Thymosin Beta-4 Research

Thymosin beta-4 is a 43-residue peptide found across most human tissues. It is registered under CAS 77591-33-4, PubChem CID 16132341, and carries the International Nonproprietary Name timbetasin. Its molecular weight is 4963.44 g/mol.

Thymosin beta-4 research is unusual among peptides sold for laboratory use. The problem is not an absence of human data. A large clinical programme has run for two decades, and most of what has read out has failed. This article reports what has been investigated, not what any reader should expect. This is a research compound.

What thymosin beta-4 is, and what it is not

Thymosin beta-4 is the product of the TMSB4X gene, a small and highly conserved member of the beta-thymosin family.

Two other compounds are routinely confused with it.

The first is TB-500. TB-500 is a synthetic seven-residue peptide corresponding to residues 17 to 23 of thymosin beta-4, registered under CAS 885340-08-9. It carries under a fifth of the parent's mass. In a 2026 briefing document prepared for its Pharmacy Compounding Advisory Committee, the FDA stated that thymosin beta-4 and TB-500 are not the same substance. None of the clinical evidence in this article was generated with the fragment.

The second is thymosin alpha-1, also called thymalfasin. That is a 28-residue peptide from a different precursor gene, with immunomodulatory rather than actin-binding biology. It is marketed in a number of countries outside the United States. That marketing status belongs to thymosin alpha-1 alone. It is not an approval of thymosin beta-4.

Proposed mechanism

Inside cells, thymosin beta-4 is the principal sequestering protein for monomeric G-actin. Goldstein and colleagues reviewed that role in 2005.^[1] Binding G-actin influences how the actin cytoskeleton assembles and disassembles.

The literature proposes effects on cell migration, new blood vessel formation, and cell survival signaling. Those proposals rest on cell culture and animal work. No human trial has tested any of those proposed mechanisms directly.

The preclinical foundation is contested

Smart and colleagues reported in Nature in 2011 that thymosin beta-4 primed adult mouse hearts to generate new cardiomyocytes after injury.^[2] That animal result made the molecule widely known. Two later direct tests did not support it.

Zhou and colleagues used genetic lineage tracing in mice to test the same claim. Treatment after infarction thickened the epicardium and raised capillary density in those animals. Epicardium-derived cells did not adopt a cardiomyocyte fate.^[3] The authors note the original protocol required treatment before the infarct, which they call clinically impractical.

Banerjee and colleagues built global and cardiac-specific knockout mice and reported the result in Circulation Research in 2012. The global knockouts were born at Mendelian ratios with normal hearts and vessels. The authors concluded that thymosin beta-4 is dispensable for murine heart development and adult cardiac function.^[4]

That knockout picture is not settled. Six months later the same journal published Rossdeutsch and colleagues, reporting that thymosin beta-4 is required for vascular smooth muscle development in mice.^[5] The two papers have not been reconciled.

The Nature paper has not been retracted and carries no correction notice. It stands unreplicated rather than withdrawn.

Human trials in eye disease

Most human data on thymosin beta-4 comes from a topical eye drop, RGN-259. Two Phase 2 trials in dry eye came first.

One reported significant improvements in signs and symptoms of severe dry eye.^[6] It treated nine patients at two sites, and its registered primary outcome was safety. The second Phase 2, in 72 patients, missed both of its prespecified primary endpoints.^[7] Its published conclusion argued for efficacy on secondary endpoints.

Three larger dry eye trials followed, randomizing 1,618 patients between them. ARISE-1 (n=317), registered as Phase 2/3, was reported positive by its sponsor and never published. ARISE-2 (n=601) posted registry results in which both co-primary endpoints numerically favored placebo. ARISE-3 (n=700) failed its co-primary endpoints, in the words of the RegeneRx annual report for fiscal 2022. No peer-reviewed publication exists for any of the three.

After ARISE-3, the trial sponsor pooled all three trials and announced significant improvements across a stated 1,500 to 1,600 patients. That pooled figure still circulates widely. The same annual report states that the FDA would not accept the analysis for a Biologics License Application.

Neurotrophic keratopathy is the second eye indication. SEER-1 was a Phase 3 trial that terminated at 18 patients for a stated business decision. Its prespecified primary analysis returned p=0.0656 by Fisher's exact test, which is a miss. The published paper presents that trial as a positive Phase III result.^[8] The annual report discloses that a different statistical test on the same endpoint returned p=0.0400. Changing the test after seeing the result exploits an analytical degree of freedom.

