TB4-FRAG

TB4-FRAG designates the biologically active fragments derived from thymosin beta-4 (Tβ4), a ubiquitous 43-amino acid G-actin-sequestering protein encoded by TMSB4X on the X chromosome. Two fragments are principally studied in research and performance contexts.

The heptapeptide Ac-LKKTETQ (amino acids 17-23, MW 889 g/mol), marketed as TB-500, contains the conserved actin-binding motif LKKTET. It forms 1:1 complexes with monomeric G-actin, sequestering unpolymerized actin and directing it toward sites of cell migration. Downstream signaling includes activation of integrin-linked kinase (ILK), upregulation of VEGF and matrix metalloproteinases, stimulation of angiogenesis, and attenuation of NF-κB-driven inflammation. A 2024 in vitro characterization study identified the TB-500 metabolite Ac-LKKTE — not the intact heptapeptide — as the primary wound-healing active species, suggesting that biological activity may be partly metabolite-mediated rather than intrinsic to the parent fragment.

The second fragment, Ac-SDKP (N-acetyl-Ser-Asp-Lys-Pro, amino acids 1-4), is cleaved endogenously from Tβ4 by prolyl oligopeptidase and degraded by angiotensin-converting enzyme. It circulates at nanomolar plasma concentrations. Ac-SDKP modulates TGF-β and MEK-ERK pathways to suppress fibroblast-to-myofibroblast differentiation and reduce extracellular matrix accumulation, conferring anti-fibrotic activity across cardiac, renal, hepatic, and pulmonary tissue. It also regulates hematopoietic stem cell entry into the cell cycle and is reported to cross the blood-brain barrier, distinguishing it mechanistically from the actin-binding fragment.

No human clinical trials have been conducted specifically on the isolated TB4-FRAG peptide fragments (Ac-LKKTETQ or Ac-SDKP) for performance enhancement, injury recovery, or therapeutic indications. Fragment-specific evidence derives almost entirely from controlled animal models.

For Ac-SDKP, Conte et al. (2016, Oncotarget; PMID 27029074, n=79 mice) evaluated the fragment in a bleomycin-induced pulmonary fibrosis model. Preventive co-treatment increased 21-day survival from approximately 60% to 80%, significantly reduced collagen deposition, and suppressed lung expression of IL-17, TGF-β, and alpha-SMA. Delayed administration one week post-injury also reduced myofibroblast differentiation, suggesting therapeutic as well as preventive utility. Zuo et al. (2013, Kidney International; PMID 23739235) found Ac-SDKP significantly decreased renal fibrosis in a unilateral ureteral obstruction model in both wild-type and PAI-1-knockout mice, with effects independent of PAI-1 signaling — a distinction from full Tβ4.

For the actin-binding heptapeptide in the TB-500 context, a 2003 mouse study reported that the LKKTETQ sequence promoted wound repair in aged animals at a level comparable to the intact parent molecule, though this study used the full Tβ4 as primary comparator.

Human evidence exists only for the complete 43-amino acid Tβ4 molecule. Two Phase 2 ocular trials of topical 0.1% Tβ4 (RGN-259) showed improvement in dry eye signs and symptoms with no adverse events (PMID 26241398). In a murine dry eye model, RGN-259 produced a 9.3-fold increase in tear volume and 51-60% improvement in corneal irregularity scores, matching or exceeding efficacy of cyclosporine A and lifitegrast (Kim et al., 2018, Scientific Reports; PMID 30002412). Phase 2 trials in pressure ulcers and epidermolysis bullosa also demonstrated efficacy and tolerability. A rat stroke dose-response study (Morris et al., 2014; PMID 25060418; n=50) reported 24-35% relative neurological improvement at 2-12 mg/kg. Whether full-molecule human findings extrapolate to isolated fragments remains unestablished by clinical evidence.

Typical dosage: 2-5 mg (weekly).

No FDA-approved dosage exists for any TB4-FRAG preparation. The following reflects protocols reported in preclinical research and compounding or performance-enhancement communities. These are not clinical recommendations.

