LL-37 Cathelicidin: Immune Peptide Research and PH Status

Abstract still life: a ring of thirty-seven small frosted glass beads forming a loose coil on grey stone — **LL-37.** Illustration, not a product photo.

Research compound. Not approved by FDA Philippines for human use unless stated. Nothing here is a dosing recommendation or medical advice.

What it is

LL-37 is a 37 amino acid peptide and the only member of the cathelicidin family that humans produce. It is cut from a larger precursor protein, hCAP18, by enzymes at the site where it is needed. Neutrophils store it and release it during infection; keratinocytes in the skin, and epithelial cells lining the gut, airway and other mucosal surfaces, make it as part of everyday barrier defence.

Unlike most peptides in this library, LL-37 is not a synthetic analogue of a hormone. It is a molecule the body already makes constantly, and the research interest is in understanding what it does naturally and whether a manufactured version can be used as a topical treatment.

How it works

LL-37 has two separate jobs that show up throughout the literature.

Direct antimicrobial activity. The peptide inserts into microbial cell membranes and disrupts them, giving it activity against a broad range of bacteria, some viruses and fungi, and the ability to break down biofilms that protect bacteria from other defences.

Immune signalling. Beyond killing microbes directly, LL-37 acts as a chemoattractant, drawing neutrophils, monocytes and T cells to a site of injury or infection, and it can modulate how strongly those cells respond. This dual role, at the boundary between the fast-acting innate immune system and the slower adaptive immune system, is why reviews describe it as a bridge between the two.

In skin specifically, LL-37 is part of the barrier response to wounds and infection. The same signalling activity that helps fight infection is also implicated in inflammatory skin disease: in psoriasis research, LL-37 has been shown to bind self-DNA and self-RNA released from stressed or damaged skin cells, and the resulting complexes can over-activate plasmacytoid dendritic cells. That is a mechanism researchers use to explain part of psoriasis, not a therapeutic use of the peptide.

What the research found

Human studies

The published human work on LL-37 as a product is in chronic wound healing, specifically hard-to-heal venous leg ulcers, using a topical formulation rather than injections.

Grönberg and colleagues (2014) ran a randomized, placebo-controlled trial testing the safety and dose-response of topical LL-37 gel added to standard compression therapy in patients whose venous leg ulcers had not responded to standard care.
Mahlapuu and colleagues (2021) followed up with a larger multicentre phase IIb trial, double-blind and placebo-controlled, enrolling 148 patients across multiple sites, again testing topical LL-37 combined with compression therapy against hard-to-heal venous leg ulcers.

Both trials are about wound healing in an already-injured, poorly healing wound bed, not about general immune support or infection prevention in healthy people. No published human trial has tested LL-37 injected, taken orally, or used for a purpose outside chronic wound care.

Cell and mechanistic research

The much larger body of LL-37 literature is laboratory work characterising its antimicrobial mechanism, its role in innate immunity across different tissues, and its involvement in inflammatory skin conditions such as psoriasis and rosacea. This research explains why LL-37 is of interest, but it does not by itself establish that a manufactured LL-37 product is safe or effective for any use beyond the wound trials above.

Side effects reported in studies

The venous leg ulcer trials tested a topical formulation under medical supervision as part of a structured clinical trial, with safety monitoring built into the design. Detailed adverse event tables are in the published papers rather than summarised on drug information sites, since LL-37 is not an approved drug with a public label. There is no safety data for injectable or oral LL-37 use, because no such human trial has been published.

LL-37 vs other peptides studied for wound and skin repair

Compound	Nature	Route studied in trials	Human evidence
LL-37	Endogenous human antimicrobial peptide	Topical	Two placebo-controlled trials, venous leg ulcers
BPC-157	Synthetic fragment of a gastric protective protein	Mostly animal studies; limited human data	Sparse human data, mostly small or unpublished
GHK-Cu (topical)	Copper-binding tripeptide, naturally occurring	Topical	Cosmetic dermatology trials
Thymosin alpha-1	Thymic peptide, approved abroad as Zadaxin	Subcutaneous injection	Approved in some countries for hepatitis B and C

Status in the Philippines

No LL-37 product is registered with FDA Philippines. It is not a controlled substance under RA 9165. It has never been approved as a medicine anywhere, including in the countries running its clinical trials, so it is not available anywhere as a prescribed drug. Local availability through research-chemical suppliers is limited, and community use is a niche compared with better-known repair peptides.

Storage

As a peptide, manufactured LL-37 for research use should be treated the same as other lyophilised peptides: keep it refrigerated before and after reconstitution, protect it from light and heat, and avoid repeated freeze-thaw cycles. See the beginner guide for handling basics.

Common questions

What is LL-37?

LL-37 is the only cathelicidin-family antimicrobial peptide the human body makes. It is cleaved from a precursor protein called hCAP18 and is produced by neutrophils, skin cells and the linings of the gut, lungs and other mucosal surfaces.

Has LL-37 been tested in people?

Yes, as a topical formulation for hard-to-heal venous leg ulcers. A phase I/II placebo-controlled trial and a larger 148-patient phase IIb trial have both been published, alongside compression therapy. It has not been tested as an injectable or oral product in a published human trial.

Is LL-37 an approved medicine?

No. It is not approved by any medicines regulator, including FDA Philippines. The venous leg ulcer formulation is an investigational drug still in clinical development.

Does LL-37 cause skin problems like psoriasis?

LL-37 is implicated in psoriasis biology because it can bind the body's own DNA and RNA released from damaged cells, forming complexes that overstimulate immune cells in the skin. That is a disease-mechanism finding from research into psoriasis, not evidence that using LL-37 as a product causes the disease.

Is LL-37 legal in the Philippines?

It is not a scheduled controlled substance, but no LL-37 product is registered with FDA Philippines. Where it is sold locally, it is sold as a research compound, not as a medicine.