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Lexicon
Immune system

LL-37

The only human cathelicidin, an endogenous antimicrobial peptide with a role in immune defense and wound healing.

2 min read 10 sources

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LL-37 vial with white powder and the black TPD label on a white podium, next to a large blue 3D peptide chain in a bright lab
Phase 2, only tested locally · not approved

Summary

At a glance

Sources checked on 26 Oct 2026

Length
37 amino acids

the only human cathelicidin, starts with two leucines (LL)1

Faster wound healing
~6 -fold

Healing rate versus placebo, lowest concentration, applied to leg ulcers, 34 participants2

Smaller wound area
-68 %

mean area after 4 weeks, lowest concentration, no comparison value for placebo2

Phase 2b without overall effect
148 participants

no significant difference from placebo, benefit only in a post hoc subgroup with large wounds3

Phase 3 in the registry
0 trials

no phase 3 trial with LL-37 on ClinicalTrials.gov, as of 26 Oct 20269

  • LL-37 is the only human cathelicidin: the body's own peptide of 37 building blocks that attacks bacterial membranes directly and attracts immune cells.15
  • The body makes a lot of LL-37 in skin wounds, at the edge of chronic ulcers it is missing. Blocking it in a skin model makes wounds close worse.4
  • Applied to hard-to-heal leg ulcers, wounds healed about three to six times faster than with placebo at the two lower concentrations in a first study with 34 participants. A larger phase 2b trial with 148 participants found no overall difference.23
  • Too much LL-37 can also increase inflammation: in rosacea it is strongly raised in the skin, in psoriasis it makes the body's own DNA visible to immune cells.76
  • LL-37 is not approved and has only been tested locally in humans, never injected under the skin. Everything here describes the state of research, not a recommendation.92
Profile

Updated May 2026

Class
Antimicrobial peptide

cathelicidin

Use
Subcutaneous

in mcg

Half-life
short
Status
Phase 2, only tested locally

on wounds, not approved

Sizes
5 to 10 mg

2 sizes in the shop

Contents 15 sections
  1. At a glance
  2. What is LL-37?
  3. How it works
  4. What you need
  5. Reconstitution
  6. Dosing
  7. Injection
  8. What the research shows
  9. Research status
  10. Side effects
  11. Storage
  12. Related compounds
  13. FAQ
  14. Glossary
  15. Sources

What is LL-37?

LL-37 is the only human member of the cathelicidin family, a group of endogenous antimicrobial peptides. It is part of the innate immune defense, fights bacteria directly and plays a role in inflammation and wound healing.

Comparison

Peptides for defence and wounds in the lexicon

Where the peptides act according to the TPD lexicon. Tap a card to open its entry.

Endogenous antimicrobial peptide LL-37 FPR2
Anti-inflammatory tripeptide KPV PepT1 Peptide from the thymus Thymosin Alpha-1 Copper peptide, focus on skin GHK-Cu

5

How it works

LL-37 directly disrupts the membrane of bacteria and so acts antimicrobially. It also modulates the immune response and is involved in tissue repair. This dual role makes it interesting in research but also complex, because too much LL-37 can also fuel inflammation.

Where it acts

Where LL-37 acts

Tap a point or a card. Colour = receptor, grey = observed.

  1. Skin and wounds

    Strongly produced in fresh wounds, missing at the edge of chronic ulcers4

    Observed in trials
  2. Immune cells in the blood

    Attracts neutrophils, monocytes and T cells via FPR2 (cell study)5

    FPR2

Known targets5

Compound PubChem CID 16198951: two described targets, type of action on each card.

Formyl peptide receptor 2 (FPRL1)
Gene
FPR2
UniProt
P25090
ChEMBL
CHEMBL4227
Action
Agonist, attracts immune cells (cell study)
Toll-like receptor 9
Gene
TLR9
UniProt
Q9NR96
ChEMBL
CHEMBL5804
Action
Indirect: LL-37 binds the body's own DNA and hands it to TLR9 in immune cells
A translucent rod-shaped bacterium with blue 3D molecules docking onto its membrane and breaking it open, glowing immune cells around it
Symbolic image.

What you need

Supplies

2 things, numbered as in the picture

Tick off what you already have. Vial and BAC water are our real shop photos at the same size.

LL-37 vial and bacteriostatic water at the same size with the black TPD label, next to two U-100 insulin syringes, two alcohol swabs and a sharps container with a blue lid

  1. LL-37 vial

    The LL-37 vial (lyophilized powder).

    In the shop

  2. Water and supplies

    Bacteriostatic water, U-100 insulin syringes, alcohol swabs.

    In the shop

Reconstitution

Use the peptide calculator. A 5mg vial with 2ml water gives 2500mcg per ml, 250mcg is then 10 units on the U-100 syringe. Just add the water to the vial and let it dissolve.

Step by step

Dissolving the powder

Four steps, the same for every vial size.

  1. 1Wipe the cap

    Clean the rubber stopper with an alcohol swab.

  2. 2Add BAC water

    Just add it, it does not matter if it hits the powder directly.

  3. 3Swirl or shake

    Until everything has dissolved, both are fine.

  4. 4Draw up

    With the U-100 insulin syringe.

