TB-500 / Thymosin beta-4 frag

TB-500 is a laboratory-made peptide marketed in the research-chemical and grey-market space as a "fragment" of thymosin beta-4 (Tβ4). Thymosin beta-4 itself is a naturally occurring 43-amino-acid, roughly 5 kDa polypeptide found throughout human and animal cells, and is regarded as the body's main G-actin sequestering peptide, involved in cell migration, tissue repair, angiogenesis and inflammation modulation. TB-500 corresponds to the short acetylated actin-binding motif of that protein (the seven-residue sequence Ac-LKKTETQ, residues 17-23), the segment shown in lab studies to be essential for Tβ4's pro-migratory and angiogenic activity. It is important to note that most published clinical and preclinical "thymosin beta-4" data used the full-length protein or recombinant Tβ4 — not the short TB-500 fragment sold to consumers.

Education only. This page summarises published research and regulatory status. It is not medical advice and gives no dosing, preparation or administration instructions; any decision about use belongs with an independent qualified professional.

Also known as

TB-500, TB500, Thymosin beta-4 fragment, Tβ4 fragment, Ac-LKKTETQ, actin-binding domain peptide (residues 17-23 of thymosin beta-4)

Class

Synthetic actin-binding fragment of thymosin beta-4 (G-actin sequestering / regenerative peptide)

How does TB-500 work?

Thymosin beta-4 binds monomeric (G) actin with high affinity and "sequesters" it, preventing premature polymerization and creating a reservoir of actin that cells can mobilize to remodel their cytoskeleton. This regulation of actin dynamics underlies its downstream effects on cell motility, endothelial cell migration and new blood-vessel formation (angiogenesis), and wound closure. In preclinical work the same actin-binding motif that TB-500 represents was sufficient to promote endothelial migration, vessel sprouting and tubule formation at low nanomolar concentrations, and Tβ4 has been reported to reduce inflammation and activate survival signaling (e.g. integrin-linked kinase / Akt pathways) in injured tissue.

What has TB-500 been studied for?

• Dermal and corneal wound healing (preclinical and, for full-length Tβ4 eye drops, clinical dry-eye trials)
• Cardiac repair after myocardial infarction (animal models)
• Angiogenesis and endothelial cell migration (in vitro / in vivo)
• Venous stasis ulcers (topical full-length Tβ4 gel, Phase 2)
• Soft-tissue, tendon and muscle repair (animal models; popularly promoted but not clinically proven for the TB-500 fragment)
• Anti-inflammatory and anti-fibrotic effects (preclinical)

What does the evidence show?

Evidence is mixed and must be read carefully. The strongest data are preclinical (cell-culture and animal) for the full-length thymosin beta-4 protein — including landmark mouse cardiac-repair work (Bock-Marquette and colleagues) and dermal/corneal wound-healing models. Human clinical trials exist almost exclusively for full-length Tβ4 developed as a drug candidate by RegeneRx: a randomized Phase 2 dry-eye trial of RGN-259 ophthalmic solution (PMID 25826322) showed statistically significant improvement in signs and symptoms with good tolerability, and a Phase 2 RGN-137 topical gel was tested for venous stasis ulcers. Critically, there are no completed, peer-reviewed human randomized controlled trials of the short 17-/7-residue TB-500 fragment itself for muscle, tendon, joint or systemic recovery — the popular athletic "healing/recovery" claims rest on extrapolation from animal data and the full-length protein, not on human evidence for the marketed fragment. The field also carries the usual caveats: small studies, animal-only mechanistic work, publication bias toward positive findings, and no long-term human outcome data for the fragment.

What is known about the safety of TB-500?

Human safety data are limited and pertain mostly to full-length thymosin beta-4 in controlled trials, where it was generally well tolerated with mostly mild-to-moderate, self-limiting adverse events and no dose-limiting toxicities reported. For TB-500 as sold in the grey market, the key risks are: (1) no controlled human safety studies of the fragment for the touted recovery uses; (2) unstudied long-term effects; (3) a theoretical concern that a strongly pro-angiogenic, pro-migratory agent could in principle promote growth or spread of existing tumors or abnormal vasculature — not established in humans but a reason caution is urged; and (4) product-quality hazards because grey-market vials are unregulated and have been associated with endotoxins, truncated/incorrect sequences, residual solvents, heavy metals and microbial contamination. Anyone with a personal or family cancer history, or who is pregnant or breastfeeding, should be especially cautious and only consider this in a clinical context. This is general safety education, not medical advice.

What is the legal status of TB-500?

Not approved by the FDA for any human medical indication; regarded as an unapproved drug / research chemical and not lawful to sell or label for human consumption. It is prohibited at all times (in and out of competition) under the WADA Prohibited List in the growth-factor / peptide-hormone area (Section S2) and is treated as a non-Specified Substance; the U.S. Department of Defense applies the WADA list to service members. Only the full-length protein has been studied as an investigational drug (e.g., RegeneRx's RGN-259/RGN-137); none is an approved product. US compounding status, October 2026: on 15 April 2026 the FDA removed TB-500 (thymosin beta-4 fragment) from Category 2 of its interim 503A bulk-substances list, where it had been listed over safety concerns, and on 23-24 July 2026 the FDA's Pharmacy Compounding Advisory Committee voted to recommend adding it to the 503A bulks list. The vote is a non-binding recommendation, not a drug approval, and does not by itself let pharmacies compound it. On 14 October 2026 the FDA sent an “Interim Access Policy for Compounding Using Certain Peptides” to the White House Office of Information and Regulatory Affairs for review; as of 21 October 2026 its text and scope were not public and it was not in effect.

What is the half-life of TB-500?

No published human pharmacokinetic data exist for the short TB-500 fragment specifically. For full-length recombinant thymosin beta-4 given intravenously in Phase 1 studies, terminal half-life was short and dose-dependent, reported in the range of roughly 0.5 to 2.1 hours, with no notable accumulation on repeat dosing (PMID 34346165 / PMC8419156). Longer-acting effects attributed to Tβ4 are thought to reflect downstream tissue signaling rather than prolonged drug exposure.

Sources

1. Philp D, et al. The actin binding site on thymosin beta4 promotes angiogenesis. FASEB J. 2003.
2. Sosne G, et al. Thymosin β4 significantly improves signs and symptoms of severe dry eye in a phase 2 randomized trial. Cornea. 2015. PMID 25826322.
3. Wang X, et al. A first-in-human, randomized, double-blind, single- and multiple-dose, phase I study of recombinant human thymosin β4 in healthy volunteers. J Cell Mol Med. 2021. PMID 34346165.
4. Bock-Marquette I, et al. Thymosin beta4 activates integrin-linked kinase and promotes cardiac cell migration, survival and cardiac repair. Nature. 2004. PMID 15565145.
5. BSCG: TB-500 — Status, Risks, and Bans in Sport and Military.
6. World Anti-Doping Agency Prohibited List (Section S2: peptide hormones, growth factors, related substances and mimetics).
7. NCPA: FDA advisory committee nominates six peptides for pharmacies to compound (31 Jul 2026)
8. Frier Levitt: FDA to remove 12 peptides from the Category 2 list (April 2026)
9. Buchanan Ingersoll & Rooney: FDA submits interim peptide compounding policy for White House review (16 Oct 2026)