LL-37 – Learning Health

What it is

LL-37 is 37 residues long, named for its two leading leucines. It is the active fragment of human cathelicidin, cut out of a precursor protein called hCAP18 by proteases in neutrophils and epithelial cells. It is part of innate immunity — one of the molecules the body uses to kill bacteria directly, before the adaptive immune system engages.

As a drug candidate it has been developed under the non-proprietary name ropocamptide, for topical use on chronic wounds.

What it does in the body

LL-37 is cationic and amphipathic, which lets it insert into and disrupt bacterial membranes — the classic antimicrobial-peptide mechanism. But its signalling roles turned out to be at least as important as its killing: it is chemotactic for immune cells, it promotes angiogenesis and re-epithelialisation in wounds, and it modulates cytokine production.

It also does something that makes it genuinely dangerous in excess, and this is one of the better-established findings in dermatological immunology. LL-37 binds host DNA and converts it from inert material into a potent immune trigger: the peptide condenses self-DNA into aggregated structures that are retained in early endocytic compartments of plasmacytoid dendritic cells, where they activate Toll-like receptor 9 and drive type I interferon production. That is the mechanism by which LL-37 breaks innate tolerance to self-DNA, and it was identified as the key factor mediating dendritic cell activation in psoriasis.

In rosacea, patients express abnormally high levels of cathelicidin in facial skin, and the processed forms differ from those in healthy skin because of increased stratum corneum tryptic enzyme activity. Injecting those rosacea-type cathelicidin peptides into mouse skin increased inflammation; deleting the cathelicidin gene prevented protease-driven inflammation.

What the human evidence shows

Two randomised trials in hard-to-heal venous leg ulcers, and they tell a familiar story.

First-in-man, 2014. Thirty-four participants went through a three-week open-label placebo run-in, then four weeks of double-blind twice-weekly topical LL-37 at 0.5, 1.6 or 3.2 mg/mL or placebo, then four weeks of follow-up. Healing rate constants were roughly six-fold and three-fold higher than placebo at 0.5 and 1.6 mg/mL (p=0.003 and p=0.088). Mean ulcer area fell 68% and 50% in those groups. The highest dose, 3.2 mg/mL, showed no difference from placebo — an inverted dose-response that is biologically interesting and statistically awkward. No safety concerns.

Phase 2b, 2021. HEAL LL-37 enrolled 148 patients with hard-to-heal venous leg ulcers, mean age 67.6, median ulcer duration 20.3 months, mean wound area 11.6 cm², randomised to 0.5 or 1.6 mg/mL or placebo on top of compression therapy. Efficacy analysis in the full study population found no significant improvement over placebo. A post-hoc analysis found statistically significant improvement in several interrelated healing parameters in the subgroup with target wounds of at least 10 cm² — a known negative prognostic factor. The drug was well tolerated at both doses. The authors called the subgroup result an interesting observation warranting a properly powered study.

Why the subgroup finding is a hypothesis, not a result

When a trial misses its primary endpoint and a subgroup is significant on post-hoc analysis, the honest description is that a new hypothesis has been generated. Subgroup analyses carry a high false-positive rate, and the authors said as much themselves. It may well be that LL-37 helps large ulcers; that now needs a trial designed to answer it.

Claims and what backs them

Claim as usually stated	Verdict	What the published evidence actually shows
Kills bacteria	Supported	Direct membrane-disrupting activity against a broad range of organisms is well established in vitro, and is the original reason it was characterised.
Heals chronic wounds	Mixed	The phase 2b trial in 148 patients found no significant benefit overall. The earlier 34-patient trial found strong effects at the two lower doses but none at the highest.
Helps large, hard-to-heal ulcers specifically	Unproven	A post-hoc subgroup finding in the phase 2b trial. The investigators themselves framed it as warranting further study.
Boosts the immune system	Not supported	The best-documented consequence of excess LL-37 in human tissue is pathological inflammation — psoriasis and rosacea. More is not better here.
Safe to use	Mixed	Topically on ulcers, well tolerated in both trials with no safety concerns. Systemically, in people prone to autoimmune skin disease, the mechanism is directly implicated in disease.
Anti-cancer	Not supported	The literature is explicitly bidirectional: LL-37 overexpression promotes development and progression of ovarian, lung and breast cancers while suppressing tumorigenesis in colon and gastric cancer. The direction depends on the tissue.

