What Is TB-500? Thymosin Beta-4 Fragment Explained

TB-500 is a synthetic seven-amino-acid peptide, N-acetyl-Leu-Lys-Lys-Thr-Glu-Thr-Gln (Ac-LKKTETQ), with a molecular weight of about 889.0 g/mol. Its sequence is residues 17 to 23 of thymosin beta-4, a 43-residue protein that binds actin monomers inside cells, and this stretch forms part of that protein's actin-binding site. TB-500 is not an approved drug anywhere, and it is prohibited in sport under the World Anti-Doping Agency's growth factor class.

The name causes confusion, because "TB-500" has been used loosely for both the short fragment and full-length thymosin beta-4. This profile sets out which molecule the name refers to in the chemical and regulatory record, how the fragment relates to its parent protein, what the literature says about the actin-binding motif, and what to check when the material arrives in a lab.

Identity at a glance

PubChem assigns the name TB-500 to the acetylated heptapeptide (CID 62707662), and FDA's compounding records use the same definition: "Thymosin beta-4, fragment (LKKTETQ), also known as TB-500." The values below are from the PubChem record.

Two features of the sequence matter in practice. The N-terminal acetyl cap removes the free amine, so the net charge at neutral pH comes from the side chains and the C-terminal carboxylate: two lysines (positive) against one glutamate and the C-terminus (negative), which roughly cancel. And the peptide contains no cysteine, methionine or aromatic residue, which keeps it simple chemically but means it absorbs weakly at 280 nm; UV detection in HPLC relies on the peptide bond region near 214 nm instead.

The parent protein: thymosin beta-4

Thymosin beta-4 was sequenced in 1981 by Low, Hu and Goldstein, who isolated it from calf thymus as one of the peptides in a preparation called thymosin fraction 5. They reported 43 residues, a blocked (acetylated) N-terminus and a molecular weight of 4982 for the bovine protein. At the time it was studied as a possible thymic hormone.

Ten years later, Safer, Elzinga and Nachmias showed that a 5 kDa actin-binding peptide from human platelets, which they had called Fx, was identical in sequence to thymosin beta-4. The peptide forms a 1:1 complex with actin monomers (G-actin) and inhibits their polymerization, and at least half the actin in resting platelets is held in this unpolymerized state. That finding reframed thymosin beta-4 from a thymic hormone to what it is now mainly known as: the principal G-actin sequestering peptide in many mammalian cells.

UniProt's entry for the human protein (P62328, gene TMSB4X) gives the sequence as 44 residues including the initiator methionine, which is removed; the mature chain begins with an N-acetylserine. PubChem lists thymosin beta-4 at C212H350N56O78S, about 4963 g/mol. The mature protein also contains a second, unrelated bioactive fragment at its N-terminus, the tetrapeptide Ac-SDKP, which UniProt describes as an inhibitor of bone marrow stem cell differentiation that is inactivated by angiotensin-converting enzyme. Ac-SDKP (residues 1 to 4) and TB-500 (residues 17 to 23) come from different parts of the protein and are different molecules.

A note on numbering

Literature numbering for thymosin beta-4 counts from the mature N-terminal serine, so the TB-500 sequence is residues 17 to 23. UniProt counts from the initiator methionine, so the same stretch appears there as positions 18 to 24. Both describe the same seven residues.

The actin-binding motif

The rationale for making a short fragment comes from mapping studies of the parent protein.

• Van Troys and colleagues (1996) used chemically synthesized variants of full-length thymosin beta-4 to map its actin-binding site. They found two structural elements that both contribute: an N-terminal region (residues 1 to 16) that must adopt an alpha-helix to bind, and a hexapeptide motif at residues 17 to 22, LKKTET. Lysine 18 in the motif made an important electrostatic contact with actin. The critical residues are conserved across the beta-thymosin family.
• Philp and colleagues (2003) compared native thymosin beta-4, proteolytic fragments and synthetic peptides in endothelial cell assays. The seven-residue actin-binding motif showed near-identical activity to the full protein at about 50 nM in human umbilical vein endothelial cell migration and in chick aortic ring sprouting assays, while peptides lacking any part of the motif were inactive. Adding soluble actin inhibited the adhesion and sprouting activity of the full protein, consistent with the actin-binding site being the active region.

