What it is
Thymosin alpha-1 is a short chain of 28 amino acids (a peptide). The body makes it naturally. Scientists first purified it in the 1970s from calf thymus. The thymus is the organ where T cells grow up. T cells are a type of white blood cell. Today’s version is made in a lab. Its generic drug name is thymalfasin.
Researchers have studied it as a drug that adjusts the immune system. They tested it in long-lasting virus infections. They also tested it in severe infections such as sepsis (a life-threatening reaction to infection). It has also been studied alongside vaccines or cancer treatment. A brand called Zadaxin is approved in China, Italy and some other countries. It is not approved by the FDA in the United States.
Thymalfasin is Ac-SDAAVDTSSEITTKDLKEKKEVVEEAEN-OH (C129H215N33O55, 3108.3 g/mol, CAS 62304-98-7), a highly acidic, N-terminally acetylated peptide identical to the N-terminal segment of prothymosin α (residues 2–29 of the precursor, after removal of the initiator methionine; UniProt P06454). Goldstein et al. (1977) sequenced it from thymosin fraction 5, a partially purified calf-thymus extract.
Its immunomodulatory activity is attributed at least partly to Toll-like receptor 9 signalling on dendritic cells and lymphoid progenitors (FDA, 2024). In rodent models it has been reported to suppress tumour growth and viral infection, to reduce sepsis and to raise antibody titres to some vaccines, usually at fixed doses whose human equivalents are well above the doses used clinically; dose-response relationships have not been established.
Pharmacokinetics: after 900 µg/m² subcutaneously in nine healthy volunteers, tmax was 1–2 hours, elimination half-life under 3 hours and apparent volume of distribution 30–40 L, with no accumulation over 5 days of daily dosing (Rost et al. 1999). In rats given it intravenously, the half-life was about 2–3 minutes. Trial regimens have ranged from 1.6 mg subcutaneously twice weekly (hepatitis B) to 1.6 mg every 12 hours (sepsis).
Who made it and when
Allan Goldstein and his team found thymosin alpha-1 in calf thymus in 1977. They also worked out the order of its amino acids. It was one of several active parts of a thymus extract they were studying. Later, chemists learned to make it in a lab.
A US company, Alpha 1 Biomedicals, tested it for hepatitis B in the United States. That trial’s result was not strong enough to rule out chance (not statistically significant). Then SciClone Pharmaceuticals bought the rights and named the product Zadaxin. It won approvals in other countries, mainly in Asia, the Middle East and Latin America. Most sales were in China. Later US trials in hepatitis C did not succeed. It was never approved in the US.
Goldstein et al. (1977) reported the 28-residue sequence and proposed the thymosin nomenclature. According to SciClone’s 2004 annual report (Form 10-K), Alpha 1 Biomedicals completed a 99-patient US phase 3 trial in chronic hepatitis B in 1994; 25% on thymosin α1 versus 13% on placebo reached the endpoints, which was not statistically significant, and the data were never submitted to FDA. Mutchnick et al. (1999, n=97) report the same 25% versus 13% for sustained loss of HBV DNA with HBeAg negativity. SciClone stated that Zadaxin was approved in over 30 countries, “primarily in Asia, the Middle East and Latin America”, principally for hepatitis B, with some approvals for hepatitis C, as a vaccine adjuvant or as a chemotherapy adjuvant, and that 91% of 2004 sales were in China; Sigma-Tau held marketing rights in most of Western Europe. FDA (2024) notes it is not approved in the US, Japan or Europe except Italy, and that it could not verify every claimed approval. Two US phase 3 trials of Zadaxin with peginterferon in hepatitis C were reported by the company in 2005–2006 as not meeting their primary endpoints.
FDA’s orphan database lists a designation for hepatocellular carcinoma (SciClone, 2000-03-06), never approved for that use.
What the data say
There are many human studies. Most come from China. Many were small. In many, people knew which treatment was given.
For long-lasting hepatitis B, results were mixed. A trial in Taiwan assigned patients by chance but used no dummy treatment (placebo). It reported that more patients cleared the virus. A US trial that did use a placebo did not show a clear benefit. Its result could have been due to chance.
