# TB-500 Legal — The regulatory record on Ac-LKKTETQ-OH, read like a passport > A customs-form reading of the regulatory record on TB-500: FDA 503A Category 2, WADA S2 and S0, IFHA Article 6, USADA enforcement. Mechanism, evidence, and jurisdictional status of the seven-amino-acid fragment of Thymosin Beta-4. Ac-LKKTETQ-OH, fragment of Thymosin Beta-4. A reading of the customs forms — every clearance, every refused entry. ## The short version TB-500 is a synthetic seven-amino-acid peptide — Ac-LKKTETQ-OH — derived from the actin-binding region of a protein your body already makes in every cell. Researchers study the parent protein (Thymosin Beta-4) for tissue repair, but TB-500 itself, the short fragment, has never been tested in a registered human clinical trial. The FDA placed it on its list of substances too risky for compounding pharmacies to use. The World Anti-Doping Agency bans it for all athletes, in and out of competition. This site documents those regulatory findings jurisdiction by jurisdiction — not to endorse any use, but to read the record straight. [See what people report and the honest cautions on the effects page.](/effects) ## What this site documents TB-500 is a synthetic seven-amino-acid peptide with the sequence N-acetyl-Leu-Lys-Lys-Thr-Glu-Thr-Gln (Ac-LKKTETQ-OH). The sequence corresponds to residues 17–23 of full-length human Thymosin Beta-4 (Tβ4), a 43-amino-acid intracellular peptide encoded by the X-linked TMSB4X gene [1]. The fragment carries the conserved LKKTET central motif that mediates binding to monomeric actin in the parent peptide. The N-terminal acetyl group blocks aminopeptidase cleavage and improves solution stability. The distinction matters at every level of the regulatory record. Every registered human clinical trial of 'thymosin beta-4' has used the full 43-amino-acid recombinant protein. The synthetic seven-amino-acid fragment marketed as TB-500 has never been evaluated in a registered human clinical trial. Vendor literature treats the two molecules as interchangeable; the published structural work treats them as related but pharmacologically distinct [22]. This site reads the regulatory record on TB-500 the way a customs officer reads a passport: jurisdiction by jurisdiction, stamp by stamp. The FDA placed it on Category 2 of the interim 503A bulks list in 2023 [20]. The World Anti-Doping Agency lists it under sections S2 and S0 of the 2025 Prohibited List [21]. The International Federation of Horseracing Authorities prohibits it under Article 6 of the International Agreement on Breeding, Racing and Wagering. The U.S. Anti-Doping Agency has issued four-year ineligibility sanctions for possession alone [22]. The site documents these findings as observational record, not as prescription or proscription. ## Where TB-500 stands today TB-500 is not approved for any human indication by any major regulatory authority — not the FDA, EMA, MHRA, TGA, PMDA, or NMPA. The molecule is not currently scheduled as a controlled substance under the federal Controlled Substances Act or under the controlled-substance acts of California, Texas, New York, or Florida. The FDA's 2023 placement of TB-500 on the Category 2 interim 503A bulks list was the formal finding that the substance presents 'significant safety risks' that licensed compounding pharmacies are prohibited from using [20]. The classification followed a multi-year FDA review of bulk peptides nominated for compounding and warning-letter enforcement actions issued at intervals between 2017 and 2023. The World Anti-Doping Agency lists TB-500 and thymosin-β4 explicitly in the 2025 Prohibited List under section S2 (Peptide Hormones, Growth Factors, Related Substances and Mimetics), prohibited at all times — in competition and out of competition — for athletes under the World Anti-Doping Code [21]. The S0 catch-all also applies, because the molecule has no marketing authorization in any jurisdiction. The S0 status carries a mandatory four-year ineligibility period on first offense unless the athlete establishes no significant fault. Mere possession is sufficient grounds for sanction under the Code. The USADA case against weightlifter Michael Nackoul — a four-year ineligibility for possession of prohibited growth factors including Thymosin Beta-4 — was published as a deterrent precedent [22]. ## What the science says, briefly The biology of the parent peptide is extensively characterized. Full-length Tβ4 is the principal G-actin-sequestering protein in eukaryotic cells, binding monomeric actin in a 1:1 complex through its central LKKTET motif [4]. Beyond actin sequestration, Tβ4 induces angiogenesis by stabilizing HIF-1α and inducing VEGF transcription [1], suppresses NF-κB-driven inflammatory cytokine expression [3], activates the PINCH–ILK–Akt cell-survival axis in cardiomyocytes [10], and is enzymatically cleaved to release the antifibrotic tetrapeptide AcSDKP [19]. The most-developed clinical program for full-length Tβ4 is RGN-259, a 0.1% topical ophthalmic solution. A Phase III trial in neurotrophic keratopathy (NCT02600429, n=18) reported complete corneal healing at day 29 in 60% of treated patients versus 12.5% on placebo and statistically significant healing at day 43 [14]. Subsequent Phase III trials in dry eye (ARISE-3) and European neurotrophic keratitis (SEER-3) missed their prespecified primary endpoints. Phase IIb cardiac trial NCT05984134 in 90 acute-MI patients post-PCI completed in 2023; primary results have not yet been published [13]. What the synthetic TB-500 fragment does in a human body is not documented in the peer-reviewed literature. The class of LKKTET-containing peptides shows preserved corneal-healing activity in rodent models [16], and equine doping-control work confirms systemic exposure after IV administration in horses [15]. Beyond that, the fragment's human pharmacology is the territory of vendor literature, not science. ## How the site is organized The remaining pages follow the customs-form structure. /research documents the mechanism and the published evidence, organized by tissue system and citation. /dosage catalogs the dose ranges that appear in the published research literature — these are study-attributed values, not recommendations. /faq answers the questions readers actually arrive with: is TB-500 legal in the United States, is it controlled, why did the FDA classify it, is it banned by WADA, what sanction follows. /references is the customs manifest — every citation, sortable and searchable. /about establishes who publishes the site and on what basis. /contact provides editorial correspondence channels. The site does not sell anything. It is not a clinic. It is not affiliated with any vendor, supplier, compounding pharmacy, anti-doping organization, or regulatory authority. The content is editorial commentary on publicly available science and publicly published regulatory documents. --- An independent editorial survey of the published regulatory and research record — not a clinic, not a vendor, not legal counsel.