LL-37

LL-37 is the sole human cathelicidin, a 37-amino acid cationic amphipathic alpha-helical peptide generated by proteolytic cleavage of the hCAP18 precursor protein. It is stored in neutrophil secondary granules and constitutively or inductively produced by epithelial cells of the skin, gut, and respiratory tract.

Its direct antimicrobial activity depends on membrane disruption. LL-37 selectively targets negatively charged bacterial outer surfaces, binding lipopolysaccharide of Gram-negative organisms and teichoic acid of Gram-positive organisms before inserting into the lipid bilayer via barrel-stave, toroidal-pore, or carpet mechanisms, all resulting in pore formation and lysis. At sub-lethal concentrations as low as 0.5 mcg/mL it also disrupts quorum-sensing circuits and inhibits biofilm formation through three distinct pathways: reducing initial bacterial attachment, promoting bacterial motility, and downregulating quorum-sensing gene expression.

Beyond direct killing, LL-37 acts on the G-protein-coupled receptor FPR2/FPRL1 to chemoattract neutrophils, monocytes, and T lymphocytes to sites of infection. It activates P2X7 receptors, promotes neutrophil extracellular trap (NET) release, and suppresses pro-inflammatory pyroptosis of macrophages. Expression of the CAMP gene encoding hCAP18/LL-37 is transcriptionally upregulated by 1,25-dihydroxyvitamin D3 acting on a vitamin D response element. LL-37 additionally activates MAPK and PI3K/Akt signaling in keratinocytes, inducing matrix metalloproteinase activity and transcription factors Snail and Slug to drive epithelial migration and wound re-epithelialization.

The most rigorous human evidence for exogenous LL-37 comes from two randomized controlled trials in topical wound healing. A Phase I/IIa trial by Gronberg et al. (Wound Repair and Regeneration, 2014; PMID 24888447) enrolled 34 participants with hard-to-heal venous leg ulcers in a three-week open-label placebo run-in, followed by four weeks of twice-weekly topical application of LL-37 at 0.5, 1.6, or 3.2 mg/mL versus placebo, and a four-week follow-up. Healing rate constants at the 0.5 mg/mL dose were approximately sixfold higher than placebo, and mean ulcer area decreased by roughly 68% in this arm. No safety concerns were identified.

The subsequent Phase IIb HEAL trial (Mahlapuu et al., Wound Repair and Regeneration, 2021; PMC9298190) enrolled 148 patients at 15 sites across Poland and Sweden with chronic venous leg ulcers (median duration 20.3 months, mean wound area 11.6 cm²). LL-37 was applied at 0.5 or 1.6 mg/mL twice weekly for 13 weeks alongside compression therapy. The primary endpoint of complete wound closure was not met in the full population (26.5% with 0.5 mg/mL versus 25.3% with placebo, p=0.45). A post-hoc subgroup analysis of patients with large refractory wounds of 10 cm² or greater showed a statistically significant benefit (28.8% complete healing versus 8.1% for placebo), a finding the investigators stated warrants a dedicated adequately powered trial.

In the preclinical domain, Heilborn et al. (Journal of Investigative Dermatology, 2008; PMID 17805349) demonstrated that adenoviral delivery of LL-37 in ob/ob diabetic mice significantly improved re-epithelialization and granulation tissue formation via MAPK and PI3K/Akt signaling.

A Phase I dose-finding study in 15 healthy volunteers (Rekha et al., PLOS ONE, 2013; PMC3637063) showed that oral phenylbutyrate 500 mg twice daily combined with vitamin D3 5000 IU once daily for four days significantly increased LL-37 expression in macrophages and enhanced intracellular killing of Mycobacterium tuberculosis. This provides human proof-of-concept for nutritional induction of endogenous LL-37.

Typical dosage: 2-5 mg (twice weekly).

In completed clinical trials, topical LL-37 has been studied at concentrations of 0.5 mg/mL, 1.6 mg/mL, and 3.2 mg/mL in polyvinyl alcohol solution applied directly to chronic wounds twice weekly for 4 to 13 weeks alongside standard compression therapy. The 0.5 mg/mL dose demonstrated the most consistent efficacy signal across both Phase I/IIa and Phase IIb investigations.

For induction of endogenous LL-37 through nutritional means, the Phase I dose-finding trial by Rekha et al. used oral phenylbutyrate 500 mg twice daily plus vitamin D3 5000 IU once daily for four consecutive days in healthy adult volunteers, producing significant upregulation in macrophages. This represents the only orally administered protocol with published human evidence.

