TB-500 and Thymosin Beta-4 in Human Clinical Research: What Has Been Studied

The clinical literature attached to the name TB-500 is almost entirely literature on full-length thymosin beta-4. This article separates the two, then goes trial by trial through the phase 1, phase 2 and phase 3 human record and what it leaves unresolved.

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Two different molecules travel under one name in this literature, and separating them is the precondition for reading any of it.

Thymosin beta-4 is a 43-residue naturally occurring protein, the principal actin-sequestering molecule of most mammalian cells. It has entered clinical development several times, under the code RGN-259 for ophthalmic formulations and as synthetic or recombinant protein for systemic administration. Every trial described in this article administered that molecule.

TB-500 is commonly described as a short synthetic fragment corresponding to residues 17 to 23 of thymosin beta-4 — the actin-binding region. It is a different chemical entity with a different molecular weight, and it is not what the clinical trials below tested.

The consequence is direct: a phase 2 result for thymosin beta-4 eye drops is not a result for a seven-residue fragment administered by another route, and the two are treated as interchangeable almost everywhere except in the primary literature. This article follows the primary literature.

What follows sets out the human record trial by trial — population, design, duration, endpoint, results as reported, adverse events and limitations — then the preclinical work that sits beneath it, then what the record does not establish. It describes published research and contains no guidance of any kind on handling any material.

What the molecules are

In vitro research

Thymosin beta-4's defining biochemical property is the sequestration of monomeric actin. The actin-sequestering ability of the full protein, of its fragments, and of thymosin beta-4-like peptides was assessed by DNase I inhibition assay in 1993 [1], and the actin-binding site was mapped by mutational analysis in 1996 [2]. That second paper is the origin of the interest in a short central fragment: the binding determinants are concentrated in a small part of the sequence.

Concentrating the binding determinants in a fragment is not the same as reproducing the parent molecule's behaviour. A 43-residue protein and a 7-residue fragment differ in conformation, in the other interactions they can make, in proteolytic stability and in distribution, and the 1993 assay compared fragments precisely because they do not behave identically [1]. Reviews of the field describe a range of proposed activities for the full-length protein beyond actin binding [9].

Phase 1: healthy-volunteer safety and pharmacokinetics

Human clinical research

Results from pharmaceutical clinical trials describe the investigational material and populations used in those studies and should not be interpreted as establishing the effects of research-grade materials offered for laboratory use.

Synthetic thymosin beta-4, intravenous. Four cohorts of 10 healthy volunteers each received a single intravenous administration of placebo or synthetic thymosin beta-4 in ascending amounts of 42, 140, 420 or 1260 mg. After safety review, the same regimen was repeated daily for 14 days. Endpoints were safety evaluations, treatment-emergent adverse events and pharmacokinetic parameters [6].

Adverse events were infrequent and mild or moderate in intensity. There were no limiting toxicities and no serious adverse events. Single-administration pharmacokinetics were proportional to the amount given, with half-life increasing as the amount increased. The authors concluded that further development for cardiac ischaemia should be considered [6].

Recombinant human thymosin beta-4. A first-in-human, randomised, double-blind study in healthy Chinese volunteers enrolled 54 participants across seven single-administration cohorts at 0.05 to 25.0 μg/kg, observed for 28 days, plus 30 participants across three cohorts receiving 0.5, 2.0 or 5.0 μg/kg daily for 10 days with 28 days of observation. Endpoints were safety, tolerability, pharmacokinetics and anti-drug antibodies. Adverse events were mild to moderate, with no limiting toxicities and no serious adverse events; exposure increased with the amount given and no obvious accumulation followed continuous administration [11].

Limitations. Both are phase 1 trials in healthy volunteers: 40 and 84 participants respectively, designed to characterise tolerability and exposure rather than to measure any effect. Neither enrolled a patient population, neither had an efficacy endpoint, and a 14-day or 10-day administration period says nothing about longer exposure. The two also differ by three orders of magnitude in the amounts studied — milligrams in one, micrograms per kilogram in the other — which is itself a reason not to pool them.

The ophthalmic programme

Human clinical research

Results from pharmaceutical clinical trials describe the investigational material and populations used in those studies and should not be interpreted as establishing the effects of research-grade materials offered for laboratory use.

The largest and most advanced clinical programme for this molecule is topical rather than systemic.

