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Peptide library · Synthetic peptide

TB-500

A synthetic seven-amino-acid fragment, Ac-LKKTETQ, associated with the actin-binding region of thymosin beta-4. Most repair research used the full 43-amino-acid protein, not TB-500 itself.

Last reviewed October 8, 2026

The short version

TB-500 is the synthetic, N-terminally acetylated sequence Ac-LKKTETQ. It is much shorter than thymosin beta-4, the natural 43-amino-acid protein from which that motif comes. This distinction controls how the evidence reads: most animal repair experiments and the small human safety study tested full-length thymosin beta-4, not TB-500.

TB-500 is not FDA-approved. FDA’s current table lists its compounding nomination as withdrawn and retains a safety note. A licensed provider’s clinical decision does not change those FDA facts.

What it is

TB-500 is a synthetic heptapeptide, meaning seven amino acids, with an acetyl group attached at the beginning. Its sequence corresponds to residues 17 through 23 of thymosin beta-4 and includes the parent protein’s LKKTET actin-binding motif. Full-length thymosin beta-4 occurs widely in human cells and contains 43 amino acids. TB-500 itself is a synthetic fragment, not the natural full protein.

The sequence is Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH, commonly shortened to Ac-LKKTETQ. Its molecular formula is C38H68N10O14 and reported molecular weight is 889.02 daltons. Anti-doping analytical literature explicitly defines TB-500 as the short fragment.1

Where the research stands

Direct TB-500 evidence is largely chemical identification and anti-doping detection. The widely cited tissue-repair studies used full-length thymosin beta-4. They can explain why the short fragment attracted interest, but they do not show that TB-500 reproduces the parent protein’s effects.

How far the evidence has climbed

  1. Cell studiesLimitedMostly full-protein or structural work
  2. Animal studiesLimitedRepair studies largely use full protein
  3. Small human pilotsNoneFull protein studied, not TB-500 fragment
  4. Controlled human trialsNoneNo controlled fragment trial
  5. FDA approvalNoNot approved for any use

Here is what selected studies looked at. Every card says whether it tested TB-500 or full-length thymosin beta-4.

  • Structural2004

    Partial-protein hybrid: actin binding

    X-ray crystallography examined a hybrid containing the C-terminal half of thymosin beta-4 fused to gelsolin G1 and bound to actin. The molecule studied was not full-length thymosin beta-4 or TB-500.2

  • Rats1999

    Full protein: skin wounds

    Rats with full-thickness skin wounds received full-length thymosin beta-4. Researchers measured re-epithelialization, contraction, collagen and blood-vessel growth; the fragment was not tested.3

  • Mice2004

    Full protein: heart model

    In a mouse coronary-ligation model, researchers measured ILK/Akt signaling, myocyte survival and cardiac function after full-length thymosin beta-4. Cell migration was measured separately in embryonic-heart tissue and cultured cardiomyocytes. The study cannot establish a human or TB-500 outcome.4

  • Humans2010

    Full protein: phase 1 safety

    A placebo-controlled phase 1 study gave full-length synthetic thymosin beta-4 to 40 healthy volunteers and measured short-term tolerability and pharmacokinetics. It did not test the TB-500 fragment.5

  • Horses2012

    TB-500: anti-doping detection

    An equine analytical study defined TB-500 as the acetylated LKKTETQ fragment and developed a mass-spectrometry method to detect it and its metabolites. It did not test clinical benefit.1

  • Mice2010

    Full protein: a null functional result

    In dystrophin-deficient mice, the study measured regenerating muscle fibers, muscle strength, heart function and fibrosis after full-length thymosin beta-4. It did not test the TB-500 fragment or people.6

What the research can’t tell you

  • Fragment and parent are not interchangeable

    TB-500 contains only seven residues from a 43-amino-acid protein. A full-protein result cannot establish that the fragment behaves the same way.1

  • No controlled human fragment trial

    The human phase 1 study used full-length thymosin beta-4. It cannot establish TB-500 safety or effectiveness.5

  • More repair markers may not mean function

    The mouse muscular-dystrophy study found more regenerating fibers without better strength or heart function.6

  • A theoretical tumor question

    Full-length thymosin beta-4 influences cell migration and angiogenesis in models. Concern about tumor biology is mechanism-based, not a finding in people using TB-500.