For sepsis, an early Chinese trial reported fewer deaths. That trial was not fully blinded, so some people knew who got what. A much larger trial of 1,106 patients came out in 2025. In it, neither patients nor doctors knew who got what. It found no difference in survival at 28 days.
During COVID-19, a small study from Wuhan looked back at old records. It reported fewer deaths. But later studies that pooled many results mostly did not find this. In 2024, the FDA reviewed the evidence. It found the evidence was not good enough for any of the 12 uses it looked at.
Hepatitis B: Chien et al. (1998, Taiwan, n=98) reported complete virological response at 18 months in 40.6% after a 26-week course of 1.6 mg twice weekly versus 9.4% in untreated controls (P=.004), without placebo or blinding. The US multicentre, double-blind trial (Mutchnick et al. 1999, n=97) found 14% versus 4% complete response (P=0.084); the authors wrote that the results “do not confirm” earlier efficacy reports.
Sepsis: ETASS (Wu et al. 2013, n=361, single-blind) reported 28-day mortality of 26% versus 35%. The confirmatory TESTS trial (NCT02867267, n=1,106, double-blind, 1.6 mg every 12 hours for up to 7 days) found 23.4% versus 24.1% (HR 0.99, 95% CI 0.77–1.27); a prespecified subgroup analysis suggested a possible differential effect by age, with an HR of 1.67 in patients under 60 and 0.81 in those 60 or older. The trial was partly funded by SciClone.
COVID-19: Liu et al. (2020, retrospective, n=76 severe cases) reported mortality of 11.1% versus 30.0%; of four meta-analyses FDA reviewed, three found no mortality reduction. FDA’s 2024 review concluded there was a lack of evidence supporting subcutaneous thymosin α1 for any evaluated use.
Regulatory picture
Approval: No thymosin alpha-1 product is approved by the FDA. Approvals in China, Italy and other places do not count in the United States. The FDA gave it orphan-drug status for liver cancer. This status is a reward meant to encourage work on rare diseases. It is not an approval.
Compounding (pharmacies making their own versions): In 2023, the FDA put it in “Category 2.” That is the group of substances pharmacies should not compound from. In 2024, it came off that list because the person who suggested it withdrew the request. The FDA still reviewed it. In December 2024, an FDA expert panel voted 17 to 4 against letting pharmacies compound it. It is not on the list of allowed ingredients. It was also not among the six peptides in the US health department’s October 2026 draft policy easing enforcement.
Enforcement: The FDA has sent warning letters about it. A 2020 letter covered “Thymosin Alpha 1” sold online with COVID-19 claims. A 2021 letter covered compounded thymosin alpha.
Approval: none in the US; no BLA or NDA approval exists. FDA’s orphan database shows a designation for hepatocellular carcinoma (2000). SciClone announced in March 2006 that thymalfasin had also been granted orphan designation for stage IIb–IV malignant melanoma; that record could not be retrieved from FDA’s database for this article, and other designations cited online were not verified.
Compounding: “Thymosin-Alpha 1 (Ta1)” was listed in 503A Category 2 in FDA’s update of 2023-09-29. The update of 2024-09-27 removed it because “the nomination was withdrawn by the nominator”, while announcing that FDA would consult the Pharmacy Compounding Advisory Committee anyway. On 2024-12-04, with FDA proposing exclusion for uses including hepatitis B and C, HIV, COVID-19, vaccine response, melanoma, hepatocellular carcinoma, NSCLC, sepsis, post-transplant infection, COPD and ME/CFS, the committee voted 4 yes and 17 no for both the free base and the acetate. FDA’s safety-risk page (2026-04-22) still lists Ta1’s immunogenicity concerns among withdrawn substances. Ta1 was not on the July 2026 agenda, and no proposed or final rule on it was found in the Federal Register. It is not on the 503A bulks list, and it is not among the six peptides in the HHS draft enforcement-discretion policy of 2026-10-30, so this article records compounding status as unknown.
