TB-500 & Thymosin Beta-4 — Molecular Weight, Sequence & Identity Reference

01What is TB-500 (as Thymosin β4)?

Thymosin beta-4 is a 43-residue, N-terminally acetylated peptide (UniProt P62328) and the principal G-actin sequestering protein in mammalian cells. TB-500, as originally described, is something much smaller: a synthetic seven-residue fragment, Ac-LKKTETQ, corresponding to the actin-binding motif within that larger peptide. Full-length TB4 has an average mass of 4,963.44 g/mol; the heptapeptide fragment is 889.01 g/mol. These are not two names for one substance.

In practice the labels have collapsed into each other. The overwhelming majority of research material sold as "TB-500" is full-length thymosin beta-4, which means published data on one molecule is routinely attributed to the other in both directions. The only reliable resolution is the certificate of analysis: check the stated residue count, the sequence, and the mass-spectrometry result. A COA reporting a mass near 4,963 describes full-length TB4; one near 889 describes the fragment.

Two mass figures require care even within the full-length form. The 4,921 Da value in wide circulation — including in this database's own hub table before this revision — is the des-acetyl 43-mer; the mature N-acetylated peptide is 4,963.44 Da. Separately, UniProt reports 44 residues and about 5,053 Da for the translated precursor including the initiator methionine, which is not the mature chain. All three numbers are defensible for different species, and none of them is interchangeable.

02Molecular information

03Amino acid sequence

Structural modifications

FULL-LENGTH TB4: the initiator methionine of the 44-residue translated product is removed and the resulting N-terminal serine is N-alpha-acetylated, giving the mature 43-residue acetylated peptide (UniProt P62328). UniProt also annotates phosphorylation at Ser2, Thr23, Ser31 and Thr34, lysine acetylation at several positions, SUMO2 conjugation at Lys12 and one O-linked glycosylation site - these are regulatory PTMs on the endogenous protein, not features of synthetic material. TB4 contains no cysteine and therefore no disulfide bonds, and is intrinsically disordered in solution. Proteolysis of the N-terminus releases Ac-Ser-Asp-Lys-Pro (Ac-SDKP), an antifibrotic tetrapeptide, which UniProt records as a separate active entity. TB-500 FRAGMENT: N-terminally acetylated, free acid C-terminus, no other modification, all L-amino acids, no cyclisation. The acetyl cap is the only modification and is essential to the identity - unacetylated LKKTETQ (846.97 Da) is a different compound.

04Pharmacokinetic information

Half-life context

This is a genuine gap, not an omission. FULL-LENGTH TB4, human: the best available source is Ruff et al. (2010), a randomised placebo-controlled single- and multiple-dose study of INTRAVENOUS thymosin beta-4 in healthy volunteers at 42, 140, 420 and 1260 mg, which reported 'an increasing half-life with increasing dose' - i.e. non-linear, dose-dependent kinetics - without a single generalisable value [2]. A later first-in-human phase I study of recombinant human TB4 (NL005), also intravenous, reported that Cmax and AUC rose with dose and that there was no obvious accumulation on repeat dosing, but the retrievable record did not state a terminal half-life [3]. Both human studies used the intravenous route; NO human subcutaneous pharmacokinetic data were located, which matters because subcutaneous administration is the route assumed in almost all non-clinical use. TB-500 FRAGMENT: we located no human pharmacokinetic study whatsoever for Ac-LKKTETQ. Any half-life quoted for 'TB-500' - values from a few hours to two or three days circulate widely - traces to vendor marketing copy rather than to a published study, and should be treated as unsourced. Three distinct quantities are routinely conflated and must be kept apart: plasma half-life of exogenously administered peptide; the turnover of the large endogenous intracellular TB4 pool, which is a different process entirely since TB4 is one of the most abundant intracellular proteins in many cell types; and the duration of any downstream tissue-repair effect observed in animal models, which can persist long after the peptide is cleared. Rodent and rabbit data should not be extrapolated to humans.

