Thymosin alpha-1

Thymosin alpha-1 (thymalfasin), marketed as Zadaxin, is a 28-amino-acid synthetic peptide identical to the endogenous thymic peptide. Authorizations outside the United States, including in China and Italy, are reported in the reviewed sources, but this atlas did not independently confirm a current count across all national registers. It has been studied in hepatitis C, cancer immunotherapy, and as a vaccine adjuvant. No FDA-approved product was identified. FDA review concluded insufficient evidence of effectiveness for hepatitis B, noting that available studies showed mixed results and that FDA-approved therapies with established efficacy exist.

Thymosin alpha-1 is an immunomodulator that acts through Toll-like receptors (TLR9 and TLR2) on dendritic cells and precursor T-cells. It enhances MHC class I and II expression, promotes T-cell differentiation and NK cell cytotoxicity, and modulates cytokine production. It has been shown to prevent pro-inflammatory cytokine storm and reduce apoptosis in preclinical models. Its half-life after subcutaneous administration is approximately 2 hours, with renal clearance.

Approved labeled regimen

In approved jurisdictions (e.g., Singapore label): Zadaxin 1.6 mg (900 mcg/m²) administered subcutaneously twice weekly for 6-12 months for chronic hepatitis B. For patients under 40 kg: 40 mcg/kg.

The label summary below is product-, indication-, and jurisdiction-specific; consult the full current label and a licensed clinician/pharmacist.

Studied regimens (not recommendations)

• Chronic hepatitis C: 1.6 mg SC twice weekly for 6-12 months (in combination with interferon-based therapy).

• Cancer/immune adjuvant: 1.6 mg SC, variable schedules.

• COVID-19/sepsis: 1.6 mg SC daily or twice weekly in published observational studies.

What is not established

No FDA-approved dosing exists. This atlas did not verify a current, product-specific approved label for the other studied uses; authorization must be checked by product, indication, and jurisdiction.

Established label risks

In approved markets: generally well tolerated. Injection-site reactions are the most common adverse event. Transient ALT flare can occur during hepatitis B treatment (TA1 should be continued unless signs of liver failure emerge). Contraindicated in known hypersensitivity.

Human-study signals

Across over 4,400 patients studied in clinical trials internationally, thymalfasin has been associated with a favorable safety profile. The most common AEs are injection-site reactions, mild fever, and transient ALT elevation. This atlas did not compare current warning formats across every approving jurisdiction.

Unknowns and product-quality risks

• Immunogenicity risk identified by FDA as a concern due to insufficient characterization.

• US compounding pharmacies operate outside FDA-approved manufacturing; product quality and consistency are not guaranteed by regulatory oversight.

• FDA review noted insufficient evidence of effectiveness across all 12 evaluated uses.

No clinically significant drug interactions identified in labeling. No adequate data in pregnancy or lactation. Use in pediatric patients under 18 years for hepatitis B is based on limited data.

• FDA status: no approved product was identified. SciClone submitted NDA data for hepatitis B in the early 2000s, and FDA requested additional Phase III data that were not pursued. Public sources do not establish whether a confidential IND or NDA remains active.

• US compounding: the July 2026 PCAC meeting is a dated advisory event, not by itself a final eligibility or availability determination. Current access requires a substance-, facility-, prescription-, and product-specific assessment under sections 503A and 503B.

• WADA: not identified by exact name in the 2026 List. Because this page records governmental human-use approvals outside the US, S0 cannot be inferred merely from lack of FDA approval. Athletes should obtain a current case-specific classification.

• International: authorizations in China and Italy are reported in the reviewed sources; current status elsewhere requires national-register verification.

• US FDA approval has never been obtained; two Phase III programs failed to meet efficacy endpoints.

• No adequately powered RCT has confirmed benefit for any indication beyond hepatitis B.

• FDA review concluded: "insufficient evidence to determine effectiveness" for hepatitis B.

• Current standard-of-care for hepatitis B (entecavir, tenofovir) is more effective than TA1.

• No head-to-head trials against modern antiviral agents exist.

• Databases and registries: PubMed, ClinicalTrials.gov, FDA Drugs@FDA, FDA PCAC documents, Singapore HDA label

• Search terms: thymosin alpha-1, thymalfasin, Zadaxin, hepatitis B

• Last searched: 2026-08-06

• Inclusion emphasis: FDA review documents, approved-label data, controlled trials

1. https://www.fda.gov/media/183892/download (FDA PCAC review materials — thymosin alpha-1)

2. https://pubmed.ncbi.nlm.nih.gov/18078676/ (Yang et al. 2008 — meta-analysis TA1 vs interferon for CHB)

3. https://www.ndf.gov.sg/about-drugs/product-information/sin07483p/ (Singapore HDA label — Zadaxin)

4. https://precision.fda.gov/uniisearch/srs/unii/W0B22ISQ1C (FDA Substance Registration System — thymalfasin)

5. https://pubmed.ncbi.nlm.nih.gov/15992078/ (Rost et al. 2005 — Zadaxin for viral hepatitis review)

6. https://pubmed.ncbi.nlm.nih.gov/11381492/ (Ancell et al. 2001 — thymosin alpha-1 clinical review)

7. https://pmc.ncbi.nlm.nih.gov/articles/PMC4688733/ (You et al. 2015 — RCT TA1 vs interferon for CHB)