Compound Profile
Thymosin Alpha-1: An Immune Peptide in the Orphan Drug Register
Published: August 24, 2026
TL;DR
Thymosin alpha-1 occupies an unusual place in the regulatory record. It has received four separate US orphan drug designations since 1991, each tied to a rare condition and each reflecting formal interest in further development. None became an approval. In a December 4, 2024 briefing, FDA stated that it had approved no drug products containing thymosin alpha-1.
The same review made an unusually direct distinction between reported international availability and what the agency could confirm. FDA described narrow exceptions to the compound's general lack of approval in major markets, then wrote that it was "unable to independently verify those claims of approval" made for several other regions. That combination of recognition, incomplete development, and explicit regulatory uncertainty is the central story.
Curo does not sell thymosin alpha-1. This is a neutral reference profile, not a product listing. Curo's broader research-material catalogue, lot-specific COA lookup, and Research Use Only policy cover materials that are listed. Its standard uses exactly HPLC, LC-MS, and endotoxin testing, as outlined in the quality and testing standard. The guide to responsible handling of research-use-only materials explains the intended-use boundary.
An immune signalling peptide
Thymosin alpha-1 is a linear, N-terminally acetylated peptide discussed in research for its role in immune signalling. Thymalfasin is the international nonproprietary name for its synthetic form. Experimental and clinical literature has examined immune-cell maturation, cytokine signalling, and related responses, but a proposed mechanism does not establish clinical effectiveness for a condition.
FDA's 2024 briefing identifies thymosin alpha-1 free base under UNII W0B22ISQ1C and CAS number 62304-98-7. It gives the molecular formula as C129H215N33O55 and the molecular weight as approximately 3108.3 g/mol. Those identifiers matter because similar names can refer to different molecular substances, salt forms, or members of the broader thymosin family.
Four orphan drug designations
FDA's orphan drug database records four designations for thymosin alpha-1, all sponsored by SciClone Pharmaceuticals:
- Chronic active hepatitis B, designated May 3, 1991
- DiGeorge anomaly with immune defects, designated January 8, 1998
- Hepatocellular carcinoma, designated March 6, 2000
- Stage IIb through stage IV malignant melanoma, designated March 13, 2006
An orphan drug designation is an incentive status granted early in development for a candidate associated with a rare condition. It is not a finding that the candidate is safe or effective, and it is not permission to market an approved drug. Four entries in the orphan register can therefore coexist with no US approval.
That distinction separates thymosin alpha-1 from tesamorelin's path to an FDA drug label. One record shows multiple development incentives that did not culminate in US approval. The other provides a useful contrast with a peptide that did.
What FDA said in its own review
FDA's December 4, 2024 briefing stated that the agency had approved no drug products containing thymosin alpha-1. The document also stated that thymosin alpha-1 was not approved in the United States, Japan, or Europe, with the exception of Italy, and that it was not recognised in the European or Japanese Pharmacopoeias.
Regarding claims of approval across Asia-Pacific, Latin America, Eastern Europe, and the Middle East, FDA wrote that it was "unable to independently verify those claims of approval".
The immediate regulatory question concerned pharmacy compounding. A 503A Bulks List nomination for thymosin alpha-1 related substances, docket FDA-2015-N-3534-0470, was withdrawn by its nominator. FDA elected to proceed with advisory committee review anyway.
At the December 4, 2024 meeting, FDA's Pharmacy Compounding Advisory Committee first voted 20 yes, 1 no, and 0 abstentions to cast one combined vote for the free base and acetate forms. The substantive vote on whether to include those forms on the 503A Bulks List was 4 yes, 17 no, and 0 abstentions. The published minutes say the majority cited no compelling evidence of effectiveness.
The agency had addressed a different marketing issue four years earlier. In a warning letter dated December 7, 2020, FDA cited sections 505(a) and 502 of the Federal Food, Drug, and Cosmetic Act after a US seller marketed thymosin alpha 1 as an unapproved product for COVID-19.
The clinical trial record
The registered studies show that thymosin alpha-1 moved beyond preliminary laboratory work, but they do not convert orphan designations into approvals.
NCT00040027 was a SciClone-sponsored Phase 3 study of thymalfasin with peginterferon in non-cirrhotic hepatitis C non-responders. It enrolled 500 participants, began in April 2002, and is listed as completed.
A separate Phase 3 study enrolled 552 hepatitis C non-responders between 2004 and 2009. Its analysis of all randomized participants found that sustained virological response did not reach statistical significance, at 12.7 percent against 10.5 percent with p = 0.407. Among participants who completed the course, the difference did reach significance, at 41.0 percent against 26.3 percent with p = 0.048. The two analyses answer different questions, so the completer result does not erase the non-significant primary population result.
The later COVID-19 record is smaller and unfinished. NCT04487444 was a Phase 2 study of thymalfasin in COVID-19 with lymphocytopenia, run at Rhode Island Hospital from September 10, 2020 to February 3, 2023. It was terminated for slow enrolment. Because enrolment ended the study early, this record should not be presented as evidence of benefit.
Not the same as thymosin beta-4
Thymosin alpha-1 and thymosin beta-4 are different peptides. Their shared family name does not make them interchangeable. Alpha-1 is discussed primarily in immune-signalling research, while beta-4 has different molecular biology involving actin and cell structure. The distinction between TB-500 and thymosin beta-4 adds another naming layer: TB-500 is associated with beta-4, not alpha-1.
The anti-doping record preserves that separation. WADA's Prohibited List does not name thymosin alpha-1. Section S2.3, the growth factors category, names thymosin-beta-4 and its derivatives, for example TB-500. A status statement about beta-4 or TB-500 should not be transferred to alpha-1 simply because all three names contain "thymosin."
Common questions
Is thymosin alpha-1 FDA approved?
No. FDA stated in its December 2024 briefing that it had approved no drug products containing thymosin alpha-1. An orphan designation and a clinical trial registration are both development records, not approvals.
Why are four orphan designations important?
They show sustained formal interest across four rare-condition development programs from 1991 through 2006. Their meaning is limited but specific: each made regulatory incentives available for development. None established safety, effectiveness, or approval.
What is thymalfasin?
Thymalfasin is the international nonproprietary name for synthetic thymosin alpha-1. The naming difference does not create a separate approval status.
Does Curo stock thymosin alpha-1?
No. Curo does not have a thymosin alpha-1 product page or list the compound for sale. This article documents its identity, orphan designations, FDA review, trial history, and distinction from thymosin beta-4.
What is the clearest reading of the record?
Thymosin alpha-1 is neither an unexamined research idea nor a US-approved drug. It is a documented immune peptide with four orphan designations, several substantial clinical studies, no US approval, and a 2024 FDA review that set unusually clear limits on what the agency could verify.