B tier · viable
It is the only compound here with genuine marketing approval abroad and a properly powered phase 3 trial, but that trial (TESTS, n=1106) missed its primary endpoint and FDA found the evidence inadequate across all twelve conditions it reviewed.
thymosin alpha-1
Thymosin alpha-1 is a synthetic copy of a naturally occurring 28-residue thymic peptide, marketed as Zadaxin and approved in a number of countries outside the US and most of Europe, mainly for chronic hepatitis B and C and as a vaccine adjuvant. Its largest and most rigorous trial, a phase 3 study in sepsis, found no mortality benefit.
// we supply this one
available as a research reagent
≥ 99% HPLC · lyophilised powder · batch certificate published. Grade B above is our own and is not adjusted because we stock it.
the explanation
Thymosin alpha-1 is a lab-made version of a natural immune-signalling molecule from the thymus gland, and it is an approved medicine in dozens of countries though not in the US. The biggest and best-run trial, involving over a thousand patients with sepsis, found it did not reduce deaths.
regulatory status
not FDA-approved; approved in numerous countries abroad
Marketed as Zadaxin with approvals across Asia-Pacific, Latin America, Eastern Europe and the Middle East plus Italy, but unapproved in the US, Japan and most of Europe, and FDA declined to place it on the 503A compounding bulks list citing inadequate evidence and immunogenicity risk.
how it works · proposed mechanism
Thymosin alpha-1 acts as an upstream innate-immune signal rather than a direct effector, which is why its clinical effect is context-dependent.
Toll-like receptor agonism
It signals through TLR2 and TLR9 on dendritic cells and monocytes, activating the MyD88-dependent pathway. This is the best-characterised molecular action of the peptide.
T-cell maturation and Th1 shift
It promotes dendritic cell maturation, T-cell differentiation and a Th1-polarised cytokine profile including IL-2 and interferon-gamma. This underlies its use as a vaccine adjuvant and in chronic viral hepatitis.
Restoration in immunoparalysis
In lymphopenic and immunosuppressed states it has been reported to restore circulating T-cell counts and function. This was the explicit rationale for testing it in sepsis and in COVID-19.
Effect depends on immune state
Because it amplifies an existing immune response rather than supplying one, benefit is expected only in patients who are measurably immunosuppressed. Trials that did not stratify on immune status may have diluted any real effect.
what’s reported
evidence shape
5 sourcesThe composition of the cited literature by study type. This describes the shape of the evidence, not its quality — and it does not by itself set the grade.
⚠ the catch
Its one properly powered, multicentre, double-blind phase 3 trial found a hazard ratio of 0.99 for 28-day mortality, essentially ruling out the large effect earlier small trials had suggested. The favourable meta-analytic signal disappears as soon as the analysis is restricted to high-quality or multicentre studies, which is the classic signature of small-study bias.
key published findings
- TESTS phase 3 trial (BMJ, 2025; 388:e082583) randomised 1,106 adults with sepsis (1,089 in modified intention-to-treat) and found 28-day all-cause mortality of 23.4% (127/542) with thymosin alpha-1 versus 24.1% (132/547) with placebo, hazard ratio 0.99 (95% CI 0.77-1.27), P=0.93.
- Systematic review and meta-analysis (Frontiers in Cellular and Infection Microbiology, 2025) of 11 RCTs and 1,927 patients found reduced 28-day mortality overall (OR 0.73, 95% CI 0.59-0.90, P=0.003), but the effect did not hold in high-quality (OR 0.82, 95% CI 0.65-1.03, P=0.09) or multicentre (OR 0.86, 95% CI 0.68-1.08, P=0.20) subgroups.
- FDA's bulk drug substance review evaluated thymosin alpha-1 across twelve conditions and concluded evidence was inadequate for all, noting that three of four COVID-19 meta-analyses showed no mortality reduction and that hepatitis B/C studies showed no clear advantage over standard therapy.
- Pharmacokinetic characterisation of thymalfasin reports rapid absorption with peak serum levels at approximately 2 hours, an elimination half-life of about 2 hours, and return to basal levels by 24 hours.
- FDA recommended against inclusion on the 503A Bulks List, stating the substances are not well characterised from a physicochemical perspective and citing immunogenicity risk for injectable administration.
limitations of the evidence
- The single properly powered phase 3 trial was neutral, and no indication has a confirmatory positive trial with a hard endpoint.
- Much of the supporting literature comes from small single-centre studies in a limited number of countries, with meta-analytic benefit vanishing under quality restriction.
- Approvals abroad predate modern evidentiary standards for several indications, and FDA considers even the marketed material poorly physicochemically characterised.
identity
| full name | Thymalfasin (thymosin alpha 1), N-acetylated 28-residue thymic peptide |
| category | Healing & Repair |
| modality | peptide |
| formula | C129H215N33O55 |
| molar mass | 3108.28 g/mol |
| cas | 62304-98-7 |
| half-life | ~2 h |
| sequence | Ac-SDAAVDTSSEITTKDLKEKKEVVEEAEN |
laboratory handling
| storage | Supplied as a white lyophilised powder stored at -20 °C for long-term laboratory holding; reconstituted solutions are kept at 2-8 °C for short-term use and are not intended for repeated freeze-thaw cycles. |
| solubility | Water-soluble; reconstituted in sterile water or aqueous buffer for laboratory work, with commercial vials supplied for reconstitution in sterile water. |
| co-studied with | Most often co-studied with interferon-alpha in chronic hepatitis regimens and with standard-of-care antivirals or chemotherapy as an adjuvant. |
Handling information describes laboratory practice for a research reagent. It is not a preparation guide for use in humans.
the receipts
5 citedUS Food and Drug Administration · 2025 · official
US Food and Drug Administration · 2026 · official
BMJ · 2025 · peer-reviewed
Frontiers in Cellular and Infection Microbiology · 2025 · review
BMC Infectious Diseases · 2016 · review
others in Healing & Repair