Thymosin Alpha-1

A natural immune-signaling peptide cleaved from prothymosin alpha, a thymus-derived protein. The synthetic drug thymalfasin is identical to the human peptide and was developed by SciClone.

The mechanism is not fully understood. Cell and animal work suggests it acts on Toll-like receptors 2 and 9 on antigen-presenting cells, activating downstream immune signaling. Studies report increased T-cell maturation and activity, interleukin-2 and interferon-gamma, and natural killer cell activity.

Thymalfasin has been studied in chronic hepatitis B and C, sepsis, vaccine non-response, as a cancer add-on and in COVID-19. A 1,106-patient phase 3 sepsis trial found no survival difference versus placebo, and two US hepatitis C phase 3 trials missed their endpoints. Older hepatitis B trials showed some higher virologic response, but reviewers now consider that use outdated. Small studies in older adults and dialysis patients reported better vaccine antibody responses, and COVID-19 results are mixed. Use in healthy people for general immune support is essentially unstudied.

Trials report a good safety record; injection-site irritation is the most common complaint, and in the sepsis trial adverse events were about equal on drug and placebo. Reviews of the approved product list hypersensitivity and deliberate immunosuppression (such as transplant patients) as contraindications. FDA flags possible immunogenicity and inadequate safety information for compounded versions; pregnancy data were not found.

Not FDA-approved; it held US orphan designations (melanoma, liver cancer), which are not approvals. FDA lists it as 'nominated but withdrawn' for compounding with an immunogenicity warning, and it is in no 503A category, so it is not lawfully compoundable. Not named on the 2026 WADA Prohibited List.