Thymalfasin (Zadaxin): immune-boosting peptide for hepatitis B and C
A lab-made copy of a natural thymus hormone that strengthens the immune system; approved in over 35 countries to treat long-term hepatitis B and C, but not approved in the United States.
A researcher, an agent, or an algorithm wrote down the sequence and picked a target to hit.
An AI model like OpenFold3 or AlphaFold built a 3D structure and scored how well it fits the binding site.
A second contributor repeated the computation on their own hardware and the scores matched.
A chemistry service or a researcher ordered the sequence, it was manufactured, and mass spectrometry confirmed the right molecule was produced.
A binding or activity measurement confirmed that it actually does what the computer predicted — or didn't.
What this is
Thymalfasin (also called Thymosin Alpha-1 or Tα1, sold as Zadaxin) is a 28-amino-acid peptide that the thymus gland produces naturally to regulate immune function. It has been approved in more than 35 countries — including Italy, China, and much of Asia and Latin America — for the treatment of chronic hepatitis B and C and as a vaccine adjuvant in immunocompromised patients. It is not FDA-approved in the United States. Thymalfasin should not be confused with Thymosin Beta-4 (TB-500): despite sharing a family name, they are distinct peptides with different sequences, mechanisms, and indications.
History
Thymosin Alpha-1 was isolated in the 1970s by Goldstein and colleagues at the Albert Einstein College of Medicine, working from a thymus extract preparation called thymosin fraction 5 (Goldstein and colleagues, PNAS 1977). The 28-residue sequence — corresponding to the N-terminal segment of prothymosin alpha — was characterized as the principal immunologically active component of that fraction and shortly after produced synthetically. Goldstein subsequently co-founded SciClone Pharmaceuticals, which commercialized the synthetic peptide as Zadaxin (thymalfasin) for hepatitis B, hepatitis C adjunct therapy, and immune reconstitution. Regulatory approvals in more than 30 countries followed over the subsequent decades. In the United States, Thymosin Alpha-1 was never approved by the FDA despite multiple attempted indications; it remained accessible through compounding pharmacies until 2023, when the FDA removed it from the 503A compounding bulk substances list based on its size-and-complexity risk criteria — not on any specific harm signal.
What it does
Thymalfasin acts on the immune system rather than on a single receptor or tissue. It promotes the maturation and activation of T-cells — the immune system's primary adaptive fighters — and enhances the killing activity of natural killer (NK) cells. At the same time it modulates cytokine production to favor a coordinated Th1-type immune response without simply amplifying inflammation. The practical effect in clinical use is meaningful in specific immunocompromised or dysregulated-immunity contexts: improving the immune response to viral infections like hepatitis B and C, enhancing vaccine responsiveness in elderly or immunocompromised patients, and helping to restore immune homeostasis in conditions like severe sepsis and acute pancreatitis. It is more accurately described as an immune modulator than a generic immune booster — its strongest evidence is in states of immune dysfunction rather than in healthy people with vague complaints.
Mechanism
Thymalfasin acts on toll-like receptors TLR2 and TLR9 on dendritic cells, promoting their maturation and antigen-presentation capacity. This upstream effect drives T-cell differentiation and enhances both NK cell activity and cytotoxic T-lymphocyte function. Thymalfasin also upregulates MHC class I molecule expression, improving immune surveillance. The combined effect is a shift toward Th1-type cytokine profiles while modulating excessive inflammatory responses — the mechanistic rationale for its use in both infectious disease (hepatitis, sepsis) and as a vaccine adjuvant. The structural biology of the peptide has been characterized: NMR and molecular dynamics studies show Tα1 is intrinsically disordered in solution, adopting transient helical elements rather than a fixed fold (Elizondo-Riojas and colleagues, Biochemistry 2011).
Evidence
- Human: Strong. Multiple randomized controlled trials and meta-analyses support efficacy in chronic hepatitis B (as an adjunct to entecavir or interferon-based regimens), chronic hepatitis C (adjunct to peginterferon/ribavirin in non-responders), severe sepsis, and acute pancreatitis. A large randomized trial in metastatic melanoma found no survival benefit when combined with interferon alfa and dacarbazine. RCT-level evidence also exists for vaccine adjuvant effects in elderly patients receiving influenza vaccination. For COVID-19, multiple observational reports and meta-analyses have been published but no large definitive randomized trial has produced a conclusive result. Garaci and colleagues (Annals of the New York Academy of Sciences, 2007) reviewed the full bench-to-bedside clinical trajectory.
- Animal: Well-characterized immunology in preclinical models underpins the human trial program.
- In vitro: Receptor-level mechanism (TLR2/TLR9 activation, dendritic cell maturation, MHC class I upregulation) established in cell-based studies.
Known effects
- Hepatitis B immune response enhancement — Strong (multiple RCTs and meta-analyses; basis for international approval as Zadaxin)
- Hepatitis C adjunct response — Moderate (RCTs in non-responders to interferon/ribavirin; adjuvant role, not monotherapy)
- Severe sepsis immune restoration — Moderate (multicenter RCT evidence, ulinastatin combination protocols)
- Acute pancreatitis immune support — Moderate (double-blind RCT evidence; reduced infection rates in severe acute pancreatitis)
- Vaccine adjuvant in elderly/immunocompromised — Moderate (RCT evidence for influenza vaccination response enhancement)
- COPD exacerbation reduction — Emerging (meta-analysis evidence)
- Long COVID / post-viral immune restoration — Emerging (observational data showing restored T-cell homeostasis; no large RCT)
- Cancer (metastatic melanoma) — Preclinical rationale not confirmed in large RCT; no benefit found in combination with interferon alfa + dacarbazine
Safety signals
Thymalfasin has a favorable safety profile in published trials. Injection site reactions are the most commonly reported adverse effect and are typically mild. The peptide has been well tolerated in trials lasting up to 6–12 months, including in elderly populations and patients with advanced liver disease. No withdrawal or rebound syndrome has been described. Formal drug-interaction studies are limited; available data derive from co-administration with antivirals and immunomodulators in hepatitis and sepsis trials, where no clinically significant pharmacokinetic interference was reported.
