LL-37

LL-37 is the only cathelicidin-family antimicrobial peptide produced by the human body (most other mammals produce several cathelicidins). It is a 37-amino-acid, cationic, amphipathic peptide cleaved from the C-terminal end of hCAP18, a single precursor protein encoded by the CAMP gene and stored in the secondary granules of neutrophils and produced by keratinocytes, epithelial cells, and several other cell types. First characterized in 1995 under the name FALL-39 by Agerberth and colleagues, it was later renamed LL-37 once the mature, active fragment was isolated and found to begin with two leucine residues across 37 total amino acids. Unlike most peptides in this queue, LL-37 is not a synthetic analog of an animal hormone or a laboratory-designed mimetic; it is a naturally occurring component of the human innate immune system that has subsequently been studied, in synthetic topical and injectable formulations, as a potential therapeutic agent.

LL-37 acts through two distinct mechanisms. First, its cationic, amphipathic structure allows it to insert directly into and disrupt the membranes of bacteria, some fungi, and enveloped viruses, killing microbes through a physical mechanism rather than by inhibiting a specific bacterial enzyme, which is part of why bacterial resistance to it is less easily acquired than resistance to conventional antibiotics. Second, LL-37 acts as a signaling molecule, binding formyl peptide receptor-like 1 (FPRL1, also called FPR2) to recruit neutrophils, monocytes, and T cells to sites of infection or injury, and separately promoting keratinocyte migration, proliferation, and angiogenesis, functions relevant to wound repair. Production of LL-37 is under direct transcriptional control of the vitamin D receptor, which binds a response element in the CAMP gene promoter upon activation by 1,25-dihydroxyvitamin D3, mechanistically linking vitamin D status to innate antimicrobial defense, a connection demonstrated in human myeloid cells and now well established in the vitamin D and immunity literature.

Topical LL-37 has been tested in genuine human randomized controlled trials for chronic, hard-to-heal wounds, giving it a more developed clinical evidence base than most other peptides in this queue, though the results have been mixed rather than uniformly positive. An early phase 1/2 trial in venous leg ulcers found a multi-fold improvement in healing rate with LL-37 over placebo, but the larger, better-powered phase 2b confirmatory trial did not meet its primary endpoint across the full study population, succeeding only in a post-hoc subgroup of larger wounds. A separate small trial in diabetic foot ulcers found faster granulation tissue formation with LL-37 cream but no significant difference in overall wound area reduction. This pattern, a promising early signal not fully confirmed at larger scale, recurs across several peptides in this queue and is an important context for interpreting any specific LL-37 wound-healing claim.

LL-37 also has a well-documented, mechanistically established role in triggering autoimmune skin and systemic disease that is not present for most peptides marketed for tissue repair. Bound to self-DNA and self-RNA released from dying or stressed cells, LL-37 converts these otherwise poorly immunogenic molecules into potent triggers of plasmacytoid dendritic cell type I interferon production, a mechanism central to psoriasis pathology and implicated in systemic lupus erythematosus, demonstrated directly in human skin and immune cell studies rather than inferred from animal models. Separately, in melanoma, LL-37 has shown a genuinely two-sided profile: tested as a potential antitumor immune activator in an early human trial, but also shown, in cell culture, mouse, and human tissue studies, to stimulate tumor cells and tumor-associated macrophages toward more invasive, pro-angiogenic behavior correlated with tumor stage. No LL-37 product is approved as a medicine anywhere; it is available outside formal trials through unregulated research-peptide and cosmetic-adjacent vendors, and its overall profile is best understood as that of a genuine, well-characterized, double-edged host defense molecule rather than a straightforward healing supplement.

Mechanism of Action

An Endogenous Host Defense Peptide, Not a Designed Mimetic

LL-37 is unusual among the peptides in this queue because it is not a synthetic analog of an animal hormone, a receptor-targeted mimetic, or a fragment engineered for a specific pharmacological property. It is a naturally occurring human antimicrobial peptide, the only cathelicidin-family member humans produce, cleaved from the C-terminal end of hCAP18, a single precursor protein encoded by the CAMP gene. hCAP18 is stored in neutrophil secondary granules and is also produced directly by keratinocytes, epithelial cells lining the airway and gut, and several other cell types, where extracellular proteases cleave it to release the mature 37-amino-acid LL-37 fragment at sites of infection, injury, or inflammation. This endogenous origin means LL-37 biology has been studied both as a natural component of human innate immunity, with direct relevance to conditions like vitamin D deficiency and chronic wounds where its production or activity may be insufficient, and as a therapeutic candidate in synthetic topical and injectable formulations.

