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Peptide Insider

Actin-sequestering peptide and derived fragment

TB-500 / Thymosin Beta-4

Also known as Thymosin β4 · Tβ4 · TMSB4X (gene) · RGN-259 (ophthalmic formulation)

InvestigationalEARLY CLINICALFDA approved: No

This profile covers two related but distinct things, and keeping them apart is the single most useful thing a reader can take from it. Thymosin β4 is a naturally occurring peptide of 43 amino acids found in most human cells, where it binds monomeric actin and participates in cell migration and wound repair. It has been developed as a pharmaceutical — most substantially as a topical ophthalmic solution, RGN-259, which has completed randomised placebo-controlled trials in dry eye disease and neurotrophic keratopathy. "TB-500" is something else: a shorter synthetic fragment, typically described as containing the actin-binding motif LKKTETQ, sold through research-chemical channels and injected systemically by people who are not in a trial. The published human evidence belongs almost entirely to the first of those, in a formulation applied to the eye.

Written by Peptide Insider Editorial TeamPublished Last reviewed

The Insider Summary

What it is
Thymosin β4 is a naturally occurring 43-amino-acid peptide involved in actin regulation and cell migration. TB-500 is a shorter synthetic fragment marketed as a research chemical; it is a different molecule.
Why researchers are interested
Thymosin β4 sits at the centre of cell migration and wound repair. Animal models report accelerated repair in cornea, skin, heart and other tissues, and it is one of very few compounds in this space to have reached randomised placebo-controlled human trials.
Evidence
Early clinical — but narrowly. The human evidence is for thymosin β4 as a topical eye drop in ocular surface disease, with mixed results against primary endpoints. There is no comparable human evidence for systemic administration, and effectively none for the TB-500 fragment specifically.
Regulatory status
Not approved by the FDA for any indication. Thymosin β4 remains in active clinical development for ophthalmic use. FDA lists thymosin beta-4 fragment (TB-500) among substances whose 503A compounding nominations were withdrawn.
Bottom line
Thymosin β4 has a real clinical file, and it is an ophthalmology file. Extending that evidence to injected TB-500 for tendon or muscle repair means changing the molecule, the route, the dose and the indication simultaneously — which is not a small extrapolation, it is an unstudied one.

Quick reference

Compound
Peptide fragment
Research areas
Tissue & Recovery ResearchImmune Research
Evidence level
EARLY CLINICAL
Research status
Investigational
FDA approved
No
Human clinical evidence
Moderate
Sequence
Thymosin β4: 43 amino acids (Ac-SDKPDMAEIEKFDKSKLKKTETQEKNPLPSKETIEQEKQAGES). TB-500 is commonly described as a shorter fragment containing the actin-binding motif LKKTETQ.
Last reviewed
17 September 2026

What is TB-500, and how does it differ from thymosin β4?

Thymosin β4 is a real, naturally occurring human peptide, encoded by the TMSB4X gene and present in most cell types. It is 43 amino acids long.

TB-500 is a name used in the research-chemical market. Products sold under it are generally described as a synthetic fragment containing the actin-binding sequence LKKTETQ, not the full 43-residue peptide. Sellers frequently present TB-500 and thymosin β4 as the same thing, and cite the thymosin β4 clinical literature in support of the fragment.

They are not the same thing. A fragment and its parent peptide can differ in stability, distribution, receptor interactions and duration of action. Where this page describes human clinical evidence, that evidence concerns thymosin β4 — and, in every published randomised trial we have identified, thymosin β4 applied topically to the eye.

What is thymosin β4 being studied for?

Ocular surface disease. This is where the compound has been taken furthest. RGN-259, a preservative-free thymosin β4 ophthalmic solution, has been evaluated in randomised, placebo-controlled trials in dry eye disease and in neurotrophic keratopathy. A Phase II dry eye study using a controlled adverse environment model did not meet its co-primary endpoints, though several secondary measures favoured treatment [1]. Later work extended into Phase III in neurotrophic keratopathy [2].

