Thymosin beta-4
TB-500, Tbeta4, TB4, TMSB4X gene product, RGN-259, RGN-137, Ac-LKKTETQ
Thymosin beta-4 is a naturally occurring 43-amino-acid actin-sequestering peptide found in nearly all human cells and in wound fluid. It is the most clinically tested compound in this class, having reached Phase 3 in dry eye disease, but the large trials did not produce a clean primary endpoint win. The product sold online as TB-500 is usually a short fragment, not the peptide that was tested.
Mechanism
Thymosin beta-4 is the major G-actin sequestering protein in mammalian cells. It binds actin monomers through a WH2 (WASP-homology 2) fold, holding a large intracellular reservoir of unpolymerised actin and thereby setting the pool available for rapid filament assembly. This is not a passive buffer: by controlling monomer availability it governs how quickly a cell can remodel its cytoskeleton, which is what cell migration requires. Keratinocyte and endothelial cell migration into a wound bed, endothelial tube formation and angiogenesis all follow from this. Separate activities include downregulation of NF-kappaB-driven inflammatory signalling, reduced myofibroblast differentiation and scarring, and promotion of laminin-5 production in the corneal and dermal basement membrane.
Two derived peptides matter. The N-terminal tetrapeptide AcSDKP, a documented processing product annotated in UniProt, is an antifibrotic and haematopoietic regulator in its own right and is a substrate of angiotensin-converting enzyme. The central heptapeptide LKKTETQ carries the actin-binding function and reproduces several, though not all, of the parent peptide's biological activities. This heptapeptide is what most vendors actually supply under the name TB-500, which means the human trial evidence described below was generated with a different molecule from the one most people buy.
What the research shows
Preclinically, thymosin beta-4 is well characterised and independently replicated. Smart, Riley and colleagues showed it is essential for coronary vessel development and reactivates adult epicardium to form new vasculature after ischaemic injury; corneal, dermal and cardiac repair effects have been reproduced across several laboratories. The actin biochemistry is settled science.
The clinical record is the more instructive part and it is a story of consistent near-misses. RegeneRx ran topical Phase 2 trials in venous stasis ulcers (NCT00832091, registered enrolment 72) and pressure ulcers (NCT00382174, registered enrolment 72). Guarnera's published report of the venous ulcer study describes 73 patients randomised across eight European sites, five in Italy and three in Poland; the safety profile was comparable to placebo and a 0.03% dose was suggested as possibly accelerating healing, with complete healing within three months in about 25% of patients, concentrated among those with small-to-moderate or mild-to-moderate wounds. That is a dose-finding signal, not a demonstration of efficacy. A Phase 2 trial in epidermolysis bullosa (NCT00311766, enrolment 30) was terminated. In ophthalmology the programme went furthest: Sosne's 2015 Phase 2 in severe dry eye was positive but enrolled only nine patients, and ReGenTree subsequently ran ARISE-1 (NCT02597803, registered as Phase 2/3, n=317), ARISE-2 (NCT02974907, Phase 3, n=601) and ARISE-3 (NCT03937882, Phase 3, n=700). No peer-reviewed full report of that programme has been published. Sponsor announcements describe statistically significant improvement in ocular grittiness, a prespecified secondary symptom endpoint, at one and two weeks, rather than success on the co-primary endpoints. In neurotrophic keratopathy, SEER-1 (NCT02600429) was terminated after 18 participants and SEER-2 (NCT05555589, Phase 3, planned n=70) is registered as recruiting; no published results exist for either. Separately, a Chinese developer has run a Phase 1a study in 54 healthy volunteers (NCT04555824) and two Phase 2 trials of recombinant thymosin beta-4 in acute myocardial infarction (NCT05485818, n=62; NCT05984134, n=90). The overall picture is a compound with real, plausible biology that has repeatedly failed to convert into a published, regulatory-grade efficacy result.
Evidence assessment
Mixed evidence
Genuine randomised, placebo-controlled Phase 2 and Phase 3 trials exist in venous ulcers, pressure ulcers, dry eye and myocardial infarction, involving well over a thousand registered participants in total, but the large ophthalmic programme has not delivered a published primary endpoint success and no regulator has approved the peptide for any indication.
Tiers are applied consistently across the library and re-checked when new trials read out. Read the grading method.
Key studies
The effect of thymosin treatment of venous ulcers Mixed evidence
Safety across all doses was comparable to placebo; the 0.03% dose showed a possible signal for accelerated healing, with complete healing within three months in about 25% of patients, mainly those with small-to-moderate or mild-to-moderate ulcers.