05Research focus

Primary research areas

• G-actin sequestration and cytoskeletal regulation
• cutaneous and corneal wound repair
• angiogenesis and endothelial cell migration
• cardiac repair after experimental myocardial infarction
• anti-inflammatory and antifibrotic signalling (via Ac-SDKP)
• dry eye disease and neurotrophic keratopathy (clinical, ophthalmic)

Research context

Thymosin beta-4 is a 43-residue, intrinsically disordered, N-terminally acetylated peptide that is among the most abundant intracellular proteins in many mammalian cell types. Its principal characterised biochemical function is sequestration of monomeric G-actin, buffering the pool available for filament assembly and thereby regulating cytoskeletal dynamics and cell migration [4]. Structure-function work has mapped several functionally distinct domains onto the sequence: the N-terminal Ac-SDKP tetrapeptide is reported to reduce inflammation and fibrosis, the first 15 residues to support cell survival and block apoptosis, and the central actin-binding motif at residues 17-23, LKKTETQ, to promote angiogenesis, wound healing and cell migration in experimental systems [1][4]. That central heptapeptide, N-acetylated, is precisely the molecule originally described as 'TB-500' [1]. In animal models, TB4 has been reported to promote corneal and dermal wound repair, stimulate angiogenesis, upregulate VEGF, inhibit apoptosis via caspase downregulation and Bcl-2 upregulation, and reduce NF-kB-mediated inflammation, acting through PI3K/Akt/eNOS, Notch, TGF-beta/Smad and Wnt pathways [4]. Cardiac studies in rodents have reported cardioprotective and reparative effects after experimental myocardial infarction [4]. Human clinical work is real but limited and largely ophthalmic: phase 2 trials have been completed in dry eye syndrome, pressure and venous stasis ulcers and acute myocardial infarction, with the ophthalmic formulation RGN-259 advancing to phase 3 in neurotrophic keratopathy [4]. Two intravenous phase 1 studies in healthy volunteers reported that TB4 was well tolerated, with adverse events mild to moderate and no dose-limiting toxicities or serious adverse events across a 42-1260 mg range [2][3]. Regulatory and anti-doping status is unambiguous and important: thymosin beta-4 is not approved for human use in any major jurisdiction, is listed by WADA under S2.3 Growth Factors and Growth Factor Modulators, is prohibited in sport at all times both in and out of competition, and Sport Integrity Australia states that a Therapeutic Use Exemption would not be granted for any medical condition [5].

06Research evidence and limitations

Regulatory status

Not approved for human therapeutic use in any major jurisdiction, in either form. Sport Integrity Australia states thymosin beta-4 is not approved by the TGA, lacks approval for human use globally, and is not considered suitable for human use [5]. It is not an FDA-approved drug; the ophthalmic formulation RGN-259 remains investigational, having reached phase 3 in neurotrophic keratopathy [4]. ANTI-DOPING: thymosin beta-4 is prohibited under WADA Prohibited List section S2.3, Growth Factors and Growth Factor Modulators, and is banned in sport AT ALL TIMES - both in-competition and out-of-competition. Sport Integrity Australia states explicitly that a Therapeutic Use Exemption would not be granted for the use of thymosin beta-4 in any medical condition [5]. Athletes subject to anti-doping rules should treat any product labelled TB-500 or thymosin beta-4 as prohibited regardless of which molecular form it contains. Material sold online is research-use-only and is not manufactured, tested or regulated to pharmaceutical standards.

07Research material and sourcing notes

Reported research-use reconstitution information

Laboratory handling only, from chemical-supplier technical documentation - no clinical or manufacturer reconstitution protocol exists because neither form is an approved drug. TB-500 FRAGMENT (MedChemExpress HY-P0170): water solubility greater than 200 mg/mL (>=224.97 mM); approximately 1.1248 mL of solvent per mg gives a 1 mM solution. FULL-LENGTH TB4 (ChemicalBook): water 50 mg/mL (approximately 10.07 mM), may require ultrasonication; isoelectric point approximately 5.1. Both are highly water-soluble, consistent with their strongly hydrophilic, charge-rich composition. No authoritative source specifies an optimal buffer, pH range or a stability-indicating reconstitution method. The clinical ophthalmic formulation RGN-259 is a 0.1% TB4 solution, but its composition is proprietary and is not a laboratory reconstitution reference.

Reported storage and stability information

From chemical-supplier technical documentation. TB-500 FRAGMENT (MedChemExpress HY-P0170): lyophilised powder approximately 2 years at -80 C or 1 year at -20 C; stock solutions approximately 6 months at -80 C or 1 month at -20 C, sealed, protected from moisture and light, under nitrogen. FULL-LENGTH TB4 (ChemicalBook): store in a freezer below -20 C, in the dark, under inert atmosphere. No peer-reviewed stability study for either form was located, and no data on freeze-thaw tolerance or in-solution degradation pathways.

