LL-37: Side Effects

Safety Notice#

The safety profile of LL-37 in humans has not been established through controlled clinical trials. The information below is derived primarily from animal studies and should be interpreted accordingly.

Documented Adverse Effects#

Topical administration in humans (venous leg ulcers). In a multicenter, double-blind, randomized, placebo-controlled phase IIb trial (LL-37 0.5 mg/mL, n=46; 1.6 mg/mL, n=48; placebo, n=50), local reactions at the target ulcer (redness, edema, warmth) occurred more frequently with LL-37 than placebo: 42.6% (0.5 mg/mL) and 39.2% (1.6 mg/mL) versus 20.3% with placebo. Adjacent-skin reactions (redness, scaling, papules, pustules, vesicles) were also common with LL-37, reported in up to 54.1%, while lower with placebo. Most events were mild or moderate; severe local reactions were rare (≤3.4% at study visits). Target-ulcer infections were infrequent and similar across arms (2.1% and 4.1% with LL-37 vs 5.9% with placebo). Across the study, 12 serious adverse events occurred in 11/148 patients (7.4%), largely judged unrelated to the investigational product; six AEs in four patients (2.7%) were judged possibly related, and none were judged probably related. No deaths were reported. Discontinuations were not specified in the excerpted text (mahlapuu2021evaluationofll‐37 pages 8-10).

Systemic administration in animals (intravenous LL-37 in rat sepsis models). In Wistar rat models of endotoxemia, E. coli peritonitis, and cecal ligation and puncture, LL-37 given systemically at 1 mg/kg (intravenous in at least the LPS model) was monitored for toxicity over 72 hours with prespecified clinical and physiological parameters. Investigators reported no drug-related adverse effects; in an uninfected supplementary cohort receiving 1 mg/kg LL-37, no changes in physiological parameters were observed. No frequencies beyond the statement of no observed drug-related AEs were provided.

Oral administration in animals and humans via engineered Lactococcus lactis (cas001). Preclinical oral toxicity studies in rats administered approximately 100× the estimated clinical dose of LL-37-producing L. lactis (6 × 10^10 CFU/kg/day) for 2–3 weeks found no adverse effects on body weight, intake, hematology, serum chemistry, organ weights/histology, or observed behavior; the LD50 exceeded the 100× dose. Pharmacokinetics showed transient serum LL-37 elevations peaking at 2 hours post-dose. In a small, single-arm exploratory human study in 11 adults with mild COVID-19 treated for 3 weeks with oral cas001, the report states 0/11 adverse reactions, with good tolerance and no discomfort noted; a volunteer PK substudy reported no laboratory abnormalities. No graded severity data, discontinuations, or SAEs were detailed for the human cohort in the provided excerpts.

Overall interpretation. Across routes, the clearest dose–event characterization comes from topical LL-37 in venous ulcers, where local cutaneous reactions were common (approximately 39–43%) but generally mild/moderate, with severe reactions uncommon (≤3.4%) and few events judged possibly related (2.7% of patients). Systemic intravenous dosing at 1 mg/kg in rats was not associated with observable drug-related toxicity over 72 hours, and high-dose oral exposure via engineered bacteria showed no toxicity in rats and no adverse reactions in a small human cohort; however, these latter human data are limited by single-arm design and small sample size.

Evidence gaps. We did not retrieve detailed adverse-event data for intratumoral injections of LL-37 in melanoma (NCT02225366) or comprehensive safety tables from the diabetic foot ulcer LL-37 cream trial in our excerpts; therefore, frequency/severity for those routes cannot be summarized here (mahlapuu2021evaluationofll‐37 pages 8-10).

Animal Study Safety Data#

We synthesized adverse-effect data for exogenous LL-37 from controlled animal experiments and available human administrations, stratified by route. A structured summary is embedded below, followed by narrative context and limitations.

Contraindications#

Exogenous LL-37 has proinflammatory and alarmin properties, forms immunostimulatory complexes with nucleic acids, is cationic and binds polyanions and lipoproteins, and shows dose-dependent local toxicity. Contraindications (or strong cautions) include disorders driven by NETs/type I IFN or mast cells (psoriasis, SLE), rosacea, and thromboinflammatory states. Important interactions include neutralization by heparin/glycosaminoglycans and serum components (e.g., lipoproteins), modulation by vitamin D (endogenous induction), broad antibiotic synergies, and amplification with TLR agonists; conversely, proteases (host and microbial) degrade LL-37.

