TB-500: the peptide with a naming problem, and what the evidence shows

Three names, one confusion

Ask what TB-500 is and you can get three different answers. Some sellers call it "synthetic thymosin beta-4" ^[11]. Some papers use the name loosely ^[15]. And FDA, in its 2026 review, defined it as a specific seven-residue fragment ^[11].

FDA also noted that "TB-500" has no official nonproprietary name, that it had encountered several salts and derivatives sold under that one label, and that many websites use "TB-500" and "thymosin beta-4" interchangeably even though they are different substances ^[11].

When a US university laboratory tested products labelled TB-500 for the racing industry, many contained no thymosin beta-4 or related peptide, and most contained no protein, peptide or amino acid at all ^[5].

Here is how the three things relate.

Our data card for this article shows Ac-LKKTETQ, which matches the description of TB-500 used on this site: a synthetic peptide based on the active region of thymosin beta-4. The difference matters because adding an acetyl group changes a peptide's charge and shape, so FDA warns that results for the plain fragment cannot simply be transferred to TB-500 ^[11].

From a thymus hormone to the cell's scaffolding

The story starts in 1966, when Allan Goldstein, F. D. Slater and Abraham White described a factor from the thymus gland that stimulated lymphocyte production. They called it thymosin ^[1]. One of the peptides later isolated from calf thymus was thymosin beta-4 ^[11].

At first it was thought to be a thymus hormone. Then researchers found it nearly everywhere: in most cell types and in blood, saliva, tears and wound fluid ^[11]. Its main day job turned out to be structural. Thymosin beta-4 is the major protein that binds and stores actin, the building block of the cell's internal scaffolding ^[2].

Think of actin as bricks and thymosin beta-4 as a site manager holding a stack of them in reserve. A cell that needs to move, such as a skin cell crawling over a wound, has to build and dismantle actin filaments fast. Keeping a ready pool of loose actin is part of how that happens ^[2,11]. The actin-binding motif, LKKTET, sits at positions 17 to 22. Add the next residue and you get LKKTETQ, the sequence behind TB-500 ^[2,11].

Thymosin beta-4 has other active regions too. A four-residue piece at its start, Ac-SDKP, is linked to reduced inflammation and scarring ^[2]. That piece is not part of TB-500, and a racing-industry bulletin noted that the anti-inflammatory effects attributed to thymosin beta-4 seem to come from a region not found in TB-500 products ^[5].

What the research shows

Full-length thymosin beta-4 in animals

A large body of work from the 1990s onward found that thymosin beta-4 sped up skin and corneal wound healing, promoted new blood vessels and helped cells migrate in animal models ^[2]. These are the studies most often quoted for TB-500.

The fragment itself

The evidence for the seven-residue fragment is small. In a 2003 study in aged mice, the unacetylated LKKTETQ peptide applied to skin wounds improved closure and collagen at day 7, similar to the full protein ^[11]. There was no dose-response testing ^[11].

For acetylated TB-500, a 2024 study from a Korean doping-control laboratory opened with a blunt statement: the biological effects of TB-500 "have not been documented" ^[6]. In a fibroblast scratch-wound test, TB-500 itself did not speed closure. One of its breakdown products, Ac-LKKTE, did ^[6]. The authors suggested that any wound-healing activity may come from that metabolite rather than from TB-500 ^[6]. FDA's review, completed in May 2026, found no animal studies testing TB-500 for wound healing ^[11].

One has appeared since. In a July 2026 study, 32 rats with a repaired Achilles tendon received BPC-157, TB-500, both, or nothing for four weeks ^[15]. The TB-500 group was the only one whose tendons withstood significantly more load than controls, and it also had better tissue scores ^[15]. The authors describe their material as "synthetic thymosin beta-4 (TB-500)", so which molecule was tested is not fully clear, and it is a single study ^[15].

Human trials of thymosin beta-4

RegeneRx Biopharmaceuticals and ReGenTree, its US joint venture with HLB Therapeutics, have developed thymosin beta-4 as an eye drop called RGN-259 ^[8].

