Thymosin α1

Thymosin α1 is a naturally occurring 28-residue peptide whose synthetic counterpart, thymalfasin, has been studied for its effects on immune cells. Experiments show changes in how cells activate and restrain immune responses. Clinical trials then ask whether those changes help patients with particular diseases. The largest selected sepsis trial, TESTS, did not establish a reduction in deaths; its corrected 2025 results are central to interpreting the earlier studies.

Overview

The thymus helps developing T cells acquire the ability to respond to threats while limiting attacks on the body's own tissues. Thymosin α1 entered research through work on thymic peptides, but its experimental effects extend to mature immune cells outside the thymus. The synthetic peptide has therefore been investigated in conditions where immunity is disrupted by infection, chronic disease or intensive treatment.

The name identifies a defined amino-acid chain. Thymalfasin describes its chemically synthesized counterpart; Zadaxin is a finished medicinal product used in some clinical studies. A finished product also has excipients, a manufacturing specification and a route of administration that matter to interpretation. FDA's chemistry review distinguishes the free peptide from acetate preparations whose counterion content is variable. FDA identity review.

Molecular profile

Identity: Thymosin α1; thymalfasin is the chemically synthesized peptide Source
Length: 28 amino-acid residues Source
Free-peptide sequence: Ac-Ser-Asp-Ala-Ala-Val-Asp-Thr-Ser-Ser-Glu-Ile-Thr-Thr-Lys-Asp-Leu-Lys-Glu-Lys-Lys-Glu-Val-Val-Glu-Glu-Ala-Glu-Asn-OH Source
Free-peptide formula: C129H215N33O55 Source
Free-peptide molecular mass: 3108.3 g/mol Source
Free-peptide CAS: 62304-98-7 Source

Forms and derivatives

Thymalfasin and finished clinical products

Thymalfasin names chemically synthesized thymosin α1. Zadaxin is a finished medicinal product used in some studies; formulation and manufacturing information accompany the peptide identity when interpreting its results.

Thymosin α1 acetate

FDA records the acetate form with variable acetic-acid content, written C129H215N33O55·xCH3COOH. Its review notes inconsistencies between some submitted substance names and identity fields. The free-peptide mass above therefore identifies only the peptide portion.

Uses and research areas

Sepsis provides a demanding test of immune modulation. Patients can develop damaging inflammation at the same time as impaired immune-cell function, and these states change during illness. The ETASS and TESTS trials added thymosin α1 to conventional sepsis care and followed survival. Their design connects an immune hypothesis with an outcome that matters directly to patients. ETASS, TESTS.

Chronic hepatitis B poses a different problem: investigators measured sustained changes in viral markers after a defined treatment period. The older placebo-controlled trial described below tested thymosin α1 in patients who were both HBV-DNA-positive and hepatitis-B-e-antigen-positive. Its response definition belongs to that disease and that study. Laboratory work on dendritic cells provides a biological rationale for immune modulation, while each clinical setting needs its own controlled outcome evidence. Hepatitis-B trial.

Mechanism of action

Dendritic cells connect the detection of foreign material to the activation of T cells. They take up material, display antigens and release signals that influence what kind of response follows. In Romani and colleagues’ 2004 experiments, thymosin α1 promoted maturation and increased interleukin-12 production in dendritic cells exposed to the fungus Aspergillus fumigatus. Interleukin-12 is a signal that helps direct a T-cell response. Separate experiments found better antifungal protection in susceptible mice after blood-forming-cell transplantation. The experiments used immune-cell systems and animal models; their useful contribution is the cellular explanation. Romani et al., 2004.

A later study examined how this response could be restrained. The investigators linked thymosin α1 to indoleamine 2,3-dioxygenase, or IDO, an enzyme that metabolizes tryptophan. In the tested dendritic-cell systems, IDO activity depended on Toll-like-receptor-9 and type-I-interferon signaling and accompanied interleukin-10 production and regulatory T-cell generation. These findings describe a coordinated immune response that includes tolerance. They help explain why a single phrase such as “stronger immunity” gives an incomplete account of the peptide's biology. Romani et al., 2006.

An immune marker can show that a cell changed its behavior without establishing that the whole organism benefited. Survival also depends on organ injury, infection control, timing and the rest of clinical care. This connection between cellular response and patient outcome is the main question running through the sepsis trials.