The severe dry eye phase 2 trial. A small multicentre, randomised, double-masked, placebo-controlled 56-day trial at two sites in the United States, including a 28-day follow-up. Nine patients with severe dry eye, including disease associated with graft-versus-host disease, received thymosin beta-4 0.1% eye drops or vehicle six times daily for 28 days. At day 56 the treated group (12 eyes) showed a 35.1% reduction in ocular discomfort against vehicle (6 eyes; P=0.0141) and a 59.1% reduction in total corneal fluorescein staining (P=0.0108) [7].

The controlled adverse environment phase 2 trial. A single-centre, prospective, double-masked, placebo-controlled trial randomised 72 subjects with moderate to severe dry eye 1:1 to thymosin beta-4 0.1% or placebo for 28 days across six visits over 32 days. The primary efficacy endpoints were ocular discomfort scores and inferior corneal staining at day 29. Neither primary endpoint showed a significant difference between groups. Several secondary endpoints did: discomfort in the challenge model at day 28 was 27% lower than placebo (P=0.0244), and central and superior corneal staining differed significantly (P=0.0075 and P=0.0210). No adverse events were observed [8].

That pairing is the most instructive thing in the ophthalmic record. One trial with nine patients reported significant results on its measures; a larger trial with a prespecified primary endpoint missed it, and the paper's own conclusion nonetheless reads as confirmatory. A trial that misses its primary endpoint and reports positive secondary endpoints is a hypothesis-generating trial, whatever the discussion section says.

The larger trials. Three ophthalmic trials in dry eye syndrome have completed: one phase 2/3 with 317 participants [16], one phase 3 with 601 [18] and one phase 3 with 700 [19]. A phase 3 trial in neurotrophic keratopathy was terminated with 18 participants [17]. Between them the three completed trials randomised 1,618 people, and no primary results publication for any of them appears in the indexed literature. Completed and unpublished is a specific state, and it is not the same as either positive or negative.

Dermatology and cardiac trials

Human clinical research

Results from pharmaceutical clinical trials describe the investigational material and populations used in those studies and should not be interpreted as establishing the effects of research-grade materials offered for laboratory use.

Dermatology. A phase 2 trial of thymosin beta-4 in epidermolysis bullosa was terminated with 30 participants [13]; the clinical rationale for that indication was set out in the same period [22]. Phase 2 trials in pressure ulcers and in venous stasis ulcers, each with 72 participants, are registered as completed [14, 15]. Neither has an indexed primary publication.

Cardiac. A pilot study randomised 10 patients with acute ST-elevation myocardial infarction to transplantation of autologous endothelial progenitor cells, with the cells in the experimental group pre-treated with thymosin beta-4 for 24 hours before transplantation. At six months the average six-minute walking distance had increased by 38.2 m in the control group and 75.7 m in the experimental group, an average difference of 37.5 m (95% CI 28.7 to 56.3; P<0.01), with no severe procedure-related complications in either group [10].

Two things about that trial deserve emphasis. It enrolled five patients per group. And no patient received thymosin beta-4 — the protein was applied to cells in culture before those cells were transplanted, which is an ex vivo conditioning step, not administration of the molecule to a person.

A phase 2 trial of thymosin beta-4 in acute myocardial infarction with 62 participants is registered as completed [20] and has no indexed primary publication.

The one trial of the fragment. A phase 1/2 trial named for the thymosin beta-4 17-23 fragment, examining cardiovascular biomarkers in stable atherosclerotic cardiovascular disease with an estimated 80 participants, is registered and listed as recruiting [21]. It is the only registered clinical study of the fragment itself, and it has reported nothing.

The preclinical literature

Animal research

The claims most often attached to this molecule originate in animal work, and that work is real.

Thymosin beta-4 was reported in 1999 to accelerate wound healing in a rodent model [3], and a 2003 study reported that both the full protein and a synthetic peptide containing its actin-binding domain promoted dermal wound repair in aged animals [4] — one of the few direct comparisons of parent and fragment in vivo. In a cardiac model, thymosin beta-4 was reported to activate integrin-linked kinase and to promote cardiac cell migration, survival and cardiac repair [5]. A 2026 rat study compared thymosin beta-4 with BPC-157 in an Achilles tendon model using histopathological and biomechanical assessment [12].

These are rodent models of surgically created acute injury in otherwise healthy animals. They are the reason the clinical programmes above were started, and the clinical programmes above are the reason they are not the end of the question.

