Bottom line
Thymosin beta-4 (Tβ4) is an endogenous 43-amino-acid protein and the principal intracellular G-actin-sequestering peptide in humans. A synthetic ophthalmic formulation (RGN-259, INN timbetasin) has completed Phase 3 trials for neurotrophic keratopathy (NK) and dry eye disease, with FDA orphan drug designation for NK. No systemic formulation is approved. The name is frequently conflated with TB-500, which may refer either to full-length Tβ4 or a synthetic 7-residue fragment — distinct chemical entities.
Identity and composition
| Field | Verified information |
|---|---|
| Preferred name | Thymosin beta-4 |
| Key aliases | TB4, Tβ4, Timbetasin (USAN), TMSB4X, FX gene product |
| Molecular/sequence identity | 43-amino-acid N-acetylated protein: Ac-SDKPDMAEIEKFDKSKLKKTETQEKNPLPSKETIEQEKQAGES |
| Modifications/form | N-terminally acetylated; no disulfide bonds; MW ~4,963 Da |
| Stable identifiers | UniProt P62328; PubChem CID 45382195; CAS 77591-33-4; FDA UNII 549LM7U24W |
| Identity caveats | Not the same as TB-500 (which may be full-length Tβ4 or a 7-residue fragment Ac-LKKTETQ). Verify identity against certificate of analysis. |
Development and approval status
| Jurisdiction | Status and indication | Product/source | As of |
|---|---|---|---|
| United States (FDA) | Not approved for any indication; RGN-259 has orphan drug designation for NK | N/A (investigational) | 2026-08-06 |
| European Union (EMA) | No centralized marketing authorization | N/A | 2026-08-06 |
| Other jurisdictions | Status of synthetic versions requires current national-register review; endogenous status is not product authorization | — | 2026-08-06 |
Mechanism and pharmacology
Tβ4 is the principal G-actin-sequestering protein in human cells. It binds monomeric actin and regulates actin polymerization dynamics. Beyond actin binding, Tβ4 promotes cell migration, angiogenesis (via VEGF and PI3K/Akt/eNOS signaling), stem cell recruitment, production of laminin-332, and cytoprotection through reduced oxidative stress and inflammation. The LKKTETQ motif (residues 17-23) constitutes the actin-binding domain. Tβ4 also releases Ac-SDKP on processing, which has anti-fibrotic and angiogenic activities.
Evidence by claim
| Claim/indication | Stage | Grade | Best human evidence | Main result | Important limitations |
|---|---|---|---|---|---|
| Neurotrophic keratopathy (NK) | Phase 3 | B | SEER-1 RCT: n=18 NK patients; 0.1% RGN-259 vs placebo × 28 days | Complete healing at 4 weeks: 6/10 vs 1/8 (p=0.0656); significant improvements in ocular discomfort | Small sample; primary endpoint missed statistical significance |
| Dry eye disease | Phase 3 | B | ARISE trials: >1600 patients across 3 Phase 3 RCTs | Pooled data showed significant improvement in signs and symptoms in subgroups | Results mixed across trials; full analysis pending |
| Wound healing/ophthalmic | Compassionate use | C | Open-label n=6 NK patients | 4/6 complete healing at 28 days; 2/6 by days 55-60 | No control, small N |
| Systemic wound healing | Preclinical | D | None | Rodent models only: dermal, cardiac, CNS | No controlled human trials for systemic use |
Key studies
| Study | Design/population | Exposure studied | Endpoints and result | Limitations |
|---|---|---|---|---|
| SEER-1 (NCT02600429) | Phase 3, DBPC, n=18 stages 2-3 NK | 0.1% RGN-259 ophthalmic solution 5×/day × 28 days | Complete healing at day 28: 6/10 vs 1/8 (p=0.0656); no recurrence in treated group; significant comfort improvements | Did not meet statistical significance on primary; small sample |
| ARISE-3 (NCT03937882) | Phase 3, DBPC, n>1600 DED patients | 0.1% RGN-259 ophthalmic QID × 14 days | Statistically significant improvements in signs and symptoms in pooled subgroups | Mixed individual trial results; subgroup analyses |
| Sosne & Ousler 2015 | Phase 2, DBPC, n= DED patients | 0.1% RGN-259 BID × 28 days | Safety and preliminary efficacy established | Phase 2; not powered for efficacy |
Dose and administration evidence
Approved labeled regimen
None. No Tβ4-containing product has received FDA or EMA approval.
Studied regimens (not recommendations)
Ophthalmic: 0.1% RGN-259 (timbetasin acetate) eye drops, one drop per eye, 2-5× daily for 14-28 days in clinical trials.
Systemic: No established systemic regimen has been studied in controlled human trials.
What is not established
No established or recommended human dose. This applies to systemic use; the ophthalmic candidate and any marketed research material are not interchangeable.
Safety
Established label risks
None — no approved label.
Human-study signals
In ophthalmic trials (total >1600 patients), RGN-259 was well tolerated. Adverse events were generally mild and ocular (instillation-site reactions). No systemic safety concerns were identified. A Phase 2 DED study and SEER-1 NK study both confirmed the safety profile.
Unknowns and product-quality risks
Systemic safety in humans is unstudied.
Products sold outside the RGN-259 clinical program are not pharmaceutical-grade.
Market confusion with TB-500 fragment creates identity/quality risks.
WADA classifies Tβ4 and its derivatives as prohibited substances (2026 Prohibited List).
Interactions and special populations
No systemic drug-interaction studies exist. No data in pregnancy, lactation, or pediatric populations (except Barth syndrome, which involves Tβ4 investigation as exploratory).
Regulatory, compounding, and sport notes
FDA: Not approved. RGN-259 has orphan drug designation for NK. FDA-listed bulk drug substance "Thymosin beta-4, fragment (LKKTETQ), also known as TB-500" — distinct from full-length Tβ4.
WADA: Prohibited under S2.3 (growth factors and growth-factor modulators), which explicitly names thymosin-β4 and derivatives such as TB-500 on the 2026 Prohibited List.
The name "TB-500" is used for both full-length Tβ4 and a 7-residue fragment; regulatory records distinguish them.
ReGenTree LLC (US/EU joint venture) is developing RGN-259 through Phase 3.
Evidence gaps
No FDA-approved systemic formulation.
No pharmacokinetic or safety data for SC/IM/injectable routes in humans.
The NK Phase 3 SEER-1 trial did not meet its primary endpoint with statistical significance.
SEER-2 and SEER-3 Phase 3 NK trials are ongoing.
Most marketed claims for musculoskeletal/systemic healing are extrapolated from animal studies and do not reflect the human evidence base.
Search notes
Databases and registries: PubMed, ClinicalTrials.gov, FDA UNII, UniProt, WADA Prohibited List
Search terms: thymosin beta-4, timbetasin, RGN-259, TB4, neurotrophic keratopathy
Last searched: 2026-08-06
Inclusion emphasis: Controlled human trials prioritized; distinction from TB-500 maintained