Risks and unknowns

This is the clearest case in the atlas where more of an endogenous peptide is demonstrably harmful. LL-37 is central to psoriasis pathogenesis via the self-DNA/TLR9 mechanism, and elevated, abnormally processed cathelicidin is central to rosacea. Anyone with psoriasis, rosacea, lupus or another interferon-driven condition is contemplating a molecule their disease already overproduces.

Cancer biology is tissue-dependent and unpredictable. A review of LL-37 in cancer development describes two contradictory effects — promotion or inhibition — with mechanisms that are tissue-specific and depend on which membrane receptors a given cancer expresses. There is no way to know in advance which direction applies to a particular person.

The dose-response is not monotonic. In the first-in-man trial the highest dose performed no better than placebo while lower doses performed well. That is a real signal that more is not better, and it is poorly understood.

Route matters enormously. All human evidence is topical, on open wounds, under medical supervision. Systemic administration of a membrane-disrupting cationic peptide has an obvious haemolysis and cytotoxicity concern and has not been established.

Where regulators stand

United StatesNot approved for any use. “Cathelicidin LL-37” appears in the FDA’s table of bulk drug substances previously in Category 2 whose nominations were withdrawn by the nominators. It was not among the seven peptides voted on in July 2026.

Development statusDeveloped as ropocamptide for topical wound treatment. It has completed phase 2b, which missed its primary endpoint in the full population.

SportNot named on the WADA 2026 Prohibited List. As an unapproved pharmacological substance, use would nonetheless engage the S0 catch-all for substances with no approval for human therapeutic use.

The bottom line

LL-37 is a real human antimicrobial peptide with a legitimate development programme behind it, and it is one of the few compounds here to reach a properly sized phase 2b trial. That trial did not show benefit in the population it enrolled.

The more important point is about direction. For most peptides in this atlas the open question is whether they do anything. For LL-37 we know what too much of it does in human skin — it drives psoriasis and rosacea through a well-mapped interferon mechanism. That makes “supplementing” an endogenous antimicrobial peptide a very different proposition from topping up something inert.

References

Grönberg A, Mahlapuu M, Ståhle M, Whately-Smith C, Rollman O. Treatment with LL-37 is safe and effective in enhancing healing of hard-to-heal venous leg ulcers: a randomized, placebo-controlled clinical trial. Wound Repair Regen. 2014;22(5):613–621 PMID 25041740
Mahlapuu M, Sidorowicz A, Mikosinski J, et al. Evaluation of LL-37 in healing of hard-to-heal venous leg ulcers: a multicentric prospective randomized placebo-controlled clinical trial. Wound Repair Regen. 2021;29(6):938–950 PMID 34687253
Lande R, Gregorio J, Facchinetti V, et al. Plasmacytoid dendritic cells sense self-DNA coupled with antimicrobial peptide. Nature. 2007;449(7162):564–569 PMID 17873860
Yamasaki K, Di Nardo A, Bardan A, et al. Increased serine protease activity and cathelicidin promotes skin inflammation in rosacea. Nat Med. 2007;13(8):975–980 PMID 17676051
Piktel E, Niemirowicz K, Wnorowska U, et al. The role of cathelicidin LL-37 in cancer development. Arch Immunol Ther Exp. 2016;64(1):33–46 PMID 26395996
Vandamme D, Landuyt B, Luyten W, Schoofs L. A comprehensive summary of LL-37, the factotum human cathelicidin peptide. Cell Immunol. 2012;280(1):22–35 PMID 23246832