TB-500 Fragment (17-23)

TB-500 Fragment (17-23) refers to Ac-LKKTETQ, a short seven-amino-acid piece of the larger protein thymosin beta-4 (Tβ4) that contains its actin-binding site. It is a laboratory research tool used to study how Tβ4 interacts with actin filaments. This is important to know because most vials sold online labeled simply “TB-500” are marketed and described as full-length thymosin beta-4 (43 amino acids), not this small fragment — the two are chemically different molecules with different, and very unevenly studied, evidence bases.

Research snapshot

Peptide category	Short synthetic fragment (residues 17-23) of thymosin beta-4, the actin-binding motif
Primary research interest	Laboratory research tool for studying thymosin beta-4/actin interaction; distinct from full-length thymosin beta-4 sold as “TB-500”
Highest available evidence	Tier 4 — mutational/structural laboratory research on the parent protein’s actin-binding site; very unevenly studied compared to full-length thymosin beta-4
Human research available	No — no human clinical trial of this specific 7-amino-acid fragment identified
Development status	Laboratory research reagent; no active clinical development program identified
Regulatory status	Not an approved drug anywhere. FDA staff proposed against adding it to the 503A bulks list; an advisory committee recommended it 8-6-1 on 23 July 2026 (non-binding, no FDA action found). Sold labeled “research use only, not for human consumption.” WADA prohibits thymosin beta-4 and related peptides (including material described as TB-500) in competitive sport

Technical identity

Primary name	TB-500 Fragment (17-23)
Alternative names	Ac-LKKTETQ; thymosin beta-4 actin-binding fragment. Distinct from full-length thymosin beta-4 (43 amino acids), which is what most vials labeled “TB-500” actually contain
Peptide sequence	Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln (Ac-LKKTETQ)
Amino-acid length	7 (heptapeptide fragment of the 43-residue parent protein)
Molecular formula	C38H68N10O14 (free base, per FDA briefing document; the acetate salt is C38H68N10O14·CH3COOH)
Molecular weight	889.01 g/mol (free base); 949.1 g/mol (acetate salt), per FDA briefing document
CAS Registry Number	885340-08-9 (free base, as listed in the FDA briefing document)
PubChem CID	Not reliably established
Chemical modifications	N-terminal acetylation (Ac-)
Peptide class	Synthetic fragment peptide (actin-binding motif of thymosin beta-4)
Primary biological target	Monomeric (G-actin) actin, via the mapped actin-binding site of the parent thymosin beta-4 protein
Developer or originator	Academic/laboratory research tool derived from mutational and structural mapping of thymosin beta-4; not a commercial drug candidate
Development status	Research reagent; not in drug development

What it is

Thymosin beta-4 (Tβ4) is a small, naturally occurring 43-amino-acid protein found in nearly all human and animal cells, where it binds and sequesters monomeric (G-actin) actin and plays a role in cell movement, cytoskeletal regulation, and wound-healing biology. Its actin-binding activity was mapped by mutational and structural studies to a specific internal stretch of the molecule, the sequence Leu-Lys-Lys-Thr-Glu-Thr-Gln, corresponding to residues 17 through 23 of the full protein. Researchers synthesized this short segment on its own, typically N-acetylated as Ac-LKKTETQ, as a laboratory tool to isolate and study the actin-binding function of Tβ4 without the rest of the molecule.

This is where naming becomes a genuine problem for anyone researching “TB-500.” The name TB-500 was originally used, including in some regulatory and anti-doping literature, to refer to this synthetic Ac-LKKTETQ fragment specifically. However, the peptide research-chemical marketplace that grew up around Tβ4 in the 2010s largely adopted “TB-500” as a brand-style name for full-length, 43-amino-acid synthetic thymosin beta-4 — the same molecule studied in human trials for pressure ulcers, dry eye, and cardiac repair under names like Tβ4 or RGN-137. Many vendor product listings, community forum threads, and even some educational summaries use “TB-500” to describe full Tβ4, not the seven-amino-acid fragment, and do not always disclose which molecule is actually in the vial. Because the fragment and the full protein have different molecular weights, different (and non-overlapping) evidence bases, and were never established as interchangeable in the same studies, this entry treats them as distinct compounds and focuses specifically on the evidence for the Ac-LKKTETQ fragment. Where a claim traces only to full Tβ4 research, that is stated explicitly rather than folded into “TB-500.”

The Ac-LKKTETQ fragment itself has a comparatively thin, mostly older and mechanistic literature, concentrated on how it binds actin and what that does to cell motility and actin polymerization dynamics in cell-based assays. It has not undergone the kind of dedicated human clinical development that full-length Tβ4 received from RegeneRx Biopharmaceuticals. No developer or regulatory record of a formal clinical program for the isolated fragment was identified in this research pass.

