TB-500research overview.

Overview

TB-500 is a name used for a synthetic peptide related to thymosin β4 (Tβ4). The material OzPept supplies, and the material described in the anti-doping literature, is Ac-LKKTETQ: the seven residues at positions 17 to 23 of Tβ4, with an acetyl group added to the N-terminus ^6,7.

This distinction matters when reading the literature. Tβ4 itself is a 43-residue protein, first sequenced from bovine thymus in 1981 ¹, and most published research uses the full-length protein. Only a small number of studies have tested the 17⁠–⁠23 fragment on its own. In this overview, every study is labelled as either full-length Tβ4 or the fragment.

Sport Integrity Australia lists thymosin β4, also known as TB-500, as prohibited at all times under section S2.3 (growth factors and growth factor modulators) of the World Anti-Doping Agency Prohibited List ⁸.

Structure and chemistry

Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln (Ac-LKKTETQ). Molecular formula C38H68N10O14, molecular weight 889.0 g/mol, CAS 885340-08-9. It is a linear peptide of L-amino acids with an acetylated N-terminus and a free C-terminal carboxyl.

Full-length Tβ4 is acetylated at its own N-terminus, on serine 1 ¹. In the native protein, leucine 17 sits inside the chain and carries no acetyl group. The acetyl group on TB-500 is therefore added during synthesis; Ho et al. describe it as an artificial acetylation ⁷. With two lysines, one glutamate, a free C-terminal carboxyl and a capped N-terminus, the fragment is close to neutral overall at neutral pH.

The 17⁠–⁠23 sequence is the central actin-binding motif of Tβ4. Safer et al. (1991) showed that Tβ4 is identical to Fx, a peptide that holds much of the unpolymerised actin in human platelets. It forms a 1:1 complex with actin monomers and inhibits their polymerisation ².

Esposito et al. (2012) analysed a product sold as TB-500 by high-performance liquid chromatography and high-resolution mass spectrometry. They identified its active content as Ac-LKKTETQ and synthesised reference material for anti-doping laboratories ⁶.

Mechanisms investigated

Studies are grouped below by the molecule tested. Findings for full-length Tβ4 should not be assumed to apply to the fragment.

• Fragment: endothelial assays. Philp et al. (2003) compared Tβ4, proteolytic fragments and synthetic peptides in human umbilical vein endothelial cell migration assays and chick aortic arch sprouting assays. They reported that the seven-residue actin-binding motif showed activity close to that of full-length Tβ4, while peptides lacking any part of the motif were inactive ³. The abstract does not say whether the synthetic motif was N-acetylated.
• Fragment and full-length: protease expression. Philp et al. (2006) measured matrix metalloproteinase (MMP-1, -2 and -9) expression with Tβ4 in an animal wound model and in cultured keratinocytes, endothelial cells and fibroblasts. They reported that the 17⁠–⁠23 region accounted for the MMP induction they measured ⁴.
• Fragment: detection in horses. Ho et al. (2012) developed a liquid chromatography⁠–⁠mass spectrometry method for N-acetylated LKKTETQ and its metabolites in equine urine and plasma. They confirmed the parent peptide and metabolites in samples from horses given a TB-500 preparation ⁷.
• Full-length Tβ4: actin sequestering. Safer et al. (1991) showed that full-length Tβ4 binds actin monomers and keeps them unpolymerised ². This is the best-characterised biochemical property of the parent protein.
• Full-length Tβ4: wider literature. Goldstein et al. (2012) review the full-length protein, including its release from platelets and macrophages, cell migration and survival studies, and clinical programmes run with full-length Tβ4 ⁵.

Research areas

• Sequence of full-length bovine Tβ4 (in vitro) ¹
• Actin monomer sequestering by full-length Tβ4 (in vitro) ²
• Endothelial migration and sprouting assays with the actin-binding motif (in vitro) ³
• MMP expression with full-length Tβ4 and the 17⁠–⁠23 region (in vitro and animal) ⁴
• Identification of Ac-LKKTETQ in a TB-500 product (analytical) ⁶
• Detection of Ac-LKKTETQ and metabolites in horses (animal, analytical) ⁷
• Full-length Tβ4 biology and clinical programmes (review) ⁵

Limits of the evidence

Most of the biology attributed to TB-500 in general sources comes from studies of full-length Tβ4, a protein more than five times larger by mass. The fragment studies cited here are cell and tissue assays ^3,4, and they used the motif without stating N-acetylation, so they may not have tested exactly the same molecule as Ac-LKKTETQ.

The fragment-specific literature that does name Ac-LKKTETQ is analytical. It describes how to identify the peptide and its metabolites for anti-doping purposes, not its biology ^6,7. This summary did not identify controlled studies of Ac-LKKTETQ in people.

The name TB-500 has not always been used consistently. Some sources use it for full-length synthetic Tβ4, so the material tested in any study described as using TB-500 needs to be checked against its reported sequence.

OzPept’s TB-500 is a research chemical, not a medicine, and is not for human use.

Key studies

1. 1981In vitro

   Sequence of full-length bovine Tβ4 ¹

2. 1991In vitro

   Full-length Tβ4 sequesters actin monomers ²

3. 2003In vitro

   Actin-binding motif in endothelial assays (fragment) ³

4. 2006Animal

   MMP expression and the 17⁠–⁠23 region ⁴

5. 2012In vitro

   Ac-LKKTETQ identified in a TB-500 product ⁶

6. 2012Animal

   Detection of Ac-LKKTETQ and metabolites in horses ⁷

Questions

References

1. 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):1162-1166. Source
2. Safer D, Elzinga M, Nachmias VT. Thymosin beta 4 and Fx, an actin-sequestering peptide, are indistinguishable. J Biol Chem. 1991;266(7):4029-4032. Source
3. Philp D, Huff T, Gho YS, Hannappel E, Kleinman HK. The actin binding site on thymosin beta4 promotes angiogenesis. FASEB J. 2003. Source
4. Philp D, et al. Thymosin beta4 promotes matrix metalloproteinase expression during wound repair. J Cell Physiol. 2006;208(1):195-200. Source
5. Goldstein AL, Hannappel E, Sosne G, Kleinman HK. Thymosin β4: a multi-functional regenerative peptide. Basic properties and clinical applications. Expert Opin Biol Ther. 2012;12(1):37-51. Source
6. 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;4(9):733-738. Source
7. 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. Source
8. Sport Integrity Australia. Thymosin beta-4 peptide hormone information. Substance education. Accessed October 2026. Source