Thymosin alpha-1 is a 28-residue peptide first isolated from calf thymus in 1977 and later made synthetically. In the body it is cut from a larger protein, prothymosin alpha.2,3 Studies focus on its effects on the immune system.
As thymalfasin (Zadaxin) it is licensed in China and a number of other countries, mainly for chronic hepatitis B. It is not approved in the UK, the EU or the US.12 It is a different molecule from thymosin beta-4, the parent of TB-500.
28
3108.3 g/mol
China and others
Not named
Identity
| Value | |
|---|---|
| Sequence | Ac-SDAAVDTSSEITTKDLKEKKEVVEEAEN |
| Origin | Thymosin fraction 5 from calf thymus; cut from prothymosin alpha in the body |
| Formula | C129H215N33O55 |
| Avg mass | 3108.3 g/mol |
| CAS | 62304-98-7 |
| Brand | Zadaxin (SciClone) |
| Not to be confused with | Thymosin beta-4 and TB-500, a different peptide family |
History
Goldstein and colleagues describe "thymosin", a thymus factor that affects lymphocytes.1
Thymosin alpha-1 isolated and sequenced: 28 residues, highly acidic.2
First solid-phase synthesis.4
Prothymosin alpha identified as its natural precursor.3
Approved in China for chronic hepatitis B as Zadaxin.12
EU orphan designation for liver cancer (a designation, not a licence).13
A 1,106-patient sepsis trial finds no effect on 28-day mortality.9
What the studies measured
| Study | People or model | What was measured | Finding | Source |
|---|---|---|---|---|
| Romani 2004 | Mouse dendritic cells; transplanted mice | Immune signalling and resistance to Aspergillus | Dendritic cells activated through toll-like receptors; mice protected from aspergillosis | [5] |
| Chien 1998 | 98 people with chronic hepatitis B, Taiwan, randomised | Viral clearance at 18 months | 40.6% responded in the 26-week arm against 9.4% untreated | [6] |
| Mutchnick 1999 | 97 people with hepatitis B, US phase 3, placebo-controlled | Complete response | 14% against 4%; not significant | [7] |
| ETASS 2013 | 361 people with severe sepsis, China | 28-day mortality | 26.0% against 35.0%; borderline significance | [8] |
| TESTS 2025 | 1,106 adults with sepsis, 22 centres, double-blind | 28-day mortality | 23.4% against 24.1%; no difference | [9] |
| COVID-19 meta-analyses | Mostly observational studies | Mortality | Conflicting: one found no effect, another lower mortality | [10],[11] |
Regulatory and sport status
As of this revision. General information, not legal advice.
| Where | Status |
|---|---|
| China and others | Licensed as Zadaxin, mainly for hepatitis B; SciClone listed 13 countries by 1999, including Italy as a vaccine adjuvant.12 |
| UK | No licence found. |
| EU | No central authorisation. Orphan designation for liver cancer since 2002.13 |
| US | Not approved. |
| Sport | Not named on the 2026 WADA list. Class S2.3 names thymosin beta-4, a different peptide.15 |
Open questions
- Why the positive hepatitis B results from Asia were not confirmed in the US phase 3 trial.6,7
- Whether any group of sepsis patients benefits, after the large TESTS trial found no overall effect.9
- Exactly which receptors and cell types it acts on, and how that relates to its origin as a fragment of prothymosin alpha.14
Common questions
Is thymosin alpha-1 the same as TB-500?
No. TB-500 is a fragment of thymosin beta-4, an unrelated protein. Thymosin alpha-1 is a separate 28-residue peptide from the thymus.
Is thymosin alpha-1 an approved drug?
Yes, in some countries. As thymalfasin (Zadaxin) it is licensed in China and several others, mainly for hepatitis B. It is not approved in the UK, the EU or the US.
Did it work in sepsis?
A smaller 2013 trial suggested lower mortality, but the larger 2025 TESTS trial of 1,106 patients found no difference.
Is it banned in sport?
It is not named on the 2026 WADA list. The thymosin named in class S2.3 is thymosin beta-4.
See also
References
- Goldstein AL, Slater FD, White A. Preparation, assay, and partial purification of a thymic lymphocytopoietic factor (thymosin). Proc Natl Acad Sci USA. 1966;56(3):1010–1017. PMID 5230175
- Goldstein AL, et al. Thymosin alpha1: isolation and sequence analysis of an immunologically active thymic polypeptide. Proc Natl Acad Sci USA. 1977;74(2):725–729. PMID 265536
- Haritos AA, Goodall GJ, Horecker BL. Prothymosin alpha: isolation and properties of the major immunoreactive form of thymosin alpha 1 in rat thymus. Proc Natl Acad Sci USA. 1984;81(4):1008–1011. PMID 6583693
- Wang SS, Makofske R, Bach A, Merrifield RB. Automated solid phase synthesis of thymosin alpha 1. Int J Pept Protein Res. 1980;15(1):1–4. PMID 7358454
- Romani L, et al. Thymosin alpha 1 activates dendritic cells for antifungal Th1 resistance through toll-like receptor signaling. Blood. 2004;103(11):4232–4239. PMID 14982877
- Chien RN, et al. Efficacy of thymosin alpha1 in patients with chronic hepatitis B: a randomized, controlled trial. Hepatology. 1998;27(5):1383–1387. PMID 9581695
- Mutchnick MG, et al. Thymosin alpha1 treatment of chronic hepatitis B: results of a phase III multicentre, randomized, double-blind and placebo-controlled study. J Viral Hepat. 1999;6(5):397–403. PMID 10607256
- Wu J, et al. The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind, randomized and controlled trial. Crit Care. 2013;17(1):R8. PMID 23327199
- Wu J, et al. The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial. BMJ. 2025;388:e082583. PMID 39814420
- Shang W, et al. Thymosin alpha1 use in adult COVID-19 patients: a systematic review and meta-analysis on clinical outcomes. Int Immunopharmacol. 2023;114:109584. PMID 36527881
- Soeroto AY, et al. The efficacy of thymosin alpha-1 therapy in moderate to critical COVID-19 patients: a systematic review, meta-analysis, and meta-regression. Inflammopharmacology. 2023;31(6):3317–3325. PMID 37845598
- SciClone Pharmaceuticals. Form 10-K for fiscal year 1998, filed 1999. sec.gov
- European Medicines Agency. Orphan designation EU/3/02/110, thymalfasin for hepatocellular carcinoma. ema.europa.eu
- Garaci E. Thymosin alpha1: a historical overview. Ann N Y Acad Sci. 2007;1112:14–20. PMID 17567941
- World Anti-Doping Agency. The 2026 Prohibited List (in force 1 January 2026). wada-ama.org Classes S0 and S2.3.
Peptidarium Library. Thymosin alpha-1 (entry 0017), revision 1.0, 2 November 2026. peptidarium.com/library/thymosin-alpha-1
Revision history
| Rev | Date | Change | By |
|---|---|---|---|
| 1.0 | 2 Nov 2026 | Entry created. | [Scientific editor] |
Spotted an error or a missing paper? Send a correction. Every change is logged here.