How does it work?

Plain-English explanation

Actin is a protein that cells use like an internal skeleton and internal railway system, constantly building and dismantling filaments to move, change shape, and migrate to a site of injury. The Ac-LKKTETQ fragment is the exact piece of thymosin beta-4 that grabs onto free actin building-blocks and keeps them from immediately assembling into filaments. In laboratory cell studies, this fragment reproduces at least part of full Tβ4’s actin-binding behavior, which is part of why researchers isolated it in the first place: to determine which part of the larger molecule is doing that specific job.

Technical explanation

Mutational and peptide-mapping studies on thymosin beta-4 localized its G-actin-sequestering activity to the central region spanning approximately residues 4 to 24, with the LKKTET motif (residues 17–23) identified as a core determinant of actin binding affinity. Synthetic Ac-LKKTETQ has been used in vitro to compete with full-length Tβ4 for actin binding and to probe structure-activity relationships of the actin-thymosin interaction. Because the fragment lacks other regions of full Tβ4 implicated in its broader biological effects (including reported roles in cell migration signaling, angiogenesis-related gene expression, and anti-inflammatory activity attributed to the intact protein), the fragment cannot be assumed to reproduce those broader effects; the published work identified in this research is concentrated on actin-binding biochemistry rather than whole-organism healing outcomes.

Potential benefits and research applications

Actin-binding and cytoskeletal research tool

What is being investigated: use of the isolated fragment to define the actin-binding domain of thymosin beta-4 and to study actin sequestration biochemistry. How the effect might occur: direct competitive binding to monomeric actin, limiting its availability for filament assembly. Evidence: laboratory/biochemical (cell-free and cell-based assay) studies mapping the Tβ4 actin-binding domain. Strength: this is foundational structural biology, not a therapeutic-outcome literature. Limitation: these studies establish a binding mechanism, not a clinical benefit, and were not designed to test wound healing, muscle recovery, or injury repair as endpoints.

Extrapolated “wound healing” and “recovery” claims (vendor/community framing)

What is being investigated: vendor and community material commonly attributes wound-healing, tendon-repair, and anti-inflammatory benefits to “TB-500.” How the effect might occur: by inference from research on full-length Tβ4, which has been studied in human trials for dermal and corneal wound healing. Evidence: this benefit claim, as applied to the Ac-LKKTETQ fragment specifically, is not supported by an identifiable body of published human or animal research; the supporting studies that exist in this space were conducted on full-length Tβ4, not the fragment. Strength: unverified for the fragment. Limitation: this is a case where a real evidence base for one molecule (full Tβ4) is being applied by extension to a different, smaller molecule (the fragment) without a demonstrated basis for assuming equivalent effects.

What dosage information circulates?

Figures in this section summarize amounts and schedules reported in published research or circulating online. Their inclusion documents what is reported and does not establish that a regimen is verified, safe, effective, or appropriate.

Amounts studied in human research

No established or reliably sourced dosing information was identified for the Ac-LKKTETQ fragment specifically in human research. No clinical trial record for the isolated fragment was identified on ClinicalTrials.gov or in the PubMed literature search performed for this entry.

Amounts studied in animal research

No established or reliably sourced dosing information specific to the isolated fragment (as distinct from full-length Tβ4) was identified in animal research during this review.

Practitioner and community-reported protocols

Peptide vendor sites and injectable-peptide community forums circulate weekly and twice-weekly injectable protocols, commonly citing figures such as 2–2.5 mg per injection, one to two times weekly, for several weeks, for products labeled “TB-500.” These figures cannot be confirmed to refer specifically to the Ac-LKKTETQ fragment rather than full-length Tβ4, since vendors frequently do not specify which molecule the vial actually contains, and the underlying protocols themselves are not traceable to a published clinical or preclinical dosing study of either molecule. Widely repeated online, but its original source could not be verified. Evidence category: community/vendor reported, origin unclear.

Regulatory status

TB-500 is not an approved drug anywhere. In the United States it was nominated for the 503A bulk drug substances list as a wound-healing injectable. FDA staff reviewed it for the July 2026 Pharmacy Compounding Advisory Committee and proposed not adding it, citing poor physicochemical characterization, no identified history of compounding, no human safety or effectiveness data, and immunogenicity and impurity concerns (FDA briefing document). The briefing document defines TB-500 as the N-acetylated seven-amino-acid fragment (residues 17-23) of thymosin beta-4, not thymosin beta-4 itself.

