Thymosin Alpha-1

Thymosin Alpha-1 is a synthetic thymic peptide with 28 amino acids. The international non-proprietary name is thymalfasin. The brand name is Zadaxin, from SciClone Pharmaceuticals.

Thymosin Alpha-1 is an endogenous immunomodulator. It acts on innate immunity and on adaptive immunity.

It regulates T-cell subsets and inflammatory mediators.

Evidence tier: clinical.

Approved use. Clinicians use it for chronic hepatitis B and C, and as an adjunct in cancer immunotherapy.

The positive trial. The ETASS multicenter randomized controlled trial in China studied severe sepsis. The result suggested a benefit.

The larger negative trial. The TESTS Phase 3 trial (Wu et al., BMJ 2025) enrolled 1,106 participants. It found no overall decrease in mortality. Subgroups of elderly and diabetic patients showed possible signals.

How to read this. A larger trial replaced a smaller positive result with a null result. The subgroup signals need a new trial. They are not a conclusion.

Source: the approved label outside the United States.

• Hepatitis: 1.6 mg by subcutaneous injection, twice each week.
• Sepsis trials in intensive care: 1.6 mg twice each day, for up to 5 days.

A prescriber controls both schedules.

The route is subcutaneous. The standard schedule is twice each week.

Hepatitis research combines Thymosin Alpha-1 with an interferon. The immunostimulation is additive. That combination needs supervision.

Compare with KPV, which acts on inflammation through a different route.

A 2024 review of human trials describes good tolerability across indications. Injection site reactions occur.

• Complete blood count (CBC).
• Liver panel.
• Viral load, in a hepatitis context.

More than 35 countries approve thymalfasin (Zadaxin). These include China, India, the Philippines, Russia, and Italy. SciClone launched it in the 1990s.

The FDA does not approve it. It reaches United States patients through compounding.

The WADA Prohibited List does not name it. See the regulatory tracker.