Encyclopedia

Thymosin Alpha-1 (INN thymalfasin) is a 28-amino-acid thymic peptide and immune modulator. It is approved in more than 30-35 countries under the brand name Zadaxin for chronic hepatitis B (and used for hepatitis C, certain cancers, and as an immune adjunct), but it is NOT FDA-approved in the United States. In the US it holds 503A Category 2 status, available by prescription through licensed compounding pharmacies in some states.

Thymosin Alpha-1 was the first peptide isolated from thymic tissue (thymosin fraction 5) and is the N-terminal acetylated 28-amino-acid cleavage product of prothymosin alpha. Its sequence is highly conserved across species. As thymalfasin (Zadaxin), it has decades of international clinical use, principally for chronic hepatitis B, with additional study in hepatitis C, sepsis, certain cancers (hepatocellular, lung, melanoma), vaccine enhancement, and respiratory infections. Its immunomodulatory action is attributed to activation of toll-like receptor 9 (TLR9) on dendritic cells, driving T-helper 1 (Th1) polarization, T-cell maturation, and natural killer (NK) cell activity. It is generally well tolerated, with injection-site reactions the most common adverse effect.

Mechanism of Action

Activates toll-like receptor 9 (TLR9) signaling on dendritic and other immune cells, promoting Th1 polarization, maturation of T cells, increased NK-cell activity, and enhanced interferon production and antibody responses to T-cell-dependent antigens.