RESEARCH PROFILE
THYMOSIN ALPHA-1
Thymalfasin • Immune-Modulating Peptide
Thymosin Alpha-1, commonly abbreviated TA1, is a naturally occurring 28-amino-acid thymic peptide involved in immune-system regulation. Its synthetic pharmaceutical form, thymalfasin, has been studied extensively in chronic viral infections, cancer-treatment support, severe infections, and immune-modulation research.
Thymalfasin has decades of international clinical use and is approved as Zadaxin in more than 35 countries, but Thymosin Alpha-1 is not FDA approved in the United States.
EXTENSIVE HUMAN CLINICAL RESEARCH • INTERNATIONAL APPROVALS • NOT FDA APPROVED IN THE U.S.
THYMOSIN ALPHA-1 AT A GLANCE
Generic / Research Name
Thymosin Alpha-1 • TA1
Pharmaceutical Name
Thymalfasin
Brand Name
Zadaxin
Compound Type
28-amino-acid thymic peptide
Primary Research Focus
Immune modulation • T-cell function • Viral infection • Cancer-treatment adjunct research
Primary Research Pathways
TLR2 • TLR9 • Dendritic cells • T-cell signaling
Human Evidence
Decades of international clinical research
Regulatory Status
Approved internationally in 35+ countries • Not FDA approved in the United States
IMPORTANT DISTINCTION
Thymosin Alpha-1 is not the same compound as Thymosin Beta-4 or TB-500.
TA1 is a 28-amino-acid peptide associated primarily with immune-system regulation.
Thymosin Beta-4 is a 43-amino-acid peptide associated primarily with actin biology, cell migration, and tissue-repair research.
Although both belong to the broader thymosin family, their structures, mechanisms, evidence bases, and research applications are different.
WHAT IS THYMOSIN ALPHA-1?
Thymosin Alpha-1 is a naturally occurring peptide originally isolated from thymic tissue.
It consists of 28 amino acids and participates in regulation of immune function.
Its synthetic pharmaceutical form is known as thymalfasin.
THE THYMUS & IMMUNE FUNCTION
The thymus plays an important role in development and maturation of T lymphocytes, or T cells.
These immune cells participate in:
Recognition of infected cells
Adaptive immune responses
Immune surveillance
Coordination of cellular immunity
TA1 research has focused on how the peptide influences these immune systems rather than directly attacking pathogens or abnormal cells itself.
ZADAXIN
Synthetic thymalfasin is marketed internationally under the brand name Zadaxin.
Your Project source describes international approval in more than 35 countries, primarily involving:
Chronic hepatitis B
Chronic hepatitis C
Cancer-treatment adjunct contexts
The specific approved indications vary by country.
A DIFFERENT TYPE OF RESEARCH PEPTIDE
TA1 differs substantially from many peptides in the Croix library.
Its evidence base includes:
Randomized controlled trials
Long-term viral-hepatitis research
Cancer adjunct studies
Sepsis trials
Systematic reviews and meta-analyses
Decades of international clinical use
This makes it a substantially more human-studied compound than many research peptides.
MECHANISM OF ACTION
Thymosin Alpha-1 is best characterized as an immune modulator rather than a conventional immune stimulant.
Its proposed function is to influence coordination and responsiveness within existing immune pathways.
TLR2 & TLR9
TA1 interacts with immune pathways involving:
Toll-like receptor 2 — TLR2
Toll-like receptor 9 — TLR9
Toll-like receptors help the immune system recognize molecular patterns associated with pathogens and cellular danger.
DENDRITIC CELLS
Downstream signaling involving TLR2 and TLR9 can influence dendritic cells.
Dendritic cells function as antigen-presenting cells that help initiate and coordinate adaptive immune responses.
T-CELL FUNCTION
TA1 research has examined effects involving:
CD4+ T cells
CD8+ T cells
T-cell differentiation
Immune-cell maturation
Cell-mediated immunity
These pathways help coordinate immune responses to infection and abnormal cells.
IMMUNE BALANCE
Your Project source describes TA1 as potentially influencing immune balance toward a more effective cellular immune response.
This is why TA1 is better described as an immune modulator than simply an immune booster.
AN IMMUNE MODULATOR, NOT A DIRECT ANTIMICROBIAL
TA1 does not directly kill:
Viruses
Bacteria
Cancer cells
Instead, its research rationale involves modifying the host immune response.