For injectable TB-500 (Ac-LKKTETQ), reported protocols in compounding and performance contexts describe a loading phase of 2.0-2.5 mg by subcutaneous injection twice weekly for four to six weeks, followed by a maintenance phase of 2.0-2.5 mg once weekly or biweekly. Administration is typically via a 27-31 gauge insulin syringe into subcutaneous abdominal tissue. In preclinical rodent studies, doses ranged from 2 to 18 mg/kg; the calculated optimal dose for neurological recovery in a rat embolic stroke model was approximately 3.75 mg/kg, though rodent-to-human dose scaling is not established.

For oral Ac-SDKP (TB4-FRAG 1-4 tetrapeptide), commercially available supplement products (such as those branded as TB4-FRAG capsules) typically contain 500-1,000 mcg per capsule, with suggested use of one capsule twice daily on an empty stomach. Oral bioavailability of this short tetrapeptide is claimed to be superior to the full 43-amino acid molecule owing to its smaller size, but no formal human pharmacokinetic data have been published.

This information is provided for educational purposes only and does not constitute medical advice. TB4-FRAG preparations are not approved by the FDA for therapeutic or performance-enhancing use in humans. Any contemplated clinical use requires evaluation by a qualified healthcare provider.

No compound-specific stability data has been identified for this peptide. The general lyophilized-peptide handling framework applies — see Storage & handling for temperature, reconstitution diluent, and beyond-use dating principles.

The most widely discussed combination in the compounding and performance community is TB-500 (Ac-LKKTETQ) or TB4-FRAG with BPC-157, a pentadecapeptide derived from gastric protective protein. This pairing — sometimes called the Wolverine Stack — is premised on putatively complementary mechanisms: BPC-157 is proposed to act locally at injury sites via growth hormone receptor and nitric oxide pathways, while TB-500 is thought to provide systemic actin-mediated cellular mobilization and angiogenic support. No controlled study has evaluated this combination in humans, and no head-to-head preclinical design has tested the stack directly. The rationale and all reported benefits are anecdotal.

Some practitioners also combine TB4-FRAG with growth hormone secretagogues such as ipamorelin paired with CJC-1295, reasoning that growth hormone axis stimulation augments the anabolic and reparative milieu. This combination is similarly anecdotal, without supporting clinical or preclinical data specific to the combination.

TB4-FRAG is not currently FDA-approved for any indication. It is generally classified as a research compound. Regulatory status varies by country.

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Reported side effects: Similar to TB-500

Phase 1 dose-escalation studies of the full thymosin beta-4 molecule in humans — not specifically the isolated fragments — reported no serious adverse events at doses up to 1,260 mg per day for 14 days. The most common adverse events were mild headache and upper respiratory infection. No dose-limiting toxicities were identified.

For the isolated TB-500 heptapeptide and Ac-SDKP specifically, no controlled human safety studies have been published. Anecdotal reports from compounding-use contexts describe transient fatigue, lethargy, headache, and injection site reactions including localized redness, swelling, and discomfort.

A significant theoretical safety concern centers on anti-apoptotic and pro-angiogenic activity. Tβ4 and its fragments promote endothelial cell survival, angiogenesis, and cellular resistance to apoptosis. In vitro studies have demonstrated that Tβ4 overexpression in pancreatic cancer cells heightens their resistance to immune-mediated and chemotherapeutic killing; overexpression has also been documented in colon cancer, renal cell carcinoma, and other tumor types. No published evidence indicates that TB4-FRAG initiates cancer de novo in healthy tissue, but its pro-survival and pro-angiogenic signaling represents a theoretical contraindication in individuals with active or occult malignancy.

• WADA prohibition: TB-500 and all Tβ4 fragments are banned in sport at all times under Section S2 (Peptide Hormones, Growth Factors, Related Substances and Mimetics). Competitive athletes testing positive face standard suspension penalties. • No FDA approval exists for human therapeutic or performance use of any TB4-FRAG preparation. • No long-term controlled safety data exist for isolated fragment use in humans. • Contraindications include active or recent malignancy, pregnancy, and breastfeeding. • TB-500 is classified as a prescription medicine in Australia and New Zealand.