On 23 July 2026 the committee voted 8 to 6 with one abstention to recommend adding it, against FDA staff, in a single grouped vote that also covered BPC-157 and KPV (AJMC). The vote is non-binding, and a proposed rule, public comment and a final rule would have to come first. As of mid-August 2026 reporting, FDA had not acted. News reports on 7 to 8 October 2026 say the administration is drafting plans for temporary sales of some unapproved peptides, but the reports do not name the peptides or the mechanism, and no FDA or HHS announcement was found.

The FDA document is also a useful data source. It found no human data for TB-500 of any kind, no reports in FDA’s adverse event database, and no toxicity studies. The only pharmacokinetic data it identified came from one horse study (10 mg subcutaneous) and one rat study, with very low plasma levels that were undetectable within 6 to 10 hours. Its in vitro test of the free base at 50 µg/mL did not significantly close fibroblast scratch wounds.

Side effects, risks, and limitations

Because the isolated Ac-LKKTETQ fragment has not been through a formal human clinical safety program identified in this research, there is no established human side-effect profile specific to it. General concerns raised in the peptide-safety literature around unregulated compounded/research-chemical peptides apply here: uncertain purity and identity of vendor products, uncertain sterility of “research use only” vials intended for injection, and the previously noted naming confusion, which means a person believing they are using a small actin-binding fragment may in fact be administering full-length thymosin beta-4, or a product of unverified composition altogether. Full-length Tβ4 has its own separately studied safety profile that should not be assumed to apply to the fragment. Regulatory and anti-doping bodies, including the World Anti-Doping Agency, have prohibited thymosin beta-4 and related peptides (described under names including TB-500) in competitive sport, reflecting concern about unregulated use rather than a specific finding about the fragment’s safety.

Regulatory and developmental status

The Ac-LKKTETQ fragment is not an approved drug anywhere and has no active clinical development program that could be identified. It is sold by research-chemical vendors labeled “research use only, not for human consumption,” a labeling practice common across this category regardless of actual buyer use. Full-length thymosin beta-4 (a distinct molecule) was developed clinically by RegeneRx Biopharmaceuticals under names such as RGN-137 and TB4, with trials in dermal wound healing, dry eye disease, and epidermolysis bullosa; none of the fragment-specific literature located here traces to that clinical program.

Frequently asked questions

What is TB-500 Fragment (17-23)?

It is Ac-LKKTETQ, a synthetic seven-amino-acid piece of thymosin beta-4 spanning residues 17–23, containing the actin-binding portion of that larger protein.

Is TB-500 Fragment (17-23) the same as TB-500?

It depends who is using the name. Some sources use “TB-500” to mean exactly this fragment. Many commercial vendors and community discussions use “TB-500” to mean full-length, 43-amino-acid thymosin beta-4 instead. The two are different-sized molecules, and a buyer cannot assume which one is actually in a given vial without independent lab verification. This entry treats them as distinct and reports evidence only where it specifically concerns the fragment.

What dosage has been studied for the fragment?

No established or reliably sourced dosing information was identified for the isolated fragment in human or animal research.

Has it been studied in humans?

No human clinical trial of the isolated Ac-LKKTETQ fragment was identified. Full-length Tβ4, a different molecule, has been studied in humans.

What does the fragment actually do?

In laboratory studies, it binds monomeric actin, reproducing the actin-sequestering function that was mapped to this region of the full Tβ4 protein.

Why do vendors sell something called “TB-500” if it usually is not the fragment?

The name became a popular commercial shorthand for thymosin beta-4-related products generally. Because “TB-500” is not a regulated or standardized name, it has been applied inconsistently to both the fragment and the full protein.

Is it legal or regulated?

It is not an approved medication. It is prohibited in competitive sport by the World Anti-Doping Agency under thymosin beta-4/TB-500-related listings, and is a prescription-only substance in some countries such as Australia.

What remains unknown?

Whether the isolated fragment reproduces the broader tissue-repair effects reported for full-length Tβ4, what a meaningful human dose would be, its pharmacokinetics, and — most basically — which molecule most commercial “TB-500” products actually contain.

Bottom line

TB-500 Fragment (17-23) is a real, specific molecule — Ac-LKKTETQ — with a genuine, if narrow, role in classic actin-binding biochemistry research. It is not the same thing as the full-length thymosin beta-4 protein that has actually been through human clinical trials, even though the two are frequently conflated under the same commercial name. Anyone researching “TB-500” dosing, benefits, or safety should recognize that most of the circulating information traces to full Tβ4, not to this smaller fragment, and that the fragment itself has essentially no dedicated human safety or efficacy data.