Thymosin Alpha-1

What it is

Thymosin Alpha-1 — abbreviated TA1, sold commercially as Zadaxin and known generically as thymalfasin — is a synthetic 28-amino-acid peptide identical in sequence to a naturally occurring fragment of prothymosin alpha, a precursor protein found in the thymus and many other tissues. The peptide is acetylated at the N-terminus, which matches its endogenous form.

The molecule was identified by Allan Goldstein and colleagues in the early 1970s during a long-running effort to characterize the immunoactive components of thymic extract. The 28-amino-acid sequence was sequenced and synthesized in 1977, and the chemically synthetic version is the same as the commercial product today.

History

Goldstein's lab originally isolated a crude thymic preparation in the 1960s and progressively fractionated it through the 1970s. Thymosin Alpha-1 was the first chemically defined component to enter clinical use. SciClone Pharmaceuticals licensed the molecule in the 1990s and pursued approval initially in Asia and southern Europe.

1996 — First approved in Singapore for chronic hepatitis B.
Late 1990s–2000s — Approvals followed in Italy, China, Argentina, Russia, and others. China is by far the largest market.
2008 — A meta-analysis of Chinese hepatitis B trials (Yang et al., Antiviral Research) suggested better long-term virological response on TA1 plus interferon than interferon alone.
2020–2022 — Thymosin Alpha-1 was used in COVID-19 trials in China and Italy; results were mixed and the compound did not receive a formal COVID-19 indication.

In the United States, multiple Phase III trials in chronic hepatitis C did not reach a primary endpoint sufficient for FDA approval, and the molecule remains unapproved in the US.

Regulatory status

Thymosin Alpha-1 is approved in more than 35 countries for chronic hepatitis B, chronic hepatitis C (in some markets), and as an adjuvant to certain vaccines in immunocompromised populations. It is not FDA approved in the United States and is not on the FDA's 503A bulk substances Category 1 list, meaning compounding pharmacies cannot legally prepare it for patients. The FDA placed thymosin alpha-1 in Category 2 of the 503A list in 2023, where it remains under review.

Thymosin Alpha-1 is on the World Anti-Doping Agency's prohibited list under the S2 category (peptide hormones, growth factors).

Mechanism

Thymosin Alpha-1 is described in the published mechanism papers as an immune modulator that acts predominantly through Toll-like receptor 9 (TLR9) on dendritic cells, with downstream effects on T-cell maturation:

TLR9 activation on plasmacytoid dendritic cells, increasing interferon-alpha output and supporting antigen presentation.
T-helper cell maturation, with a shift toward Th1 responses associated with antiviral immunity.
Increased natural killer cell activity, observed in studies of cancer adjuvant use.

The mechanism is consistent with the indications in which it has demonstrated benefit (chronic viral hepatitis, vaccine response in immunocompromised patients) and with the lack of effect in indications driven by other immune pathways.

Half-life and dosing intervals

The published terminal half-life is approximately 2.8 hours after subcutaneous injection. Despite the short half-life, the approved schedule is twice-weekly subcutaneous dosing — the pharmacodynamic effect on immune cells outlasts the parent peptide.

In the approved hepatitis B regimen, the typical dose is 1.6 mg subcutaneously twice weekly (Monday and Thursday, or another consistent two-day spacing) for 6 to 12 months. In adjuvant vaccine settings, courses are typically shorter — 4 to 8 doses over 2 to 4 weeks. The 1.6 mg dose has been used consistently across the major hepatitis trials and represents the approved dose for adults in most markets.

Reconstitution example

Branded Zadaxin ships as a 1.6 mg lyophilized powder with a 1 mL water-for-injection diluent. Reconstitution yields a single-use 1.6 mg/mL solution that is injected subcutaneously.

Research-grade thymosin alpha-1 is typically supplied lyophilized in 5 mg or 10 mg vials. A 5 mg vial reconstituted with 3.1 mL of bacteriostatic water yields a concentration of approximately 1.6 mg/mL — meaning 1 mL on a U-100 insulin syringe (100 units) contains approximately 1.6 mg, the labeled dose. Vial's calculator handles the unit-to-mg conversion automatically.

What to know

Approved abroad, not in the US. Thymosin Alpha-1 is on the market in more than 35 countries but has not been approved by the FDA. US compounding pharmacies cannot legally prepare it.
Best evidence is in chronic hepatitis B. The strongest published data covers chronic hepatitis B, often in combination with interferon. Evidence in other immune indications is weaker.
Banned in competition. Thymosin Alpha-1 is on the WADA prohibited list under S2.
Common side effects in trials: injection-site reactions, mild myalgia, transient flu-like symptoms. Serious adverse events have been uncommon in the published trials.
Storage. Lyophilized Zadaxin and research-grade material are stable refrigerated. Once reconstituted, the labeled product is for single use; research-grade reconstituted material is best refrigerated and used within the stability window specified by the supplier.