Thymosin Alpha-1
28-amino-acid thymic immunomodulator marketed abroad as Zadaxin for hepatitis B; researched in sepsis and COVID-19.
Reviewed by Mei Li, PhD, Chief Science Officer
Overview
Thymosin Alpha-1 (Tα1), marketed internationally as thymalfasin under the brand Zadaxin (originally developed by SciClone), is a 28-amino-acid, N-terminally acetylated peptide corresponding to the biologically active N-terminal fragment of the larger thymic protein prothymosin alpha. First characterized from calf thymus in the 1970s, it is among the most extensively studied immunomodulatory peptides in the published literature. Rather than acting as a simple immune stimulant, research describes Tα1 as a context-dependent immune modulator reported to help restore underactive responses while tempering overactive inflammation. It is approved as a pharmaceutical in more than 35 countries for chronic hepatitis B and as an immune adjuvant, but it is not FDA-approved in the United States; in the US it has held only orphan-drug designations (for indications such as chronic active hepatitis B, hepatocellular carcinoma, malignant melanoma, and DiGeorge anomaly), which are not the same as approval. The research literature spans hepatitis B and C, cancer immunotherapy adjunct settings, vaccine response, sepsis, and COVID-19. Clinical trials have generally reported a favorable tolerability profile across thousands of participants. In the United States it remains unscheduled and available only for research use, and it is not among the compounds on the FDA PCAC July 2026 docket.
Mechanism of action
Published mechanistic work describes Thymosin Alpha-1 as acting primarily on the innate-to-adaptive immune interface rather than on a single receptor. It has been reported to signal through Toll-like receptors, particularly TLR9 (and TLR2), on dendritic cells and monocytes, promoting dendritic-cell maturation, cytokine production, and antigen presentation. Downstream effects described in the literature include enhanced differentiation and maturation of T cells, a shift toward Th1-type responses, augmentation of natural killer (NK) cell cytotoxicity, and modulation of regulatory T-cell activity. In states of immune exhaustion or lymphopenia, studies report that Tα1 can help restore T-lymphocyte counts and reverse markers of T-cell exhaustion; in hyperinflammatory states it is characterized as helping to rebalance rather than amplify the response. This bidirectional, homeostatic profile is the basis for its description as an immunomodulator rather than an immunostimulant.
What published research has examined
The largest body of clinical research concerns chronic hepatitis B and hepatitis C, where Tα1 (thymalfasin) has been studied alone and combined with interferon or antivirals, and this indication underlies its approval in many countries. It has also been investigated as an adjuvant intended to improve vaccine responses (including in older adults and dialysis patients) and to support immune function during chemotherapy in oncology research settings such as melanoma and hepatocellular carcinoma. In critical care, the multicenter ETASS trial examined Tα1 in severe sepsis. During the pandemic, numerous cohort studies evaluated Tα1 in COVID-19, with some reports of reduced mortality and restored lymphocyte counts in severe cases, though findings across studies were mixed and data in non-severe patients were inconsistent. All of these represent research and, where applicable, regulatory approvals outside the United States; none constitutes an endorsement of use, and much of the emerging non-hepatitis data remains heterogeneous.
Safety considerations in the literature
Across more than three decades of clinical study, spanning many trials and thousands of participants, the published literature generally characterizes Thymosin Alpha-1 as well tolerated, with adverse events at studied doses reported as infrequent and typically mild, most commonly local injection-site reactions. Because the peptide modulates immune function, reviews note theoretical considerations in the context of autoimmune conditions or organ transplantation, and the literature emphasizes that long-term data outside its approved hepatitis indications and short trial windows remain limited. Much of the strongest safety evidence derives from regulated hepatitis B use abroad; data quality for newer research areas such as COVID-19 is more heterogeneous, and several sepsis and COVID-19 analyses are single-blind or observational rather than large double-blind trials. In the United States the compound is unscheduled and research-use-only, and material sold outside an approved pharmaceutical channel is not subject to the same quality controls, which the literature flags as a purity and identity concern.
Key published studies
| Study | Journal | Year | Finding |
|---|---|---|---|
| The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind, randomized and controlled trial | Critical Care | 2013 | Wu J et al. Multicenter single-blind RCT in 361 ICU patients with severe sepsis; the Tα1 group showed lower in-hospital mortality (28.7% vs 39.4%, RR 0.73, 95% CI 0.54–0.98). Moderate-quality human RCT evidence. |
| Thymosin Alpha 1 Reduces the Mortality of Severe Coronavirus Disease 2019 by Restoration of Lymphocytopenia and Reversion of Exhausted T Cells | Clinical Infectious Diseases | 2020 | Liu Y et al. Retrospective cohort of 76 severe COVID-19 patients reporting reduced 28-day mortality in those receiving Tα1, associated with restored lymphocyte counts and reduced PD-1/Tim-3 expression. Observational, hypothesis-generating evidence. |
| Thymosin alpha 1: A comprehensive review of the literature | World Journal of Virology | 2020 | Dominari A, Hathaway D et al. Comprehensive narrative review summarizing Tα1 mechanism (TLR signaling, T-cell and NK modulation), its approvals for hepatitis B/C in 35+ countries, its US orphan-drug designations, and its reported tolerability across trials. Review-level synthesis. |
Regulatory status
Current status: Unscheduled / RUO. Unscheduled / RUO. PepSense tracks FDA filings and updates this record as the regulatory picture changes. See the FDA PCAC tracker for the July 2026 vote and what happens next.