A Cochrane systematic review assessed that same 18-patient dataset in 2025. It found no significant effect on corneal re-epithelialization, risk ratio 9.00, 95% confidence interval 0.57 to 141.88. That evidence was rated low-certainty.^[9] The review covered seven randomized human trials and 494 participants in this indication, and rated the field low to very low certainty overall. Five of the seven trials that reported their funding were industry-sponsored.^[9]

SEER-3, a European Phase 3 in the same indication, was reported to miss its primary endpoint in June 2025. That report comes from trade press. The trial is not registered on ClinicalTrials.gov and has not been published. SEER-2 is still recruiting and has not reported results.

A third eye indication was attempted and abandoned. A Phase 2 in diabetic corneal wounds terminated for slow recruitment after 12 human patients. Its posted results are adverse event counts only, with no efficacy outcome reported.

Dermal and injectable programmes

The topical gel programme produced no positive efficacy trial in the United States. A US pressure ulcer Phase 2 and a US venous stasis ulcer Phase 2 each registered safety and tolerability as the primary endpoint. Posted healing results were null in both of those human trials, and both numerically favored placebo. In the venous ulcer trial, 4 of 17 placebo patients healed, 23.5 percent, against 12 of 55 on thymosin beta-4, 21.8 percent. In the pressure ulcer trial the figures were 3 of 18 on placebo, 16.7 percent, against 8 of 54 on thymosin beta-4, 14.8 percent.

A third ulcer trial ran in Europe, and it is a separate study from the US venous ulcer trial. Guarnera and colleagues randomized 73 human patients across eight sites in Italy and Poland, in a double-blind, placebo-controlled, dose-escalation Phase 2 of the topical gel.^[10] Safety was reported as comparable to placebo. Its published efficacy conclusion is hedged, stating that one of the tested concentrations may have potential to accelerate healing. The paper reports complete healing within three months in about 25 percent of patients, especially those whose wounds were smaller or less severe.^[10]

Two epidermolysis bullosa Phase 2 trials terminated early, one of them after four patients. A third, open-label trial in that disease enrolled three patients. One of the terminated trials was registered to Lenus Therapeutics, a sponsor since dissolved. The RegeneRx annual report states that the rights reverted to HLB Therapeutics.

The injectable formulation, RGN-352, has a thinner human record. The sponsor's annual report describes a completed Phase 1a and Phase 1b in 60 healthy subjects in 2009, given intravenously. That completion does not appear in ClinicalTrials.gov. Two registered RGN-352 records were withdrawn with zero enrollment. One of them was registered in 2008 and describes a Phase 1 intravenous study in healthy volunteers, matching the scope of that completed programme. The public record does not reconcile that withdrawn registration with the completion the filing reports. The other was registered in 2011 and was the Phase 2 in acute myocardial infarction. In March 2011 the FDA placed that Phase 2 on clinical hold. The stated reason was the contract manufacturer's alleged failure to comply with current Good Manufacturing Practice regulations. The US injectable has been given to healthy volunteers only, never to patients.

A separate Chinese recombinant programme has gone further. Its first-in-human Phase 1 enrolled 84 healthy volunteers across single and multiple ascending dose cohorts. Adverse events were mild to moderate, with no dose-limiting toxicities and no serious adverse events reported.^[11] That is a safety and pharmacokinetic dataset, not an efficacy one.

A randomized, placebo-controlled trial in 96 human patients with ST-elevation myocardial infarction found no significant difference in infarct area overall.^[12] The significant result was confined to a 43-patient subgroup. That subgroup was defined by starting treatment within eight hours after percutaneous coronary intervention.^[12] The authors report the full-trial null explicitly and call for further rigorous studies. The paper's abstract names no registry record for the trial. Two completed Phase 2 trials in that Chinese programme enrolled 62 and 90 patients. The published figure of 96 matches neither, and the public record does not say whether this was a third study. A further Phase 2 is registered but not yet recruiting.

Regulatory status

No marketing application for thymosin beta-4 has ever been submitted to the FDA, in any formulation. The FDA drug approval database holds no record of this molecule under any of its name forms.