Pharmacokinetics

Rost and colleagues compared three subcutaneous thymosin α1 formulations in a randomized crossover study of nine healthy volunteers. Serum peptide was measured with an enzyme-linked immunosorbent assay. The indexed report describes mean times to peak of one to two hours, an elimination half-life below three hours, and formulation-dependent differences in systemic exposure. Most participants' endogenous concentrations were below the assay's quantification limit. Rost et al., 1999.

These observations describe serum exposure after the preparations studied. They do not determine how long a downstream change in an immune cell persists. The accessible primary abstract gives a half-life bound across formulations; it supplies neither a preparation-specific estimate with uncertainty nor the individual numerical observations needed to reproduce its concentration curve.

A pharmacokinetic graph is therefore omitted here. The reported timing is retained in the text so that the available human information remains visible.

Research findings

ETASS supplied a survival signal with an uncertain estimate

ETASS analyzed 361 patients with severe sepsis at six Chinese teaching hospitals. Thymosin α1 was added to conventional care for 181 patients, while 180 controls received conventional care. By day 28, 47 peptide recipients and 63 controls had died. This favored the peptide numerically: the relative risk was 0.74, with a 95% confidence interval of 0.54–1.02. The comparison of death proportions gave P=0.062; an analysis that also considered when deaths occurred, the log-rank test, gave P=0.049. Monocytes, a type of white blood cell, also showed a larger increase in HLA-DR, a protein involved in presenting material to other immune cells. Wu et al., 2013.

The mortality confidence interval included no effect, and the two analyses gave different nominal significance judgments. Together with single blinding, this made the result a reason for a larger confirmatory study. HLA-DR is involved in antigen presentation; the marker improvement supplied additional evidence of immune activity within this cohort.

The corrected TESTS trial did not confirm a mortality benefit

TESTS randomized 1,106 adults with sepsis across 22 centers in China and included 1,089 in the modified intention-to-treat analysis. In the corrected report, 28-day deaths occurred in 127 of 542 active participants (23.4%) and 132 of 547 placebo participants (24.1%). The hazard ratio was 0.97 (95% CI 0.76–1.24; P=0.82). Both groups received standard sepsis care. Corrected TESTS report.

The trial's primary question was whether the additional peptide reduced mortality. Its result did not establish that benefit. Age-related and chronic-condition subgroup findings remain exploratory and require prospective confirmation, especially because the paper examined multiple subgroups. A May 30, 2025 correction updated survival estimates and some immune measurements; the corrected hazard ratio replaces the earlier value of 0.99 still displayed in some indexed records. Correction notice.

The hepatitis-B response comparison was also inconclusive

In the 1999 phase 3 hepatitis-B trial, 97 patients received thymosin α1 or placebo for six months and were followed for another six months. Investigators tested for HBV DNA, the virus's genetic material, and HBeAg, a viral protein. A complete response required sustained absence of detectable HBV DNA, with both HBV DNA and HBeAg tests negative at month 12. Seven of 49 peptide recipients and two of 48 placebo recipients met this definition. The result favored the peptide numerically, but was too uncertain to establish a benefit (P=0.084). The investigators concluded that the trial did not confirm the efficacy reported in earlier studies. Mutchnick et al., 1999.

The numerical difference remains part of the record, alongside its uncertainty. It cannot be pooled informally with a sepsis immune marker: the populations, time scales and outcomes address different clinical questions.

Safety and tolerability

Within TESTS's 90-day follow-up, at least one adverse event was reported in 360 of 542 active participants and 370 of 547 placebo participants. Serious events occurred in 145 and 160 participants, respectively. The study reported no unexpected serious event related to thymosin α1. These observations come from severely ill patients receiving trial material and extensive concurrent care. TESTS safety results.

FDA's current compounding-risk page identifies potential immunogenicity, peptide-related impurities and difficulties characterizing the active pharmaceutical ingredient. It says the available safety information is inadequate to determine the extent of the proposed compounded product's safety issues. The official sources checked for this review did not identify a U.S.-approved thymalfasin medicinal product. International product use and an FDA orphan-drug designation have their own regulatory meanings and do not establish a U.S. approval. FDA safety context, FDA review.

