Identity a 28-amino-acid, N-terminally acetylated peptide (~3,108 Da; < 40 aa → peptide), a fragment of prothymosin alpha, produced by thymic epithelial and peripheral immune cells. Serum levels decline ~40–60% with age (thymic involution). Discovered by Allan Goldstein's lab at George Washington University (Goldstein et al., 1977) — the same lab associated with thymosin beta-4 / TB-500 (#12) — but Tα1 and Tβ4 have fundamentally different biology (Tα1 = immune modulator; Tβ4 = actin-sequestering tissue repair). ## Development & history - Isolated by Goldstein's group in the 1970s from thymic tissue (thymosin fraction 5) — one of the first demonstrations that the thymus secretes soluble immune-modulating factors. Synthetic Tα1 was made by solid-phase synthesis in the early 1980s, freeing production from tissue extraction.
- SciClone Pharmaceuticals developed the synthetic version as Zadaxin and pursued international approvals: China approved it for hepatitis B in 1996, and it went on to marketing authorisation in ~35–37 countries across Asia, South America, the Middle East and parts of Europe.
- US: the FDA granted orphan-drug designations (hepatitis B in 1991, plus melanoma, DiGeorge anomaly, hepatocellular carcinoma), and SciClone ran US Phase 3 trials (hepatitis C, melanoma) — but these did not lead to US marketing approval. (Widely attributed to commercial/regulatory economics, not a safety signal.) ## Mechanism (as proposed) an immune modulator acting largely through Toll-like receptors TLR2 and TLR9 on dendritic and myeloid antigen-presenting cells → activates T-cell / cell-mediated immunity. It promotes T-cell maturation (upregulating CD3/CD4/CD8), dendritic-cell maturation and Th1 polarisation, and enhances NK-cell function, helping restore immune competence in immunosuppressed states while also contributing to control of inflammation and tolerance.