LL-37 and antimicrobial peptides: the body's answer to resistance

One molecule, one family

Antimicrobial peptides are part of the innate immunity of almost every organism. In mammals, one of their main families is the cathelicidins, and humans have a single member: LL-37, an amphipathic, helical 37-residue peptide¹. It is expressed in skin, the respiratory tract, the gut and the testis, and in neutrophils, monocytes, T cells, B cells and NK cells¹.

Its role goes beyond killing microbes. LL-37 has been described to neutralize lipopolysaccharide, regulate the inflammatory response, recruit adaptive immune cells to the site of infection and promote re-epithelialization and wound closure¹. That dual nature, antimicrobial and immunomodulatory, explains both its therapeutic promise and its unwanted effects.

The vitamin D link

In 2006 a study in Science showed that Toll-like receptor activation in human macrophages up-regulated the vitamin D receptor and 1-hydroxylase, which induced cathelicidin and killing of intracellular Mycobacterium tuberculosis². The authors also observed that serum from African-American individuals, with low 25-hydroxyvitamin D, was inefficient at supporting cathelicidin mRNA induction².

The finding gave a mechanistic basis to an old observation about vitamin D and tuberculosis. It should be read precisely: it demonstrates a biological pathway in cells, not that vitamin D supplementation prevents infections in clinical practice².

LL-37 in venous ulcers: from enthusiasm to phase IIb

The first-in-human trial enrolled 34 patients with hard-to-heal venous ulcers, treated twice weekly for 4 weeks with topical LL-37 (0.5, 1.6 or 3.2 mg/mL) or placebo³. With 0.5 and 1.6 mg/mL, the healing rate constant was about six and three times higher than with placebo (p = 0.003 and p = 0.088; the latter not statistically significant), and mean ulcer area fell 68% and 50%³. The highest dose did not differ from placebo, and there were no local or systemic safety concerns³.

The phase IIb, with 148 patients, was more sobering: in the full population there was no significant improvement in healing with LL-37 vs placebo⁴. A post hoc analysis suggested benefit in large ulcers (at least 10 cm²), which the authors consider grounds for an adequately powered study in that subgroup⁴. The drug was well tolerated at both strengths⁴.

A reassuring point for topical development: although LL-37 is degraded by trypsin and by Pseudomonas aeruginosa elastase, ex vivo, in fluid from chronic venous ulcers, it was fairly resistant to proteolysis for up to 24 hours⁵.

The inflammatory side: psoriasis and rosacea

The same peptide that defends can perpetuate inflammation. In psoriasis, LL-37 binds self-DNA and turns it into a potent trigger for plasmacytoid dendritic cells, which produce interferon through TLR9; the authors propose that this pathway breaks tolerance to self-DNA⁶.

In rosacea, affected people express abnormally high cathelicidin levels in facial skin, with processed forms that differ from those in normal skin, linked to increased stratum corneum tryptic enzyme; in mice, those fragments reproduce the inflammation⁷. For the clinician the lesson is direct: more LL-37 is not always better.

The pressure of antimicrobial resistance

The Global Burden of Disease analysis estimated 4.71 million deaths associated with bacterial resistance in 2021, of which 1.14 million were directly attributable to it⁸. The 2050 forecast is 1.91 million attributable and 8.22 million associated deaths, and south Asia and Latin America and the Caribbean are forecast to be the super-regions with the highest resistance mortality rate in 2050⁸. Deaths associated with carbapenem resistance rose from 619,000 in 1990 to 1.03 million in 2021⁸.

Peptides already in the pharmacy

The idea is not theoretical. Of more than 3000 membrane-active antimicrobial peptides described, the FDA has approved three for therapeutic use: gramicidin, daptomycin and colistin⁹. Daptomycin, a last-line lipopeptide against Gram-positive bacteria, forms a complex with calcium and phosphatidylglycerol in the bacterial membrane and depolarizes it⁹. Polymyxins such as colistin have re-emerged as last-resort antibiotics against multidrug-resistant Gram-negative bacteria¹⁰.

Vancomycin is often mentioned alongside them, but it is a glycopeptide with a different mechanism: it binds the D-Ala-D-Ala region of lipid II and interferes with peptidoglycan maturation¹¹. These drugs show the price of the class: in a meta-analysis of 48 studies, pooled polymyxin nephrotoxicity was 45% (48% with colistin and 38% with polymyxin B)¹⁰, and gramicidin S was historically limited to topical use because of hemolytic toxicity¹².

Why so few make it: pexiganan and murepavadin

Pexiganan, a magainin analogue, was tested as a cream in mildly infected diabetic foot ulcers. In two trials with 835 patients, the combined data showed results equivalent to oral ofloxacin, with 85–90% clinical improvement, although one of the two trials failed to show equivalence; no significant resistance to pexiganan emerged¹³. Years later, the phase 3 OneStep-1 and OneStep-2 trials, with 389 patients, did not show superiority over vehicle plus standard wound care, nor any difference in ulcer closure¹⁴.

Murepavadin, a peptidomimetic targeting Pseudomonas aeruginosa, reached phase 3 in nosocomial pneumonia. In May 2019 enrollment was temporarily halted because 56% of patients treated in the PRISM-MDR study developed kidney alterations, when 25–40% was expected based on the literature and the comparator arm¹⁵.

A review in Lancet Infectious Diseases sums up the balance: antimicrobial peptides have clear advantages, such as slower emergence of resistance, antibiofilm activity and favourable modulation of the host response, but their structural and functional limitations, together with a stricter regulatory environment, have hampered their translation to the clinic¹⁶.