What it is
Thymosin β4 is a small natural protein, 43 amino acids long, found in almost every cell in the body. Its day job is to hold spare actin, the protein cells use to build their internal skeleton and to move. Because of that role it turns up wherever tissue is repairing itself.
TB-500 is not thymosin β4. It is a lab-made copy of just seven of those 43 amino acids, the stretch (positions 17 to 23) that grips actin. Vendors often blur the two names, and some sell the full protein under the TB-500 label. That matters because every clinical trial in this article used the full-length protein, made by a pharmaceutical company under the code RGN-259 or NL005. No genuine human study of the seven-amino-acid fragment was found: the one 2026 registry entry naming TB-500 describes itself as a fictional example record.
Thymosin β4 (Tβ4) is a 43-residue, 4,963 g/mol G-actin-sequestering peptide encoded by TMSB4X, present at high intracellular concentrations and released at wound sites. Its actin-binding motif, LKKTET (residues 17–22), is the basis for TB-500: the synthetic heptapeptide Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln (Ac-LKKTETQ, C38H68N10O14, 889.0 g/mol, CAS 885340-08-9), N-acetylated to slow aminopeptidase cleavage.
Full-length Tβ4 has been reported in preclinical models to promote keratinocyte and endothelial migration, angiogenesis, reduced inflammatory cytokine release, and cardiomyocyte survival after infarction (Goldstein et al., 2005). Whether the isolated 17–23 fragment reproduces those effects is unsettled: some in-vitro and animal work attributes migratory activity to the actin-binding region, while other activities of Tβ4 (for example the N-terminal tetrapeptide Ac-SDKP, a cleavage product with its own biology) reside elsewhere in the molecule. No human pharmacokinetic data for TB-500 have been published.
Who made it and when
Thymosin β4 was isolated and sequenced in 1981 by Allan Goldstein’s laboratory at George Washington University, originally from calf thymus. Goldstein went on to found RegeneRx Biopharmaceuticals, which has run the human trials of the full protein: eye drops (RGN-259) for dry eye and a nerve-damage condition of the cornea, an injection (RGN-352) for heart attack, and a gel (RGN-137) for wounds. A Chinese company, Beijing Northland Biotech, is separately testing a recombinant version (NL005) after heart attacks.
Nobody has claimed credit for the TB-500 fragment. It appears in the scientific and anti-doping literature mainly as a product sold by research-chemical vendors. The only registry entry naming it, posted in 2026 by a company called Hudson Biotech, describes itself as a fictional example study, so it is not counted as a trial.
Low, Hu and Goldstein published the complete sequence of bovine Tβ4 in PNAS in 1981. RegeneRx (later through the ReGenTree joint venture with HLB Therapeutics for ophthalmology) advanced Tβ4 as RGN-259 (ophthalmic), RGN-352 (systemic injection; a Phase 2 AMI trial was withdrawn) and RGN-137 (dermal gel, Phase 2 in pressure and venous ulcers and epidermolysis bullosa). The fragment’s origin is not documented in peer-reviewed literature; its CAS registration (885340-08-9) and PubChem record exist, but no developer, patent or IND could be verified for it. The only ClinicalTrials.gov record naming the 17–23 fragment, NCT07487363 (Hudson Biotech, posted 2026), states in its own summary that it is “a fictional study” and an example record; this article does not treat it as a trial.
What the data say
For the full protein, the human record is mixed. In two large dry-eye trials (about 600 and 700 people), thymosin β4 eye drops did no better than placebo on the main measurements. In a very small trial of people with a nerve-damage condition of the cornea, 6 of 10 on the drops healed completely versus 1 of 8 on placebo; a larger follow-up trial is still running and a parallel European trial reportedly missed its main goal. Wound-healing gels in the 2000s were safe but showed only modest healing signals. A Chinese heart-attack trial finished in 2023 without publishing results.
For TB-500 itself there is no human result of any kind. Animal data on the fragment are sparse; most of what is quoted about it is actually data on the full protein.
Full-length Tβ4: ARISE-2 (n=601, 0.1% four times daily, 28 days) and ARISE-3 (n=700, 14 days) did not meet primary endpoints for corneal staining or ocular discomfort. SEER-1 (n=18) reported complete healing of persistent epithelial defects in 60% vs 12.5% at day 29 (p=0.0656), reaching significance at day 43; SEER-2 (n=70 planned) is recruiting. Earlier RGN-137 gel trials (NCT00382174, pressure ulcers, n=72, 0.01–0.1% daily up to 84 days) reported 8 healed on active versus 3 on placebo, a secondary outcome in a safety study. NL005 Phase IIb (NCT05984134, IV 0.5 or 1.0 µg/kg daily for 7 days after PCI) is completed without posted results.
TB-500 fragment: no genuine trial was found. The only registry record naming it, NCT07487363 (a Phase 1/2 design in stable atherosclerotic cardiovascular disease), describes itself as “a fictional study” and discloses no doses. No efficacy, pharmacokinetic or adverse-event data for the fragment have been published. FDA stated in 2023 that it had “not identified any human exposure data” for the fragment.
Regulatory picture
Approval: no product containing thymosin β4 or TB-500 is approved anywhere. The full protein is still in company-sponsored trials; the fragment is not part of any approval programme.
Compounding: in September 2023 the FDA placed the seven-amino-acid fragment in Category 2, the group of substances it said raise significant safety risks, so pharmacies compounding with it could face FDA action; it said it had found no human exposure data at all. By April 2026 the FDA’s page listed it as withdrawn from that category by its nominator, and in July 2026 an FDA advisory committee voted narrowly to recommend allowing it for wound healing. That vote is not binding and the FDA had not decided when this was written.
Sport: WADA prohibits thymosin β4 and its fragments, including TB-500, at all times.
Approval: none. Full-length Tβ4 is investigational under RegeneRx/ReGenTree and Northland programmes; that status does not extend to the 17–23 fragment, which has no IND that could be verified.
Compounding: FDA placed “Thymosin beta-4, fragment (LKKTETQ), also known as TB-500” in Category 2 of the interim 503A policy on 2023-09-29, citing aggregation-related immunogenicity, peptide impurities and the absence of human exposure data. The safety-risk page updated 2026-04-22 lists it among substances withdrawn from Category 2 by the nominators. It is not on the 503A bulks list (21 CFR 216.23), so this article records compounding status as unknown. On 2026-07-23 the Pharmacy Compounding Advisory Committee reviewed TB-500 free base and acetate for wound healing and voted narrowly in favour of inclusion; a final FDA rule is required before that changes anything.
Anti-doping: thymosin β4 and its derivatives such as TB-500 have been named examples under S2 (peptide hormones, growth factors, related substances and mimetics) of the WADA Prohibited List since 2018.
