment ended, the difference was significant (P = 0.0359)3. In a trial in healthy Chinese volunteers, intravenous recombinant protein showed no dose-limiting toxicity4.

KPV: the other peptide with no human data

KPV, a tripeptide derived from α-MSH, is a similar case. The FDA wrote that "The nomination did not include, and FDA did not find any information on the use of these substances administered in humans"6. Its evidence comes from cell cultures and mouse colitis models7. The committee also voted 8–6, with one abstention, in its favor5.

Sources

  1. Ho EN, et al. Doping control analysis of TB-500, a synthetic version of an active region of thymosin β4, in equine urine and plasma by liquid chromatography-mass spectrometry. J Chromatogr A. 2012;1265:57-69. PubMed 23084823 ↗
  2. U.S. Food and Drug Administration. PCAC July 2026 briefing document: TB-500. fda.gov ↗
  3. Sosne G, et al. 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;24(1):554. PubMed 36613994 ↗
  4. Wang X, 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. J Cell Mol Med. 2021;25(17):8222-8228. PubMed 34346165 ↗
  5. Jacobus N. FDA Panel Votes to Loosen Restrictions for Four Peptides. Pharmaceutical Executive. 24 July 2026. pharmexec.com ↗
  6. U.S. Food and Drug Administration. PCAC July 2026 briefing document: KPV. fda.gov ↗
  7. Dalmasso G, et al. PepT1-mediated tripeptide KPV uptake reduces intestinal inflammation. Gastroenterology. 2008;134(1):166-78. PubMed 18061177 ↗
  8. World Anti-Doping Agency. 2026 Prohibited List (S2.3), English text as published in Tractatenblad van het Koninkrijk der Nederlanden 2026, Nr. 25. zoek.officielebekendmakingen.nl ↗
  9. 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):491-6. PubMed 25826322 ↗
  10. Bascom E. FDA committee recommends looser restrictions for several peptides. Healio. 24 July 2026. healio.com ↗
  11. Biçer O, 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. PubMed 42542926 ↗
  12. 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. Proc Natl Acad Sci U S A. 1981;78(2):1162-6. PubMed 6940133 ↗
  13. Esposito S, et al. 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;4(9):733-8. PubMed 22962027 ↗
  14. Philp D, et al. The actin binding site on thymosin beta4 promotes angiogenesis. FASEB J. 2003;17(14):2103-5. PubMed 14500546 ↗
  15. Malinda KM, et al. Thymosin beta4 accelerates wound healing. J Invest Dermatol. 1999;113(3):364-8. PubMed 10469335 ↗
  16. Bock-Marquette I, et al. Thymosin beta4 activates integrin-linked kinase and promotes cardiac cell migration, survival and cardiac repair. Nature. 2004;432(7016):466-72. PubMed 15565145 ↗
  17. Rahaman KA, et al. 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. PubMed 38382158 ↗

Informational article. It summarizes news, official statements and published studies; it is not medical or legal advice. Sources were checked on the publication date.

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