What the research means

The evidence supports a biologically interesting immune modulator with disease-specific clinical results. Dendritic-cell experiments explain how activation and restraint can occur within the same response. The sepsis trials then test whether that biology changes survival under clinical conditions. The larger, blinded TESTS study carries particular weight because it directly tested the mortality hypothesis raised by earlier work.

A useful next clinical question concerns a prospectively defined population and a specified outcome. Selecting a subgroup after examining a trial can generate a hypothesis, but confirmation requires a study designed to test it. The selected evidence leaves benefits and risks in healthy people, long-term preventive use and autoimmune conditions unresolved. A laboratory signal alone cannot settle those questions.

FAQs

What does immune modulation mean here?

It means changing how immune cells activate and regulate a response. The dendritic-cell studies measured antigen-presenting-cell behavior and tolerance-related signaling. Whether a change helps a patient depends on the disease and the outcome, which is why the clinical trials are discussed separately from the mechanism experiments.

Did thymosin α1 reduce mortality in TESTS?

TESTS compared thymosin α1 with placebo alongside standard care in adults with sepsis. By day 28, 127 of 542 peptide recipients and 132 of 547 placebo recipients had died. This small numerical difference was too uncertain to establish a mortality reduction: the corrected hazard ratio was 0.97, with a 95% confidence interval of 0.76–1.24. Read the corrected paper with its May 2025 notice; some indexed abstracts still show the earlier estimate.

Why was ETASS viewed as a reason for further study?

ETASS reported 47 deaths among 181 people given thymosin α1 and 63 among 180 controls receiving conventional sepsis care. Although fewer deaths occurred with the peptide, the estimate was imprecise: the relative risk was 0.74 with a 95% confidence interval of 0.54–1.02. Comparing death proportions gave P=0.062, while also considering when deaths occurred gave P=0.049. The trial also found a larger increase in an immune-cell marker. These results gave a reason to test the survival finding in a larger blinded study.

Is thymalfasin the same as thymosin α1?

Thymalfasin describes the chemically synthesized peptide. A brand such as Zadaxin identifies a finished drug product, including its formulation. Acetate identity also needs to be recorded separately when supplied; the molecular profile labels the free peptide explicitly.

Does the short serum half-life establish how long an immune effect lasts?

Serum measurements follow the amount of peptide detectable in blood. An immune cell can continue responding after that amount falls. The human crossover PK study describes exposure, while the cell and clinical studies examine downstream effects on their own time scales.

Does the clinical literature establish preventive benefit in healthy people?

The selected trials enrolled patients with defined diseases and used specific clinical endpoints. They leave long-term preventive use in healthy people unresolved. Clinical trial material also has a preparation and manufacturing history that cannot be inferred from a product name on a research-material label.

Thymosin α1, thymulin and thymosin β4 arose from different lines of peptide research. Their names share a thymic association, while their structures and experimental actions differ. Comparing them is useful for understanding those biological questions; it supplies no basis for substituting one study preparation for another.

References

The sources below include the primary studies, the TESTS correction, the human pharmacokinetic report and official FDA identity and safety material. Each annotation explains the source’s contribution to the biological or clinical interpretation.

