# TB-500: The Actin-Binding Fragment of Thymosin Beta-4, Cited

> TB-500 is the synthetic Ac-LKKTETQ heptapeptide fragment of thymosin beta-4. A clear-eyed, fully cited digest of the research record — what is established and where the human data stop.

Most of the efficacy data belong to the full-length parent protein, the isolated seven-amino-acid fragment has no completed controlled human trial, and this digest holds that distinction on every page.

## TB-500 in one paragraph

TB-500 is the synthetic, N-acetylated heptapeptide Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln (Ac-LKKTETQ), corresponding to residues 17–23 of the 43-amino-acid protein thymosin beta-4 [1]. Those seven residues are the conserved actin-binding motif of the beta-thymosins — the part of the parent protein that grips monomeric actin. That single structural fact governs the whole record. Thymosin beta-4 binds globular (G-) actin one-to-one and caps both ends of the monomer to hold a buffered pool of unpolymerized actin, controlling cytoskeletal dynamics and cell migration [1]. TB-500 carries that motif and nothing else.

Here is the distinction this site keeps in front of you on every page: in commerce and in the anti-doping literature, "TB-500" means the ~889-dalton heptapeptide, but the overwhelming majority of published *efficacy* studies were run with full-length recombinant or synthetic thymosin beta-4 (~4963 daltons), not the seven-mer [1]. Whether the isolated fragment reproduces the parent protein's effects at the doses used in peptide research is not established in controlled human trials. So this is a digest of two related but separable things — a well-studied parent protein and a much-less-studied fragment that borrows its name — and it labels which is which every time a number appears.

## TB-500 as a research peptide: the Ac-LKKTETQ fragment

The TB-500 peptide is a laboratory and veterinary-context construct, not an endogenous molecule. The parent thymosin beta-4 is ubiquitous — present in nearly all human cells and released by platelets and macrophages at sites of injury — but the Ac-LKKTETQ fragment itself is synthesized, supplied as a lyophilized powder, and reconstituted for research use [1]. Its molecular formula is C38H68N10O14 at roughly 889 daltons; the parent protein, by contrast, is a 43-residue chain near 4963 daltons.

That size gap matters for interpretation. The N-terminal region of full-length thymosin beta-4 can be cleaved to Ac-SDKP, a separate peptide with its own anti-fibrotic and angiogenic activity — but Ac-SDKP comes from the N-terminus, not the C-terminal-region LKKTETQ segment that constitutes TB-500, so the fragment does not generate it. When a study reports a thymosin beta-4 finding, the first question this digest asks is which molecule was actually in the syringe. For the structural and biochemical backbone of that story, see [the thymosin beta-4 research base](/research).

## What the literature establishes — and where it stops

X-ray crystallography of a gelsolin-domain-1–thymosin beta-4 hybrid bound to actin, resolved to 2 ångström, established that the protein forms a one-to-one complex with G-actin and sequesters the monomer by capping both ends, preventing polymerization [1]. That is the firmest fact in the file: a solved structure, not an inference. In a rat full-thickness wound model, topical or intraperitoneal thymosin beta-4 increased re-epithelialization by 42% at four days and up to 61% at seven days versus saline [2]. In mice, the protein activated the PINCH–ILK–Akt survival pathway and improved cardiac function after coronary artery ligation [3]. And in a randomized, placebo-controlled Phase 1 study, intravenous synthetic thymosin beta-4 was well tolerated in healthy volunteers up to 1260 mg with no dose-limiting toxicities [4].

Now the honest edge. Every one of those four findings used full-length thymosin beta-4, not the TB-500 fragment. There are no completed controlled clinical trials of the heptapeptide for any indication [4]. There is no validated human pharmacokinetic half-life for it. And the same pro-migratory, pro-angiogenic biology that aids repair is the basis of a recognized tumor/angiogenesis safety signal [12]. This digest organizes the record by confidence: a solved structure and a reproduced animal figure are bright signals; a fragment with no human efficacy trial is a clearly labeled gap. For the recovery-specific evidence, see [TB-500 and muscle recovery](/muscle-recovery).

## Definitional questions, answered

The short answers below resolve the most common confusions before you go deeper. Each is expanded in the relevant page, and every quantitative claim is cited.

## What is TB-500?

TB-500 is the synthetic N-acetylated heptapeptide Ac-LKKTETQ, corresponding to residues 17–23 — the actin-binding motif — of the 43-amino-acid protein thymosin beta-4 [1]. It is a research and veterinary-context substance with no approved human indication. It is not an endogenous molecule itself; the parent protein is.

## What does TB-500 stand for?

"TB" references thymosin beta-4 (Tβ4), the parent protein; "TB-500" is the commercial and veterinary research designation for its synthetic Ac-LKKTETQ actin-binding fragment [1]. The number is a product-line label, not a chemical descriptor. The same fragment also circulates under designations such as TB1000.

## What is TB-500 used for in research?

In animal and cell models, thymosin beta-4 and its actin-binding region are studied for wound healing, muscle and ligament repair, angiogenesis, and cardiac and neurological recovery [5][7][8]. The efficacy of the isolated seven-mer is unproven in controlled human trials [4]. Studied outcomes are properties of the parent protein far more often than of the fragment.

## How does TB-500 work?

TB-500 carries the actin-binding LKKTETQ motif of thymosin beta-4, the body's main G-actin sequestering peptide [1]. Full-length thymosin beta-4 binds monomeric actin one-to-one to regulate cytoskeletal dynamics, cell migration, angiogenesis, and anti-inflammatory signaling [5]. Whether the isolated heptapeptide reproduces these effects at research doses is not established in humans.

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A clear-eyed digest of the TB-500 and thymosin beta-4 record, read by glow intensity — the established findings bright, the human-data gaps left dim and labeled, with no clinic behind the aurora and nothing here dispensed.