Also Known As: Thymalfasin, Tα1, TA1, Ta1, Zadaxin, ZADAXIN, Thymosin α1, Thymosin Alpha 1
Thymosin alpha-1 (Tα1, INN thymalfasin) is a synthetic 28-amino-acid peptide that reproduces the N-terminally acetylated sequence of the natural pro-thymosin-α fragment — a polypeptide secreted by thymic epithelial cells and detectable in human serum. Pharmacologically, Tα1 is a pleiotropic immune modulator: it engages the innate-immune sensors Toll-like receptor 2 (TLR2) and Toll-like receptor 9 (TLR9) on dendritic cells and monocytes, drives DC maturation, polarises Th1 responses, activates natural killer (NK) cells, and supports thymopoiesis and peripheral T-cell differentiation — rather than acting as a single classical hormone-receptor agonist. Marketed as Zadaxin (originator SciClone Pharmaceuticals; Italian partner Sigma-Tau / Alfasigma), Tα1 is approved in more than 35 countries (Italy, China, Vietnam, Singapore, Mexico, Argentina and others) for chronic hepatitis B, as an adjuvant in chronic hepatitis C, and as an immunoadjuvant for influenza/H1N1 vaccination in immunocompromised populations such as haemodialysis patients. The U.S. FDA has not granted marketing approval for Tα1 — it has issued only orphan-drug designations for chronic hepatitis B (1996), non-small-cell lung cancer (2007) and Stage IIB–IV malignant melanoma (2010); orphan-drug designation is regulatorily distinct from marketing approval and confers only sponsor-side incentives (tax credits, market exclusivity if subsequently approved) for the development of drugs targeting rare diseases.
Thymosin α1 is a pleiotropic immune modulator: it engages the innate-immune sensors TLR2 and TLR9 on dendritic cells and monocytes, drives DC maturation, polarises a Th1 response, activates NK cells and supports thymopoiesis — not a classical single-receptor hormone agonist.
Thymosin alpha-1 is a genuinely pleiotropic immune modulator. The receptor-level work that clarified its mechanism appeared in a series of Romani-group Blood papers (2004 → 2006 → 2007), which showed that Tα1 functions as an endogenous regulator of dendritic-cell function via Toll-like receptor 9 (and to a lesser extent TLR2), mobilising MyD88-dependent NF-κB and IRF7 pathways to mature dendritic cells, induce IL-12 and type-I interferons, and prime Th1-polarised T-cell responses. Camerini and Garaci (2010, Annals NYAS) consolidate these findings and add that Tα1 supports thymopoiesis and peripheral T-cell differentiation, boosts NK-cell cytotoxicity, restores regulatory-T-cell balance in chronic viral infection and oncology, and modifies the cytokine milieu (↑ IL-2, ↑ IL-12, ↑ IFN-γ; ↓ IL-10) in a context-dependent manner. Because Tα1 acts on innate sensors expressed by antigen-presenting cells rather than on a single classical hormone receptor, its effects depend on the immunological state of the host: it is most active when basal immune function is impaired (chronic hepatitis, sepsis, cancer chemotherapy, vaccination of the immunocompromised) and produces only modest changes in healthy adults. This pleiotropy is why Tα1 has been investigated across an unusually broad set of indications, but it is also why mechanism-to-clinical-outcome translation is complex and why high-quality endpoint-driven trials are essential before any therapeutic conclusion is drawn.
The evidence base for thymosin α1 spans the Goldstein 1977 isolation paper, multiple randomised chronic hepatitis B trials (Andreone 2001; Wu 2015 systematic meta-analysis), the Sherman 1998 hepatitis C combination RCT, the Wu 2013 multicentre Phase III sepsis trial (ETASS, NCT00711620), the Maio 2010 Phase II metastatic melanoma study, and the Liu 2020 COVID-19 retrospective cohort. Editorial framing matters: Wu 2013 reported a 9-percentage-point absolute reduction in 28-day mortality (26 % vs 35 %) but the result was statistically non-significant (P = 0.062); Liu 2020 is a retrospective single-centre observational study and is hypothesis-generating, not confirmatory.