What the evidence base does not establish

Anything at all about the 17-23 fragment in humans. No completed trial has administered it. One is recruiting [21]. Every human result in this article was produced with full-length thymosin beta-4, and the one preclinical comparison of parent and fragment that exists is in aged rodent skin [4].

The outcome of the largest trials. Three ophthalmic trials totalling 1,618 participants are registered as completed with no indexed primary publication [16, 18, 19], as are phase 2 trials in pressure ulcers, venous stasis ulcers and acute myocardial infarction [14, 15, 20]. A programme with this much completed and unpublished activity cannot be characterised from its published fragments.

Efficacy on a prespecified primary endpoint. The one trial in this record with a clearly reported prespecified primary endpoint and adequate size for it missed that endpoint [8].

Any systemic efficacy in patients. The systemic human record is two phase 1 trials in healthy volunteers [6, 11] and one 10-patient pilot in which the molecule was applied to cells rather than to people [10].

Musculoskeletal effects in humans. The tendon and skeletal claims attached to this compound rest on rodent models [12, 3]. No human trial in this record has a musculoskeletal endpoint.

Long-term safety. The longest reported administration in a human trial is 14 days [6]. Nothing in the published record describes exposure beyond that.

Regulatory position. Neither thymosin beta-4 nor the TB-500 fragment is approved for any indication, in the United States or elsewhere.

All of the research described in this article is research into pharmaceutical product candidates and laboratory preparations, conducted under registered or laboratory protocols on material prepared for those studies. None of it is research into, or evidence about, research-grade material supplied for laboratory use.

Frequently Asked Questions

Is TB-500 the same thing as thymosin beta-4?
No. Thymosin beta-4 is a 43-residue protein; TB-500 is commonly described as a short fragment corresponding to residues 17 to 23 of that sequence. Every clinical trial described in this article administered full-length thymosin beta-4 or a formulation of it [6, 11, 7]. Only one registered trial names the fragment itself, and it is listed as recruiting rather than reported [21].
Has thymosin beta-4 been tested in a phase 1 trial in humans?
Twice. A randomised, placebo-controlled single- and multiple-ascending study gave intravenous synthetic thymosin beta-4 to four cohorts of 10 healthy volunteers at 42, 140, 420 or 1260 mg, then the same regimen daily for 14 days; adverse events were infrequent and mild or moderate, with no limiting toxicity [6]. A first-in-human study of recombinant human thymosin beta-4 enrolled 54 healthy volunteers in a single-administration phase and 30 in a multiple-administration phase [11].
What happened in the thymosin beta-4 dry eye trials?
A 9-patient phase 2 trial in severe dry eye reported significant differences against vehicle in ocular discomfort and corneal fluorescein staining at day 56 [7]. A 72-subject phase 2 trial using a controlled adverse environment model missed both of its primary endpoints at the prespecified visit, while reporting significant differences on several secondary endpoints [8]. Three larger ophthalmic trials have completed without an indexed primary publication [16, 18, 19].
Is TB-500 approved by the FDA?
No. Neither thymosin beta-4 nor the TB-500 fragment is approved for any indication in the United States or elsewhere. The clinical record consists of phase 1 and phase 2 trials and completed phase 3 trials whose primary results have not appeared in the indexed literature [18, 19].
How large are the human trials of thymosin beta-4?
The reported ones are small: 40 volunteers in the intravenous phase 1 [6], 84 in the recombinant phase 1 [11], 9 patients in the severe dry eye trial [7], 72 in the controlled-environment trial [8] and 10 in a cardiac cell-transplantation pilot [10]. The larger trials — 317, 601 and 700 participants — are registered as completed but unpublished [16, 18, 19].
What does thymosin beta-4 do at the molecular level?
It sequesters monomeric actin. Its actin-sequestering ability and that of its fragments were characterised by DNase I inhibition assay [1], and its actin-binding site was mapped by mutational analysis [2]. A body of preclinical work associates it with integrin-linked kinase activation and cell migration [5].
Are there ongoing trials of TB-500 or thymosin beta-4?
A phase 1/2 trial of the thymosin beta-4 17-23 fragment against cardiovascular biomarkers in stable atherosclerotic disease is registered with an estimated 80 participants and listed as recruiting [21]. A phase 2 trial of thymosin beta-4 in acute myocardial infarction with 62 participants is registered as completed [20]. Neither has an indexed primary publication.