Orphan drug designation was granted for neurotrophic keratopathy in 2013. A second orphan designation is associated with the topical gel programme in epidermolysis bullosa. A designation is a development incentive granted before efficacy is shown. It is not a marketing authorization.

An FDA label database record names a thymosin beta-4 fusion construct in a Korean cosmetic face pack. That entry is an unapproved listing, not an approval.

FDA's 2026 compounding briefing document evaluated TB-500 and proposed not adding it to the 503A bulks list. That evaluation concerns the fragment, not the 43-residue parent.

The US originator stopped reporting in 2023. RegeneRx Biopharmaceuticals filed to deregister its securities in August 2023, ending its SEC reporting obligations. Its fiscal 2022 annual report is the last full public account of the programme from that company. Development continues under Korean and Chinese sponsors.

Conflicts of interest in the literature

Almost every favorable human result on this molecule carries sponsor authorship. The positive neurotrophic keratopathy paper declares authors employed by the developer and by its parent company.^[8] One of those declared developer employees is also a co-author of the 2005 mechanism review.^[1] One investigator is lead or senior author on essentially the whole favorable ophthalmic literature. The Chinese Phase 1 paper opens by declaring no conflicts and then names three sponsor employees among its authors.^[11] Sponsor employees also co-author the myocardial infarction trial.^[12] The Cochrane review shares no author with any of those papers, and its verdict is negative. Its declared external funders are the National Eye Institute of the National Institutes of Health, the UK Public Health Agency, Queen's University Belfast, and Birmingham Health Partners.^[9] The review reports no internal source of support. None of its nine authors declares a commercial or industry interest. The interests they do declare are those public research grants and two Cochrane editorial roles recused from this review.^[9]

Conclusion

Thymosin beta-4 has a well-documented clinical history in humans. ARISE-2 and ARISE-3 missed their co-primary endpoints, and SEER-1 missed its prespecified primary. SEER-3 was reported to miss as well, and SEER-2 has not read out. Its most famous animal finding has not been replicated by lineage tracing or by genetic knockout. Anyone working with Thymosin Beta-4 should read the registry records and sponsor filings alongside the journal literature.

Frequently Asked Questions

Is thymosin beta-4 the same as TB-500?

No, they are different molecules. TB-500 is a synthetic seven-residue peptide corresponding to residues 17 to 23 of thymosin beta-4, registered under CAS 885340-08-9. The FDA stated in a 2026 compounding briefing document that the two are not the same substance.

Is thymosin beta-4 the same as thymosin alpha-1?

No, they are different molecules. Thymosin alpha-1, also called thymalfasin, is a 28-residue peptide from a different precursor gene, with immunomodulatory rather than actin-binding biology. It is marketed in a number of countries outside the United States, and that marketing status is not an approval of thymosin beta-4.

Has the 2011 mouse heart regeneration finding been replicated?

No, and it has also not been retracted. Genetic lineage tracing in mice found that epicardium-derived cells did not adopt a cardiomyocyte fate.^[3] Global knockout mice were born with normal hearts and vessels.^[4] The 2011 Nature paper stands unreplicated rather than withdrawn.^[2]

Has thymosin beta-4 been approved by the FDA?

No marketing application for thymosin beta-4 has ever been submitted to the FDA, in any formulation. Orphan drug designation was granted for neurotrophic keratopathy in 2013. A designation is a development incentive granted before efficacy is shown, and it is not a marketing authorization.

References

1. 1

   Goldstein AL, Hannappel E, Kleinman HK. Thymosin beta4: actin-sequestering protein moonlights to repair injured tissues. Trends Mol Med. 2005;11(9):421-429. PMID 16099219
2. 2

   Smart N, Bollini S, Dube KN, Vieira JM, Zhou B, et al. De novo cardiomyocytes from within the activated adult heart after injury. Nature. 2011;474(7353):640-644. PMID 21654746
3. 3

   Zhou B, Honor LB, Ma Q, et al. Thymosin beta 4 treatment after myocardial infarction does not reprogram epicardial cells into cardiomyocytes. J Mol Cell Cardiol. 2012;52(1):43-47. PMID 21907210
4. 4

   Banerjee I, Zhang J, Moore-Morris T, et al. Thymosin beta 4 is dispensable for murine cardiac development and function. Circ Res. 2012;110(3):456-464. PMID 22158707
5. 5