Studies report that, in a randomised trial of Tα1 1.6 mg subcutaneously twice weekly for 6 months, sustained virological response was observed in 40.6 % of Tα1-treated patients versus 9.4 % of controls at 26-month follow-up (P = 0.04).
— Andreone et al., J Viral Hepat 2001;8(3):194–201 (PMID 11380797)
Studies report, in a pooled analysis of randomised Tα1 trials in chronic hepatitis B, improved virological and biochemical response rates over follow-up versus control or interferon comparators, with the strongest signal at 12 months post-treatment; modern direct-acting antivirals are now first-line for chronic HBV.
— Wu, Jia & You, PLoS ONE 2015;10(5):e0127490 (PMID 25996374)
Studies report, in a randomised, placebo-controlled, double-blind multicentre trial in interferon-α non-responder chronic hepatitis C patients, a modest but statistically meaningful improvement in biochemical and virological end-of-treatment response with Tα1 + interferon-α combination therapy versus interferon-α monotherapy.
— Sherman et al., Hepatology 1998;27(4):1128–1135 (PMID 9537455)
Studies report, in the Wu 2013 ETASS trial (Tα1 1.6 mg subcutaneously every 12 h × 7 days, then daily for 7 days), 28-day all-cause mortality of 26.0 % under Tα1 versus 35.0 % under control — a 9-percentage-point absolute reduction that did not meet the conventional threshold for statistical significance (log-rank P = 0.062) and therefore should not be described as a positive Phase III trial; secondary endpoints (monocyte HLA-DR expression, faster SOFA-score reduction) favoured Tα1.
— Wu et al., Crit Care 2013;17:R8 (PMID 23327199)
Studies report, in an open-label Phase II randomised trial of dacarbazine + IFN-α2b ± Tα1 (3.2 mg or 6.4 mg) in stage-IV metastatic melanoma, numerically longer median overall survival and higher objective response rates in Tα1-containing arms versus dacarbazine + IFN alone; the trial did not formally meet a hierarchical statistical hypothesis for superiority.
— Maio et al., Ann Oncol 2010;21(8):1696–1701 (PMID 20146422)
Studies report, in randomised trials in haemodialysis populations, that pre-vaccination administration of Tα1 increased seroconversion and seroprotection rates after influenza (including H1N1) vaccination versus vaccine alone — mechanistically supported by TLR9-mediated DC maturation and Th1 polarisation in lymphopenic hosts.
— Camerini & Garaci, Ann NY Acad Sci 2010;1194:116–126 (PMID 20566298); ZADAXIN H1N1 study NCT01031966
Studies report, in a retrospective single-centre cohort of severe COVID-19 patients (n ≈ 76), lower 28-day mortality and faster recovery of T-lymphocyte counts in a Tα1-treated subgroup versus untreated controls; given the retrospective, unblinded, non-randomised design, this signal is hypothesis-generating, not confirmatory.
— Liu et al., Clin Infect Dis 2020;71(16):2150–2157 (PMID 32442287)
Observed in research settings
Across published Tα1 trials in chronic hepatitis B, hepatitis C adjuvant therapy, severe sepsis, oncology and vaccine-adjuvant settings, Tα1 has been described as generally well tolerated, with most adverse events reported as mild, transient and limited to the injection site. Because Tα1 is not FDA-approved in the United States, there is no official US prescribing information; the safety summary draws on published clinical-trial reports and on non-US (Italian / Chinese) Zadaxin labelling.
Goldstein AL, Low TLK, McAdoo M, McClure J, Thurman GB, Rossio J, Lai CY, Chang D, Wang SS, Harvey C, Ramel AH, Meienhofer J Thymosin alpha 1: isolation and sequence analysis of an immunologically active thymic polypeptide Proc Natl Acad Sci USA 1977;74(2):725–729. 1977 .
