TB-500 vs LL-37: Actin-Binding Fragment vs Human Cathelicidin

Q: Is there human trial data on TB-500?

Not on the TB-500 heptapeptide that we have identified. The human studies people cite used full-length thymosin beta-4 (RegeneRx's RGN-137 and RGN-259), which is a different molecule.

Q: What is the FDA status of TB-500 and LL-37 in 2026?

Neither is FDA-approved. FDA removed the thymosin beta-4 fragment from Category 2 in April 2026, and its advisory committee voted 8-6 in July 2026 to recommend adding it to the 503A bulks list, which still requires rulemaking. LL-37 was also slated for removal from Category 2 and for advisory committee review expected before February 2027.

> Research disclaimer: This article summarizes peer-reviewed literature and public regulatory records for informational and research purposes only. It is not medical advice. TB-500 and LL-37 sold online are research chemicals, not FDA-approved drugs. Nothing here is a dosing protocol, an efficacy claim, or a recommendation for human or veterinary use.

Quick answer: TB-500 and LL-37 both show up in wound-healing research, but they're unrelated molecules. TB-500, as usually sold, is the acetylated heptapeptide Ac-LKKTETQ, matching residues 17–23 of thymosin β4 (Tβ4), the region that binds actin. Its literature is about cell migration and blood-vessel growth, and most of the human data behind it actually comes from the full-length protein. LL-37 is the only human cathelicidin: a 37-residue peptide cut from the protein hCAP18. It kills bacteria by disrupting their membranes and also signals to immune and skin cells. Unlike TB-500, LL-37 itself has been tested in placebo-controlled human trials, as a topical treatment for hard-to-heal venous leg ulcers. The larger of those trials missed its primary endpoint. Neither peptide is FDA-approved, and research on one tells you little about the other.

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Why These Two Get Compared

Both peptides circulate in the same recovery-focused communities and get described with the same words: "healing," "regenerative," "anti-inflammatory." Both have been on FDA's radar in the 2026 compounding debate. And both have wound-healing papers. That overlap in vocabulary hides a real difference in biology. TB-500 is a fragment of an intracellular actin-buffering protein. LL-37 is a host-defense peptide whose main job, as far as the literature can tell, is to deal with microbes and to coordinate the immune response that follows. If you're designing an experiment or reading a vendor page, the first question is which of those two biologies you actually care about.

Where Each Peptide Comes From

TB-500: a piece of thymosin β4

Thymosin β4 is a 43-amino-acid protein found in most cells, where it binds monomeric G-actin and keeps a pool of it ready for the cytoskeleton to use. The commercial "TB-500" heptapeptide reproduces the 17–23 stretch (LKKTETQ) that sits at the center of that actin interaction. It's a synthetic fragment, not something the body is known to make on its own in that form. Our mechanism of action guide covers the actin story in detail, and our fragment vs full-length article explains why the two shouldn't be treated as interchangeable.

LL-37: the active end of human cathelicidin

Humans have a single cathelicidin gene, CAMP, which encodes the precursor protein hCAP18. Neutrophils store hCAP18 in their granules. After release, the enzyme proteinase 3 cleaves off the C-terminal 37 residues, which begin with two leucines, hence "LL-37" (Sørensen et al., Blood, 2001). Skin keratinocytes, airway epithelium, and other barrier tissues also make it, especially after injury or infection.

One regulatory detail matters for research design: the CAMP gene promoter contains a vitamin D response element, and the active form of vitamin D strongly induces cathelicidin expression in human cells (Wang et al., J Immunol, 2004; Gombart et al., FASEB J, 2005). That pathway has no counterpart in the thymosin β4 literature.

How They Act

| | TB-500 (Ac-LKKTETQ) | LL-37 |
|---|---|---|
| Origin | Synthetic fragment of thymosin β4 (residues 17–23) | Natural cleavage product of human hCAP18 |
| Length | 7 residues | 37 residues |
| Core activity in the literature | Actin binding; cell migration; angiogenesis | Membrane-disrupting antimicrobial activity; immune and epithelial signaling |
| Notable receptor work | Mostly attributed to actin interaction and downstream migration effects | Angiogenesis via formyl peptide receptor-like 1 (FPRL1) (Koczulla et al., J Clin Invest, 2003) |
| Human trials of the exact molecule | None identified for the heptapeptide | Yes: topical trials in venous leg ulcers |
| Human trials of a related molecule | Full-length Tβ4 (RegeneRx RGN-137, RGN-259) | Not applicable |

TB-500: migration and vessels

The case for thymosin β4 in wound repair comes largely from animal work on the full protein. Malinda and colleagues reported that Tβ4, applied topically or given by injection, sped up healing of full-thickness wounds in rats, with more re-epithelialization, collagen deposition, and new vessel growth (J Invest Dermatol, 1999). Later work tied those effects to cell migration and angiogenesis. How much of that the 17–23 fragment reproduces is a much thinner question, which our angiogenesis research page covers.

