LL-37

Origin and Structure

LL-37 is the active 37-amino-acid C-terminal fragment of hCAP18, the single human cathelicidin protein encoded by the CAMP gene. Its name comes from the two leucine residues at the N-terminus followed by 35 additional residues. LL-37 is cleaved from hCAP18 by neutrophil-derived serine proteases (primarily proteinase 3) at sites of infection or tissue injury, generating the mature, amphipathic α-helical antimicrobial peptide that circulates in humans.

Unlike most peptides in this library, LL-37 is not a synthetic drug candidate in search of an indication — it is a real human molecule with a central role in the innate immune system. It is expressed constitutively in epithelial cells of the skin, respiratory tract, and GI tract, and is stored in neutrophil secondary granules for rapid release.

Mechanism of Action

LL-37 has three overlapping activities that together account for its therapeutic interest:

• Direct antimicrobial activity. LL-37's amphipathic helix inserts into bacterial membranes and disrupts them. It is active against Gram-positive and Gram-negative bacteria, several fungi, some enveloped viruses, and — crucially — bacterial biofilms, which resist most conventional antibiotics.
• LPS neutralization. LL-37 binds bacterial lipopolysaccharide with high affinity, preventing LPS-induced TLR4 activation. This has made it an object of interest in sepsis and septic-shock research, where uncontrolled LPS signaling drives much of the pathology.
• Immunomodulation and wound healing. LL-37 is chemotactic for neutrophils, monocytes, and T cells; promotes angiogenesis; and accelerates re-epithelialization in wound models. In chronic wounds — diabetic foot ulcers and chronic venous ulcers — LL-37 expression in the epidermis is measurably depressed relative to healthy skin.

Clinical Research

LL-37 is a mature research target with several hundred peer-reviewed papers across microbiology, dermatology, and immunology journals. Human clinical data is thinner but real: small trials of topical LL-37 for hard-to-heal venous leg ulcers have shown improved closure rates compared to placebo, and inhaled formulations have been explored for chronic lung infections in cystic fibrosis. None of these have produced an approved drug yet, in part because synthetic manufacture of a 37-residue cationic peptide at therapeutic scale is expensive and the risk of immunogenicity from analog modifications is nontrivial.

Separately, LL-37 levels are being investigated as a biomarker — low LL-37 correlates with risk of recurrent respiratory infection and with poorer wound-healing outcomes.

Practical Considerations

In the research-chemical and wellness peptide market, LL-37 is typically sold as a lyophilized powder for subcutaneous reconstitution. Community protocols cite 100–500 mcg per injection, sometimes site-specific for localized chronic infection. These doses are not derived from clinical trial data and should be understood as extrapolation. LL-37 is unstable to several common proteases and is degraded rapidly in serum, which complicates systemic dosing; shorter analogs (e.g., IG-25, KR-12) are under active investigation for better pharmacokinetics.

LL-37 is the cleaved, mature form of hCAP18; for the full-length precursor and background on the cathelicidin family, see our Cathelicidin reference page.

Safety Considerations

LL-37 is cytotoxic to mammalian cells at high concentrations — the same membrane-active properties that kill bacteria will damage host cells above a threshold. In serum, LL-37 activity is partially inhibited by binding to apolipoproteins, which may explain why endogenous concentrations are tolerated. Exogenous dosing bypasses some of this homeostasis, and no long-term human safety data exists for injectable LL-37 outside the tightly controlled conditions of a clinical trial.

LL-37 has also been implicated as a self-antigen in rosacea and in psoriasis, where dysregulated LL-37 production contributes to disease. That pathway is a reason for caution in patients with active autoimmune or autoinflammatory skin conditions.

Regulatory Status

LL-37 is not FDA-approved for any indication. It is not a scheduled substance and is not on the WADA prohibited list. Western availability is through research-chemical suppliers only.

The Bottom Line

LL-37 is one of the more scientifically serious antimicrobial peptides on this library — a real human innate-immune molecule with decades of mechanistic research, credible chronic-wound and anti-biofilm potential, and legitimate ongoing drug-development interest. Its clinical promise is tempered by manufacturing cost, immunogenicity concerns, and a dose-response in which therapeutic and cytotoxic concentrations are uncomfortably close. For now, most sensible use cases are topical and investigational.