Summary
"TB-500" is a trade name rather than an international non-proprietary name or a USAN, and the molecules it can denote are not one substance 1. Three distinct peptides are involved. Full-length thymosin β4 is the 43-residue endogenous actin-sequestering peptide encoded by the human gene TMSB4X and given the international non-proprietary name timbetasin 234. The substance FDA registers under the name TB-500 is a synthetic heptapeptide, N-acetyl-Leu-Lys-Lys-Thr-Glu-Thr-Gln, corresponding to residues 17 to 23 of that protein 51. Ac-SDKP is the N-terminal tetrapeptide of the same protein and a separate substance with its own identifiers and its own international non-proprietary name 62. FDA states that websites use "thymosin beta-4" and "TB-500" interchangeably although they are not the same substance 1.
Full-length thymosin β4
Thymosin β4 is a 43-residue, roughly 5 kDa, acidic and heat-stable peptide, annotated in UniProt as P62328 and described there as organising the cytoskeleton by binding and sequestering monomeric actin 2. The mature chain is precursor residues 2 to 44, formed by removal of the initiator methionine, with the resulting N-terminal serine acetylated; its sequence is SDKPDMAEIEKFDKSKLKKTETQEKNPLPSKETIEQEKQAGES 23. In solution the peptide is intrinsically disordered, adopting two α-helices at residues 5 to 16 and 31 to 39 in trifluoroethanol; helix I and the LKKTET segment at residues 17 to 22 are the two principal actin-contact elements 7. Functionally it forms a 1:1 complex with actin monomers and inhibits salt-induced polymerisation, and the platelet peptide "Fx" was shown to be indistinguishable from it 8.
Its own sequence therefore contains both of the other two molecules discussed here, which is the root of the confusion: residues 2 to 5 are the Ac-SDKP tetrapeptide and residues 17 to 23 are the heptapeptide, so the three are nested rather than unrelated 265.
The N-acetylated 17–23 fragment
The fragment is residues 17 to 23 carrying an artificial N-terminal acetyl group on Leu17, a leucine that is internal and unacetylated in the parent protein, and a free C-terminal acid 91. Esposito and colleagues prepared the reference peptide by Fmoc solid-phase synthesis on Wang resin, with acetylation of the N-terminal leucine as the final on-resin step 9.
That acetyl group is not a bookkeeping detail. FDA notes that N-acetylation irreversibly alters charge, hydrophobicity and size, so the pharmacology of the non-acetylated LKKTETQ used in most published experiments cannot be directly extrapolated to the acetylated substance 1. The distinction matters because the best-known motif experiments used the unmodified sequence: in endothelial migration and chick aortic-arch sprouting assays, thymosin β4 and its seven-residue actin-binding motif showed near-identical activity at about 50 nM, peptides lacking any part of the motif were inactive, and the motif was identified as the major cell-binding site 10.
For the acetylated substance itself the position is weaker. FDA's evaluation identified no in vivo pharmacology study of TB-500; it reports an in vitro fibroblast scratch assay in which the substance did not induce closure, and nonclinical pharmacokinetic data showing that it reaches the circulation and is hydrolysed to smaller peptides 1. Rahaman and colleagues attribute the activity seen in fibroblast assays to the metabolite Ac-LKKTE rather than to the parent substance 11. No membrane receptor has been established for either the full-length peptide or the fragment 1012.
Ac-SDKP
Ac-SDKP, also known as goralatide and seraspenide, corresponds to precursor residues 2 to 5 and carries CAS 120081-14-3, UNII H041538E9P, PubChem CID 65938, the formula C20H33N5O9 and a molecular weight of 487.5 g/mol 62. It is released from thymosin β4 in vivo by successive hydrolysis involving meprin-α and prolyl oligopeptidase 12. Thymosin β4 is therefore described as acting indirectly through it, in addition to its direct actin-sequestering role 1012. It is neither TB-500 nor thymosin β4, and its literature is not interchangeable with either 62.