Contraindications and high‑caution settings

• Psoriasis and psoriatic arthritis: LL-37 binds self-DNA/RNA to form complexes that activate pDCs and myeloid TLRs (TLR9, TLR7/8), fueling type I IFN and NETosis; LL-37-specific T cells are described. Exogenous LL-37 could exacerbate disease activity (theoretical but strongly mechanistic).
• Systemic lupus erythematosus and NET-driven autoimmunity: LL-37 coats NETs and protects NET DNA from degradation, promoting pDC IFN-α production and inflammasome activation; this supports avoiding exogenous LL-37 in SLE and related interferonopathies.
• Rosacea: Pathology includes aberrant processing of cathelicidin to LL-37-like inflammatory fragments via KLK5, with angiogenesis and cytokine release; exogenous LL-37 could aggravate cutaneous inflammation in rosacea.
• Mast cell–mediated disorders and anaphylactoid risk: LL-37 can activate/degranulate mast cells and drive pruritogenic cytokines; use cautiously where mast cell activation worsens symptoms (theoretical based on mechanism and human cell data).
• Thromboinflammatory states/atherothrombosis: LL-37 can activate endothelium, promote adhesion molecules, and participates in NETosis; while not definitive clinical contraindication, prothrombotic amplification is a theoretical risk.
• Local dose-related toxicity: High topical doses in humans produced ulcer necrosis, edema, and severe inflammation; indicates a narrow therapeutic window and supports avoiding high-concentration formulations or inflamed/compromised tissue without careful dosing.

Drug/biomolecule interactions (known or strongly plausible)

• Polyanions (heparin, dermatan sulfate, glycosaminoglycans): These anionic biomolecules bind LL-37 and attenuate its antimicrobial activity in wound/plasma environments; heparinized contexts may diminish efficacy.
• Serum lipoproteins (e.g., HDL/ApoA‑I) and mucins: Binding/sequestration of LL-37 by lipoproteins and mucins can reduce free, active peptide; expect reduced bioavailability in serum- or mucus-rich sites.
• LPS/TLR ligands: LL-37 binds and neutralizes LPS and modulates TLR signaling; co-exposure to TLR agonists may yield complex effects (attenuation of TLR4 by LPS binding but amplification of endosomal TLR7/8/9 when complexed with nucleic acids). Co-administration of nucleic acid–containing agents could enhance inflammatory signaling via LL-37 complexes.
• Vitamin D and CAMP inducers: 1,25-dihydroxyvitamin D3 induces CAMP/LL-37 expression; concomitant vitamin D or agents that raise calcitriol may elevate endogenous LL-37, potentially augmenting effects or inflammatory risk (context-dependent).
• Antibiotics (synergy): LL-37 and derivatives often show synergy with conventional antibiotics (e.g., ciprofloxacin, vancomycin, daptomycin; combinations can lower biofilm MBEC and increase bacterial susceptibility); however, subinhibitory LL-37 may promote bacterial adaptive responses and potential cross-resistance.
• Proteases (host and microbial): Host mast cell tryptase and microbial proteases (e.g., Pseudomonas elastase) degrade LL-37, reducing activity; peptide stabilization or protease inhibition may be needed.

Additional safety considerations

• Hemolysis/cytotoxicity: LL-37 is membrane-active and can lyse host cells at higher concentrations; formulation and dosing must minimize host cytotoxicity.
• Delivery matrix/biofluids: Rapid binding to surrounding matrices and inactivation is common; nanoparticles or co-formulations can modulate pharmacodynamics and mitigate toxicity, but also change interactions.

Practical implications and monitoring

• Avoid or use extreme caution in patients with active psoriasis, SLE, or rosacea; monitor for disease flares if exposure is unavoidable.
• Anticipate reduced activity in heparinized patients or GAG-rich wound beds; consider timing relative to heparin and formulation strategies to overcome polyanion binding.
• If co-administering with vitamin D (or in replete states), anticipate higher endogenous LL-37; weigh benefits (antimicrobial) vs risks (inflammation) in autoimmunity-prone settings.
• For anti-infective use, leverage synergistic combinations with antibiotics for biofilm or resistant organisms; avoid prolonged sub-MIC exposure that could promote adaptive resistance.
• Limit concentration at application sites and avoid inflamed, highly vascular lesions without careful dosing, given the documented dose-dependent local toxicity.