IMPORTANT MECHANISM LIMITATION
Effects on Toll-like receptors, dendritic cells, cytokines, or T-cell activity do not establish that TA1 prevents infection, cures immune disease, or improves immunity in otherwise healthy people.
Mechanistic evidence and demonstrated clinical outcomes should remain separate.
WHAT THE RESEARCH SHOWS
TA1 has one of the longest clinical research histories among compounds in the Croix peptide library.
However, the strength of evidence differs substantially by indication.
CHRONIC HEPATITIS B
Your Project source characterizes chronic hepatitis B as one of the stronger historical evidence areas.
A pooled analysis of three randomized controlled trials involving Zadaxin reported sustained virologic-response advantages compared with placebo.
The source reports approximately:
16% response with TA1
versus
6% with placebo at six months
and approximately:
12 months after therapy: 36% with TA1 versus 11% with placebo
These findings come from older hepatitis B treatment contexts and should not be interpreted as establishing TA1 as current first-line HBV therapy.
CHRONIC HEPATITIS C
TA1 has also been studied in chronic hepatitis C, particularly in combination with interferon.
Your Project source cites pooled analyses involving randomized controlled trials and historical controls.
One analysis reported sustained ALT normalization in approximately:
22.4% with TA1 plus interferon
versus
9.3% with interferon alone
Modern direct-acting antiviral therapy has largely replaced these historical HCV treatment approaches.
CANCER-TREATMENT ADJUNCT RESEARCH
TA1 has been investigated as an adjunct during cancer treatment.
Your Project profile identifies research involving:
Hepatocellular carcinoma
Non-small-cell lung cancer
Melanoma
Some randomized and observational studies reported findings involving:
Infection rates
Immune-cell preservation
Treatment tolerance
Clinical outcomes
However, these studies are heterogeneous, and TA1 is not an FDA-approved cancer treatment.
SEVERE SEPSIS — 2015 TRIAL
Your Project source identifies a multicenter randomized controlled trial involving 1,068 adults across 22 Chinese centers.
The study reported lower 28-day all-cause mortality in the thymosin-alpha-1 group than placebo.
This contributed to substantial interest in TA1 for severe infection.
SEVERE SEPSIS — TETIS TRIAL
The evidence became more complicated with the newer TETIS trial.
Your Project profile identifies the trial as enrolling approximately 1,100 adults with sepsis across 22 Chinese centers.
The primary endpoint was negative.
The source reports 28-day all-cause mortality of approximately:
0.94 hazard ratio
with a 95% confidence interval of 0.76 to 1.16
and p = 0.54.
This means the newer large randomized trial did not demonstrate a statistically significant mortality benefit.
SEPSIS META-ANALYSIS
Your Project source also cites a 2025 meta-analysis of 11 randomized controlled trials involving approximately 1,827 patients.
That analysis reported a pooled mortality risk ratio of approximately 0.73.
However, the authors identified substantial heterogeneity and credibility concerns.
POST-COVID & LONG-COVID RESEARCH
TA1 has also been investigated following COVID-19.
Your Project source cites a randomized trial involving 819 adults.
The study reported lower mortality in some subgroups but did not demonstrate consistent improvement in length of stay.
Dedicated long-COVID research remained limited in the Project source.
GENERAL IMMUNE SUPPORT & ANTI-AGING
This distinction is especially important for the Croix website.
TA1's mechanism makes it attractive for generalized “immune support” or anti-aging claims.
However, your Project profile states that dedicated trials in healthy adults using TA1 for general immune enhancement or longevity are limited.
IMPORTANT EVIDENCE LIMITATION
TA1 has substantial human research.
But the evidence is indication specific.
Historical hepatitis research does not establish modern first-line treatment.
Cancer-adjunct findings do not establish anticancer efficacy.
Mixed sepsis results do not establish a universal survival benefit.
And clinical research in disease populations does not prove that TA1 improves immune function in healthy people.
SAFETY & RESEARCH CONSIDERATIONS
Thymosin Alpha-1 has accumulated a substantial human safety record through decades of international clinical use and research.
Your Project source describes its overall safety profile as generally favorable.
INJECTION-SITE REACTIONS
The most commonly reported effects involve the injection site.
