Thymosin Alpha-1

The two share a name and a tissue of origin, and little else. Thymosin alpha-1 is a 28-residue, N-acetylated peptide of 3108.28 g/mol, studied in immunology: dendritic cell signalling and cytokines. TB-500 is the research-market name for synthetic thymosin beta-4, a 43-residue actin-binding protein of 4963.4 g/mol. Its literature concerns actin, cell migration and angiogenesis, built around the seven-residue LKKTETQ actin-binding motif (Philp et al., FASEB J, 2003). Choosing between them means choosing between an immunology tool and a cytoskeletal one.

Thymosin alpha-1, also written thymosin α1 or TA1, and known by its INN thymalfasin, is a 28-residue peptide with an acetylated N-terminus, sequence Ac-Ser-Asp-Ala-Ala-Val-Asp-Thr-Ser-Ser-Glu-Ile-Thr-Thr-Lys-Asp-Leu-Lys-Glu-Lys-Lys-Glu-Val-Val-Glu-Glu-Ala-Glu-Asn (Ac-SDAAVDTSSEITTKDLKEKKEVVEEAEN), molecular formula C₁₂₉H₂₁₅N₃₃O₅₅, molecular weight 3108.28 g/mol and CAS number 62304-98-7. It is the N-terminal 28-residue fragment of prothymosin alpha, the acidic nuclear protein from which it is released by proteolysis, and it was first isolated from bovine thymus — specifically from calf thymosin fraction 5 — and sequenced by Goldstein and colleagues (Proc Natl Acad Sci USA, 1977). The N-terminal acetylation is the molecule's defining chemical feature: it blocks aminopeptidase degradation and is considered necessary for full activity. The sequence contains no cysteine and no methionine, so TA1 carries no disulfides and is spared the methionine-oxidation liability that dominates handling guidance for many other research peptides; what it does carry is a C-terminal asparagine, which makes deamidation the principal degradation route to design against.

The regulatory history is unusual for a peptide in this catalogue and worth stating precisely. Under the brand name Zadaxin, developed by SciClone Pharmaceuticals, thymosin alpha-1 (thymalfasin) is licensed as a prescription medicine in more than 35 countries, principally in Asia and the Middle East, for immunomodulatory indications most notably including chronic hepatitis B. The evidence base includes randomised controlled trials such as Chien and colleagues' 98-patient trial in chronic hepatitis B (Hepatology, 1998), and the history is reviewed by Camerini and Garaci (Expert Opin Biol Ther, 2015). It has never been licensed in the United Kingdom, the European Union or the United States. That asymmetry matters for researchers: a compound with a genuine pharmaceutical dossier in some jurisdictions remains an unlicensed substance in others, and no thymosin alpha-1 product is an approved medicine in the UK.

Mechanistically, the most cited modern work centres on toll-like receptor signalling rather than a classical single receptor. Romani and colleagues reported in Blood (2004) that thymosin alpha 1 activates myeloid dendritic cells for Th1-type responses through a TLR9-dependent, MyD88-mediated signalling pathway, an observation that reframed TA1 as an endogenous regulator of innate immune sensing rather than a simple T-cell maturation factor. Around that anchor sit the older T-cell literature — maturation, differentiation and function of thymocytes and peripheral T cells — alongside reports of natural killer cell activity and broad cytokine regulation, reviewed by Garaci (Ann N Y Acad Sci, 2007). The most active current direction is oncology: recent studies examine TA1 in combination with immune checkpoint inhibitors, including a 2025 retrospective analysis in platinum-resistant recurrent ovarian cancer (Wang et al., Cancer Manag Res, 2025).

The honest position on evidence follows. The licensed indications for Zadaxin are prescription medicine uses in specific jurisdictions, and nothing on this page describes or endorses any human use; the material supplied here is for in-vitro laboratory research only. Much of the older immunology literature is small, heterogeneous and in places unreplicated, and the recent checkpoint-inhibitor combination data are largely retrospective. Thymosin alpha-1 is supplied as lyophilised powder, batch UKPL-9504, at 99.76% purity by HPLC with the supplier certificate of analysis, published on the product page, with independent third-party verification pending.