Thymosin α-1

Thymosin α-1 (Tα1) is a 28-amino-acid peptide derived from prothymosin α, naturally produced by thymic epithelial cells, that modulates innate and adaptive immunity by activating TLR signalling in dendritic cells and monocytes and promoting Th1-type T-cell responses. The thymus undergoes substantial involution from early adulthood, and declining thymic output with age contributes to T-cell senescence and immune ageing; Tα1 supplementation is proposed to partially compensate for this deficit. Thymalfasin (Zadaxin, SciClone Pharmaceuticals) is approved in over 35 countries — but not the United States — as an adjunct therapy for chronic hepatitis B and C and as an immunostimulant in patients with impaired immunity; it has been used off-label during COVID-19 outbreaks in several countries. Human evidence for anti-aging or longevity benefit is absent; trials in cancer and infection support immunomodulatory activity, but effect sizes and consistency vary. Tα1 is widely marketed by anti-aging clinics; its regulatory classification ranges from approved drug to unregulated peptide depending on jurisdiction. FDA added thymosin α-1 to Category 2 of the interim 503A bulks list on 29 September 2023; the substance was removed from Category 2 effective 27 September 2024 after the nominator withdrew its nomination, and on 4 December 2024 the FDA Pharmacy Compounding Advisory Committee (PCAC) voted against its inclusion on the 503A allowed bulks list, leaving it not eligible for use in 503A compounding in the US.