The three sets of identifiers
| Field | Heptapeptide fragment (TB-500) | Full-length thymosin β4 | Ac-SDKP |
|---|---|---|---|
| Registered name | N-acetyl thymosin β4(17-23) 51 | Thymosin β4; INN timbetasin, USAN GH-159 34 | Goralatide; seraspenide 6 |
| CAS | 885340-08-9 free base; none for the acetate 51 | 77591-33-4 for timbetasin; a second record carries 77642-24-1 313 | 120081-14-3 6 |
| UNII | QHK6Z47GTG 5 | 2D5MRE3SSY, 549LM7U24W, 945CRJ0XZ0 313 | H041538E9P 6 |
| PubChem CID | 62707662 14 | 45382195 by name lookup 15 | 65938 6 |
| Formula | C38H68N10O14 free base 14 | C212H350N56O78S 15 | C20H33N5O9 6 |
| Weight (g/mol) | 889.0 free base; 949.1 acetate 141 | 4963 153 | 487.5 6 |
| Residues | 7 116 | 43 210 | 4 6 |
Several of these fields are internally inconsistent in the public record. The substance registry's calculated mass for the TB-500 record, 846.981 Da, corresponds to the non-acetylated heptapeptide and disagrees with the record's own N-acetyl name, while its thymosin β4 record gives 4920 Da against 4963 elsewhere 51315. The UniProt figure of 5053 Da is the unprocessed 44-residue precursor including Met1 and is not the mature peptide 2. CAS 77591-33-4 is assigned to full-length timbetasin in the substance registry but was cited in FDA's 2026 briefing document from a supplier page describing TB-500 acetate 31. And the withdrawn 503A nomination quoted a CAS number, 476014-70-7, and a formula, C38H66N10O14, matching neither form 1.
What the anti-doping analytical literature reports about products
The contents of material sold under the TB-500 name have been established directly, twice, by doping-control laboratories. Esposito and colleagues analysed a TB-500 formulation by high-resolution Orbitrap mass spectrometry, identified the N-terminally acetylated 17-23 fragment of human thymosin β4, synthesised the reference peptide and proposed a liquid chromatography triple-quadrupole method for plasma and urine 9. Ho and colleagues describe a veterinary preparation "known as TB-500" whose key ingredient is LKKTETQ with artificial N-terminal acetylation, and detected the parent and its metabolites in equine urine and plasma, with confirmation limits of 0.02 ng/mL in plasma and 0.01 ng/mL in urine after ion-exchange solid-phase extraction 16.
The more recent picture is that both molecules circulate under the name. Delcourt and colleagues report that intelligence and doping-control laboratories have encountered numerous online products claiming to contain either the synthetic acetylated fragment or thymosin β4 itself, established an endogenous equine plasma range for thymosin β4 that did not depend on sex, age or breed, and detected a non-natural synthesis impurity after a single administration of a thymosin β4-containing product 17.
Metabolite work distinguishes the two analytically. For the fragment, the principal early metabolite in rats and in vitro systems is Ac-LK, with Ac-LKK persisting to 72 hours, quantified by ultra-high-performance liquid chromatography Orbitrap tandem mass spectrometry against synthesised standards 11. For the full-length peptide, thirteen in vitro metabolites were identified and Ac-Tβ1-14, absent from blank human urine, was proposed as a urinary marker of exogenous administration, with a limit of detection of 0.19 ng/mL 18.
Both molecules fall within the same anti-doping entry despite the chemical distinction: section S2.3 of the Prohibited List in force, covering growth factors and growth factor modulators, names "Thymosin-β4 and its derivatives e.g. TB-500", and class S2 is prohibited at all times and is non-Specified 19.
Full-length thymosin β4: phase 2 and phase 3 trials in ophthalmic and dermal formulations, two terminated early, with no authorisation retrieved in any jurisdiction 2021. Acetylated 17-23 fragment: no published human exposure data in FDA's evaluation, one phase 1/2 trial registered in 2026, and in vitro evidence that activity may reside in a metabolite 12211.
Open questions
Which molecule a given material is cannot be settled by its name: the analytical literature identifies the acetylated heptapeptide as the ingredient of TB-500 products, while products claiming full-length thymosin β4 also circulate, so a certificate carrying mass-spectrometric identity is required before either set of identifiers can be used 917. Free base or acetate is likewise undetermined, FDA treating them as different bulk drug substances and reporting suppliers applying the free-base CAS number to the salt 1. No United States Pharmacopeia, National Formulary or European Pharmacopoeia monograph exists for TB-500, and FDA considers both forms not well characterised, flagging peptide-related impurities, aggregation and immunogenicity for injectable routes 1. Only one identity database yielded a CAS number for the full-length peptide, so the dossier's two-source rule is unmet for that field 3.