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For research use only. Not for human or veterinary use. Not a drug, food, or dietary supplement.
AMIRAVOResearch Library

Thymosin alpha-1 vs TB-500

Thymosin alpha-1 and TB-500 share the word 'thymosin' but are different molecules: thymosin alpha-1 is a thymic peptide studied as an immune modulator, while TB-500 is a synthetic acetylated seven-residue fragment of thymosin beta-4 1,7. Thymosin alpha-1 has randomized trials in sepsis and a synthetic form approved in many countries outside the US, whereas FDA found no human exposure data for TB-500 1,3,7. Neither is FDA-approved as of 22 October 2026 5,7.

Unbranded reference vial illustrating Thymosin Alpha-1; not an actual product
Thymosin Alpha-1
Approved outside the US
Unbranded reference vial illustrating TB-500; not an actual product
TB-500
Not approved — research use only

Side by side

FieldThymosin Alpha-1TB-500
ClassThymic peptide immunomodulator 1,2Synthetic N-acetylated fragment (residues 17-23) of thymosin beta-4 7,8
Origin and structureNaturally occurring thymic peptide; the synthetic form is thymalfasin 1Ac-LKKTETQ, identified in a product sold as TB-500 7,8
Proposed mechanismDescribed as a biological response modifier that activates and regulates immune cells 2Not established; contains the actin-binding region of thymosin beta-4 7
Half-lifeNot identified in the sources reviewedNot identified; FDA found no human data 7
Highest human trial phasePhase 3 randomized trial in sepsis (1,106 participants) 3No human studies identified 7
FDA statusNot FDA-approved; no DailyMed label; on FDA's significant-safety-risk compounding page (nomination withdrawn) 5,10Not approved; on FDA's significant-safety-risk compounding page (nomination withdrawn); reviewed by FDA's advisory committee in July 2026 7,10
Published human outcomesIn TESTS, 28-day mortality was 23.4% versus 24.1% with placebo (hazard ratio 0.99) 3None; FDA found no case reports or clinical studies of TB-500 in humans 7
Safety signalsFDA cites immunogenicity risk and inadequate safety information for compounding 10FDA states human safety risks are unknown 7
WADA statusNot named in the 2026 List 11Named in S2.3 as a thymosin-beta-4 derivative; prohibited at all times 11
Evidence levelRandomized human trials plus reviews 2,3,4Analytical chemistry and in vitro data; no human data 7,8

Thymosin Alpha-1: what the data says

Thymosin alpha-1 is a naturally occurring thymic peptide, and its synthetic form, thymalfasin, is described in a 2020 review as approved in more than 35 countries for hepatitis B and C and as an immune enhancer in other diseases 1. A 2015 historical review describes it as a biological response modifier that activates and regulates various immune cells and notes open questions about trial end-points and combinations 2.

In the phase 3 TESTS trial (1,106 adults with sepsis at 22 centres in China), 28-day mortality was 23.4% with thymosin alpha-1 and 24.1% with placebo (hazard ratio 0.99) 3. An earlier 361-patient single-blind trial (ETASS) reported 26.0% versus 35.0% mortality with a marginal P value of 0.062 4. No FDA-approved thymalfasin label was found in DailyMed 5.

TB-500: what the data says

TB-500 is the N-acetylated heptapeptide Ac-LKKTETQ, residues 17-23 of the 43-amino-acid protein thymosin beta-4 7. FDA states that thymosin beta-4 and TB-500 are not the same substance 7. High-resolution mass spectrometry identified this acetylated fragment in a commercial product sold as TB-500 8.

FDA's 2026 briefing found no clinical studies or human exposure data for TB-500 by any route and stated that its human safety risks are unknown 7. In an in vitro fibroblast scratch assay that FDA reviewed, wound closure with TB-500 did not differ significantly from vehicle 7. Studies of full-length thymosin beta-4, such as a 40-volunteer intravenous safety study, concern the parent protein rather than TB-500 9.

Key research differences

Where the evidence is limited

No study comparing thymosin alpha-1 and TB-500 was identified. The largest thymosin alpha-1 trial, in sepsis, did not show a mortality reduction, and earlier positive results were marginal 3,4. For TB-500, FDA identified no human exposure data, no nonclinical toxicity studies and no in vivo studies of the acetylated fragment, so claims based on full-length thymosin beta-4 do not apply to it 7.

Frequently asked questions

Are thymosin alpha-1 and TB-500 the same kind of peptide?
No. Thymosin alpha-1 is a thymic immunomodulatory peptide, while TB-500 is a synthetic acetylated fragment of thymosin beta-4 1,7.
Is TB-500 the same as thymosin beta-4?
FDA states that thymosin beta-4 and TB-500 are not the same substance; TB-500 is the acetylated 17-23 fragment 7.
Did thymosin alpha-1 reduce sepsis mortality?
In the 1,106-participant TESTS trial, 28-day mortality was 23.4% versus 24.1% with placebo (hazard ratio 0.99) 3.
Is either FDA-approved?
No. No DailyMed label for thymalfasin was found, and TB-500 is not approved; both appear on FDA's significant-safety-risk compounding page as withdrawn nominations 5,7,10.
Are they prohibited in sport?
TB-500 is named in S2.3 of WADA's 2026 List; thymosin alpha-1 is not named 11.

References

  1. Thymosin alpha 1: A comprehensive review of the literature World Journal of Virology. 2020
  2. Historical review of thymosin alpha 1 in infectious diseases Expert Opinion on Biological Therapy. 2015
  3. The efficacy and safety of thymosin alpha1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial BMJ. 2025
  4. The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind, randomized and controlled trial Critical Care. 2013
  5. DailyMed drug label search for thymalfasin (zero labels returned on 22 October 2026) DailyMed (NLM). 2026
  6. beta-Thymosins Annals of the New York Academy of Sciences. 2007
  7. FDA Briefing Document: TB-500-Related Bulk Drug Substances, Pharmacy Compounding Advisory Committee, July 23-24, 2026 FDA. 2026
  8. Synthesis 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 Testing and Analysis. 2012
  9. A randomized, placebo-controlled, single and multiple dose study of intravenous thymosin beta4 in healthy volunteers Annals of the New York Academy of Sciences. 2010
  10. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks (content current as of 04/22/2026) FDA (Human Drug Compounding). 2026
  11. International Standard: Prohibited List 2026 WADA. 2026

For research use only. Not for human or veterinary use. Not a drug, food, or dietary supplement. This page compares published research and is not medical advice or a recommendation.