Identity and sourcing notes

THIS IS THE MOST IMPORTANT SOURCING ENTRY IN THIS DATABASE. The name 'TB-500' does not identify a molecule. As originally described in the literature, TB-500 is the synthetic N-acetylated heptapeptide Ac-LKKTETQ (CAS 885340-08-9, C38H68N10O14, MW 889.01), corresponding to residues 17-23 of thymosin beta-4 [1]. However, the overwhelming majority of research material sold today as 'TB-500' is FULL-LENGTH thymosin beta-4 (CAS 77591-33-4, C212H350N56O78S, MW 4963.44) - roughly 5.6 times the mass. A buyer ordering 'TB-500' should expect, in most cases, to receive full-length TB4. HOW TO TELL FROM A COA: the molecular weight is decisive and unmistakable. A COA reporting ~889 Da, formula C38H68N10O14, or CAS 885340-08-9 indicates the genuine heptapeptide fragment. A COA reporting ~4963 Da (or ~4921, the des-acetyl value, or ~5053, the precursor value), formula C212H350N56O78S, or CAS 77591-33-4 indicates full-length TB4. Note that ChemicalBook lists 'TB500' among the synonyms for full-length thymosin beta-4 acetate, which shows the ambiguity is embedded even in chemical reference databases. Check also that the MS data match the stated identity rather than merely being present, and confirm N-terminal acetylation - the 42 Da difference between acetylated and des-acetyl material is easy to miss and distinguishes a correctly synthesised peptide from an incomplete one. Because the two forms differ enormously in mass, any mass-based quantity comparison between them is meaningless. Establish salt form (acetate is usual) and demand peptide content by amino-acid analysis alongside HPLC purity; HPLC area-percent alone does not establish which molecule is in the vial.

08References and evidence

Primary sources

1. Biological activities of thymosin beta4 defined by active sites in short peptide sequencesprimary literatureSosne G, Qiu P, Goldstein AL, Wheater M — FASEB Journal, 2010 (DOI 10.1096/fj.09-142307)
2. A randomized, placebo-controlled, single and multiple dose study of intravenous thymosin beta4 in healthy volunteersclinical trialRuff D, Crockford D, Girardi G, Zhang Y — Annals of the New York Academy of Sciences, 2010 (DOI 10.1111/j.1749-6632.2010.05474.x)
3. A first-in-human, randomized, double-blind, single- and multiple-dose, phase I study of recombinant human thymosin beta4 in healthy Chinese volunteersclinical trialWang X, Liu L, Qi L, Lei C, Li P, Wang Y, Liu C, Bai H, Han C, Sun Y, Liu J — Journal of Cellular and Molecular Medicine, 2021 (DOI 10.1111/jcmm.16693)
4. Progress on the Function and Application of Thymosin beta4reviewMultiple authors — Frontiers in Endocrinology, 2021 (DOI 10.3389/fendo.2021.767785)
5. Thymosin Beta-4 - Peptide Hormone Information (WADA Prohibited List S2.3, prohibited at all times)regulatorySport Integrity Australia — Sport Integrity Australia, accessed 2026
6. TYB4_HUMAN - Thymosin beta-4 (P62328): sequence, mature chain, N-terminal acetylation and PTM annotationregulatoryUniProt Consortium — UniProtKB, accessed 2026

Chemical and molecular databases

• PubChem CID 45382195 (Thymosin Beta 4, C212H350N56O78S)
• UniProt P62328 flat file (raw sequence and feature records)
• ChemicalBook - Thymosin beta 4, CAS 77591-33-4 (MW 4963.4408)
• MedChemExpress HY-P0170 - TB500 (Ac-LKKTETQ), CAS 885340-08-9, MW 889.01
• DrugBank DB12003 (Thymosin beta-4) - clinical trial listings

09Related compounds

• Thymosin β4 FragmentDirectly the same naming problem - the Ac-LKKTETQ heptapeptide is itself the fragment, and any separate 'fragment' listing must be reconciled against this record to avoid a third overlapping entry.Reference page in preparation
• BPC-157The other peptide most commonly marketed for tissue repair and most often stacked with TB-500; shares the same evidentiary profile of extensive rodent data and near-absent human trials.
• GHK-CuAnother short endogenous peptide studied for wound healing, angiogenesis and remodelling, offering a mechanistic comparison from the copper-binding tripeptide class.
• Thymosin Alpha-1Shares the thymosin name and thymic origin but is a functionally unrelated 28-residue immunomodulator - included specifically because the shared name causes buyer confusion.Reference page in preparation
• Pentadeca ArginateA newer synthetic peptide marketed on the same tissue-repair positioning as TB-500 and BPC-157, with an even thinner published evidence base.

10Frequently asked questions