LL-37

LL-37 is a 37-amino-acid cationic, amphipathic, alpha-helical peptide that is the only cathelicidin family member produced by humans. It is the C-terminal mature peptide cleaved from the precursor protein hCAP-18 by neutrophil proteinase 3 and epithelial kallikreins. The sequence begins with two leucine residues (hence the "LL" prefix), followed by a 37-residue helical region with both hydrophobic and cationic faces. LL-37 is expressed in epithelial cells of the skin, gastrointestinal tract, respiratory tract, and testis, and in leukocytes including neutrophils, monocytes, T cells, NK cells, and B cells. The peptide is investigational and has no FDA approval for any indication, though it is widely studied as a candidate antimicrobial and immunomodulatory therapeutic.

The molecule's structural design is the basis for its multiple functions. The cationic charge (positive at physiologic pH) drives initial attraction to negatively charged bacterial membranes. The amphipathic helical structure allows the peptide to insert into and disrupt microbial lipid bilayers, producing the antimicrobial effect. The same structural features mediate binding to host cell receptors and intracellular targets that produce the immunomodulatory effects.

LL-37 sits at the intersection of innate immunity research and translational therapeutic development. The protective antimicrobial function (against pathogens including methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci, and other drug-resistant organisms) is part of why it has been studied as an antibiotic alternative. The immunomodulatory effects (which can be both anti-inflammatory and pro-inflammatory depending on context) are part of why translational development has been challenging.

LL-37 has multiple distinct mechanisms that operate in different contexts and at different concentrations.

Membrane disruption antimicrobial activity. The classical mechanism. The cationic LL-37 is attracted to negatively charged bacterial outer membranes (gram-negative bacteria have negatively charged LPS) or cell walls. The amphipathic helix inserts into the lipid bilayer, forming transient pores or producing carpet-model disruption that lyses the bacterial cell. Activity against gram-positive bacteria, gram-negative bacteria, fungi, and some viruses.

LPS neutralization. LL-37 binds bacterial lipopolysaccharide (LPS, endotoxin) and prevents LPS from activating TLR4-mediated inflammation. This dampens the inflammatory response to gram-negative infections while still killing the bacteria.

Chemotaxis. LL-37 acts as a chemokine, attracting neutrophils, monocytes, T cells, and other immune cells to sites of infection. The chemotaxis is mediated through binding to FPRL1 (formyl peptide receptor-like 1) on immune cells.

Wound healing and epithelial repair. LL-37 promotes keratinocyte proliferation, angiogenesis, and epithelial wound repair through multiple receptor-mediated mechanisms. The molecule is upregulated at sites of skin injury and is part of the normal wound healing response.

Self-DNA and RNA presentation in autoinflammation. A paradoxical pro-inflammatory mechanism. LL-37 forms complexes with extracellular self-DNA and self-RNA that bind to scavenger receptors and toll-like receptors (TLR3, TLR7, TLR9), enabling inert nucleic acids to become inflammatory stimuli. This is the mechanism behind LL-37's role in psoriasis pathogenesis and other autoinflammatory diseases.

Cytotoxicity to host cells at high concentrations. At concentrations of 1 to 10 µM, LL-37 is cytotoxic to many human cell types. At sites of severe inflammation, LL-37 concentrations can reach 300 µM in psoriatic skin lesions, producing local cytotoxic effects. This concentration-dependent cytotoxicity is part of why systemic therapeutic use has been limited.

Citrullination and inactivation. Arginine residues in LL-37 can be converted to citrulline by peptidyl-arginine deiminases (PAD2, PAD4) at inflammatory sites. Citrullination impairs the bactericidal activity and abrogates immunomodulatory functions. This is part of the regulatory mechanism that prevents excessive LL-37 activity.

The concentration-dependent dual nature (protective at moderate concentrations, pro-inflammatory at high concentrations) is the central challenge for therapeutic development.

Regulatory status

No FDA approval, no EMA approval, no marketing authorization in any country for any LL-37-based therapeutic.

Clinical trial status. Several small Phase 1 and Phase 2 trials have investigated LL-37 or LL-37 analogs for wound healing, antibiotic-resistant infection, and skin disease applications. None have progressed to Phase 3.

Modified analogs in development. Multiple research groups have developed LL-37 analogs with improved properties (citrullination resistance, reduced cytotoxicity, enhanced antimicrobial potency). hArg-LL-37 (with arginine replaced by homoarginine) is one example of citrullination-resistant variants. None have reached approval.

Compounding status. LL-37 is not on the FDA Category 2 bulks list as of May 2026.

WADA status. LL-37 is not currently on the WADA Prohibited List. The mechanism does not directly support sport performance.

Cosmetic use. LL-37 and modified analogs have been incorporated into some experimental cosmetic formulations for skin barrier support and antimicrobial claims. The cosmetic positioning is separate from any pharmaceutical claim.

Research-chemical availability. LL-37 is widely available from peptide synthesis vendors for research use with "for laboratory use only" labeling.

Dosing protocols and literature-reported ranges are documented in the approved label or trial publications referenced above.

The LL-37 safety profile is complex because of the dual antimicrobial / pro-inflammatory nature.

Concentration-dependent cytotoxicity. At concentrations above 1 to 10 µM, LL-37 is cytotoxic to multiple human cell types. Systemic administration risks producing serum concentrations in this range with broad cytotoxic effects.

Pro-inflammatory effects in disease. Systemic LL-37 administration could theoretically drive autoimmune or autoinflammatory disease (psoriasis-like, lupus-like) through the self-DNA presentation mechanism.

Hemolysis risk. Cationic peptides have documented hemolytic activity at high concentrations.

Limited pharmacokinetic data. The few clinical studies have used topical or local administration to limit systemic exposure.

No long-term safety database. The absence of Phase 3 trials means no large-scale safety database exists for any LL-37-based product.

The safety challenges have been a major obstacle to clinical development. Modified analogs with reduced cytotoxicity and pro-inflammatory effects are an active area of research.

The antimicrobial peptide field has multiple candidate molecules.

LL-37 vs Defensins. Defensins are the other major family of human antimicrobial peptides. Alpha-defensins (HNP-1 through HNP-4) are produced by neutrophils. Beta-defensins are produced by epithelial cells. Different structural class (beta-sheet rather than alpha-helix) but overlapping antimicrobial spectrum. The two families work together as innate immune effectors.

LL-37 vs Magainins. Magainins are frog-derived antimicrobial peptides that have been studied extensively. Different species origin but similar broad-spectrum activity. Pexiganan (a magainin analog) was developed for diabetic foot infection but did not receive FDA approval.

LL-37 vs Nisin. Nisin is a bacterial-derived lantibiotic used as a food preservative. Different structural class. Approved for food use, not therapeutic use.

LL-37 vs Bacitracin / Polymyxin / Vancomycin. Conventional antibiotic peptides. Approved and widely used. LL-37 has been positioned as a candidate for antibiotic-resistant infections where conventional options fail.

LL-37 vs KPV / BPC-157 / TB-500. Different mechanism classes. KPV is anti-inflammatory through NF-κB. BPC-157 and TB-500 are healing peptides. LL-37 is primarily antimicrobial with secondary immunomodulatory effects.

The antimicrobial peptide therapeutic field has been challenging across all candidates due to manufacturing costs, pharmacokinetic limitations, and safety considerations. No member of this peptide class has achieved broad FDA approval for systemic antimicrobial use.