Thymosin Alpha-1

What is thymosin alpha-1?

Thymosin alpha-1 is an N-terminally acetylated 28-amino-acid peptide first isolated from an immunologically active bovine thymus fraction. Once its sequence was established, the molecule could be synthesized chemically. Thymalfasin is the drug name for synthetic Tα1, and Zadaxin is a finished thymalfasin product marketed in parts of Asia-Pacific, Latin America, Eastern Europe, and the Middle East.

Tα1 is generally described as an immunomodulator, not as a direct antimicrobial and not as a growth-hormone secretagogue. Proposed actions include dendritic-cell activation, Toll-like-receptor signaling, antigen presentation, T-cell differentiation, natural-killer activity, and context-dependent cytokine signaling. These effects are not equivalent to a predictable whole-body “immune boost.”

The free-base chemical record lists formula C129H215N33O55 and molecular mass 3,108.3 Da. Tα1 acetate contains the same active peptide plus variable acetate counterion. A label stating only “10 mg thymosin alpha-1 acetate” may not reveal whether the amount is peptide free-base equivalent, total salt mass, or assay-corrected amount. ([PubChem thymalfasin](https://pubchem.ncbi.nlm.nih.gov/compound/16130571); [FDA Tα1 briefing](https://www.fda.gov/media/183820/download))

Identity and naming checks

Confusing Tα1 with thymosin beta-4 or TB-500 is a common search and labeling error. The two molecules have different sequences, targets, clinical histories, and doses. A “thymosin protocol” without a complete identity statement is not reproducible.

An HPLC chromatogram labeled “99% purity” cannot establish peptide quantity, counterion content, sterility, endotoxin, aggregation, or usable shelf life. FDA's 2024 review noted that public bulk-substance certificates lacked important impurity, aggregate, bioburden, and endotoxin information.

U.S. and international status

There is no U.S. prescribing label or FDA-approved dosage for thymosin alpha-1. Thymalfasin has been licensed or marketed in parts of Asia-Pacific, Latin America, Eastern Europe, and the Middle East, with indications varying by country. Zadaxin is not approved in Japan or most of Europe; Italy is a notable exception for a vaccine-related indication.

In December 2024, the U.S. Pharmacy Compounding Advisory Committee reviewed Tα1 free base and acetate. The committee voted 4 yes, 17 no on adding each substance to the section 503A bulks list. The vote was an advisory compounding-policy recommendation — not a finding that no pharmacology exists. Main concerns were weak effectiveness evidence for proposed U.S. uses, incomplete bulk characterization, concentration/solubility questions, aggregation, and potential immunogenicity. ([FDA meeting page](https://www.fda.gov/advisory-committees/advisory-committee-calendar/updated-meeting-time-and-public-participation-information-december-4-2024-meeting-pharmacy); [final minutes](https://www.fda.gov/media/185642/download))

PCAC vote ≠ revocation of foreign approvals. Dose familiarity from international labels and trials is not the same as proven indication-specific benefit in contemporary U.S. wellness settings.

International product dosage

The traditional single-use Zadaxin vial contains 1.6 mg, is reconstituted with 1.0 mL sterile water for injection immediately before use, and produces 1.6 mg/mL. Product materials instruct immediate use after reconstitution. These facts cannot be transferred to multidose 5 mg, 10 mg, or 15 mg research vials.

Dosage used in human clinical trials

The doses below are exposures evaluated in trials or specified by study registries. They are not a single treatment algorithm, and one study's dose does not establish a dose for another condition. A seven-day ICU sepsis exposure, a 24-week hepatitis schedule, and a one-year oncology program cannot be merged without losing clinical context.

Pharmacokinetics and what they do not prove

In a randomized three-formulation crossover study, nine healthy volunteers received 900 mcg/m² SC. Mean time to peak was one to two hours, maximum concentrations were about 30–80 mcg/L, elimination half-life was under three hours, and exposure after repeated short-term dosing did not show accumulation. ([Rost et al., 1999](https://pubmed.ncbi.nlm.nih.gov/10027483/))

This supports that Tα1 reaches circulation quickly and declines quickly, that repeated exposure over several days did not accumulate in this small study, and that formulation can change exposure even when nominal peptide dose is similar. It does not show that a two-hour half-life requires dosing every few hours, that morning or bedtime is superior, or that serum presence equals duration of downstream immune signaling.

Chronic hepatitis B: where 1.6 mg twice weekly became established

The clearest historical dosing program used 1.6 mg SC twice weekly for six months, often written as 900 mcg/m². The Chien randomized trial and other older studies reported delayed improvements in HBV DNA, HBeAg, or ALT endpoints after treatment ended. Other trials were negative or inconclusive.

One Japanese randomized dose comparison enrolled 316 participants and used an intensive first two weeks: 0.8 or 1.6 mg SC six times per week, followed by twice-weekly dosing for 22 weeks. Outcomes were similar between dose groups. Twenty-two participants experienced transient ALT flares. That design was studied — not evidence that every Tα1 protocol should start with frequent dosing. ([Iino et al., 2005](https://pubmed.ncbi.nlm.nih.gov/15850471/); [Chien et al., 1998](https://pubmed.ncbi.nlm.nih.gov/9581695/))

Current chronic-HBV care relies on specialist staging and potent nucleos(t)ide analogues. Tα1 should not replace antiviral therapy or hepatocellular-carcinoma surveillance.

