Thymosin alpha-1

Definition

Thymosin alpha-1 is a 28-amino-acid peptide originally isolated from thymic tissue, where it participates in the maturation and regulation of T cells. It is an immunomodulator rather than an antiviral or a cytotoxic agent, and its proposed clinical value has always rested on that indirect mechanism.

It is licensed as a medicine in a number of countries outside the United States. It has never been approved here, and the distance between those two facts is what this page records.

Evidence

The trial literature is genuine and it is not small. Across four name variants PubMed returns 870 indexed records, of which 553 carry the Humans subject heading, 95 the clinical-trial publication type, and 65 the randomized-controlled-trial type ^[1]. That is a larger controlled-trial record than any other molecule in this index without an approved United States product.

It is also a literature whose center of gravity sits outside the United States, in chronic viral hepatitis, sepsis, and oncology. A multicenter single-blind randomized controlled trial of the peptide in severe sepsis is representative of the strongest end of it ^[2]. The uses FDA reviewed span that whole range: hepatitis B, hepatitis C, HIV, COVID-19, depressed response to vaccination, adjuvant use with influenza vaccines, malignant melanoma, hepatocellular carcinoma, non-small cell lung cancer, sepsis, and infection after hematopoietic stem cell transplantation ^[3].

A large record and a refusal are not in contradiction. The advisory committee that reviewed the dossier concluded that the evidence did not establish clinical effectiveness and safety in the uses under review, which is a judgment about what the trials show rather than a claim that they do not exist ^[3].

Lawful access

There is no lawful United States supply route and no approved product. On December 4, 2024 the Pharmacy Compounding Advisory Committee voted 4 in favor and 17 against placing Thymosin alpha-1 (free base) on the section 503A bulks list, and 4 to 17 again on Thymosin alpha-1 acetate. The minutes record that members voting against found no compelling evidence demonstrating clinical effectiveness and safety in the uses under review, and that members voting in favor argued the substance should be accessible to patients as an option ^[3].

The nomination was later withdrawn. Thymosin alpha-1 now appears in FDA's table of substances nominated but withdrawn, in no category of the 503A bulks list published on May 14, 2026, and in Category 3 of the 503B list — the category for substances nominated without adequate support ^[4][5]. FDA's published safety finding is that the safety-related information is inadequate for the agency to sufficiently understand the extent of any safety issues raised by the proposed compounded drug ^[5].

A national registration elsewhere confers nothing here. Under section 503A a pharmacy may compound from a bulk substance only where the substance is the subject of an applicable USP or NF monograph, is a component of an FDA-approved drug, or appears on the 503A bulks list; thymosin alpha-1 meets none of the three ^[4]. Material sold under this name in the United States is research-use-only chemical supply. The research-only page sets out the consequences.

Related molecules

The index files thymosin alpha-1 with the immune peptides beside LL-37, which never reached a committee at all. Its refusal belongs with the December 2024 pair — CJC-1295 and AOD-9604 — reviewed on the same day; the nomination tracker files all of them, and the human trial record shows how far this molecule's literature stands above the rest of the unapproved set.

Elsewhere in this index