By: Laura Mildon
TB-500 is a synthetic heptapeptide: a chain of seven amino acid residues joined by peptide bonds. The chemically defined form discussed here is N-acetyl-LKKTETQ, a modified fragment corresponding to residues 17 through 23 of thymosin beta-4. Its research spans peptide identification, metabolism, and experimental cell behavior.
The most useful starting point is its identity. TB-500 is frequently discussed alongside full-length thymosin beta-4, but those names should not be treated as interchangeable. This article explains the chemistry, historical milestones, selected original studies, and evidence gaps without turning laboratory findings into claims about a research product.
| Property | Description |
| Name discussed | TB-500 |
| Chemical class | Synthetic N-acetylated heptapeptide |
| Length | 7 amino acid residues |
| Sequence | Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH |
| Abbreviated sequence | Ac-LKKTETQ |
| Relationship to thymosin beta 4 | Corresponds to residues 17 through 23 with an added N-terminal acetyl group |
| Molecular formula | C₃₈H₆₈N₁₀O₁₄ |
| Molecular weight | Approximately 889.0 g/mol |
| CAS number | 885340-08-9 |
| PubChem identifier | CID 62707662 |
Ac indicates an acetyl group at the amino terminus. The final OH indicates a free carboxylic acid terminus. These features form part of the chemical identity. The formula and weight above exclude salt counterions and associated water.
PubChem chemical identity and authentic molecular structure
Acetylation adds a chemical group to the beginning of the peptide. The unacetylated sequence LKKTETQ has the same seven amino acid residues but is a different chemical entity. Removing the acetyl group changes composition and mass, and may change how a peptide behaves in an experimental system. A paper that specifies only LKKTETQ should not automatically be counted as a study of Ac-LKKTETQ.
The linked PubChem record supplies a two-dimensional structure showing the peptide bonds, side chains, terminal groups, and stereochemistry. A formula alone counts atoms and cannot uniquely determine their arrangement. This verified structure is a more useful reference than a decorative molecular illustration.
| Substance | Chemical distinction |
| Thymosin beta 4 | The full-length 43-residue peptide |
| LKKTETQ | An unacetylated seven-residue sequence corresponding to positions 17 through 23 |
| TB-500 as defined here | Ac-LKKTETQ with N-terminal acetylation |
A fragment can preserve a sequence involved in a larger peptide’s activity while lacking other parts of that molecule. The relationship supplies a reason to investigate it; it does not establish identical binding, metabolism, distribution, or biological outcomes.
Descriptions of a 43-residue molecule under the name TB-500 should be reconciled against the actual sequence and analytical identity. Likewise, a study of a formulated thymosin beta-4 medicine or investigational product is evidence about that tested material, not automatically about the seven-residue TB-500 fragment.
Low, Hu, and Goldstein published the complete amino acid sequence of thymosin beta-4 isolated from calf thymus in 1981. They described a 43-residue peptide with an acetylated amino terminus. This is part of the parent peptide’s history, rather than the discovery date of the short TB-500 fragment.
Low and colleagues in Proceedings of the National Academy of Sciences 1981
Philp and colleagues investigated an actin-binding sequence within thymosin beta-4 using synthetic peptides, endothelial cells, and chick aortic tissue. Their work helped establish scientific interest in the short sequence. The acetylation state of the fragment matters when comparing that work with later TB-500 studies.
Philp and colleagues in The FASEB Journal 2003
Esposito and colleagues at Ghent University identified Ac-LKKTETQ in a preparation called TB-500 and synthesized the same peptide for characterization. This provides a documented identification milestone. It does not establish the exact date of the compound’s first synthesis or identify every preparation subsequently sold under that name.
Esposito and colleagues in Drug Testing and Analysis 2012
Actin is a protein that forms part of the cytoskeleton, the internal framework involved in cell shape and movement. Researchers studying the thymosin beta-4 sequence examined cell migration and vessel sprouting. These are experimental endpoints; neither is equivalent to a clinical demonstration of tissue repair.
