Class
These are peptides that act on immune function rather than on a metabolic or structural target: one an endogenous antimicrobial peptide the body makes at every barrier surface, one a thymic peptide that participates in T-cell maturation, and one the thymic hormone also known as serum thymic factor. None has a lawful United States route, and the reasons differ in an instructive way.
One carries a substantial controlled-trial literature accumulated outside the United States and was refused by an advisory committee that read it. Another carries a trial count that looks large and is almost entirely a record of the body's own peptide being measured rather than a drug being given. The third was nominated to the 503A list without adequate support, and its small trial count is mostly the same kind of measurement.
Molecules in this class
| Molecule | Class | US status | Position |
|---|---|---|---|
| LL-37Cathelicidin LL-37; hCAP18 | Immune peptides | No lawful route | Nomination withdrawn in 2026; on no list. FDA's safety finding cites nonclinical evidence of harm to male reproduction and of pro-tumorigenic effects in some tissues. Verified August 2026. |
| Thymosin alpha-1Thymalfasin; Ta1 | Immune peptides | Nomination withdrawn | Licensed as a medicine in other countries and never approved here; the advisory committee voted 4–17 against listing it in December 2024, and the nomination was withdrawn. Verified August 2026. |
| ThymulinThymulin acetate; serum thymic factor | Immune peptides | No lawful route | Category 3 of the 503A bulks list — nominated without adequate support; no approved product and no lawful compounding route. |
Cathelicidin LL-37; hCAP18
Nomination withdrawn in 2026; on no list. FDA's safety finding cites nonclinical evidence of harm to male reproduction and of pro-tumorigenic effects in some tissues. Verified August 2026.
Thymalfasin; Ta1
Licensed as a medicine in other countries and never approved here; the advisory committee voted 4–17 against listing it in December 2024, and the nomination was withdrawn. Verified August 2026.
Thymulin acetate; serum thymic factor
Category 3 of the 503A bulks list — nominated without adequate support; no approved product and no lawful compounding route.
The measured cases are at LL-37, where sixty-eight of seventy-two trial-type records administer nothing, and thymosin alpha-1, refused 4 to 17 by the committee that reviewed it. Thymulin sits in Category 3, and its trial records show one of eight administering anything thymulin-related. The human trial record prints all three counts with the searches behind them.
The 2026 WADA Prohibited List names no member of this class. It does not name LL-37, Thymosin alpha-1 and Thymulin, and that is not the same as permitted: the relevant sections are expressly non-exhaustive, and section S0 covers any substance with no current approval for human therapeutic use.
The vote ledger and the WADA table carry the sources.
Evidence reviews
Reference tables
Definitions