Chronic hepatitis C: studied doses, outdated treatment backbone

Older HCV trials commonly added 1.6 mg SC twice weekly to interferon or peginterferon, sometimes with ribavirin, for six to twelve months. In a 552-participant randomized trial of prior nonresponders, Tα1 plus peginterferon/ribavirin produced SVR of 12.7% versus 10.5% with placebo — not significant. ([Pockros et al.](https://pubmed.ncbi.nlm.nih.gov/22233415/)) Direct-acting antiviral combinations now cure most HCV without interferon.

Sepsis: the large confirmatory trial matters most

The 2013 ETASS trial enrolled 361 adults with severe sepsis. The Tα1 group received 1.6 mg SC twice daily for five days, then once daily for two days. Twenty-eight-day mortality was 26.0% with Tα1 and 35.0% with control — conventional comparison P = 0.062. ([ETASS](https://pubmed.ncbi.nlm.nih.gov/23327199/))

The confirmatory TESTS Phase III trial enrolled 1,106 adults at 22 centers, double-blind, placebo-controlled. Participants received 1.6 mg SC every 12 hours for up to seven days. Twenty-eight-day mortality was 23.4% with Tα1 and 24.1% with placebo; hazard ratio 0.99, 95% CI 0.77–1.27, P = 0.93. No secondary or safety outcome differed significantly. ([Wu et al., BMJ 2025](https://pubmed.ncbi.nlm.nih.gov/39814420/))

1.6 mg twice daily is an ICU trial exposure, not an at-home acute-infection protocol, and the best Phase III evidence did not show a sepsis mortality benefit. Suspected sepsis requires emergency assessment — not a peptide cycle.

Vaccine, oncology, and other human programs

Older vaccine studies used 900 mcg/m² twice weekly for two to four weeks or one/two high doses around vaccination. The Carraro hemodialysis pilot gave 3.2 or 6.4 mg SC seven days before and on H1N1 vaccination day — mixed assay results. FDA concluded available studies did not establish an optimal dose or reliable benefit with contemporary U.S. vaccines.

Tα1 has been evaluated as an adjunct in melanoma, hepatocellular carcinoma, NSCLC, HIV, COPD exacerbations, and post-transplant immune recovery. Doses ranged from 1 mg to 6.4 mg per day in some melanoma studies to 900 mcg/m² twice weekly in NSCLC. Many studies were small, open-label, or combined Tα1 with outdated therapies. Post-HSCT literature is especially unsuitable for community borrowing.

Formal dose escalation and dose-intensity studies

The highest tested single dose is not a recommended maximum. FDA noted no well-defined minimum pharmacologic dose and no clear dose-response relationship across the program.

Thymosin alpha-1 research dosage

The community landscape is easier to interpret when each schedule is converted to total weekly exposure. Repetition documents convention; it does not validate efficacy, safety, or cycle length.

Clinical research versus community reports

The biggest discrepancy is frequency, not the nominal 1.5-versus-1.6 mg amount. A 1.5 mg five-on/two-off protocol provides 7.5 mg per week — about 2.34 times the 3.2 mg weekly exposure of 1.6 mg twice weekly. Calling these schedules “the same dose” because each injection is near 1.6 mg is mathematically incorrect.

Reported research dosage range

Dosage by weight

The fixed 1.6 mg adult dose dominates the literature. International materials use 40 mcg/kg for people under 40 kg, which converges on 1.6 mg at 40 kg. The table reconstructs that foreign product rule — not a general weight-based dosing tool. Applying 40 mcg/kg above 40 kg would create unsupported doses (e.g., 4 mg at 100 kg).

Complete community-anchored 12-week research protocol

The protocol below is a prospective, fixed-exposure research design studying the dominant short community schedule at the best-documented per-dose amount and frequency. It is not a personal treatment plan. Any human implementation requires qualified investigator oversight, ethics review, regulatory authorization where applicable, verified parenteral product, and predefined endpoints.

Reconstitution and U-100 syringe math

Reconstitution arithmetic answers only how much labeled peptide is present in a stated final volume. It does not establish that the material dissolves at that concentration, stays monomeric, remains sterile, or has a validated beyond-use date.

Concentration (mg/mL) = peptide in vial (mg) ÷ final volume (mL) · Dose volume (mL) = target dose (mg) ÷ concentration · U-100 units = dose volume (mL) × 100.

Adding 2 mL to a 10 mg vial gives 5 mg/mL — more than twice the highest clinical strength FDA identified and above published free-base solubility (~2 mg/mL). A clear solution still does not exclude soluble aggregates or potency loss. 10 mg + 2 mL is not included as a validated preset.