Direct Membrane Disruption

LL-37’s cationic charge and amphipathic alpha-helical structure allow it to insert directly into the negatively charged membranes of bacteria, some fungi, and enveloped viruses, disrupting membrane integrity and causing cell death through a physical mechanism rather than by inhibiting a specific bacterial enzyme or metabolic pathway. This mode of action is shared broadly across antimicrobial peptides and is one reason bacterial resistance to LL-37 is less easily acquired through the single-point mutations that commonly confer resistance to conventional antibiotics targeting a specific protein. This mechanism has been demonstrated extensively in vitro against a broad range of gram-positive and gram-negative bacteria, including some drug-resistant strains, but LL-37 has not been developed or tested as a systemic antibiotic in humans.

Immune Cell Recruitment via FPRL1/FPR2 and Vitamin D-Dependent Production

Separate from its direct killing activity, LL-37 functions as an immune signaling molecule. It binds formyl peptide receptor-like 1 (FPRL1, also called FPR2) on neutrophils, monocytes, and T cells, chemoattracting these cells to sites of infection or injury (De Yang et al., 2000). Production of LL-37 itself is under direct transcriptional control of the vitamin D receptor (VDR): activated VDR, bound to the biologically active form of vitamin D, binds a vitamin D response element in the CAMP gene promoter and strongly upregulates its expression, a mechanism demonstrated in human myeloid cells (Gombart et al., 2005). CYP27B1, the enzyme that converts circulating 25-hydroxyvitamin D into its active hormonal form, is often locally upregulated in immune cells at sites of infection, allowing on-demand local vitamin D activation and CAMP transcription rather than relying solely on systemic vitamin D levels. This VDR-CAMP axis is the best-established molecular explanation for the long-observed clinical association between vitamin D deficiency and increased susceptibility to certain infections.

Wound Healing-Relevant Functions

In addition to its antimicrobial and immune-signaling roles, LL-37 has been reported to promote keratinocyte migration and proliferation and to stimulate angiogenesis, functions directly relevant to tissue repair and the rationale for testing it as a topical wound-healing agent. Chronic wounds, particularly diabetic and venous ulcers, often exist in a protease-rich, poorly perfused local environment that may degrade endogenous LL-37 faster than it can act or impair the local vitamin D activation needed to sustain its production, providing the biological rationale for supplementing LL-37 topically in these conditions, though this specific causal chain (impaired local LL-37 activity leading to impaired healing) has not been directly measured in the human trials described in Clinical Evidence.

The Self-DNA Complex Mechanism and Autoimmune Disease

LL-37 has a second, mechanistically distinct signaling role that stands in direct tension with its host defense function. When cells die or are stressed, they release self-DNA and self-RNA into the extracellular space; on their own, these self-nucleic acids are poorly immunogenic and are efficiently cleared without triggering a strong immune response. LL-37 changes this: its cationic charge allows it to bind and condense self-DNA (and self-RNA) into complexes that are efficiently taken up by plasmacytoid dendritic cells, where they access endosomal Toll-like receptor 9 (for DNA) or TLR7/8 (for RNA), triggering a large burst of type I interferon production (Lande et al., 2007). This mechanism, demonstrated directly in human psoriatic skin and dendritic cell cultures, is considered central to psoriasis pathology, and a related process, together with LL-37 acting as a target autoantigen for pathogenic T cells, has been implicated in systemic lupus erythematosus. This is not a preclinical hypothesis derived from animal models; it is a mechanism established in human tissue and cells, and it means LL-37 is mechanistically part of the disease process in two major autoimmune conditions.

Clinical Evidence

Venous Leg Ulcers: An Early Positive Trial Not Confirmed at Larger Scale

The most substantial human efficacy evidence for LL-37 comes from trials in venous leg ulcers. A phase 1/2 randomized, placebo-controlled trial (n=34) found that topical LL-37 solution (0.5, 1.6, or 3.2 mg/mL, applied twice weekly for 4 weeks) produced healing rate constants roughly six-fold (at 0.5 mg/mL) and three-fold (at 1.6 mg/mL) higher than placebo (Grönberg et al., 2014). This promising early result led to a larger, phase 2b confirmatory trial (HEAL LL-37, n=148 to 149) using the same two concentrations. In the full study population, this trial did not meet its primary endpoint: complete wound closure rates were statistically indistinguishable across the LL-37 0.5 mg/mL (26.5 percent), LL-37 1.6 mg/mL (24.7 percent), and placebo (25.3 percent) arms (Mahlapuu et al., 2021). A significant benefit did emerge in a post-hoc subgroup of patients with larger wounds (28.1 percent closure with LL-37 0.5 mg/mL versus 8.1 percent with placebo, an odds ratio of 4.454, p=0.0458), but post-hoc subgroup findings are hypothesis-generating rather than confirmatory, and this larger, more rigorous, more recent trial should be weighed more heavily than the smaller, earlier, uniformly positive one when assessing LL-37’s overall efficacy for this indication.