Regulatory status
- US: Not FDA-approved for any indication. Removed from the 503A compounding bulk substances list in 2023 (risk-based size-and-complexity criteria, not a safety signal). 503B outsourcing facilities may compound under more restrictive conditions.
- International: Approved in more than 35 countries as Zadaxin or thymalfasin for chronic hepatitis B, hepatitis C adjunct therapy, immune reconstitution in immunocompromised patients, and vaccine adjuvant use. Approved markets include Italy, China, the broader ASEAN region, parts of Latin America, and the Middle East. EU status is mixed — available in some member states under national authorizations, without centralized EMA approval.
- WADA: Not listed by name on the WADA Prohibited List. Thymalfasin is an immune modulator rather than a performance or recovery enhancer in the WADA framework. Athletes subject to WADA code should be aware that WADA's S0 category can capture substances not approved for human therapeutic use in the athlete's jurisdiction.
Myths and misconceptions
- "Thymalfasin is FDA-approved" — It is not FDA-approved for any indication. Approval in more than 35 other countries is a meaningful clinical track record, but approved-elsewhere is not equivalent to FDA-approved, and since 2023 the US compounding pathway has also narrowed.
- "The 2023 FDA compounding decision means it was found to be unsafe" — The decision removed Thymalfasin from the 503A bulk substances list based on size-and-complexity risk criteria for compounded peptides, not on any specific harm signal. Its international safety record is favorable.
- "Tα1 is a general immune booster for anyone with a weak immune system" — Its strongest evidence is in specific dysregulated-immunity contexts (chronic hepatitis, severe sepsis, vaccine response in the immunocompromised). Evidence for general "wellness" use in healthy individuals is substantially thinner.
- "Thymosin Alpha-1 and Thymosin Beta-4 (TB-500) do similar things because they share a name" — They share only a family designation. Tα1 is a 28-residue immune modulator; Thymosin Beta-4 is a 43-residue actin-sequestering peptide primarily associated with tissue repair. They have distinct sequences, mechanisms, and regulatory histories.
- "Tα1 was shown to cure COVID-19 based on Italian and Chinese data" — Early-pandemic observational reports suggested possible benefit in severe COVID-19, but no large randomized trial produced a definitive positive result, and no regulatory body authorized Tα1 for COVID-19.
Related peptides
- LL-37 — a human cathelicidin antimicrobial peptide with innate immune and TLR-modulating activity; related through the innate immunity interface but distinct in origin and primary function.
- Thymosin Beta-4 (TB-500) — the other prominent thymosin-family peptide in clinical and biohacker discussion; different sequence, different mechanism (actin sequestration and tissue repair rather than immune modulation), frequently confused with Tα1 due to the shared name.
Research directions for this peptide, selected from the current sources — hypotheses you can explore and model. None of it is proven yet; tap any one to see the full thinking.
Could thymalfasin help sepsis survivors whose immune systems shut down and leave them vulnerable to new infections?
After surviving sepsis, many patients face weeks of dangerous immune weakness with no approved treatment. If thymalfasin could safely restart their immune response, it might prevent the secondary infections that kill many ICU patients after the initial crisis has passed.
Could thymalfasin restart an immune response in patients whose cancer has become resistant to immunotherapy?
Checkpoint inhibitors help many cancer patients but stop working over time. If thymalfasin reactivates immune cells through a different pathway, it could give those patients a second chance at immune-driven remission.
Does thymalfasin only activate the immune cells that fight viruses without triggering the ones that cause inflammatory storms?
If thymalfasin is selective for the right immune cell type, it could be developed into a safer adjuvant for vaccines and immune therapies, particularly for elderly or immunocompromised patients who need a strong response without inflammation risk.
Could a much shorter version of thymalfasin be just as effective as the full peptide?
If the active region is small, scientists could make a shorter, cheaper version that behaves the same way, making treatments more affordable and easier to deliver for patients in lower-income countries where thymalfasin is already widely used.
Could thymalfasin's real target be a human immune sensor rather than a bacterial membrane?
If true, drug developers could design sharper versions that tune the immune system more precisely, potentially helping patients with infections, cancer, or immune deficiencies without the risk of direct antimicrobial resistance pressure.
▸full evidence table1 metrics
| metric | value | tool |
|---|---|---|
| ranking score | 0.7594963908195496 | boltz-2 |
▸3-letter notation
▸recipeboltz-2 2.2.1
| parameter | value |
|---|---|
| model | boltz-2 2.2.1 |
| weights | — |
| hardware | vast_v100_32gb |
| mlx version | — |
| python | — |
| random seed | 1 |
| msa strategy | none_monomer |
| runtime | — |
| predicted by | — |
| predicted at | 2026-05-23 |
▸citationbibtex
@peptide{pep04429,
sequence = {SDAAVDTSSEITTKDLKEKKEVVEEAEN},
target = {antimicrobial},
author = {peptidemodel},
year = {2026},
status = {bioassayed}
}