TB-500

TB-500 (also written TB500 or TB 500) is a research name tied to thymosin beta-4, a 43-residue actin-binding peptide found throughout mammalian tissue (Philp et al., 2003). The name does not point to one molecule. Analysis of material sold as TB-500 identified a seven-residue fragment, residues 17 to 23 of thymosin beta-4 with an acetylated N-terminus, Ac-LKKTETQ (Esposito et al., 2012), and a racing laboratory's detection method targets the same fragment (Ho et al., 2012).

Most biological data, however, concern the full-length protein. This guide keeps the two apart, naming the molecule and model behind every finding.

Published work falls into five areas. Each summary names the model and the molecule tested.

What the name TB-500 refers to

A doping-control laboratory analyzed a TB-500 preparation by liquid chromatography and high-resolution mass spectrometry, identified Ac-LKKTETQ, and confirmed it by independent synthesis (Esposito et al., 2012). A racing laboratory described TB-500 as a veterinary preparation whose key ingredient is LKKTETQ with an artificially acetylated N-terminus (Ho et al., 2012). The name is not a reliable guide to content.

Analysis of internet products sold as TB500 and TB1000 found contents not systematically consistent with their descriptions (Delcourt et al., 2023); a 2025 hydrogel study in human corneal cells and an alkali burn model used "TB500" for the unacetylated sequence (Lu et al., 2025); and a 2026 rat study described its TB-500 as synthetic thymosin beta-4 (Biçer et al., 2026).

Metabolism of the acetylated fragment

In vitro incubations identified likely metabolites of Ac-LKKTETQ, and the parent peptide and its metabolites were then detected in urine and plasma from horses that had received TB-500 (Ho et al., 2012). A later study followed TB-500 in human serum, in vitro enzyme systems and rats: in rat urine Ac-LK was the main metabolite over the first six hours, and Ac-LKK remained detectable for up to 72 hours (Rahaman et al., 2024). Models: in vitro enzyme systems, horses and rats.

The 17 to 23 actin-binding motif

Comparing natural protein, fragments and synthetic peptides, one group reported that the seven-residue motif matched the full protein in endothelial cell migration and chick aortic arch sprouting assays at about 50 nM, while peptides lacking part of it were inactive (Philp et al., 2003).

In a second report the same year, a synthetic LKKTETQ peptide promoted repair of full-thickness skin wounds in aged mice comparably to the parent molecule (Philp et al., 2003). In human hepatic stellate cells, the 17 to 23 peptide, but not the 1 to 15 peptide, reduced PDGF-BB driven proliferation and migration (Shah et al., 2018). Models: human and chick cell and tissue assays, mice.

According to a 2024 study, such activities were reported for the unacetylated form, and biological effects of acetylated TB-500 itself had not been documented. In fibroblast culture that study found no cytotoxicity, and a significant scratch-wound effect only for the metabolite Ac-LKKTE (Rahaman et al., 2024).

Full-length thymosin beta-4

Thymosin beta-4 was sequenced from calf thymus in 1981 as a 43-residue peptide with an acetylated N-terminus (Low et al., 1981), and later identified as the peptide holding most unpolymerized actin in resting human platelets, binding actin monomers 1:1 and inhibiting their polymerization (Safer et al., 1991). In a rat full-thickness wound model it increased re-epithelialization, contraction, collagen deposition and angiogenesis, and it stimulated keratinocyte migration in a chamber assay (Malinda et al., 1999).

It formed a complex with PINCH and integrin-linked kinase that activates Akt, and after coronary artery ligation in mice the study reported greater early myocyte survival and better cardiac function (Bock-Marquette et al., 2004). Models: purified protein, cell culture, rats and mice. None of these used the fragment, and results for the full protein should not be assumed to carry over to it.

Human studies of full-length thymosin beta-4

A randomized, placebo-controlled study in 40 healthy volunteers, with single and then repeated administration over 14 days, reported infrequent mild or moderate adverse events, none serious, and pharmacokinetics proportional to the amount given (Ruff et al., 2010). In a small phase 2 randomized trial in nine patients with severe dry eye, a thymosin beta-4 eye drop reduced ocular discomfort and corneal staining against vehicle (Sosne et al., 2015).

Models: healthy volunteers and a phase 2 trial. We found no published human study of the acetylated fragment.