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Peptide Spencer

Synthetic peptide. It is an acetylated 7-residue fragment of thymosin beta-4 (actin-binding region). Not an FDA-approved drug.

TB-500

Also known as TB500, Ac-LKKTETQ, N-terminal acetylated thymosin beta-4 (17-23) fragment, thymosin beta-4 fragment, TB-500 acetate, TB-500 (free base)

Preclinical onlyLowest tier. No efficacy data exist for the fragment itself, in animals or humans. The tier describes the evidence for TB-500, not for full-length thymosin beta-4.

Not FDA-approved. Not on FDA's 503A category lists (May 14, 2026). Source, May 14, 2026

Identity. A doping-control group identified the N-terminal acetylated 17-23 fragment of human thymosin beta-4 (Ac-LKKTETQ) in a product sold as TB-500 (PMID 22962027). FDA's compounding page lists the substance as 'Thymosin beta-4, fragment (LKKTETQ), also known as TB-500'. Some vials are sold as full-length thymosin beta-4 (43 amino acids) instead. Every human trial identified used the full-length protein (RGN-259 eye drops, IV synthetic Tbeta4, topical dermal Tbeta4), not the 7-residue fragment. Those doses do not transfer to a TB-500 vial. Mass-based doses are not interchangeable between the fragment (about 0.9 kDa) and full-length Tbeta4 (about 4.96 kDa). These molecular weights are clinician-review figures, not quoted from a cited source.

At a glance

These dosages are a helpful guide, not a prescription. Every clinician is expected to do their own due diligence before prescribing.

No established human dose

No human or animal efficacy study of TB-500 (Ac-LKKTETQ) itself was identified. Trade press reports that FDA acknowledged 'there are no studies on TB500 itself'. Published work on the fragment is analytical, rat metabolism and in-vitro. Human trials exist only for full-length thymosin beta-4: an IV phase 1 study in healthy volunteers, ophthalmic RGN-259 dry-eye phase 2 trials (mixed efficacy, no safety issues reported), and topical dermal phase 2 trials in stasis and pressure ulcers. These data do not establish TB-500 dosing, safety or efficacy. Related full-length work not used for dosing includes a venous-ulcer design report and a recombinant human Tbeta4 STEMI study.

See the human studies

No allowed source gives TB-500 injectable dosing; human trials used full-length thymosin beta-4 by other routes and do not transfer.

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Dosing detail

Human studies

Human study

  • FULL-LENGTH synthetic thymosin beta-4 (NOT the TB-500 fragment). Healthy volunteers, phase 1 single-ascending then 14-day multiple-dose safety study (cardiac-ischemia development program).

    42, 140, 420, or 1260 mg1

    Route
    intravenous
    Frequency
    Single dose, then the same dose daily after safety review
    Duration
    14 days (multiple-dose phase)
    n
    40 (4 cohorts of 10 healthy subjects each, placebo or Tbeta4)
    Design
    Randomized, double-blind, placebo-controlled, phase 1; four ascending-dose cohorts
    Source quote
    Four cohorts, with 10 healthy subjects each, were given a single intravenous dose of placebo or synthetic Tbeta4. Cohorts received ascending doses of either 42, 140, 420, or 1260 mg. Following safety review, subjects were given the same dose regimen daily for 14 days.

    Finding. Adverse events were infrequent, mild or moderate. There were no dose-limiting toxicities or serious adverse events, and pharmacokinetics were dose-proportional. This was a safety study with no efficacy outcome. The unit is as printed in the abstract (full text not opened).

  • FULL-LENGTH thymosin beta-4 ophthalmic solution RGN-259 (NOT the TB-500 fragment). Moderate to severe dry eye, controlled adverse environment model.

    0.1 % (w/w) ophthalmic solution, 1-2 drops per eye2

    Route
    ophthalmic (topical, both eyes)
    Frequency
    twice a day
    Duration
    28 days
    n
    72 (36 per arm)
    Design
    Single-center, randomized, double-masked, placebo-controlled phase 2
    Source quote
    Subjects received their first dose of randomized test agent in-office at visit 2, and were then instructed to instill 1–2 drops of study drug (0.1% Tβ4 or placebo) into each eye twice a day for a total of 28 days.

    Finding. Neither co-primary endpoint significant; several secondary corneal staining and discomfort endpoints favored 0.1% Tbeta4. No safety issues reported.

  • FULL-LENGTH thymosin beta-4 ophthalmic solution RGN-259 (NOT the TB-500 fragment). Severe dry eye including graft-vs-host disease.