These can include:
Redness
Tenderness
Swelling
Local irritation
They are generally described as mild and transient.
TRANSIENT ALT FLARE
Your Project profile identifies a transient increase in ALT as a documented finding in hepatitis B treatment contexts.
The Zadaxin label describes this as an observed treatment-related phenomenon rather than automatically indicating treatment failure.
Persistent liver-enzyme elevation, jaundice, or signs of liver dysfunction require appropriate medical evaluation.
FATIGUE
Mild transient fatigue has occasionally been reported.
FLU-LIKE SYMPTOMS
Some participants have reported mild flu-like symptoms, potentially reflecting immune-system activation.
HEADACHE
Headache has been reported but appears relatively uncommon in the published clinical record.
GI DISCOMFORT
Mild gastrointestinal discomfort has been reported in a minority of participants.
RARE REACTIONS
Your Project source identifies rare reports involving:
Polyarthralgia with hand edema
Hypersensitivity or allergic reaction
DRUG-INTERACTION PROFILE
The Project profile describes TA1 as having a relatively clean documented drug-interaction profile.
Published trials and the Zadaxin label have not identified major conventional pharmacokinetic interactions.
However, that does not mean interactions are impossible.
IMMUNOSUPPRESSIVE THERAPY
This is one of the most important interaction considerations.
Because TA1 modifies immune-cell activity, combining it with:
Immunosuppressants
Biologic immune therapies
Other immune-modulating drugs
can create theoretically opposing or unpredictable effects.
AUTOIMMUNE DISEASE
Because TA1 can enhance aspects of cell-mediated immunity, the Project source identifies active autoimmune disease as an area requiring caution and clinical oversight.
ORGAN TRANSPLANTATION
TA1 should not be treated casually in organ-transplant recipients receiving immunosuppressive therapy.
Its immune-modulating mechanism could theoretically interfere with the therapeutic goal of suppressing immune rejection.
PREGNANCY & BREASTFEEDING
Human safety data during pregnancy and lactation are limited.
REGULATORY CONTEXT
THYMOSIN ALPHA-1 IS NOT FDA APPROVED
Thymosin Alpha-1 has never received FDA approval for a therapeutic indication in the United States.
Your Project source notes that SciClone advanced TA1 through U.S. clinical development during the 1990s and 2000s, but those programs did not result in FDA approval.
INTERNATIONAL APPROVAL
The regulatory picture is different outside the United States.
Your Project source identifies thymalfasin as approved under the brand name Zadaxin in more than 35 countries.
Approvals have historically included indications involving:
Chronic hepatitis B
Chronic hepatitis C
Cancer-treatment adjunct use
Specific labeling varies by jurisdiction.
U.S. COMPOUNDING STATUS
Your Project profile documents an evolving FDA compounding history.
In September 2023, FDA placed TA1 among substances in Category 2 of the interim 503A bulk-drug-substances framework, citing potential significant safety concerns.
Litigation subsequently challenged aspects of that process.
SEPTEMBER 2024
Your Project source states that FDA announced TA1 was being removed from Category 2 because its original nominators withdrew their nominations.
DECEMBER 2024 PCAC REVIEW
The FDA Pharmacy Compounding Advisory Committee subsequently reviewed TA1-related bulk-drug-substance questions.
Your Project profile states that FDA had presented safety considerations but had not made a final determination on TA1's ultimate 503A status as of the profile's update.
IMPORTANT REGULATORY DISTINCTION
Removal from an interim FDA compounding category does not mean:
FDA approval
FDA confirmation of efficacy
FDA confirmation of safety for general use
Approval of research-grade products
RESEARCH-GRADE TA1
A research-grade lyophilized TA1 vial is not the same thing as the internationally approved Zadaxin pharmaceutical product.
IMPORTANT REGULATORY DISTINCTION
International approval ≠ FDA approval
Clinical use abroad ≠ U.S. approval
Compounding review ≠ therapeutic approval
Zadaxin ≠ unrelated research-grade TA1
ANALYTICAL DOCUMENTATION
Croix maintains available third party analytical documentation only for specific samples and lots that can be matched to corresponding laboratory reports.
LOT SPECIFIC ANALYTICAL DOCUMENTATION
Analytical documentation is displayed only when Croix has a corresponding laboratory report that can be matched to the specific product and lot.