Why 1.6 mg and twice-weekly dosing recur

1. BSA development: 900 mcg/m² ≈ 1.53 mg for a 1.7 m² adult, close to the 1.6 mg vial size.
2. Finished presentation: the commercial single-use vial standardized one full administration at 1.6 mg.
3. Clinical replication: hepatitis, oncology, vaccine, and infection programs repeatedly borrowed the established formulation.
4. Tolerability: local injection reactions were usually mild at common trial doses.
5. Lack of clear dose-response: higher doses did not consistently produce greater effects — escalation lacked empirical advantage.

Mechanism and dose-response limits

1. Dendritic-cell signaling: nonclinical work implicates TLR9 and, in some systems, TLR2, with downstream MyD88, NF-κB, and interferon-regulatory pathways.
2. Antigen presentation: Tα1 can influence dendritic-cell maturation and MHC expression in some cell models.
3. T-cell and NK effects: changes in T-cell differentiation, CD4/CD8 function, Th1 cytokines, and cytotoxic responses reported in some models.
4. Regulatory feedback: effects on IDO, regulatory T cells, and IL-10 suggest context-dependent damping as well as stimulation.
5. No reliable concentration-response curve linking dose to validated clinical outcome in humans — “more” cannot be assumed to mean “better.”

Animal and preclinical research dosage

Animal / preclinical research only. These doses are not human protocols and should not be converted into community doses.

FDA noted many pharmacology experiments used animal exposures markedly higher than the usual 1.6 mg human SC dose. Human-equivalent-dose calculations do not repair species, route, and formulation differences.

What results can research reasonably measure?

Dosage evidence ladder

The 1.6 mg SC amount and twice-weekly schedule are well established as human exposures and have more clinical history than most peptides marketed by U.S. wellness clinics. What remains poorly established is how that exposure should be used for contemporary diseases, short wellness cycles, daily protocols, nasal products, or peptide stacks.

Safety and tolerability

Across many trials, the most consistent Tα1-attributed events were injection-site pain, redness, occasional rash, and transient flu-like symptoms. Many systemic adverse events occurred in combination trials with interferon, chemotherapy, critical illness, or transplantation — causality is difficult to isolate.

Seek urgent evaluation for anaphylaxis signs, chest pain, severe shortness of breath, fever with rigors or suspected sepsis, spreading injection-site reaction, suspected autoimmune flare, or pregnancy during exposure.

Storage and handling

Do not repeatedly freeze-thaw. Quarantine material after temperature excursion, haze, visible particles, or precipitation. Absence of visible change does not prove stability.

Anti-doping status

Thymosin alpha-1 is not a growth-hormone secretagogue and is not the same as TB-500. However, the 2026 WADA Prohibited List includes S0 Non-Approved Substances, which can cover pharmacologic substances without current approval by a governmental regulatory health authority for human therapeutic use. Athletes should obtain a written determination from their anti-doping organization before any use. ([2026 WADA Prohibited List](https://www.wada-ama.org/sites/default/files/2025-09/2026list_en_final_clean_september_2025.pdf))

Bottom line

Thymosin alpha-1 has a real human clinical history, but its dosage evidence is more specific than marketing suggests. 1.6 mg SC twice weekly, three or four days apart, is the clearest label- and trial-anchored schedule. Under-40-kg materials use 40 mcg/kg; daily and twice-daily regimens belong to narrow acute or hospital studies. The largest sepsis trial found no mortality benefit.

For a rigorous short-course study, the cleanest design is fixed 1.6 mg twice-weekly exposure for eight weeks with four weeks off, verified sterile product, predefined endpoints, no loading or escalation, no stacks, and no automatic repeat cycle. Reconstitution math prevents volume errors but cannot validate a concentrated bulk formulation.

Tα1 ≠ TB-500 / thymosin beta-4. Core dose: 1.6 mg SC BIW. Short half-life ≠ daily is better. TESTS sepsis trial negative. Warn strongly against 10 mg/2 mL (5 mg/mL).

Frequently asked questions

References

• FDA

  Thymosin Alpha-1 Related Bulk Drug Substances Briefing Document, 2024

  Identity, chemistry, compounding review.

• FDA

  Pharmacy Compounding Advisory Committee Final Summary Minutes, December 2024

  4 yes / 17 no on 503A bulks list.

• Rost et al.

  Pharmacokinetics after SC injection of three formulations

  Clin Pharmacokinet. 1999.

• Chien et al.

  Randomized controlled chronic hepatitis B trial

  Hepatology. 1998.

• Iino et al.

  0.8 versus 1.6 mg chronic hepatitis B study

  J Viral Hepat. 2005.

• Pockros et al.

  Tα1 with peginterferon/ribavirin for HCV

  Aliment Pharmacol Ther. 2012.

• Wu et al.

  ETASS severe-sepsis randomized trial

  Crit Care. 2013.

• Wu et al.

  TESTS Phase III sepsis trial

  BMJ. 2025.

• Gravenstein et al.

  Influenza vaccine study in older men

  J Infect Dis. 1989.

• Maio et al.

  Randomized melanoma combination study

  Cancer Immunol Immunother. 2010.

• PubChem

  Thymalfasin, CID 16130571

  Compound record.

• World Anti-Doping Agency

  2026 Prohibited List

  S0 Non-Approved Substances.