    0.1 % (RGN-259 eye drops)3

    Route
    ophthalmic (topical)
    Frequency
    6 times daily
    Duration
    28 days of treatment (56-day trial including 28-day follow-up)
    n
    9 patients (12 treated eyes, 6 vehicle eyes at day 56)
    Design
    Small multicenter randomized, double-masked, placebo-controlled phase 2
    Source quote
    Nine patients with severe dry eye were treated with either RGN-259 (0.1%) or vehicle control 6 times daily over a period of 28 days.

    Finding. At day 56, 35.1% reduction in ocular discomfort and 59.1% reduction in total corneal fluorescein staining versus vehicle; drops safe and well tolerated in this small trial.

What clinics and vendors publish

These ranges come from clinic and vendor publications, not studies.

Community-reported

No peer-reviewed review, compounding-pharmacy protocol, professional education or textbook source with TB-500 clinical-practice dosing was identified. Every practice number above comes from commercial pages: an affiliate-funded protocol site and a clinic-locator marketing page. Both state the schedules are not validated in a human trial of the fragment, and they disagree with each other. The clinic-locator page says its figures come mainly from analytical detection studies, not therapeutic trials. Doses are quoted by mass for a molecule that is not confirmed. The fragment (about 0.9 kDa) and full-length Tbeta4 (about 4.96 kDa) are not interchangeable mg for mg. Confirm the molecule from the pharmacy certificate of analysis before using any figure, and gate any dosing display on that confirmation. Treat these figures as unverified market convention, not a dosing recommendation.

StartCommunity-reported
About 2 to 2.5 mg (one community-referenced protocol); another page cites 2-3 mg in weeks 1-2. Neither is a clinician-authored or trial-derived dose. Both are unvalidated vendor-protocol figures.
TitrationCommunity-reported
Community-referenced: about 2-2.5 mg twice weekly in a loading phase (weeks 1-6), then about 2-2.5 mg once weekly for maintenance. These are unvalidated vendor-protocol figures.
CeilingCommunity-reported
No established maximum dose exists in any source found.
FrequencyCommunity-reported
Twice weekly loading, once weekly maintenance (protocol site); 2-3 times per week (clinic-locator page).
RouteCommunity-reported
subcutaneous injection
CycleCommunity-reported
4-12 weeks common, with 4-8 weeks off (protocol site); clinic-locator page lists 2-8 week cycles by goal and states long-term safety beyond 8-12 weeks does not exist.
  • Community-reportedVendor page4
    about 2 to 2.5 mg twice weekly during an initial loading phase.
  • Community-reportedVendor page4
    followed by about 2 to 2.5 mg once weekly during maintenance.
  • Community-reportedVendor page4
    Most research-use protocols use subcutaneous injection.
  • Community-reportedVendor page4
    Common research-use cycles are 4-12 weeks, with longer maintenance phases discussed less consistently.
  • Community-reportedOther web source5
    starting with lower doses of 2-3 mg to evaluate initial response
  • Community-reportedOther web source5
    No established maximum dose limits exist due to the research-only status.

Safety

Serious warnings

  • No FDA label exists, so there is no boxed warning or labeled warning. FDA states it 'has not identified any human exposure data drug products containing Thymosin Beta-4, Fragment' and 'lacks important information regarding any safety issues raised by this drug.'6
  • Trade-press coverage of the July 2026 FDA advisory committee meeting reports that FDA and several panelists focused on a potential cancer signal, although FDA acknowledged there are no studies on TB500 itself. Dissenting members cited thin efficacy data and 'concerns about overlap with tumor progression.' Not confirmed against the FDA briefing document.7
  • Active or past malignancy: avoid. Safety is not established. Preclinical work on full-length thymosin beta-4 found that expression increased with glioma grade, and that gene silencing inhibited migration and invasion and enhanced survival of glioma-bearing mice. No oncologic safety data exist for TB-500. The signal is not settled. This is a theoretical concern, not a demonstrated human harm.8
  • Long-term safety is unknown. A clinic-locator page states 'Long-term safety data beyond 8-12 weeks of use do not exist' (commercial page, not a trial).5

Contraindications

  • No contraindications are established. Absence of data does not mean absence of risk.
  • Active or past malignancy: avoid until oncologic safety data exist (see warnings).8
  • Pregnancy and lactation: no data; not established.

