TB-500 and the FDA 503A Decision: The Thymosin β-4 Fragment FDA Declined to List

The Short Answer

At the July 23, 2026 PCAC meeting, FDA proposed that TB-500 (free base) and TB-500 acetate not be added to the 503A Bulks List. The nomination (later withdrawn, with FDA proceeding) sought TB-500 for wound healing. FDA found no adequate effectiveness or safety evidence for that use.

What TB-500 Is

TB-500 is described as a seven-amino-acid peptide with an acetylated N-terminus (Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH), corresponding to a fragment of the protein thymosin β-4. It is marketed primarily for tissue repair and wound healing. See our TB-500 peptide page for background.

Characterization: "Not Well-Characterized"

FDA concluded both forms are not physically and chemically well-characterized, citing inconsistent naming conventions (INN, USAN, IUPAC) and missing critical characterization data specific to each form. The free base is reported stable below −20°C and the acetate below −15°C under specific storage conditions, but the quality data needed to establish identity and purity were incomplete.

Historical Use: Interest, but No Documented Compounding

Interest in TB-500 has grown since 2010, and websites discuss and sell TB-500 products. However, FDA found it unclear whether those are compounded products, and no pharmacies were found compounding TB-500-related substances. The agency judged the available data too limited to understand historical compounding use.

Effectiveness: No Human Evidence — and a Negative In-Vitro Result

FDA found a lack of evidence to evaluate effectiveness for wound healing. The nomination included no clinical data, and FDA found no medical literature where TB-500 was administered to patients for any condition, including wound healing. Notably, FDA identified an in-vitro study in which TB-500 (free base) at 50 µg/mL did not induce wound healing in scratch-wounded fibroblast cultures. Pharmacokinetic studies indicated subcutaneous and intraperitoneal TB-500 reaches circulation and is hydrolyzed into smaller peptides.

Safety: No Human Data, Injectable Immunogenicity Concern

FDA cited a lack of clinical and nonclinical safety information and was "particularly concerned" about the absence of human data by any route. Because the proposed routes were intramuscular and subcutaneous injection, the agency flagged a significant potential for immunogenicity, potentially amplified by aggregation and peptide-related impurities — risks the nomination did not address.

What It Means for Physicians

TB-500 was not FDA-approved and remains so. With no monograph and no approved-drug status, listing was the open question — and FDA proposes to keep it closed. See the pillar overview and confirm access with your 503A compounding partner.

Key Takeaways

• FDA is proposing that TB-500 (free base and acetate) not join the 503A Bulks List.
• No human effectiveness data for wound healing were found — and one in-vitro study showed no wound-healing effect.
• No pharmacies were documented compounding TB-500; historical compounding use is unclear.
• Injectable routes drove specific immunogenicity concern with no human safety data to offset it.

Frequently Asked Questions

Is TB-500 banned by the FDA?
No. FDA is proposing that TB-500 not be added to the 503A Bulks List — an advisory-committee proposal, not a final rule. TB-500 was never FDA-approved.

What is TB-500 related to?
It is a seven-amino-acid peptide corresponding to a fragment of thymosin β-4.

Did any study show TB-500 works for wounds?
FDA found no human effectiveness data, and it cited an in-vitro study in which TB-500 did not induce wound healing in fibroblast cultures.

Why is the injectable route a concern?
Intramuscular and subcutaneous peptide injection can carry immunogenicity risk from aggregation and impurities, and FDA found no data showing TB-500 does not present such risk.

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