Evidence gaps

• Robust human data on systemic contraindications are limited; current recommendations are primarily mechanism-based and supported by human ex vivo/clinical observations in skin and by disease biology.

Toxicology#

Summary of findings

• LD50: We did not find a traditional LD50 determination for purified LL-37 in mammals. One preclinical safety report of an oral recombinant LL-37–producing Lactococcus lactis (cas001) stated that in rats the LD50 exceeded a dose 100-fold higher than the projected clinical dose (delivered as CFU of the bacterial vector), but this is not a peptide mass-based LD50 and cannot be directly translated to purified LL-37. No LD50 for systemic administration of synthetic LL-37 was reported in the retrieved primary studies.

• Organ/system toxicity in vivo: In adult Wistar rats subjected to gram-negative sepsis models, intravenous LL-37 at 1 mg/kg reduced lethality and inflammatory biomarkers. In a peptide-only control subgroup (no infection), clinical monitoring (vitals, leukocytes, serum creatinine) revealed no drug-related adverse signs over 72 h; however, no histopathology was presented, and the study was not a formal GLP toxicology evaluation. For oral cas001 in rats, a 3-week subchronic study reported no adverse effects on body weight, food/water intake, serum biochemistry, organ weights, or H&E histology at 100× the intended clinical CFU dose. Together, available in vivo data suggest short-term tolerability at studied doses/routes but lack comprehensive organ-specific toxicology for purified LL-37.

• Mutagenicity/genotoxicity: We found no reports of standard mutagenicity or genotoxicity testing for LL-37 (e.g., Ames, in vitro micronucleus, or comet assays) in the retrieved literature. Thus, genotoxic potential remains uncharacterized in these sources.

• Dose–response relationships: • Human topical application: In a clinical wound study, LL-37 exhibited a clear dose–response for efficacy and local toxicity. Application twice weekly for four weeks at 110 µM achieved the best wound closure (68% vs 27% placebo), while higher concentrations (350 and 710 µM) reduced efficacy (50% and 19%, respectively) and increased local adverse effects at 710 µM (ulcer necrosis, edema, severe inflammation, pronounced pruritus), indicating local dose-limiting toxicity. NOAEL/LOAEL were not defined. • In vivo systemic dosing: Rat sepsis models used a single intravenous dose of 1 mg/kg; this was efficacious and clinically tolerated in the monitoring parameters captured, but a formal toxicological dose–response (multiple dose levels, NOAEL/LOAEL) was not reported. • In vitro cytotoxicity: Reviews and mechanistic summaries indicate LL-37 is cytotoxic to various mammalian cell types at micromolar concentrations, via membrane perturbation and apoptosis-related pathways, with cytotoxicity influenced by cellular factors and peptide stereochemistry; however, standardized IC50/EC50 values are not consistently reported in the excerpts we retrieved.

Evidence-based conclusions

• LD50: No peptide mass-based LD50 for purified LL-37 in mammals was identified; one oral vectorized LL-37 study in rats reported LD50 greater than a very high CFU dose of the producer strain, which is not directly comparable to purified peptide LD50.
• Organ toxicity: Short-term clinical monitoring after 1 mg/kg IV LL-37 in rats revealed no overt adverse signs, and oral LL-37-producing bacterial delivery was well tolerated in rats with normal histology/biochemistry over 3 weeks; nonetheless, comprehensive target-organ toxicology for purified LL-37 is not established in these studies.
• Mutagenicity: No Ames, micronucleus, or comet data for LL-37 were located; mutagenicity remains unassessed in the provided corpus.
• Dose–response: Human topical dosing shows a therapeutic window with increased local toxicity at higher concentrations; systemic in vivo dose–response and formal NOAEL/LOAEL were not reported; in vitro indicates micromolar cytotoxicity but lacks harmonized EC50/IC50 values in the retrieved excerpts.

Gaps and recommendations

• A formal GLP toxicology package for purified LL-37 (single- and repeat-dose studies with full clinical chemistry and histopathology), route-specific LD50 determinations, and genotoxicity testing (Ames, in vitro micronucleus, in vitro/in vivo comet) were not found in the retrieved literature and should be sought or conducted for comprehensive risk assessment.

Evidence Gaps#

• Human adverse event data is limited to anecdotal reports
• Systematic adverse event monitoring has not been conducted
• Drug interaction studies are incomplete
• Long-term safety profiles are unknown

Related Reading#

• LL-37 overview and research guide
• LL-37 dosing protocols
• LL-37 risks and legal status