No parenteral (subcutaneous or intramuscular) LL-37 protocol exists in the peer-reviewed clinical literature. In the research peptide community, injectable compounded LL-37 is anecdotally described at doses of 100 to 200 mcg administered subcutaneously once daily or two to three times per week in cycles of four to twelve weeks; these figures originate entirely from informal sources and bodybuilding or biohacking forums and are without formal clinical trial support or regulatory approval in any jurisdiction.

This content is provided for educational and informational purposes only. It does not constitute medical advice, a diagnosis, or a recommendation for any therapeutic use. Consult a qualified and licensed healthcare provider before initiating any peptide or supplement regimen.

No compound-specific stability data has been identified for this peptide. The general lyophilized-peptide handling framework applies — see Storage & handling for temperature, reconstitution diluent, and beyond-use dating principles.

The most clinically supported combination approach involves inducing endogenous LL-37 rather than administering the peptide exogenously. Oral vitamin D3 and the short-chain fatty acid analogue sodium phenylbutyrate act synergistically via the CAMP gene promoter, with human Phase I evidence (n=15) demonstrating that 500 mg phenylbutyrate twice daily plus 5000 IU vitamin D3 daily for four days maximally upregulates macrophage LL-37 expression for antimicrobial applications, particularly tuberculosis.

At the epithelial surface, LL-37 functions synergistically with beta-defensins (HBD-1, HBD-2, HBD-3) as part of the coordinated innate barrier defense, with in vitro data showing combined antimicrobial activity exceeding either peptide alone.

In the research peptide community, LL-37 is anecdotally combined with BPC-157 or thymosin beta-4 (Tβ4) for wound healing and tissue repair, based on the hypothesis that complementary regenerative mechanisms offer additive benefit. No controlled human data exist for any such injectable combination; these uses are entirely anecdotal and cannot be evaluated for safety or efficacy on current evidence.

LL-37 is not currently FDA-approved for any indication. Effective April 22, 2026, the FDA removed LL-37 from Category 2 of its Section 503A bulk drug substances list after the original nominators withdrew their nominations. This removal lifts the prior “significant safety risk” designation but does not place LL-37 on the 503A Bulks List. Compounding pharmacies may prepare it with a valid physician prescription and pharmaceutical-grade API from an FDA-registered manufacturer. The FDA’s Pharmacy Compounding Advisory Committee (PCAC) is scheduled to review this substance at its July 23–24, 2026 public meeting. Removed from FDA Category 2 effective April 22, 2026. NOT selected for July 23-24, 2026 PCAC meeting. Deferred to second PCAC meeting before February 2027. NOT currently on 503A Bulks List — requires physician Rx. Source: FDA Advisory Committee Calendar / Lengea Law, May 2026.

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Reported side effects: Generally safe, minimal side effects

In completed human topical trials, the safety profile of LL-37 has been generally favorable. Local reactions at the wound site — erythema and edema — were reported in 20 to 43% of subjects across treatment and placebo groups in the Phase IIb trial, the majority mild to moderate and self-limiting. Six adverse events were judged possibly drug-related; twelve serious adverse events occurred across 148 participants, none of which were considered drug-related by investigators.

A critical safety consideration is LL-37's paradoxical role in autoimmune pathogenesis. At elevated or sustained concentrations, LL-37 forms stable complexes with extracellular self-DNA and self-RNA released from dying cells. These complexes activate plasmacytoid dendritic cells via TLR7 and TLR9, triggering type I interferon cascades that drive psoriasis and systemic lupus erythematosus (SLE). Approximately 45% of SLE patients develop LL-37-specific T cells and circulating autoantibodies. LL-37 overexpression is similarly documented in psoriatic skin and synovial fluid of psoriatic arthritis. Individuals with personal or family histories of these conditions may face heightened risk from exogenous LL-37 administration.

LL-37 activates platelets via FPR2/FPRL1 and augments thrombus formation in vitro, a potential concern for individuals with cardiovascular or thrombotic risk factors.

In vitro hemolysis becomes significant above 25 to 50 mcg/mL, well above topical clinical concentrations but potentially relevant at high parenteral doses. Animal data indicate organ toxicity at very high doses (approximately 3000 mcg/kg). The long-term theoretical concern that exogenous antimicrobial peptide administration could select for microbial resistance to innate host defenses has been noted in the literature, though not yet documented clinically.

Parenteral use in humans lacks any approved safety characterization.