Diabetic Foot Ulcers: A Positive Surrogate Finding Without a Clear Clinical Endpoint Benefit

A small randomized, double-blind, placebo-controlled trial (n=25) tested topical LL-37 cream (0.5 mg/g, applied at 0.025 mL per square centimeter of wound, twice weekly for 4 weeks) in diabetic foot ulcers (Miranda et al., 2023). The LL-37 group showed significantly faster granulation tissue formation at every measured timepoint (days 7, 14, 21, and 28), reaching complete granulation roughly two weeks before the placebo group. However, overall wound area reduction, the endpoint most directly relevant to clinical wound closure, showed only a nonsignificant trend favoring LL-37. This pattern, a statistically significant improvement in a histological surrogate marker without a corresponding significant improvement in the primary clinical measure, in a very small trial, means this study should be read as a preliminary positive signal rather than established evidence of a meaningful clinical healing benefit.

Melanoma: A Genuinely Two-Sided Evidence Picture

LL-37 has also been tested as an antitumor immune activator: a completed early-phase human trial injected LL-37 directly into cutaneous melanoma metastases with the goal of activating plasmacytoid dendritic cells against the tumor, an approach that produced documented dermatologic toxicity at and near injection sites in some patients (Dolkar et al., 2018). Separately, and more recently, a 2023 study combining melanoma and macrophage cell culture, a mouse melanoma model, and immunohistochemical analysis of 50 human melanoma tissue samples found that LL-37 stimulated tumor cells and tumor-associated macrophages to increase pro-angiogenic and invasion-associated factors, including CXCL5, IL-23p19, and matrix metalloproteinases, and that higher tumor LL-37 expression correlated with more advanced tumor stage in patients (Ohuchi et al., 2023). These two lines of evidence point in opposite directions: one testing LL-37 as a tool to fight melanoma, the other suggesting LL-37, whether endogenous or administered, may promote melanoma invasion in some contexts. The honest reading of this evidence is that LL-37’s role in cancer is genuinely context-dependent and unresolved, not that it is a demonstrated cancer therapy or a compound known to be safe in the context of active or prior skin cancer.

Autoimmune Disease: A Documented Risk, Not a Theoretical One

Unlike the theoretical safety concerns that apply to many research peptides, LL-37’s role in triggering autoimmune skin and systemic disease is supported by direct human evidence. In psoriatic skin, LL-37-self-DNA complexes activate plasmacytoid dendritic cells and drive the type I interferon signature considered central to psoriasis pathology (Lande et al., 2007), and related mechanisms, along with LL-37 acting as a T cell autoantigen, have been implicated in systemic lupus erythematosus. This means that, for a person with psoriasis, lupus, or a related interferon-driven autoimmune condition, LL-37 exposure is not a hypothetical risk requiring extrapolation from animal or cell-culture data; it is a documented participant in the disease mechanism itself, established in human tissue and immune cell studies.

Safety and Tolerability

Across the completed topical wound-healing trials, LL-37 was generally well tolerated over the trial durations studied (weeks), with no serious treatment-related adverse effects reported in the venous leg ulcer or diabetic foot ulcer trials. Dermatologic toxicity has been specifically documented in the melanoma intratumoral injection context, a different route and patient population than the wound trials. No large-scale, long-duration, or systemic human safety database exists for LL-37, and self-directed use outside a formal, monitored trial protocol, particularly in individuals with autoimmune skin disease or active or prior skin cancer, has no corresponding safety data and carries the specific, mechanistically grounded risks described above.

Regulatory Status

No LL-37-based product has received regulatory approval from the FDA, the EMA, or any other national regulatory authority for any indication, including wound healing, despite reaching phase 2b human trials for venous leg ulcers. Outside of completed or ongoing formal clinical trials, LL-37 is available through unregulated peptide research vendors and some cosmetic-adjacent topical product lines, a supply channel with no independent verification of purity, concentration, or accurate labeling, and no pharmaceutical manufacturing oversight. LL-37 is not a controlled substance and is not specifically addressed by anti-doping prohibited lists.

Dosing Guidance

No LL-37 dose has been established or approved for general use; the figures here describe doses used in specific completed clinical trials only. The venous leg ulcer trials used topical LL-37 solution at 0.5, 1.6, or 3.2 mg/mL, applied twice weekly for 4 weeks. The diabetic foot ulcer trial used a 0.5 mg/g cream applied at 0.025 mL per square centimeter of wound, twice weekly for 4 weeks. The melanoma trial used direct intratumoral injection, with specific dosing not comprehensively reported in the sources reviewed for this page. No oral, subcutaneous, or other systemic human dosing regimen has been established, validated, or approved for LL-37 for any indication.