Philp and colleagues reported endothelial-cell migration and vessel-sprouting activity associated with a seven-residue actin-binding motif. Angiogenesis means the formation of new blood vessels. These experiments concerned cultured cells and chick aortic tissue, and the unacetylated fragment should be distinguished from TB-500. The findings do not establish a treatment outcome or make angiogenesis uniformly beneficial in every biological setting.
Read the original actin binding motif study
Esposito and colleagues used chromatography and high-resolution mass spectrometry to identify the acetylated fragment. They also synthesized a reference peptide. Chromatography separates components of a sample; mass spectrometry measures ions and fragmentation patterns that can help identify a compound. This is evidence about chemical identity and detection, rather than proof of biological effectiveness.
Read the original identification and synthesis study
Ho and colleagues developed liquid chromatography–mass spectrometry methods for TB-500 in horse urine and plasma. The study belongs to the analytical and doping-control literature. Its detection findings should not be presented as a demonstration of tissue repair, clinical effectiveness, or human safety.
Ho and colleagues in Journal of Chromatography A 2012
Rahaman and colleagues measured TB-500 and its breakdown products in laboratory enzyme systems, human serum samples, and rat urine. They also compared biological activity in cultured fibroblasts. A metabolite is a compound produced when another substance is chemically transformed.
In their fibroblast assay, the metabolite Ac-LKKTE showed significant wound-closure activity compared with the control; the parent TB-500 did not. A scratch assay measures how cells fill a gap in a cell layer. It does not reproduce all the features of an injury in a living organism. Human serum used in a laboratory experiment does not make the work a human clinical trial.
Rahaman and colleagues in Journal of Chromatography B 2024
A paper published in Biomaterials Research on September 30, 2026 examined N-acetyl-L-leucyl-L-lysine, abbreviated Ac-LK, as a TB-500 metabolite. Its experiments included endothelial-cell functional assays, protein profiling, and an in-vivo wound-healing model. The authors reported stronger activity for Ac-LK on selected endpoints and proposed a peptide prodrug mechanism, in which a breakdown product contributes to activity.
This is a preclinical mechanism proposal. Ac-LK is different from Ac-LKKTE examined in the 2024 assay; the studies should not be collapsed into a single claim about an active metabolite. Their different systems and endpoints leave further questions about how each observation relates to the intact parent peptide.
Functional Characterization of N acetyl L Leucyl L Lysine in Biomaterials Research 2026
The research record raises useful questions about the intact peptide, related fragments, and metabolites. It does not establish that a change in one cell assay predicts a clinical outcome, or that every preparation labeled TB-500 contains the same chemical material. Results depend on the specified sequence, chemical modifications, experimental model, and outcome measured.
FDA’s 2026 compounding assessment reported that it had not identified clinical information in which TB-500 was used to treat a disease or condition. It also identified gaps in toxicology and clinical safety evidence, alongside potential immune-response concerns involving aggregation and peptide-related impurities. Absence of a reported problem in a small laboratory experiment does not resolve these questions.
FDA scientific assessment of TB-500 and TB-500 acetate prepared for the 2026 committee
TB-500 is not an FDA-approved medicine. The July 2026 Pharmacy Compounding Advisory Committee materials concern TB-500 and TB-500 acetate as proposed bulk substances for drug compounding. A scientific review, advisory recommendation, or compounding-list decision should not be represented as approval of a finished drug or validation of a research product’s safety and effectiveness.
FDA meeting materials for July 23 and 24 2026
The scientific assessment concerns specific substances and a regulatory framework. It should not be summarized as a blanket prohibition on laboratory research. Equally, research labeling does not authorize a separate intended use.
A result labeled “purity” does not, by itself, establish whether the sample contains the acetylated fragment, the unacetylated sequence, or full-length thymosin beta-4. A meaningful analytical record connects the sample and lot to the intended sequence, expected mass, testing methods, and actual results.
Purity, identity, quantity, and endotoxin testing answer different questions. An endotoxin result does not establish sterility or clinical safety. Findings from a characterized research reagent also do not authenticate a different supplier’s material. The 2012 identification study illustrates why the molecule must be established before biological claims can be interpreted.
Research and regulatory sources checked October 15, 2026.
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