Actin-binding peptide · tissue repair
A 43-amino-acid peptide made naturally in the body; it is the most abundant beta-thymosin in mammalian cells. It has been studied as eye drops, intravenous infusions and skin treatments for tissue repair, but no product is approved. TB-500, a name often linked to it, is a different molecule: a short synthetic fragment.
Inside cells, thymosin beta-4 binds single actin molecules and holds them in reserve, controlling how quickly they are added to the filaments that let cells change shape and move8. Cell movement is central to repair, and in laboratory and animal studies the peptide stimulated migration of skin cells (keratinocytes), formation of new blood vessels and collagen deposition1. A short stretch of its chain, LKKTETQ, is the actin-binding site tied to these migration effects; TB-500 is a synthetic, acetylated copy of that stretch rather than the whole peptide7. Thymosin beta-4 can also be trimmed to release Ac-SDKP, a small fragment studied for anti-inflammatory and anti-scarring actions in kidney models8. Most of this mechanism comes from cell and animal work, and how much of it applies when the peptide is given to people is not established.
Animal work came first: in rats with full-thickness skin wounds, thymosin beta-4 applied to the skin or injected sped up wound closure and increased new blood vessel and collagen formation1. In humans, a phase 1 study in healthy volunteers found intravenous synthetic thymosin beta-4, given once or repeatedly for two weeks, well tolerated with no serious adverse events2.
In a randomized trial of 96 people whose ST-elevation heart attack was treated with an artery-opening procedure (PCI), a recombinant form did not significantly reduce infarct size versus placebo across the whole trial, though the 43 patients who began treatment within 8 hours of the procedure had smaller infarcts at 90 days3. Eye-drop studies are the most advanced: in a phase 3 trial in 18 people with neurotrophic keratopathy, corneal defects closed fully in 6 of 10 on the drops versus 1 of 8 on placebo, a difference that fell short of statistical significance4, and in a 700-person dry-eye trial, posted registry results show near-identical changes with drops and placebo on both primary measures5.
Not FDA-approved for any use. An eye-drop formulation (RGN-259) remains in phase 3 trials. FDA's compounding-risk page, updated April 2026, now lists the TB-500 fragment among substances whose nominators withdrew them from its significant-risk category, while still noting possible immune reactions and no human exposure data6.
Human data come from a handful of trials. In healthy-volunteer studies, intravenous thymosin beta-4 caused infrequent, mild-to-moderate adverse events and no serious ones2, the keratopathy eye-drop trial reported no significant adverse effects4, and the 700-person dry-eye trial recorded no serious adverse events in either group5. For the TB-500 fragment, the FDA says it has found no human exposure data and flags possible immune reactions and unwanted peptide by-products in compounded versions6. Long-term effects in people are unknown.
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