Dermal and cardiac repair. Animal models report accelerated wound closure and, in cardiac injury models, effects on cell survival and vascularisation. Preclinical.

Tendon, ligament and muscle repair. This is the use most discussed in consumer settings and the one with the least supporting evidence: we have not identified published, controlled human trials of thymosin β4 or TB-500 for musculoskeletal repair.

How does TB-500 / Thymosin Beta-4 work?

Thymosin β4 is the principal actin-sequestering molecule in mammalian cells: it binds monomeric G-actin and regulates the pool available for polymerisation, which in turn governs cell motility. Because directed cell migration is central to wound closure, this gives a coherent mechanistic basis for the repair effects reported in injury models. Additional reported activities include promotion of endothelial cell migration and modulation of inflammatory signalling. The LKKTETQ motif present in TB-500 is the region implicated in actin binding, which is the rationale for using the fragment — but a rationale for why a fragment might work is not evidence that it does, and the pharmacokinetics of the fragment have not been characterised in people.

Research and clinical evidence

The table below grades the published record separately for each research area, because a compound can be well studied in one context and entirely unstudied in another. Grades follow the Peptide Insider evidence taxonomy.

Evidence level by research area
Research areaEvidence levelWhat the published record shows
Ocular surface disease (topical thymosin β4)EARLY CLINICALRandomised, placebo-controlled human trials completed, including Phase III work in neurotrophic keratopathy. Results across the programme have been mixed, with some studies missing primary endpoints while showing secondary-endpoint improvements.
Dermal and cardiac tissue repairPRECLINICALAnimal and cell-model evidence for accelerated repair and cell migration. No controlled human trials identified.
Tendon, ligament and muscle repair (systemic TB-500)INSUFFICIENTThe most commonly claimed use has the least evidence behind it. We have identified no published controlled human trials of systemically administered thymosin β4 or the TB-500 fragment for musculoskeletal injury.

Human clinical studies

Studies conducted in people. These carry the most weight, and their absence is itself a finding.

Thymosin β4 ophthalmic solution for dry eye: randomised, placebo-controlled Phase II trial using the controlled adverse environment model[1]

2015
Design
Single-centre, prospective, double-masked, placebo-controlled; 28 days of treatment
Population
Adults with moderate to severe dry eye
Subjects
72 randomised (36 active, 36 placebo)

FindingsThe co-primary endpoints — ocular discomfort and inferior corneal staining — did not differ significantly between groups. Several secondary measures favoured thymosin β4, including discomfort in the controlled adverse environment and central and superior corneal staining. No adverse events were observed.

LimitationsSingle centre, small sample, short duration, and a negative result on the co-primary endpoints. Secondary endpoints in a trial that missed its primaries are hypothesis-generating, not confirmatory.

0.1% RGN-259 (thymosin β4) ophthalmic solution in neurotrophic keratopathy — randomised, placebo-controlled, double-masked Phase III trial[2]

2022
Design
Randomised, placebo-controlled, double-masked Phase III
Population
Patients with neurotrophic keratopathy

FindingsReported improvements in corneal healing and patient comfort versus placebo in this rare ocular surface condition.

LimitationsA rare-disease ophthalmic indication with a topical formulation. Results do not transfer to systemic administration or to musculoskeletal repair. The full publication record for the wider Phase III programme should be read as a whole rather than trial by trial.

FDA and regulatory status

Neither thymosin β4 nor TB-500 is approved by the FDA for any indication. Thymosin β4 remains an investigational drug in ophthalmology, which means it is being studied under regulatory oversight — not that it is available or authorised for use.

FDA's list of bulk drug substances nominated for compounding under section 503A includes "thymosin beta-4 fragment (TB-500)" among substances whose nominations were withdrawn [3].

Material sold online as TB-500 is not a pharmaceutical product, is not the formulation used in the clinical trials described above, and is not manufactured under the controls that apply to investigational drug supply.