LL-37: microbes first, then signaling

LL-37 is cationic and amphipathic, so it binds to and disrupts negatively charged bacterial membranes. That's the classic antimicrobial-peptide mechanism. But much of the more recent literature is about what LL-37 does to human cells:

Re-epithelialization. Heilborn and colleagues found LL-37 strongly expressed in the healing edge of normal human skin wounds and largely absent from the epithelium of chronic ulcers. In an ex vivo human skin wound model, antibodies against LL-37 blocked re-epithelialization (J Invest Dermatol, 2003).

Angiogenesis. Koczulla and colleagues showed LL-37 stimulates endothelial cells and induces vessel growth through FPRL1 (J Clin Invest, 2003). This is the one place where the two peptides' research interests clearly overlap, though through different mechanisms.

Immune activation. LL-37 can form complexes with the body's own DNA that activate plasmacytoid dendritic cells, a mechanism Lande and colleagues linked to psoriasis (Nature, 2007). In rosacea, Yamasaki and colleagues found abnormally high cathelicidin levels and abnormally processed peptide forms in affected skin (Nat Med, 2007).

That last point is worth sitting with. LL-37 isn't simply "anti-inflammatory." Depending on context, it can drive inflammation, and its overexpression is part of the biology of at least two inflammatory skin diseases.

> Sourcing note for researchers: whichever literature you're working from, confirm the identity of what's in the vial first. A 7-residue fragment and a 37-residue peptide have very different masses and should be easy to tell apart on a certificate of analysis. Our peptide buying guide lists what a COA should show.

The Human Data, Read Carefully

LL-37: two venous leg ulcer trials

LL-37 is one of the few research peptides with placebo-controlled human trials of the exact molecule. The Swedish company Promore Pharma developed it as a topical drug candidate, ropocamptide, for hard-to-heal venous leg ulcers.

First-in-human trial. A small randomized, placebo-controlled phase I/II study tested topical LL-37 at 0.5 and 1.6 mg/mL alongside compression therapy (Grönberg et al., Wound Repair Regen, 2014). The authors reported a better healing rate than placebo, with the lower concentration looking at least as good as the higher one.

HEAL LL-37 (phase IIb). This larger double-blind trial randomized 148 patients with hard-to-heal venous leg ulcers to LL-37 0.5 mg/mL, 1.6 mg/mL, or placebo, all on top of compression. In the full study population, it found no significant improvement in healing versus placebo. The primary endpoint, confirmed complete wound closure, came out at roughly a quarter of patients in every arm. A post hoc analysis of patients with large ulcers (10 cm² or more) found better results with LL-37, and the authors called for a new trial powered for that group. They reported the drug was well tolerated at both strengths (Mahlapuu et al., Wound Repair Regen, 2021; EudraCT 2018-000536-10).

The honest reading: a negative primary result, a promising subgroup that was found after the fact, and good tolerability for a topical product used in a clinical setting. Promore's November 2020 announcement led with the subgroup finding. The published paper is the better summary.

TB-500: borrowed data

There are no human trials of the TB-500 heptapeptide that we've been able to identify. The human data people cite comes from full-length thymosin β4, developed by RegeneRx as RGN-137 (topical gel for skin wounds) and RGN-259 (eye drops). Our pages on the pressure and venous stasis ulcer trials and human clinical trial history go through what those studies did and didn't show. The key point for this comparison: venous leg ulcers are the one condition where both literatures have placebo-controlled human data, and in both cases the data came from the parent or exact molecule given topically, under medical supervision. That's very different from injecting a vendor's research-grade fragment.

Safety Signals in the Literature

Neither peptide has the safety record of an approved drug, and both carry specific open questions.

LL-37. At higher concentrations in lab studies, LL-37 can damage human cells as well as bacteria, which is one reason concentration matters so much in its literature. Its role in psoriasis and rosacea shows it can promote inflammation. And its relationship with cancer is mixed: it has been reported to promote growth in some preclinical tumor models, such as ovarian cancer (Coffelt et al., PNAS, 2009), while appearing suppressive in others.

TB-500. FDA's concerns about the thymosin β4 fragment have centered on immunogenicity, impurities, and the lack of human exposure data. At FDA's July 2026 advisory committee meeting, a possible cancer signal was debated, though FDA acknowledged it had no studies on TB-500 specifically. Our cancer and tumor research page covers the thymosin β4 literature behind that concern.