Together these papers explain why LKKTETQ was chosen. They also mark the limit of what is established: the mapping work shows that the motif is part of a larger binding surface in the intact protein, and the fragment alone lacks the N-terminal region that the 1996 study found necessary for binding in the intact protein. Findings made with full-length thymosin beta-4, which make up most of that literature, cannot simply be transferred to the fragment.

How TB-500 entered the literature

TB-500 did not originate in an academic paper. It entered the scientific record through anti-doping laboratories. In 2012, Esposito and colleagues, writing in Drug Testing and Analysis, analyzed a product sold as TB-500 using liquid chromatography coupled to high-resolution (Orbitrap) mass spectrometry, and identified its peptide as the N-terminal acetylated 17–23 fragment of human thymosin beta-4, Ac-LKKTETQ. They synthesized the peptide by solid-phase synthesis as a reference standard and proposed an LC–MS/MS strategy to detect it in plasma and urine.

This history is why regulators and PubChem define TB-500 as the fragment, and it is also why identity testing matters: a vial labeled TB-500 could in principle contain the heptapeptide, the full 43-residue protein, or something else, and only mass spectrometry distinguishes them cleanly (about 889 Da against about 4963 Da).

Regulatory status

FDA. TB-500 is not approved as a drug and is not on FDA's 503A bulks list, the list of bulk substances without a monograph or approved-drug status that compounders may use. FDA had placed "Thymosin beta-4, fragment (LKKTETQ), also known as TB-500" in Category 2 of its interim policy, citing a risk of immunogenicity for certain routes because of the potential for aggregation and peptide-related impurities, and noting that it had not identified any human exposure data. That entry now appears under nominations withdrawn by the nominator. FDA still presented TB-500 to its compounding advisory committee (PCAC) on July 23, 2026, and its briefing document states that FDA proposed that neither TB-500 free base nor TB-500 acetate be included on the 503A bulks list. The committee's vote is advisory, and FDA has said it will not issue a final determination until the advisory process and all reviews are complete.

Sport. The World Anti-Doping Agency's 2026 Prohibited List names "Thymosin-ß4 and its derivatives e.g. TB-500" under S2.3, growth factors and growth factor modulators, prohibited at all times.

Research supply. Anhydrolabs supplies TB-500 for in-vitro laboratory research only, not for human or veterinary diagnosis, treatment, or consumption. The terms are on the research-use page.

TB-500 in the lab

Anhydrolabs supplies TB-500 as a lyophilized powder in vacuum-sealed vials, and as 10-vial kits when a group wants a single lot for a longer series. It is also offered pre-combined with BPC-157 as BPC-157 / TB-500; the two compounds are profiled separately, and the profile of BPC-157 covers the other half of that vial.

Practical points:

• Storage. Keep sealed powder frozen, dry and away from light. Lyophilization removes most of the water that drives hydrolysis, which is why the dry form keeps far longer than a solution; how freeze-drying works explains the process. Temperatures and handling rules are in the storage and handling guide.
• Solubility. The peptide is short and hydrophilic (PubChem's computed XLogP is about −8.6), with two lysines and three hydroxyl or amide side chains, and it is typically dissolved in sterile water or an aqueous buffer for in-vitro stocks. At 889.0 g/mol, 1 mg of peptide is 1.12 µmol.
• Net peptide content. Calculate molar concentrations from the net peptide content on the certificate. An acetate or trifluoroacetate counter-ion and residual water add to the gross mass of the powder but not to the peptide.
• Identity and purity. Reversed-phase HPLC reports purity as the area of the principal peak relative to all detected signal. Mass spectrometry should show the monoisotopic mass of 888.49 Da, typically observed as the singly protonated ion near m/z 889.5 or the doubly protonated ion near m/z 445.3. How to read those numbers on a certificate is covered in peptide purity.

What is not known

• There are no published, controlled human studies of TB-500, and FDA has stated that it has not identified human exposure data.
• Most of the biological literature concerns full-length thymosin beta-4, not the heptapeptide, and the two cannot be assumed to behave alike.
• The fragment's affinity for actin on its own, outside the intact protein, is not well characterized in the literature cited here.
• Commercial material has historically been inconsistent in what "TB-500" means, which makes identity testing of each lot essential.