No matching Thymosin Alpha-1 COA is currently displayed.
IMPORTANT COA LIMITATION
When analytical documentation becomes available, results should apply only to the specific sample identified in the corresponding laboratory report.
A Certificate of Analysis for one sample does not establish the composition, purity, sterility, safety, efficacy, or characteristics of another sample, lot, batch, or product.
Croix requires the product, strength, lot or batch, sample ID, laboratory, and report date to be matched before analytical documentation is represented as supporting a product. Croix_Peptides_Brand_Pricing_Op…
A passing COA does not establish FDA approval, clinical efficacy, medical suitability, or the characteristics of unrelated lots.
SELECTED RESEARCH & REFERENCES
Wu J, Pei F, Zhou L, et al.
The efficacy and safety of thymosin α1 for sepsis (TETIS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial.
BMJ. 2025.
Large contemporary Phase 3 sepsis trial.
Gu B, Zhou Y, Nie Y, et al.
Efficacy of thymosin α1 for sepsis: a systematic review and meta-analysis of randomized controlled trials.
Frontiers in Cellular and Infection Microbiology. 2025.
Systematic review and meta-analysis of randomized sepsis research.
Dominari A, et al.
Thymosin alpha 1: A comprehensive review of the literature.
World Journal of Virology. 2020.
Broad review of TA1 biology and clinical research.
Tao N, et al.
Thymosin α1 and Its Role in Viral Infectious Diseases: The Mechanism and Clinical Application.
Molecules. 2023.
Review of immune mechanisms and infectious-disease research.
Dinertz E, Lee E.
Comprehensive Review of the Safety and Efficacy of Thymosin Alpha 1 in Human Clinical Trials.
Alternatives Therapies in Health and Medicine. 2024.
Human clinical evidence and safety review.
SciClone Pharmaceuticals
Zadaxin (thymalfasin) Professional Monograph.
International pharmaceutical information concerning approved indications and clinical use.
Rost KL, Wierich W, Masayuki F, Tutih C, Horwitz D, Herrmann W.
Pharmacokinetics of thymosin α1 after subcutaneous injection of three different formulations in healthy volunteers.
International Journal of Clinical Pharmacology and Therapeutics. 1999.
Human pharmacokinetic research.
U.S. Food & Drug Administration
Bulk Drug Substances Nominated for Use in Compounding Under Section 503A.
U.S. compounding regulatory context.
U.S. Food & Drug Administration
PCAC Topic 1: Thymosin alpha-1-related bulk drug substances — December 4, 2024.
FDA advisory-committee materials.
Tian Y, et al.
Thymosin alpha 1 alleviates inflammation and prevents infection in patients with severe acute pancreatitis through immune regulation: a systematic review and meta-analysis.
Frontiers in Immunology. 2025.
Systematic review of immune-modulating clinical research.
EVIDENCE NOTE
Thymosin Alpha-1 has substantially more human clinical evidence than many compounds commonly categorized as research peptides.
Its evidence base includes randomized trials, systematic reviews, international pharmaceutical use, and decades of clinical research.
However, results differ by disease state. The negative primary endpoint in the large TETIS sepsis trial is particularly important because it demonstrates why earlier favorable studies and pooled analyses should not be interpreted as universal proof of efficacy.
International approval of thymalfasin also should not be generalized to unrelated research-grade TA1 products or unapproved uses.
RESEARCH & EDUCATIONAL INFORMATION
This profile is provided for research and educational reference. It does not constitute individualized medical advice.
Thymosin Alpha-1 is a 28-amino-acid immune-modulating peptide with decades of international human clinical research involving chronic viral infections, cancer-treatment adjunct contexts, severe infection, and immune regulation.
Its synthetic pharmaceutical form, thymalfasin, is approved as Zadaxin in more than 35 countries, although specific approved indications vary by jurisdiction.
Thymosin Alpha-1 is not FDA approved for any therapeutic indication in the United States.
International approval, historical clinical use, or favorable findings from specific disease populations should not be interpreted as establishing efficacy for generalized immune enhancement, anti-aging, infection prevention, or other unapproved applications.
Research-grade Thymosin Alpha-1 should not be represented as equivalent to an internationally approved Zadaxin pharmaceutical product.
Analytical documentation presented by Croix applies only to the specific samples and lots identified in corresponding laboratory reports.