Romani L, Bistoni F, Perruccio K, Montagnoli C, Gaziano R, Bozza S, Bonifazi P, Bistoni G, Rasi G, Velardi A, Fallarino F, Garaci E, Puccetti P Thymosin alpha 1 activates dendritic cell tryptophan catabolism and establishes a regulatory environment for balance of inflammation and tolerance Blood 2006;108(7):2265–2274. 2006 .
Romani L, Bistoni F, Gaziano R, Bozza S, Montagnoli C, Perruccio K, Pitzurra L, Bellocchio S, Velardi A, Rasi G, Di Francesco P, Garaci E Thymosin α1 activates dendritic cells for antifungal Th1 resistance through Toll-like receptor signalling Blood 2007;109(5):1933–1941. 2007 .
Camerini R, Garaci E Historical review of thymosin α1 in infectious diseases Annals of the New York Academy of Sciences 2010;1194(1):116–126. 2010 .
Andreone P, Cursaro C, Gramenzi A, Zavaglia C, Rezakovic I, Altomare E, Severini R, Franzone JS, Albano O, Ideo G, Bernardi M, Gasbarrini G A randomised controlled trial of thymosin-α1 versus interferon alfa treatment in patients with HBeAg antibody- and hepatitis B virus DNA-positive chronic hepatitis B J Viral Hepat 2001;8(3):194–201. 2001 .
Sherman KE, Sjogren M, Creager RL, Damiano MA, Freeman S, Lewey S, Davis D, Root S, Weber FL, Ishak KG, Goodman ZD Combination therapy with thymosin alpha 1 and interferon for the treatment of chronic hepatitis C infection: a randomised, placebo-controlled double-blind trial Hepatology 1998;27(4):1128–1135. 1998 .
Wu J, Zhou L, Liu J, Ma G, Kou Q, He Z, Chen J, Ou-Yang B, Chen M, Li Y, Wu X, Gu B, Chen L, Zou Z, Qiang X, Chen Y, Lin A, Zhang G, Guan X The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicentre, single-blind, randomised and controlled trial Critical Care 2013;17(1):R8. 2013 .
Wu X, Jia J, You H Thymosin alpha-1 treatment in chronic hepatitis B: a systematic review and meta-analysis PLoS ONE 2015;10(5):e0127490. 2015 .
Maio M, Mackiewicz A, Testori A, Trefzer U, Ferraresi V, Jassem J, Garbe C, Lesimple T, Guillot B, Gascon P, Gilde K, Camerini R, Cognetti F Large randomised study of thymosin α1, interferon alfa, or both in combination with dacarbazine in patients with metastatic melanoma Annals of Oncology 2010;21(8):1696–1701. 2010 .
Liu Y, Pan Y, Hu Z, Wu M, Wang C, Feng Z, Mao C, Tan Y, Liu Y, Chen L, Li M, Wang G, Yuan Z, Diao B, Wu Y, Chen Y Thymosin alpha 1 reduces the mortality of severe coronavirus disease 2019 by restoration of lymphocytopenia and reversion of exhausted T cells Clinical Infectious Diseases 2020;71(16):2150–2157. 2020 .
DrugBank Thymalfasin (Thymosin α1) — DB04900 DrugBank Online. 2024 .
U.S. FDA Office of Orphan Products Development Orphan-drug designation database — thymalfasin (chronic hepatitis B 1996; non-small-cell lung cancer 2007; Stage IIB–IV malignant melanoma 2010) FDA OOPD listings. 2024 .
Actin-binding β-thymosin / healing & repair peptide (slug: healing-repair). Do NOT conflate with Thymosin α1, a TLR2/TLR9-mediated immune modulator from a separate gene family.
The label "TB-500" denotes two chemically distinct molecules: in the peer-reviewed literature it refers to the synthetic 7-residue fragment Ac-LKKTETQ (CAS 885340-08-9, ~889 Da); in research-chemical commerce it is routinely sold as a synonym for full-length 43-amino-acid Thymosin β-4 (timbetasin, CAS 77591-33-4, ~4921 Da). Neither form is approved; for research use only.
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