Thymosin beta4 significantly improves signs and symptoms of severe dry eye in a phase 2 randomized trial Limited evidence
At day 56 the treated group showed 35.1% reduction in ocular discomfort (P=0.0141) and 59.1% reduction in total corneal fluorescein staining (P=0.0108) versus vehicle, with improvements in tear film break-up time and tear volume.
Assessment of the Safety and Efficacy of RGN-259 Ophthalmic Solutions for Dry Eye Syndrome: ARISE-3 Mixed evidence
Registry record confirms a completed 700-participant Phase 3 in dry eye syndrome. Sponsor communications describe statistically significant improvement in ocular grittiness, a prespecified secondary endpoint, rather than success on co-primary endpoints; no approval followed.
Biological activities of thymosin beta4 defined by active sites in short peptide sequences Preclinical only
Distinct biological activities of the parent peptide map to short internal sequences, with the actin-binding region reproducing several but not all effects.
The beta-thymosin/WH2 domain; structural basis for the switch from inhibition to promotion of actin assembly Preclinical only
Defines the structural basis by which beta-thymosins sequester actin monomers and how the same fold can switch to promoting filament assembly.
Thymosin beta-4 is essential for coronary vessel development and promotes neovascularization via adult epicardium Preclinical only
Thymosin beta-4 is required for coronary vessel formation and reactivates adult epicardial progenitors to generate new vasculature after injury.
Safety
Better characterised than anything else in this class, because hundreds of people have received it in controlled trials. Topical and ophthalmic administration was consistently reported as well tolerated with adverse event rates comparable to placebo. What is not established is the safety of chronic systemic self-administration by injection, which no trial has studied. The theoretical concern is the same one that applies to any pro-angiogenic, anti-apoptotic, pro-migratory agent: thymosin beta-4 is overexpressed in several tumour types and has been associated with increased invasiveness and metastatic potential in laboratory models. That is a laboratory observation about endogenous expression, not proof that exogenous administration causes cancer, but it means chronic dosing outside supervision is not risk-free. The practical hazard for consumers remains unregulated manufacture, compounded by the fact that products labelled TB-500 frequently contain a fragment rather than the tested peptide, at unverified purity.
Regulatory status
| Jurisdiction | Status |
|---|---|
| United Kingdom | No MHRA marketing authorisation for any thymosin beta-4 product. Not an authorised medicine, food supplement or cosmetic ingredient. Supply for human use would engage the Human Medicines Regulations 2012. |
| United States | Not approved by the FDA for any indication despite completing Phase 3 trials in dry eye disease. Investigational status only. Thymosin beta-4 and TB-500 were among the peptides placed on the FDA's interim 503A Category 2 compounding list in September 2023; that list was later revised after nomination withdrawals and peptide nominations were referred to the Pharmacy Compounding Advisory Committee. |
| WADA (sport) | Prohibited at all times. Thymosin beta-4 and its derivatives, with TB-500 named explicitly, fall under section S2.3 of the Prohibited List, covering growth factors and growth factor modulators affecting muscle, tendon or ligament protein synthesis, vascularisation and regenerative capacity. It is a non-specified substance. |
Questions
Usually not, and this is the most important thing to understand about it. Thymosin beta-4 is a natural 43-amino-acid peptide, and it is what every human clinical trial used. TB-500 as sold online is most commonly the seven-residue actin-binding fragment Ac-LKKTETQ, which is cheaper to synthesise. Research shows the fragment reproduces some but not all of the parent peptide's activities. Trial results obtained with the full-length peptide should not be read across to the fragment.
No. Three large dry eye trials were run, enrolling 317 (ARISE-1, registered as Phase 2/3), 601 (ARISE-2) and 700 (ARISE-3). No peer-reviewed report of that programme has been published and no results are posted to the registry; sponsor communications describe significance on a secondary symptom endpoint rather than the co-primary endpoints. In neurotrophic keratopathy, SEER-1 was terminated after 18 participants and SEER-2 remains registered as recruiting. No regulator has approved it.
Yes, at all times. Thymosin beta-4 and its derivatives, with TB-500 named explicitly, are listed under S2.3 of the WADA Prohibited List as growth factors and growth factor modulators. It is a non-specified substance, which affects the sanctioning framework if an athlete tests positive.
Because the trials mostly did not work, and the largest ones were never published. Having real randomised data is a much higher bar than most compounds in this class clear, but the standard for 'strong' is replicated positive human trials or approved labelling. A large, well-run Phase 3 programme that produces no published primary endpoint success is informative evidence, and what it informs us of is that the effect, if present, is smaller or less reliable than the early trials suggested.