LL-37 (Cathelicidin): Antimicrobial Peptide Uses, Evidence and Status

LL-37 is the only cathelicidin antimicrobial peptide found in humans, a 37-amino-acid peptide that is part of the innate immune system. Its name is a simple code: the two leucine residues at its start give the “LL”, and 37 is its length. It is encoded by a single gene called CAMP and is released from a precursor protein, hCAP18, when host enzymes such as neutrophil proteinase 3 cut it free.

The peptide is stored in large amounts in neutrophils, the white blood cells that rush to sites of infection, and is also made by skin and other epithelial tissues. Its importance is shown starkly in Kostmann syndrome, a congenital condition in which patients have extremely low LL-37 and can die from otherwise minor infections, evidence that this single peptide does real defensive work in the body.

Structurally, LL-37 is cationic, meaning positively charged, and amphipathic, meaning it has both water-loving and fat-loving faces. It is loosely arranged in water but folds into a curved helix when it meets a membrane, which is the shape that lets it attack bacteria.

LL-37 has two distinct jobs: it kills microbes directly and it shapes the immune response. The first is the better understood. Because its positive charge is drawn to the negatively charged surfaces of bacterial membranes, LL-37 inserts into those membranes and destabilises them, either punching pores or forming fibre-like peptide-lipid structures that break the membrane apart. It is active against a broad range of Gram-positive and Gram-negative bacteria, and shows activity against some viruses, fungi and parasites.

Immune modulation and wound repair

Beyond killing microbes, LL-37 binds and neutralises bacterial endotoxins such as lipopolysaccharide and lipoteichoic acid, calming the harmful immune over-reaction those toxins trigger. It also recruits immune cells through receptors including toll-like receptor 2, the epidermal growth factor receptor and formyl peptide receptor 2, and it promotes angiogenesis and wound healing. This dual identity, antibiotic and immune messenger at the same time, is what makes it interesting for infected and chronic wounds.

Anti-biofilm activity

One of the most cited reasons for research interest is LL-37’s ability to disrupt biofilms, the protective communities of bacteria that make many chronic infections so resistant to ordinary antibiotics. Combining anti-biofilm action with wound-healing effects is why several research groups have explored topical LL-37 and engineered analogues for polymicrobial infected wounds.

LL-37 has a deep basic-science literature and clear biological relevance, but its translation into an approved therapy has been blocked by stability, cost and safety problems. Most therapeutic work focuses on engineered analogues designed to keep the antimicrobial action while reducing toxicity, rather than on the native peptide itself.

Research area	Evidence tier	Status
Antimicrobial action	Extensive in vitro	Broad-spectrum killing well established in the lab
Infected wounds / biofilm	In vitro + animal	Topical use explored; not an approved treatment
Engineered analogues	Preclinical design work	Aimed at lower toxicity and better stability
Cancer biology	Conflicting preclinical	Tumour-promoting and tumour-suppressing depending on tissue

The cancer paradox

A striking and underreported feature of LL-37 is that it cuts both ways in cancer biology. Across the published preclinical literature, over-expression of LL-37 has been reported to promote the development and progression of ovarian, lung, breast and prostate cancers, while appearing to suppress tumour growth in colon and gastric cancer. The direction of the effect is tissue-specific and depends on which receptors a given cancer cell carries. This is not a marketing footnote, it is central to why regulators treat the peptide cautiously.

LL-37 is not approved as a drug in any country and has no USP monograph, and in the US it has spent the recent regulatory cycle on the restrictive side of the ledger. The mechanics of how this is decided are set out in our explainer on how compounding categories are decided.

Jurisdiction	Status	Notes
United States	Restricted	Category 2 since 2023; early-2027 review, may stay restricted
United Kingdom	Not approved	No USP monograph; available only as a research chemical
Australia	Not approved	Unapproved good; antimicrobial peptide not separately scheduled

The FDA placed cathelicidin LL-37 on the Category 2 list of restricted bulk drug substances in September 2023, citing safety concerns including potential negative effects on male fertility and protumorigenic effects in some tissues, and it was not among the peptides released from that list in the 2024 round. When the broader reclassification of early 2026 reshuffled the peptide landscape, LL-37 was not moved straight to the eligible list. Instead, the FDA’s April action placed it among five peptides, alongside GHK-Cu, Melanotan II, dihexa acetate and pegylated mechano growth factor, scheduled for a separate committee meeting expected before the end of February 2027 to consider whether they belong on the 503A bulks list.

The signal here matters. Industry analysts have grouped LL-37 with Melanotan II, GHRP-2, GHRP-6 and pegylated mechano growth factor as the peptides most likely to remain restricted, precisely because their documented safety concerns are harder to wave through than those of the repair peptides moving to the earlier docket. In other words, being scheduled for review is not the same as being expected to pass it. The full chronology is in our reclassification tracker, the meeting structure in our breakdown of the 2026 PCAC meetings, and the wider national context in our overview of the broad US legal picture.

Outside the US, LL-37 sits in the research-use-only market with no approval pathway, as covered in our guides to the UK’s approach and Australian peptide law.

LL-37’s safety questions are more serious than those of most recovery peptides, which is the core reason it has stayed restricted. The two concerns the FDA has emphasised are potential effects on male fertility and protumorigenic activity, the latter grounded in the cancer-biology literature where LL-37 over-expression promotes growth in several epithelial cancers. At higher concentrations the peptide is also directly cytotoxic to human cells, and chronically elevated levels in skin are linked to inflammatory disease.

Layered on top of those intrinsic concerns is a quality problem specific to LL-37’s chemistry. At 37 residues it is a long peptide that is difficult and costly to synthesise at high purity, so research-grade material is especially prone to truncated or incorrect sequences that a buyer cannot detect by eye and that can change both its activity and its toxicity. Independent purity and identity testing is the only meaningful check, which is why we cover evaluating a vendor’s testing and how to read testing documentation.

LL-37 is often listed among recovery peptides because of its wound-healing and anti-biofilm research, but it is the outlier of the group, an antimicrobial rather than a tissue-repair or anti-inflammatory signal. For the full category and how the repair peptides compare, see our recovery and healing hub. The compound that shares LL-37’s review track is covered in our guide to the copper peptide GHK-Cu.