Adverse effects

  • Full-length thymosin beta-4, IV 42-1260 mg single and 14-day daily dosing in healthy volunteers: 'Adverse events were infrequent, and mild or moderate in intensity. There were no dose limiting toxicities or serious adverse events.' Not data on the TB-500 fragment.1
  • Full-length thymosin beta-4 0.1% eye drops, 28 days: 'No safety issues arose in this adverse environment challenge study with the use of 0.1% Tβ4 or its vehicle over the duration of treatment.' Topical ophthalmic route only; not data on the TB-500 fragment.2
  • Per the PubMed abstract, no cytotoxicity of TB-500 or its metabolites was found in fibroblasts in vitro. This is not human safety data.9

Monitoring

  • No validated monitoring protocol exists for TB-500.
  • A commercial page states 'any dose modifications should occur under medical supervision with appropriate monitoring for adverse effects'.5
  • Clinician judgment (no source): confirm age-appropriate cancer screening is current and review injection sites. This is not a validated schedule.

Evidence

No human or animal efficacy study of TB-500 (Ac-LKKTETQ) itself was identified. Trade press reports that FDA acknowledged 'there are no studies on TB500 itself'. Published work on the fragment is analytical, rat metabolism and in-vitro. Human trials exist only for full-length thymosin beta-4: an IV phase 1 study in healthy volunteers, ophthalmic RGN-259 dry-eye phase 2 trials (mixed efficacy, no safety issues reported), and topical dermal phase 2 trials in stasis and pressure ulcers. These data do not establish TB-500 dosing, safety or efficacy. Related full-length work not used for dosing includes a venous-ulcer design report and a recombinant human Tbeta4 STEMI study.

Mechanism

Ac-LKKTETQ comes from the actin-binding active site of thymosin beta-4. The unacetylated LKKTETQ sequence is reported to have actin-binding, dermal wound-healing, angiogenesis and skin-repair functions. The biological effects of TB-500 itself have not been documented. A 2024 rat and in-vitro study suggests the activity may belong to a metabolite (Ac-LKKTE).9

Availability

Amounts offered

  • 10 mg

Custom strengths on request.

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References

According to PubMed

  1. 1A randomized, placebo-controlled, single and multiple dose study of intravenous thymosin beta4 in healthy volunteers.Ann N Y Acad Sci 2010;1194:223-9, 2010
  2. 2Thymosin beta 4 ophthalmic solution for dry eye: a randomized, placebo-controlled, Phase II clinical trial conducted using the controlled adverse environment (CAE) model.Clin Ophthalmol 2015;9:877-84 (PMC4445951), 2015
  3. 3Thymosin β4 significantly improves signs and symptoms of severe dry eye in a phase 2 randomized trial.Cornea 2015;34(5):491-6, 2015
  4. 4TB-500 Dosing Protocol (peptidedosingprotocols.com; affiliate-supported; last reviewed Oct 2026)peptidedosingprotocols.com, 2026
  5. 5TB-500 Fragment (17-23) Dosing: What Clinics Prescribe and What to Expect (2026) (PeptideClinicLocator editorial team, 17 Feb 2026)MyPeptideMatch, 2026
  6. 6Certain bulk drug substances for use in compounding that may present significant safety risks (FDA; page content current as of 04/22/2026)U.S. FDA, 2026
  7. 7PEPTIDE-L WAVE! PCAC Approves Four Bulk Drug Substances for the 503A List (FDA Law Blog, 24 Jul 2026; trade press, not FDA)FDA Law Blog (Hyman, Phelps & McNamara), 2026
  8. 8Thymosin β 4 gene silencing decreases stemness and invasiveness in glioblastoma.Brain 2014;137(Pt 2):433-48, 2013
  9. 9Simultaneous quantification of TB-500 and its metabolites in in-vitro experiments and rats by UHPLC-Q-Exactive orbitrap MS/MS and their screening by wound healing activities in-vitro.J Chromatogr B 2024;1235:124033, 2024
  10. 10Synthesis and characterization of the N-terminal acetylated 17-23 fragment of thymosin beta 4 identified in TB-500, a product suspected to possess doping potential.Drug Test Anal 2012;4(9):733-8, 2012
  11. 11Bulk drug substances used in compounding under section 503A of the FDC Act (FDA; page content current as of 05/14/2026)U.S. FDA, 2026
  12. 12Thymosin beta-4 and venous ulcers: clinical remarks on a European prospective, randomized study on safety, tolerability, and enhancement on healing.Ann N Y Acad Sci 2007;1112:407-12, 2007
  13. 13Recombinant human thymosin beta 4 improves ischemic cardiac dysfunction in mice and patients with acute ST-segment elevation myocardial infarction after reperfusion.Cardiovasc Res 2025;121(17):2747-2758, 2025
  14. 14The regenerative peptide thymosin β4 accelerates the rate of dermal healing in preclinical animal models and in patients.Ann N Y Acad Sci 2012;1270:37-44, 2012
  15. 152018 Prohibited List: summary of major changes (USADA athlete advisory; archived, historical)USADA, 2018

Last reviewed 2026-10-14. 28 figures checked against their sources. How this works