LL-37

What it is. Cathelicidins are a family of antimicrobial peptides of the innate immune system; humans make exactly one, cleaved from the precursor hCAP18 to give LL-37 - so named for its two leading leucines and 37 residues. It is found in neutrophil granules, keratinocytes, and epithelial surfaces throughout the body. It kills bacteria by disrupting their membranes, and it also chemoattracts immune cells, modulates cytokines, promotes angiogenesis and drives re-epithelialisation [1][5]. Its expression is driven by vitamin D, which is part of why vitamin D status and infection have been linked [6].

The trial sequence, which is the point. In 2014, a randomised placebo-controlled trial in hard-to-heal venous leg ulcers found topical LL-37 safe and well tolerated, with a marked effect on healing predictors: mean ulcer area fell 68% in the 0.5 mg/mL group and 50% in the 1.6 mg/mL group [2]. Worth noting that the lower concentration did better - LL-37 is cytotoxic to host cells above a certain concentration, so this is biologically coherent rather than noise.

In 2021 the same question was asked properly: a multicentre prospective randomised placebo-controlled trial in 148 patients with hard-to-heal venous leg ulcers, mean ulcer duration 20.3 months, three arms, alongside compression therapy. The whole-cohort analysis found no significant improvement in healing. A subgroup analysis suggested benefit in patients with large ulcers, and the authors called for a further study to test that [3]. A post-hoc subgroup in a null trial is a hypothesis, not a result.

A separate randomised double-blind trial of LL-37 cream in diabetic foot ulcers reported enhanced healing [4] - a different wound type with a different pathophysiology, and a smaller literature.

Why it is interesting anyway. Host-defence peptides kill by membrane disruption rather than by hitting a specific target, which makes resistance harder to develop, and LL-37 is active against organisms that resist conventional antibiotics [7]. The translational problem is the recurring one for this class: proteolytic instability, cytotoxicity at therapeutic concentrations, and cost [1]. It has also been explored in epidermolysis bullosa [8] and around dental implants [9].

LL-37 is cationic and amphipathic: it folds into a helix with charged residues on one face and hydrophobic residues on the other. That structure inserts into and disrupts the anionic membranes of bacteria while largely sparing cholesterol-rich mammalian membranes - at low concentrations. At higher concentrations the selectivity fails and it becomes cytotoxic to host cells too [1]. This is the most practically important fact about it, and it explains why the lower concentration outperformed the higher one in the venous ulcer trial [2].

Beyond direct killing, LL-37 acts as an alarm signal: it chemoattracts neutrophils, monocytes and T cells, modulates cytokine production, promotes angiogenesis and stimulates keratinocyte migration and proliferation [1][5]. Those immunomodulatory and reparative actions, not the antibacterial ones, are the stated rationale for the wound trials - chronic ulcers are stuck in a prolonged inflammatory phase rather than simply infected [4].

Its own regulation runs through vitamin D: the hCAP18 gene carries a vitamin D response element, and topical calcipotriol raised hCAP18/LL-37 upregulation during wounding in human skin [6].

Applied topically to the wound bed, as a solution in the venous ulcer trials and as a cream in the diabetic foot ulcer trial [2][3][4]. Concentration matters in both directions - too little to act, too much to be safe for host cells - and the trials used 0.5 to 1.6 mg/mL.

The general obstacles for antimicrobial peptides as drugs are well catalogued: susceptibility to proteases, loss of activity in physiological salt and serum, host cytotoxicity, and manufacturing cost [1][7].

Topical application has a good record. The 2014 trial described treatment of hard-to-heal venous leg ulcers as safe and well tolerated [2]; the larger 2021 multicentre trial raised no safety concerns despite finding no efficacy [3]. These are open chronic wounds in elderly patients, which is a demanding setting for tolerability.

The concentration issue is the real one. LL-37 is cytotoxic to mammalian cells above a threshold; its selectivity for bacterial membranes is relative, not absolute [1]. The trial finding that 0.5 mg/mL outperformed 1.6 mg/mL [2] is the clinical shadow of that. This is a compound where increasing the dose is a plausible way to make things worse.

There is no systemic safety data at all. Every clinical study is topical. LL-37 given systemically would face rapid proteolysis, and its immunomodulatory activity is not something whose whole-body consequences have been characterised in a trial. Material sold for injection has no published basis.

No interaction studies have been published. One relationship worth knowing is upstream rather than pharmacological: vitamin D drives endogenous LL-37 expression, and topical vitamin D analogues raise it during wounding [6].

In the venous ulcer trials LL-37 was applied alongside compression therapy, the standard of care [3], so the trial result is for the combination rather than for the peptide alone.