References

  1. Actin-sequestering ability of thymosin beta 4, thymosin beta 4 fragments, and thymosin beta 4-like peptides as assessed by the DNase I inhibition assay Biological Chemistry Hoppe-Seyler; 1993. PMID 8471179 doi:10.1515/bchm3.1993.374.1-6.117
  2. The actin binding site of thymosin beta 4 mapped by mutational analysis The EMBO Journal; 1996. PMID 8617195
  3. Thymosin beta4 accelerates wound healing The Journal of Investigative Dermatology; 1999. PMID 10469335 doi:10.1046/j.1523-1747.1999.00708.x
  4. Thymosin beta 4 and a synthetic peptide containing its actin-binding domain promote dermal wound repair in db/db diabetic mice and in aged mice Wound Repair and Regeneration; 2003. PMID 12581423 doi:10.1046/j.1524-475x.2003.11105.x
  5. Thymosin beta4 activates integrin-linked kinase and promotes cardiac cell migration, survival and cardiac repair Nature; 2004. PMID 15565145 doi:10.1038/nature03000
  6. A randomized, placebo-controlled, single and multiple dose study of intravenous thymosin beta4 in healthy volunteers Annals of the New York Academy of Sciences; 2010. PMID 20536472 doi:10.1111/j.1749-6632.2010.05474.x
  7. Thymosin β4 significantly improves signs and symptoms of severe dry eye in a phase 2 randomized trial Cornea; 2015. PMID 25826322 doi:10.1097/ICO.0000000000000379
  8. Thymosin beta 4 ophthalmic solution for dry eye: a randomized, placebo-controlled, Phase II clinical trial conducted using the controlled adverse environment (CAE) model Clinical Ophthalmology; 2015. PMID 26056426 doi:10.2147/OPTH.S80954
  9. Advances in the basic and clinical applications of thymosin β4 Expert Opinion on Biological Therapy; 2015. PMID 26096726 doi:10.1517/14712598.2015.1011617
  10. Safety and efficacy of autologous thymosin β4 pre-treated endothelial progenitor cell transplantation in patients with acute ST segment elevation myocardial infarction: A pilot study Cytotherapy; 2016. PMID 27288307 doi:10.1016/j.jcyt.2016.05.006
  11. A first-in-human, randomized, double-blind, single- and multiple-dose, phase I study of recombinant human thymosin β4 in healthy Chinese volunteers Journal of Cellular and Molecular Medicine; 2021. PMID 34346165 doi:10.1111/jcmm.16693
  12. Effects of BPC-157 and TB-500 on Achilles tendon healing in rats: A histopathological and biomechanical study Joint Diseases and Related Surgery; 2026. PMID 42542926 doi:10.52312/jdrs.2026.2951
  13. A Phase 2 Study on Effect of Thymosin Beta 4 on Wound Healing in Patients With Epidermolysis Bullosa. NCT00311766
  14. Study of Thymosin Beta 4 in Patients With Pressure Ulcers. NCT00382174
  15. Study of Thymosin Beta 4 in Patients With Venous Stasis Ulcers. NCT00832091
  16. Assessment of the Safety and Efficacy of RGN-259 Ophthalmic Solutions for Dry Eye Syndrome: ARISE-1. NCT02597803
  17. Assessment of the Safety and Efficacy Study of RGN-259 Ophthalmic Solutions for Neurotrophic Keratopathy : SEER-1. NCT02600429
  18. Assessment of the Safety and Efficacy of RGN-259 Ophthalmic Solutions for Dry Eye Syndrome : ARISE-2. NCT02974907
  19. Assessment of the Safety and Efficacy of RGN-259 Ophthalmic Solutions for Dry Eye Syndrome: ARISE-3. NCT03937882
  20. Safety and Efficacy Study of Thymosin Beta 4 in Patients With Acute Myocardial Infarction.Infarction. NCT05485818
  21. TB-500 (Thymosin Beta 4 17-23 Fragment) for Cardiovascular Biomarkers in Stable ASCVD. NCT07487363
  22. Epidermolysis bullosa: a genetic disease of altered cell adhesion and wound healing, and the possible clinical utility of topically applied thymosin beta4 Annals of the New York Academy of Sciences; 2007. PMID 17468231

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