TB-500

TB-500 is a research name most often applied to a short synthetic N-acetylated fragment of thymosin β4, Ac-LKKTETQ, but it is also used loosely for full-length thymosin β4. There are no direct human studies of the fragment.

Last reviewed 2026-10-25 by the PeptideWiki editorial team. Editorial policy.

Overview

TB-500 is not a single well-defined substance. In much of the research-chemical market and in some literature the label refers to a synthetic seven-residue peptide, Ac-LKKTETQ, which reproduces the actin-binding region of thymosin β4 around residues 17–23 with an added N-terminal acetyl group. In other contexts the same label is applied to full-length thymosin β4, a 43-residue protein. These are chemically and biologically different materials, and the conflation makes it difficult to know what a given study or sample actually contains.

This matters for interpreting evidence. Human clinical trials of thymosin β4 used the full-length peptide in specific formulations, and their results cannot be transferred to a seven-residue fragment. FDA reviewers raised exactly this point in 2026, noting unsettled identity and characterization and the absence of direct human studies of TB-500 itself. Evidence specific to the fragment is limited to in-vitro work, animal experiments and anti-doping detection studies.

Compare: BPC-157 vs TB-500, TB-500 vs thymosin β4.

Structure

In its fragment sense, TB-500 is the heptapeptide Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln (Ac-LKKTETQ), corresponding to residues 17–23 of thymosin β4, the stretch that contains the LKKTETQ actin-binding motif. The N-terminal acetyl group is a synthetic modification; in native thymosin β4 this stretch sits in the middle of the chain. Some sources describe the fragment with a free C-terminal acid and others are less specific, so the exact terminal chemistry should be confirmed for any sample. When the name is used for full-length thymosin β4, the material is a 43-residue N-acetylated peptide; see thymosin β4. Salt form, typically acetate or TFA, should also be stated.

Mechanism

The rationale for the fragment is that the LKKTETQ region of thymosin β4 is central to its binding of monomeric G-actin, and some in-vitro studies have reported that peptides containing this motif retain part of the parent molecule's activity on cell migration or actin dynamics. Whether a short acetylated fragment reproduces the broader biology attributed to full-length thymosin β4, including effects on angiogenesis, inflammation and tissue repair in animal models, has not been established. Mechanistic claims made for TB-500 are frequently borrowed from thymosin β4 studies and should not be assumed to apply to the fragment.

Research

The conflation problem

Because "TB-500" names both the Ac-LKKTETQ fragment and full-length thymosin β4, readers must check which material a paper or sample describes. Studies of full-length thymosin β4, including human trials of eye drops and topical gels, are frequently cited as if they supported the fragment. They do not. A seven-residue peptide differs from a 43-residue peptide in size, structure, binding properties and likely distribution, so evidence from one is not evidence for the other.

Fragment-specific studies

Published work specifically on Ac-LKKTETQ is sparse. It includes in-vitro experiments on cell migration and actin-related activity of short thymosin β4 fragments and a small number of animal studies. The results are preliminary and do not establish efficacy or safety for any purpose. Researchers interested in the fragment should search for studies that name the exact sequence rather than relying on reviews of thymosin β4.

Anti-doping and detection research

A notable share of fragment-specific literature comes from anti-doping science. Laboratories have developed mass spectrometry methods to detect the synthetic fragment and its metabolites in urine and plasma from humans and from animals such as racehorses. These studies characterize detection and metabolism, not therapeutic effect. They are also useful analytically, because they describe the fragment's expected mass and breakdown products.

Evidence quality

There are no direct human clinical studies of TB-500 as the Ac-LKKTETQ fragment, a point FDA reviewers made explicitly. The fragment's evidence base is preclinical, and much of what is said about it online is extrapolated from full-length thymosin β4. Sample identity is a further problem, since material sold under the name may be either compound or neither. Any conclusions should be limited to the specific material tested.

Regulatory status

TB-500 is not approved by the FDA for any use. In July 2026 the FDA's Pharmacy Compounding Advisory Committee (PCAC) voted 8-6-1 to recommend adding TB-500 to the Section 503A bulk drug substances list, although FDA's own reviewers had recommended against it. The vote is advisory; FDA must act through notice-and-comment rulemaking, and no final FDA action has been announced. FDA flagged unsettled identity and characterization, including variable sequences and salt forms, and noted that there are no direct human studies of TB-500 itself; human thymosin β4 trials are not evidence for the fragment. WADA prohibits thymosin β4 and its fragments, including TB-500, under class S2.

Status changes often. See the regulatory tracker for dated updates.

Quality and identity testing

The first question for any TB-500 sample is which compound it is. A good certificate of analysis should state the exact sequence and terminal modifications and confirm identity by mass spectrometry, with the observed mass matching the expected mass shown on this page for Ac-LKKTETQ; a full-length thymosin β4 sample will give an entirely different mass. HPLC purity should be reported with the chromatogram. Check for the presence or absence of the N-terminal acetyl group, since a non-acetylated fragment differs by 42 Da. The counter-ion should be named and net peptide content reported, because short basic peptides can carry several TFA or acetate counter-ions per molecule.

Open TB-500 in the sequence analyzer to see the expected [M+H]⁺ and multiply charged ions for an MS identity check (set the N-acetyl, amide and disulfide options to match).

For a walkthrough of each section of a certificate, see how to read a COA.

Storage and stability

Store lyophilized material sealed at -20 °C or colder, protected from light and moisture, and equilibrate to room temperature before opening. The fragment contains no cysteine, methionine or tryptophan, so oxidation risk is low, but the C-terminal glutamine can undergo deamidation and the peptide may hydrolyze in solution over time. Prepare solutions fresh where possible, refrigerate for short-term lab use, aliquot for frozen storage, and avoid repeated freeze-thaw cycles. If the sample is full-length thymosin β4, its methionine residue makes oxidation a relevant concern.

Download the TB-500 data sheet (PDF): sequence, masses, expected MS ions and handling on one printable page.

Latest research

Frequently asked questions

Related compounds

• Thymosin β4

  Thymosin β4 is a naturally occurring 43-amino-acid peptide that binds monomeric actin and is abundant in many cell types. It has been studied in animal models of tissue repair and in human trials of ophthalmic and topical formulations, and is not approved.

• BPC-157

  BPC-157 is a synthetic 15-amino-acid peptide whose sequence was derived from a protein fraction of gastric juice. It is studied mainly in rodent models of gastrointestinal, tendon and soft-tissue injury and is not approved for any use.

• Thymosin α1

  Thymosin α1 is a 28-amino-acid N-acetylated peptide derived from prothymosin α. Its synthetic form, thymalfasin (Zadaxin), is approved in several countries outside the US and has been studied as an immune modulator in clinical trials.

References

1. PubMed: all indexed literature on TB-500
2. ClinicalTrials.gov: registered studies mentioning TB-500