  • Research-product identity can vary

    Analytical identification is central to the TB-500 literature because the short fragment can be confused with the parent protein or modified products.1

  • Banned in sport

    WADA names TB-500 among thymosin beta-4 derivatives prohibited at all times.7

FDA status

Not FDA-approved for any use

FDA’s page, content current as of April 22, 2026, lists the nomination for the thymosin beta-4 fragment LKKTETQ, identified as TB-500, as withdrawn while retaining a safety note. Withdrawal does not change the other requirements that apply to compounding.8

TB-500 is not on FDA’s finalized list of additional bulk substances for patient-specific compounding.9 Before a July 2026 advisory meeting, FDA staff proposed against listing it; FDA had posted no official vote record as of September 17, 2026.10

Compounded drugs are not FDA-approved.11 Promise prescribes TB-500 now through licensed U.S. providers. A provider reviews every request and decides; not everyone qualifies.

A prescription route

Promise prescribes TB-500 now. Here is the route to a licensed U.S. provider’s review; not everyone qualifies:

  1. Start on Promise’s TB-500 page

    The page has a visit-start link and a price. A request does not guarantee a prescription.

  2. Share your health information

    A licensed U.S. provider reviews every request, including health history, medicines and goals, and can decline. Not everyone qualifies.

  3. Pharmacy review is a separate step

    A prescription does not guarantee preparation or dispensing. A licensed U.S. compounding pharmacy must review whether it can fulfill the prescription under the applicable requirements. Compounded medications are not FDA-approved.

A prescription does not make TB-500 FDA-approved or convert full-protein thymosin beta-4 findings into evidence for the fragment; it reflects an individualized clinical decision.

TB-500 at Promise

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Questions

What is the relationship between TB-500 and thymosin beta-4?

TB-500 is the synthetic acetylated sequence Ac-LKKTETQ, drawn from the actin-binding region of the full 43-amino-acid protein thymosin beta-4.1

Are there human clinical trials on TB-500?

The cited human phase 1 study used full-length thymosin beta-4, not TB-500. No controlled human fragment trial is identified here.5

Is TB-500 a steroid?

No. It is a synthetic seven-amino-acid peptide fragment. Its chemical structure and anti-doping category are different from anabolic steroids.

Sources

  1. Ho EN, Kwok WH, Lau MY, Wong AS, Wan TS, Lam KK, Schiff PJ, Stewart BD. Doping control analysis of TB-500, a synthetic version of an active region of thymosin beta-4, in equine urine and plasma. J Chromatogr A. 2012;1265:57-69. PubMed 23084823
  2. Irobi E, et al. Structural basis of actin sequestration by thymosin-beta4: implications for WH2 proteins. EMBO J. 2004. PubMed 15329672
  3. Malinda KM, et al. Thymosin beta4 accelerates wound healing. J Invest Dermatol. 1999. PubMed 10469335
  4. Bock-Marquette I, et al. Thymosin beta4 activates integrin-linked kinase and promotes cardiac cell migration, survival and cardiac repair. Nature. 2004. PubMed 15565145
  5. Ruff D, et al. A randomized, placebo-controlled, single and multiple dose study of intravenous thymosin beta4 in healthy volunteers. Ann N Y Acad Sci. 2010. PubMed 20536472
  6. Spurney CF, et al. Evaluation of skeletal and cardiac muscle function after chronic administration of thymosin beta-4 in the dystrophin deficient mouse. PLoS One. 2010. PubMed 20126456
  7. World Anti-Doping Agency. The 2026 Prohibited List (S2.3).
  8. U.S. Food and Drug Administration. Certain bulk drug substances for use in compounding that may present significant safety risks. Content current as of April 22, 2026.
  9. U.S. Food and Drug Administration. Bulk drug substances used in compounding under section 503A of the FD&C Act.
  10. U.S. Food and Drug Administration. July 23-24, 2026: Meeting of the Pharmacy Compounding Advisory Committee.
  11. U.S. Food and Drug Administration. Compounding and the FDA: Questions and Answers.