• Neurotrophic keratopathy (SEER-1): In 18 patients with a hard-to-heal corneal defect, 6 of 10 on RGN-259 healed completely at four weeks versus 1 of 8 on placebo. That primary result missed statistical significance (p = 0.0656), though a difference was significant two weeks after treatment stopped ^[7]. The trial was small, and several authors worked for the developer ^[7].
• Dry eye (ARISE-3): In 700 participants, results posted on ClinicalTrials.gov show almost identical changes on drug and placebo for both primary endpoints ^[9].

TB-500 in humans

FDA found no human exposure data for TB-500 by any route and no published case reports ^[11,12]. A phase 1/2 dose-escalation study of TB-500 in adults with stable cardiovascular disease was registered and began recruiting in February 2026; it has no results yet ^[10].

Doping: horses first, then people

TB-500 surfaced as a veterinary product. FDA dates its appearance to 2011, marketed for equine and greyhound racing ^[11]. In 2012 the Hong Kong Jockey Club's laboratory reported what it called the first identification of TB-500 and its breakdown products in samples from treated horses ^[4]. The same year, Ghent University's doping-control lab confirmed that a TB-500 product contained Ac-LKKTETQ ^[3].

The Racing Medication and Testing Consortium advised US racing regulators in 2013 to treat any finding of the peptide or its metabolites as a Class 1 substance with Class A penalties, and to test out of competition ^[5]. The published record we could verify focuses on detection methods and regulatory guidance rather than on named racing cases.

In human sport, WADA approved a research project in 2013 to work out how TB-500 is broken down and to add its metabolites to the screening methods used by testing laboratories ^[11].

What we don't know yet

What is in the vial. Because "TB-500" covers more than one substance, research findings and product labels may not refer to the same molecule ^[5,11].

Whether TB-500 does anything in a living body. Apart from one recent rat study with an unclear test material ^[15], no controlled study has shown that acetylated TB-500, rather than full-length thymosin beta-4, improves healing in animals, and none has been done in people ^[6,11].

Safety. FDA found no human safety data and no animal studies of acute or repeat-dose toxicity, genotoxicity, reproductive effects or cancer risk for TB-500 ^[11]. FDA's adverse-event database held no TB-500 reports up to March 2025, which says little given how rarely such products are reported ^[11]. Like other injected peptides, it carries a theoretical risk of immune reactions, especially if it clumps or contains impurities ^[12].

Regulatory status

• Approval: TB-500 is not approved anywhere and is not a component of any FDA-approved drug ^[11]. Thymosin beta-4 itself remains investigational ^[7,9].
• WADA: Class S2.3 lists "Thymosin-β4 and its derivatives e.g. TB-500", added as an example in the 2018 List; it is banned at all times ^[13].
• FDA, 2023: Placed "thymosin beta-4 fragment (LKKTETQ)" in Category 2 on 29 September 2023, citing immunogenicity risk from aggregation, peptide impurities and no human exposure data ^[12].
• FDA, 2026: After the nomination was withdrawn, the substance was moved to FDA's "nominated but withdrawn" list by April 2026 ^[12]. FDA staff still reviewed it for wound healing and recommended against listing ^[11]. On 23 July 2026 the advisory committee voted 8 to 6, with one abstention, in favour ^[14]. Listing would require FDA rulemaking ^[14].

In the lab: handling and storage

TB-500 is supplied as a lyophilised powder. FDA's review notes that the free base is reported to be stable below −20 °C when sealed and kept away from moisture and light ^[11], which matches common practice for research peptides: −20 °C for long-term storage and 2-8 °C for short periods. Reconstituted solutions are usually refrigerated, used within a short window, and not repeatedly frozen and thawed. Given the naming confusion, the certificate of analysis matters more than usual: the identity by mass spectrometry should match Ac-LKKTETQ (C38H68N10O14, about 889 Da). See our guide on reading a COA.

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