Thymosin beta-4

Where it comes from

Made in

Nearly every cell; platelets, white cells and wound fluid carry the most

Sent out by

Into wounds when platelets degranulate and cells die

Acts on

• Actin (keeping it unpolymerised)
• Endothelial cells and keratinocytes
• Heart and corneal cells after injury

What it does

• Holds the pool of actin monomers the cell uses to move and divide
• Speeds wound closure, vessel growth and corneal healing in animals
• Its four-residue fragment Ac-SDKP, made by prolyl oligopeptidase, restrains scarring and is raised by ACE inhibitors

In medicine

• No approved medicine. Eye drops (RGN-259) went through three phase 3 trials in dry eye, the last of which missed its main goals; a phase 3 trial in neurotrophic keratopathy finished enrolling in 2026
• In a placebo-controlled phase 3 trial in neurotrophic keratopathy, 18 patients in all, 6 of 10 on the drops healed by four weeks against 1 of 8 on placebo, short of significance on its main measure ²
• Trials in heart attack and in pressure and venous ulcers did not deliver an approval ¹

Beyond the body’s own use

• TB-500, the name under which a thymosin beta-4 fragment is traded for athletes and racehorses, is unapproved. The 2026 anti-doping list names thymosin-β4 and its derivatives, TB-500 among them, among the growth factors prohibited at all times, in and out of competition ³
• Reviewing the TB-500 fragment as a bulk substance for pharmacy compounding, the FDA said it had found no human exposure data and lacked the information to know whether the peptide would cause harm ⁴
• Unapproved and compounded preparations are not reviewed for safety, effectiveness or quality
• There are no controlled studies in people of the injury claims made for it; the repair evidence is in animals, where thymosin beta-4 improved recovery after stroke, spinal cord injury and heart attack ¹