Mutchnick MG, Lindsay KL, Schiff ER, Cummings GD, Appelman HD, Peleman RR, Silva M, Roach KC, Simmons F, Milstein S, Gordon SC, Ehrinpreis MN. Thymosin alpha1 treatment of chronic hepatitis B: results of a phase III multicentre, randomized, double-blind and placebo-controlled study. Journal of Viral Hepatitis. 1999;6(5):397–403. DOI: 10.1046/j.1365-2893.1999.00181.x.
In the placebo-controlled hepatitis-B trial, investigators measured sustained absence of detectable viral DNA and negative viral-protein tests. More peptide recipients met the complete-response definition, but the difference was too uncertain to establish benefit.
Wu J, Zhou L, Liu J, Ma G, Kou Q, He Z, Chen J, Ou-Yang B, Chen M, Li Y, Wu X, Gu B, Chen L, Zou Z, Qiang X, Chen Y, Lin A, Zhang G, Guan X. The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind, randomized and controlled trial. Critical Care. 2013;17(1):R8. DOI: 10.1186/cc11932.
The earlier sepsis trial reports a borderline survival signal and changes in monocyte HLA-DR expression. The risk and time-to-event analyses gave different nominal significance results.
Wu J, Pei F, Zhou L, Li W, Sun R, Li Y, Wang Z, He Z, Zhang X, Jin X, Long Y, Cui W, Wang C, Chen E, Zeng J, Yan J, Lin Q, Zhou F, Huang L, Shang Y, Duan M, Zheng W, Zhu D, Kou Q, Zhang S, Liu Y, Yao C, Shang M, Peng S, Zhou Q, Cheng KK, Guan X; TESTS study collaborator group. The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial. BMJ. 2025;388:e082583. DOI: 10.1136/bmj-2024-082583. Corrected May 30, 2025.
The large sepsis trial did not demonstrate a mortality reduction. The corrected publisher analysis is used because the accessed PubMed abstract still displays the earlier hazard ratio.
Romani L, Bistoni F, Perruccio K, Montagnoli C, Gaziano R, Bozza S, Bonifazi P, Bistoni G, Rasi G, Velardi A, Fallarino F, Garaci E, Puccetti P. Thymosin alpha1 activates dendritic cell tryptophan catabolism and establishes a regulatory environment for balance of inflammation and tolerance. Blood. 2006;108(7):2265–2274. DOI: 10.1182/blood-2006-02-004762.
Human and murine dendritic-cell experiments link tryptophan metabolism with regulatory immune responses, providing a specific explanation of immune modulation.
Romani L, Bistoni F, Gaziano R, Bozza S, Montagnoli C, Perruccio K, Pitzurra L, Bellocchio S, Velardi A, Rasi G, Di Francesco P, Garaci E. Thymosin alpha 1 activates dendritic cells for antifungal Th1 resistance through toll-like receptor signaling. Blood. 2004;103(11):4232–4239. DOI: 10.1182/blood-2003-11-4036.
The immune-cell and mouse studies examined dendritic-cell maturation and coordination of immune defense.
BMJ. Correction: The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial. 2025;389:r1098. Published May 30, 2025. DOI: 10.1136/bmj.r1098.
The notice updates survival analyses and immune data. The revised primary hazard ratio remains consistent with the trial's negative mortality conclusion.
Wu J and TESTS study collaborators. Corrected publisher version of the TESTS phase 3 report. BMJ. 2025;388:e082583. DOI: 10.1136/bmj-2024-082583.
This is the same trial as PMID 39814420. Its corrected Table 2, Figure 2 and safety section supply the current numerical results.
U.S. Food and Drug Administration. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks. Current page accessed October 13, 2026; thymosin-alpha 1 entry.
FDA identifies immunogenicity, peptide impurity and ingredient-characterization concerns and an inadequate safety information base for the proposed compounded product.
U.S. Food and Drug Administration. Thymosin Alpha-1-related bulk drug substances evaluation. Briefing document for the December 4, 2024 Pharmacy Compounding Advisory Committee meeting.
The official identity table distinguishes the free peptide and variable acetate form. The review also discusses product identity, safety concerns and the distinction between orphan designation and approval.
Rost KL, Wierich W, Masayuki F, Tuthill CW, Horwitz DL, Herrmann WM. Pharmacokinetics of thymosin alpha1 after subcutaneous injection of three different formulations in healthy volunteers. International Journal of Clinical Pharmacology and Therapeutics. 1999;37(1):51–57.
A nine-person crossover study measures serum exposure after three formulations and reports rapid absorption, short elimination and formulation-dependent exposure.

Glossary

Dendritic cell: An immune cell that displays antigens and helps direct how T cells respond.
Antigen: Material recognized by the immune system, including pieces displayed to T cells.
Regulatory T cell: A T-cell population that helps restrain immune responses and maintain tolerance.
IDO: Indoleamine 2,3-dioxygenase, an enzyme involved in tryptophan metabolism and immune regulation.
mHLA-DR: HLA-DR expression on monocytes, measured in these studies as an immune-state marker related to antigen presentation.
Hazard ratio: A comparison of event rates over follow-up; a value of one indicates equal estimated rates.
Modified intention-to-treat: A trial analysis based on specified eligibility after randomization; TESTS included participants who received study drug.
HBeAg: Hepatitis B e antigen, a viral marker used with HBV DNA in the cited response definition.