Thymalfasin (thymosin alpha-1): where it is approved and why

Thymalfasin, the drug name for synthetic thymosin alpha-1, has never been approved by the US FDA. SciClone Pharmaceuticals marketed it as Zadaxin and reported approval in more than 30 countries, mainly in Asia, the Middle East and Latin America, mostly for chronic hepatitis B and as an "immune system enhancer". China approved it in 1995. In Europe, only Italy authorized it, for boosting the immune response to influenza vaccine. In the US it holds orphan drug designations, which are not approvals, and an FDA advisory committee voted in December 2024 against letting pharmacies compound it. This page sets out what each of those facts means and where they come from.

What thymalfasin is

Thymalfasin is the international name for thymosin alpha-1 (Tα1), a peptide first identified in thymus tissue. The EMA's orphan summary describes it as a small protein that was originally isolated from the thymus gland of cattle and is now made by chemical synthesis. FDA's 2024 briefing document describes the free base as a 28-amino-acid peptide with an acetylated N-terminus, molecular formula C₁₂₉H₂₁₅N₃₃O₅₅, molecular weight 3108.3 g/mol, CAS number 62304-98-7 and UNII W0B22ISQ1C.

It is a different molecule from thymosin beta-4, whose fragment is sold as TB-500. Our thymosin alpha-1 vs beta-4 comparison explains the difference.

Approval status by region

The country count changes depending on the source and the year. SciClone's 10-K for 2007 said "over 34 countries". Its 10-K for 2015 said "over 30 countries". The AUTM technology-transfer case study gives 36. FDA's 2024 review noted the company's claims of approval in parts of Asia-Pacific, Latin America, Eastern Europe and the Middle East, and said it could not verify them on its own. A precise, current list of countries is not available from a primary regulator, so any site giving one should cite where it came from.

The China story

According to the AUTM case study, US and European rights to thymosin alpha-1 went to a small biotech company. Rights for the rest of the world went to SciClone, which later bought the European and US rights as well. China approved Zadaxin in 1995 as a stand-alone treatment for hepatitis B, and sales began in 1996.

China then became the product's main market. SciClone's 2015 annual report says China was about 96% of its net revenue in 2015 and that worldwide Zadaxin sales were over $146 million that year. The same filing says thymalfasin is included in Chinese Ministry of Health guidelines for liver cancer, and that it was added to China's national reimbursement catalogue as a Category B product in November 2009. The product reached China as an imported drug, with Italy serving as the reference country for its certificate of pharmaceutical product.

Why it was never approved in the US

No single document gives one reason. Several points can be verified:

• Early US trials were not decisive. The AUTM case study says US multicenter trials began in 1992. Results were positive but too limited to support firm conclusions, and the US program "went nowhere".
• The company focused on China. SciClone's filings describe a strategy of selling drugs in China that were approved in other markets. With nearly all revenue coming from China, there was little business reason to run the large US trials an FDA approval would need.
• Later programs did not lead to filings. SciClone's 2007 10-K described a 488-patient Phase 2 trial in stage IV melanoma at 64 European sites, a Phase 3 melanoma trial in planning, and a 553-patient European Phase 3 trial in hepatitis C. AUTM reports that SciClone got FDA clearance in late 2008 to begin a Phase 3 melanoma trial. We found no record of an FDA marketing application.
• FDA's own review found the evidence weak. In its December 2024 briefing, FDA concluded that the evidence on characterization, safety and effectiveness weighed against allowing compounding.

Company filings also summarize other trials, such as a 361-patient sepsis study in China. These are company summaries, not regulatory findings.

Orphan designations are not approvals

An orphan designation is granted early in development to encourage work on rare diseases. The EMA says plainly that designation is not a marketing authorization, and that quality, safety and efficacy must still be shown. Thymalfasin's designations are:

• US (FDA): malignant melanoma (described in the 2007 10-K as stage IIb to IV), chronic hepatitis B and hepatocellular carcinoma, according to SciClone's 10-K filings.
• EU (EMA): hepatocellular carcinoma, EU/3/02/110, granted 30 July 2002 after a positive opinion from the Committee for Orphan Medicinal Products on 27 June 2002.

The 2024 FDA compounding vote

Thymosin alpha-1 free base and acetate were nominated for the 503A bulks list, which would let licensed pharmacies compound them. The nomination was later withdrawn, but FDA reviewed them anyway. FDA's briefing raised several concerns: immunogenicity, a proposed 3 mg/mL strength above the free base's reported water solubility of 2 mg/mL, risks in stem-cell transplant patients, and adverse event reports. It recommended against listing.

At the 4 December 2024 Pharmacy Compounding Advisory Committee meeting, members voted 4 yes, 17 no on both the free base and the acetate. The final minutes cite "the lack of convincing effectiveness data". In 2026, thymosin alpha-1 was not among the seven peptides FDA sent back to the committee for its July meeting. It is also not among the peptides covered by the HHS enforcement-discretion policy filed on 28 October 2026. See our 2026 FDA peptide vote and HHS compounding policy pages.

What this means for researchers

Thymalfasin approvals abroad do not cover research-grade thymosin alpha-1 powder sold online. That powder is not Zadaxin, is not made under the approved product's manufacturing controls, and is not approved for human use anywhere. For laboratory work, check identity and purity against a certificate of analysis (see how to verify a peptide COA), and read the peptide legal guide for the US rules.

Frequently asked questions

Sources

1. EMA: EU/3/02/110 orphan designation, thymalfasin for hepatocellular carcinoma (2002)
2. EMA: public summary of positive opinion for orphan designation of thymalfasin (2002)
3. FDA: Thymosin alpha-1 related bulk drug substances, PCAC briefing (4 December 2024)
4. FDA: PCAC final summary minutes, 4 December 2024 meeting (approved 21 February 2025)
5. SciClone Pharmaceuticals Form 10-K, fiscal year 2015 (SEC)
6. SciClone Pharmaceuticals Form 10-K, fiscal year 2007 (SEC)
7. AUTM Better World Project: Zadaxin case study
8. National Law Review (Polsinelli): What FDA's latest actions mean for peptide compounding (22 April 2026)