   Rossdeutsch A, Smart N, Dube KN, Turner M, Riley PR. Essential role for thymosin beta4 in regulating vascular smooth muscle cell development and vessel wall stability. Circ Res. 2012;111(4):e89-e102. PMID 22723298
6. 6

   Sosne G, Dunn SP, Kim C. Thymosin beta4 significantly improves signs and symptoms of severe dry eye in a phase 2 randomized trial. Cornea. 2015;34(5):491-496. PMID 25826322
7. 7

   Sosne G, Ousler GW. Thymosin beta 4 ophthalmic solution for dry eye: a randomized, placebo-controlled, Phase II clinical trial conducted using the controlled adverse environment (CAE) model. Clin Ophthalmol. 2015;9:877-884. PMID 26056426
8. 8

   Sosne G, Kleinman HK, Springs C, Gross RH, Sung J, Kang S. 0.1% RGN-259 (Thymosin beta4) ophthalmic solution promotes healing and improves comfort in neurotrophic keratopathy patients in a randomized, placebo-controlled, double-masked Phase III clinical trial. Int J Mol Sci. 2022;24(1):554. doi:10.3390/ijms24010554. PMID 36613994
9. 9

   Kruoch Z, Choo AY, Kemp A, Gonzales M, Yim TW, McCann P, Liu SH, Ting DSJ, Kuo IC. Medical and surgical interventions for neurotrophic keratopathy. Cochrane Database Syst Rev. 2025;12(12):CD015723. PMID 41347649
10. 10

    Guarnera G, DeRosa A, Camerini R. The effect of thymosin treatment of venous ulcers. Ann N Y Acad Sci. 2010;1194:207-212. PMID 20536470
11. 11

    Wang X, Liu L, Qi L, et al. A first-in-human, randomized, double-blind, single- and multiple-dose, phase I study of recombinant human thymosin beta4 in healthy Chinese volunteers. J Cell Mol Med. 2021;25(17):8222-8228. PMID 34346165
12. 12

    Zhang Y, Dong Q, Bian X, et al. Recombinant human thymosin beta 4 improves ischemic cardiac dysfunction in mice and patients with acute ST-segment elevation myocardial infarction after reperfusion. Cardiovasc Res. 2025;121(17):2747-2758. PMID 41229390

Primary records cited without a PubMed identifier

• RegeneRx Biopharmaceuticals, Inc. Annual Report on Form 10-K for fiscal year 2022. SEC EDGAR CIK 0000707511, accession 0001410578-23-000628, filed 2023-04-11. Source of the ARISE-3 co-primary failure, the SEER-1 statistical test switch, the FDA refusal of the pooled analysis for a BLA, the RGN-352 Phase 1a/1b in 60 healthy subjects, the March 2011 clinical hold, the three-patient open-label EB trial, and the dissolution of Lenus Therapeutics.
• RegeneRx Biopharmaceuticals, Inc. Form 15-12G, filed 2023-08-14. SEC EDGAR CIK 0000707511. Source of the deregistration and the end of SEC reporting.
• ClinicalTrials.gov: NCT02597803 (ARISE-1), NCT02974907 (ARISE-2, results posted), NCT03937882 (ARISE-3), NCT02600429 (SEER-1), NCT05555589 (SEER-2), NCT01393132 (nine-patient Phase 2), NCT01387347 (72-patient Phase 2), NCT00598871 (diabetic corneal wounds, terminated), NCT00382174 (pressure ulcer), NCT00832091 (venous stasis ulcer), NCT00311766 and NCT03578029 (epidermolysis bullosa), NCT00743769 and NCT01311518 (withdrawn RGN-352), NCT04555824 and NCT04555850 (Phase 1a/1b, recombinant), NCT05485818 and NCT05984134 (completed Phase 2), NCT07586865 (registered, not yet recruiting).
• FDA Briefing Document, Pharmacy Compounding Advisory Committee meeting, 23-24 July 2026, memo dated 2026-05-15. FDA evaluation of TB-500 (free base) and TB-500 acetate. https://www.fda.gov/media/193349/download
• PubChem CID 16132341 (thymosin beta-4 / timbetasin, CAS 77591-33-4) and CID 62707662 (TB-500, CAS 885340-08-9).