Safety and known risks

Risks are separated into what is documented, what is plausible but unestablished, and what has not been studied. The third column is usually the longest one for an unapproved compound, and it is not a reassurance.

Known risks

Documented in regulatory labelling or published trial safety data.

  • In the published topical ophthalmic trials, thymosin β4 was generally well tolerated, with no adverse events reported in the Phase II dry eye study. That safety finding applies to an eye drop, not to injection.

Potential risks

Plausible on mechanism or drug-class grounds, but not established for this compound.

  • Thymosin β4 promotes cell migration and angiogenesis. Both are processes relevant to tumour behaviour, and the implications of sustained systemic exposure have not been established.
  • Research-chemical material has no guaranteed identity, purity or sterility, and is frequently sold without meaningful analytical documentation.

Unknown or insufficiently studied

Questions the published record does not currently answer.

  • No published human pharmacokinetic data for systemically administered thymosin β4 or for the TB-500 fragment.
  • No published long-term systemic safety data.
  • Whether the TB-500 fragment reproduces any of the parent peptide’s activity in humans is unstudied.

Frequently asked questions

Are TB-500 and thymosin β4 the same thing?
No. Thymosin β4 is a naturally occurring 43-amino-acid peptide. TB-500 is a shorter synthetic fragment typically described as containing the actin-binding motif LKKTETQ. Clinical trial evidence for thymosin β4 does not automatically apply to the fragment.
Has thymosin β4 been tested in humans?
Yes — as a topical eye drop. Randomised, placebo-controlled trials have been conducted in dry eye disease and neurotrophic keratopathy, including Phase III work. Results have been mixed: a Phase II dry eye trial missed its co-primary endpoints while showing improvements on several secondary measures.[1][2]
Is there human evidence for TB-500 and tendon or muscle healing?
We have not identified published, controlled human trials for that use. The musculoskeletal evidence base is preclinical, and the most widely claimed benefit is the one with the least support.
Is TB-500 FDA approved?
No. It is not approved for any indication, and FDA lists thymosin beta-4 fragment (TB-500) among substances whose 503A compounding nominations were withdrawn.[3]

Continue reading

Comparison

BPC-157 vs TB-500

Two research compounds discussed interchangeably for tissue repair. Their mechanisms, their evidence bases and their regulatory positions are not the same.

Peptide Science

What Are Peptides?

Peptides are short chains of amino acids. What separates a peptide from a protein, why the body uses them as signals, and why that makes them both useful drugs and difficult ones.

Regulation

What Does “Research Use Only” Mean?

The phrase appears on nearly every research peptide sold online. It is a statement by the seller about intended use — not a regulatory designation, and not a safety assessment.

Regulation

Are Peptides FDA Approved?

Some peptides are FDA approved drugs. Most compounds sold as "peptides" are not. Here is how to tell the difference, and what approval does and does not mean.

References

Peptide Insider cites primary sources wherever they exist — regulatory documents, trial registrations and peer-reviewed literature — in preference to secondary summaries.

  1. 1.
    Sosne G, Ousler GW. Thymosin beta 4 ophthalmic solution for dry eye: a randomized, placebo-controlled, Phase II clinical trial conducted using the controlled adverse environment (CAE) model. Clinical Ophthalmology, 2015;9:877–884.
    Peer-reviewedDOI: 10.2147/OPTH.S80954PMID: 26056426
  2. 2.
    Sosne G, Kleinman HK, et al.. 0.1% RGN-259 (thymosin β4) ophthalmic solution promotes healing and improves comfort in neurotrophic keratopathy patients in a randomized, placebo-controlled, double-masked Phase III clinical trial. PubMed Central, 2022;PMCID PMC9820614.
    Peer-reviewed

    Journal of record to be confirmed at next editorial review; linked here via the open-access PMC record.

  3. 3.
    U.S. Food and Drug Administration. Certain Bulk Drug Substances for Use in Compounding That May Present Significant Safety Risks. FDA, 2026;Page last updated 22 April 2026.
    Regulatory