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Orphan designations and research · Updated October 13, 2026
Thymosin alpha 1, also called thymalfasin, is a synthetic 28-amino-acid peptide. FDA's December 2024 briefing states that no drug product containing it had been approved. The four cited orphan designations, dated 1991 to 2006 with SciClone as sponsor, list their intended uses as not FDA approved. Orphan designation and drug approval are separate statuses.
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FDA stated in December 2024 that it has approved no drug product containing thymalfasin.
FDA's orphan drug database holds four designations, dated May 1991 to March 2006, all with SciClone Pharmaceuticals as sponsor.
In December 2024, an FDA advisory committee voted 4 to 17 against placing the free base on the 503A bulks list, and 4 to 17 against the acetate.
Registered studies include a completed 500-participant hepatitis C study and a COVID-19 study that ended early.
The thymosin alpha-1 peptide is a linear chain of 28 amino acids with an acetyl group on its N-terminal end. Thymalfasin is the international nonproprietary name for the synthetic form, and Zadaxin is its trade name outside the United States. The research on it concerns immune signaling, including immune cell maturation and cytokine signaling. A proposed mechanism is not evidence of clinical effectiveness for any condition.
FDA's advisory committee briefing from December 2024 identifies the free base by CAS number 62304-98-7 and UNII W0B22ISQ1C. It gives the formula as C129H215N33O55 and the weight as about 3108.3 g/mol. Those identifiers separate it from other thymosin family members with similar names. A longer orphan drug register profile covers the same record.
FDA's December 2024 briefing states that the agency had approved no drug product containing thymalfasin. It also reports no approval in Japan or Europe outside Italy and no recognition in the European or Japanese Pharmacopoeia. These are the findings of that dated briefing, rather than a new audit of every jurisdiction's current records.
SciClone's 2014 annual report describes approvals in the Middle East, Eastern Europe, Latin America, and parts of Asia-Pacific. The briefing says FDA could not independently verify those approvals in every country named, so this page counts them as sponsor statements.
The compounding question reached the Pharmacy Compounding Advisory Committee on December 4, 2024. A 503A bulks list nomination for the peptide had been withdrawn, but FDA still took the substances to the committee. The published committee minutes record a 20 to 1 vote on taking one combined ballot. That single no vote triggered separate ballots, and the free base and the acetate each failed by 4 yes to 17 no, with no abstentions. Several members who voted no cited a lack of compelling evidence for clinical effectiveness and safety in the uses under review. As of October 2026, the sources reviewed for this page show no later FDA action that changes this record.
FDA's orphan drug database lists four designations for thymalfasin, each with SciClone Pharmaceuticals as sponsor. The hepatocellular carcinoma record shows the format, including an orphan approval status field that marks the designated use as not FDA approved.
May 3, 1991: chronic active hepatitis B
January 8, 1998: DiGeorge anomaly with immune defects
March 6, 2000: hepatocellular carcinoma
March 13, 2006: malignant melanoma, stages IIb through IV
Orphan designation is an incentive status that FDA grants early in development for a candidate linked to a rare condition. The status does not mean FDA found the candidate safe or effective, and it carries no marketing permission.
Thymosin alpha 1 has registered studies with distinct populations and designs. NCT00040027 is a Phase 3 study sponsored by SciClone that paired thymalfasin with peginterferon in non-cirrhotic hepatitis C participants who had not responded to earlier interferon regimens. The record lists 500 participants, an April 2002 start, and completed status.
A randomized study of 552 hepatitis C participants, PMID 22233415, reported two analyses of sustained virological response. In the all-randomized analysis the thymosin alpha 1 and placebo rates were 12.7 percent and 10.5 percent, with p = 0.407, which did not reach significance. Among participants who completed the course they were 41.0 percent and 26.3 percent, with p = 0.048. The completer analysis does not replace the all-randomized finding.
NCT04487444 tested thymalfasin in participants with COVID-19 and lymphocytopenia at Rhode Island Hospital, as a Phase 2 study. Its dates run September 10, 2020 through February 3, 2023. It enrolled 56 participants and ended early because of slow enrollment and the principal investigator's transfer to another institution.
The COA reading guide covers lot matching and laboratory verification, with public lot reports as examples for the compounds covered by that library.
Material quality is a lab result, because identity, purity, and endotoxin load vary by manufacturer and by lot. For thymosin alpha 1, a useful certificate shows:
The exact lot number printed on the label.
The issuing laboratory's name and a report identifier.
HPLC purity shown with its chromatogram instead of a single percentage.
A mass-spectrometry result near 3108.3 g/mol, with the acetyl group accounted for.
An endotoxin result.
A way to verify the document through a public lookup or the laboratory itself.
The linked research compound catalogue does not list thymosin alpha 1 as of October 13, 2026. Its documented testing standard provides a reference for interpreting the reports for the compounds it does list.
FDA's December 2024 briefing states that the agency had approved no drug product containing thymosin alpha 1, also called thymalfasin. The cited orphan-designation records list their intended uses as not FDA approved. Those dated findings do not establish approval of a drug product or replace a fresh review of a later regulatory record.
Thymalfasin's orphan drug designation is a development incentive. Each cited FDA designation identifies an intended rare-disease use and lists that use as not FDA approved. The designation does not authorize marketing a drug product. The December 2024 FDA briefing and committee vote should be read separately from the orphan-designation records.
The cited FDA orphan drug records list four thymalfasin designations. SciClone Pharmaceuticals is the sponsor on each: chronic active hepatitis B in 1991, DiGeorge anomaly with immune defects in 1998, hepatocellular carcinoma in 2000, and stage IIb through IV malignant melanoma in 2006. A designation is a development incentive and does not establish safety or effectiveness.
TB-500 and thymalfasin are different molecules. TB-500 is the seven-residue fragment of thymosin beta 4; thymalfasin is synthetic thymosin alpha 1, with 28 residues. The shared family name does not make them interchangeable, and a regulatory finding about one does not determine the other's status.
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