For both: material sold as a research chemical has no regulated manufacturing standard, so identity, purity, and endotoxin contamination are open questions for every lot.

Regulatory Status in 2026

The two peptides are on parallel tracks at FDA, one step apart.

TB-500. On April 15, 2026, FDA said it would remove 12 peptides, including the thymosin β4 fragment, from its Category 2 list of bulk substances that raise significant safety concerns. On July 23, 2026, the agency's Pharmacy Compounding Advisory Committee voted 8–6, with one abstention, to recommend adding TB-500 to the 503A bulks list for wound healing. FDA staff had recommended against it. The vote is advisory, and listing would still require rulemaking. Our 503A compounding article explains the process.

LL-37. Cathelicidin LL-37 was among five additional substances FDA identified in April 2026 for removal from Category 2 and later review by the advisory committee, at a meeting expected before February 2027.

Removal from Category 2 doesn't make either substance eligible for compounding, and none of this is FDA approval. Both remain unapproved research chemicals.

Research-Design Implications

1. Pick the literature that matches the question. Antimicrobial activity, infected-wound models, vitamin D–cathelicidin signaling, and psoriasis or rosacea biology belong to LL-37. Actin dynamics, migration assays, and the thymosin β4 repair literature belong to TB-500.
2. Don't transfer human data across molecules. LL-37's ulcer trials don't support claims about TB-500, and RegeneRx's Tβ4 trials don't support claims about the heptapeptide.
3. Watch the concentration. LL-37's effects on host cells change with concentration, and the HEAL trial's lower dose looked at least as good as the higher one.
4. Control for vitamin D status in any in vivo LL-37 work, since it drives endogenous cathelicidin expression.
5. No combination data exists. We found no study testing TB-500 and LL-37 together.

Frequently Asked Questions

Is LL-37 the same kind of peptide as TB-500?

No. LL-37 is a 37-residue host-defense peptide cleaved from human cathelicidin (hCAP18). TB-500 is a synthetic 7-residue fragment matching residues 17–23 of thymosin β4. They have different origins, sequences, and primary activities.

Has LL-37 been tested in humans?

Yes, topically. A small phase I/II trial in venous leg ulcers (Grönberg et al., 2014) reported improved healing rates versus placebo. The larger phase IIb HEAL LL-37 trial in 148 patients found no significant difference from placebo in the full population, with a post hoc signal in patients with large ulcers.

Is there human trial data on TB-500?

Not on the TB-500 heptapeptide that we've identified. The human studies people cite used full-length thymosin β4 (RegeneRx's RGN-137 and RGN-259), which is a different molecule.

Is LL-37 anti-inflammatory?

It depends on context. LL-37 has immune-modulating effects, but it can also promote inflammation, and it's implicated in psoriasis (where LL-37–DNA complexes activate dendritic cells) and rosacea (where cathelicidin is abnormally high and abnormally processed).

What is the FDA status of TB-500 and LL-37 in 2026?

Neither is FDA-approved. FDA removed the thymosin β4 fragment from Category 2 in April 2026, and its advisory committee voted 8–6 in July 2026 to recommend adding it to the 503A bulks list, which still requires rulemaking. LL-37 was also slated for removal from Category 2 and for advisory committee review expected before February 2027.

Before You Source a Lot

Check identity before anything else. TB-500 and LL-37 differ by 30 residues, so the reported molecular mass on a certificate of analysis should make it obvious which one you have. Apollo Peptide Sciences publishes third-party COA documentation for its TB-500 lots. Compare the reported mass and sequence against what your protocol calls for. The peptide buying guide has the broader sourcing checklist.

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Sources: Sørensen OE et al., Blood 2001 (hCAP18 processing by proteinase 3); Wang TT et al., J Immunol 2004 and Gombart AF et al., FASEB J 2005 (vitamin D induction of CAMP); Heilborn JD et al., J Invest Dermatol 2003 (LL-37 in wound re-epithelialization); Koczulla R et al., J Clin Invest 2003 (LL-37 angiogenesis via FPRL1); Lande R et al., Nature 2007 (psoriasis); Yamasaki K et al., Nat Med 2007 (rosacea); Coffelt SB et al., PNAS 2009 (ovarian cancer models); Malinda KM et al., J Invest Dermatol 1999 (thymosin β4 wound healing); Grönberg A et al., Wound Repair Regen 2014; Mahlapuu M et al., Wound Repair Regen 2021, HEAL LL-37 (EudraCT 2018-000536-10); Promore Pharma press release, Nov. 19, 2020; Goodwin Procter, "FDA Signals Potentially Evolving Stance Toward Compounding of Certain Peptides," May 11, 2026; Sheppard Mullin, "Compounded Peptides on the Loose," Aug. 7, 2026.