Less water higher concentration fewer units on the syringe Example: 5 mg + 2 ml = 2.5 mg/ml

Dosing

Dosing

Dosing, titration and syringe

Loading the dosing information …

  • Getting started and stepping up, drawn as a staircase
  • Units on the U-100 syringe for every step
  • How long one vial lasts

Injection

The injection is subcutaneous, into the fat layer under the skin. Common spots are the abdomen (keep about two finger-widths clear of the navel), the front and the inner side of the thigh, or the back of the upper arm.

The short version:

Injection

Where and at what angle

Pick a spot or switch the angle. Subcutaneous means: into the fat layer, not the muscle.

Rotate the spot each time, never the same one twice in a row.

90°More fat under the skin

Rule of thumb: where there is more fat under the skin, insert straight at 90 degrees. In lean people with little fat, 45 degrees, so the needle stays in the fat layer. Simplified diagram.

  1. Clean the spot with an alcohol swab and let it dry

  2. Pinch up a small fold of skin

  3. Insert the needle: at 90 degrees where there is more fat under the skin, at 45 degrees in lean people with little fat

  4. Inject slowly, wait a moment, then withdraw

  5. Put the needle straight into the sharps container

Rotate the site each time so the tissue does not get irritated. Never the same spot twice in a row.

What the research shows

A widely cited review (Dürr, 2006) summarizes the structure and function of LL-37 as the only human cathelicidin: antimicrobial, immunomodulating, involved in wound healing. That is well-described basic biology.

In humans LL-37 has only been tested locally so far, mainly as a solution on hard-to-heal leg ulcers. In the first study (Grönberg, 2014, phase 1/2, 34 participants) wounds healed faster than with placebo at the two lower concentrations. The larger phase 2b trial (Mahlapuu, 2021, 148 participants) found no overall difference from placebo, only in a later analysis of large wounds. LL-37 was well tolerated in both studies. There are no studies with LL-37 injected under the skin. There is no approval either.

Study results

What the trials measured

Interactive: switch tabs, tap points, show or hide arms. Arms are named relative to each other, no dosing advice.

How fast hard-to-heal leg ulcers healed2

Healing rate relative to placebo, applied twice a week for 4 weeks, 34 participants

First study in humans2 Phase 1/2, venous leg ulcers
Placebo 1-fold
Lowest concentration 6-fold
Middle concentration 3-fold

Values about six- and threefold according to the abstract (only the first was significant). The highest concentration in the trial healed no better than placebo and is not shown here. A larger phase 2b trial with 148 participants later found no overall difference.

Mean wound area after treatment2

Change from baseline in percent, applied to leg ulcers

First study in humans2 Phase 1/2, 4 weeks
Lowest concentration -68%
Middle concentration -50%

The abstract gives no percentage for placebo, so the comparison bar is missing. Small groups, the effect was not confirmed in the larger follow-up trial.

A piece of skin tissue on a glass plate between Petri dishes with bacterial cultures in a bright lab
Symbolic image.

Research status

Where the research stands

LL-37 has been tested in humans applied to wounds and as an injection into tumours. The two wound trials in Europe are not listed on ClinicalTrials.gov, their results are in the charts. As of 26 Oct 2026. 9

  1. Preclinical Endogenous peptide, cell and animal studies
  2. Phase 1 no registered trial
  3. Phase 2 1 trial, 1 completed
  4. Phase 3 no registered trial
  5. Approval not yet
Not approved. Tested only locally: applied to leg ulcers (phase 1/2 and phase 2b) and as an injection into tumours (phase 1/2).3
Show all 1 phase 2 trial on a timelineHide timeline 1 completed 0 running 0 with results
Phase 2

Bar: start to primary endpoint (planned for running trials). Green = completed, blue = progress until 26 Oct 2026.

Side effects

  • Solid human use data are missing.
  • Local reactions at the injection site, since LL-37 is membrane-active.
  • Theoretically, too much LL-37 can amplify inflammation rather than dampen it.

Storage

Before mixing the powder is robust, normal room temperature (around 20 to 25 degrees) is fine, ideally kept in the dark. After mixing, keep in the fridge (2 to 8 degrees), do not freeze. It stays good there for at least 4 to 6 months with barely any loss of potency, in practice considerably longer. Swirl or shake to mix, both are fine.

Shelf life

How long it keeps

Undissolved (powder) Room temperature, around 20 to 25 degrees, dark

Robust, no fridge needed. Keeps for several months without trouble.

Dissolved (mixed) Fridge, 2 to 8 degrees, do not freeze

At least 4 to 6 months with barely any loss of potency. The bacteriostatic water brings a preservative.

  • Keep it dark
  • Dissolved: 2 to 8 degrees
  • Note the mixing date on the vial
Nine vials with white powder and blue caps in the door shelf of a bright fridge
Powder dark at room temperature, once dissolved into the fridge (2 to 8 degrees), stable there for at least 4 to 6 months. Symbolic image.

Related

Related compounds

A different path: all in the TPD lexicon.

LL-37
KPV Different anti-inflammatory peptide, focus on the gut Thymosin Alpha-1 Different peptide from immune defence GHK-Cu Different peptide focused on skin and wounds BPC-157 Different peptide from healing research

FAQ

Common questions about LL-37

Short answers, each backed by the sources below.

Is LL-37 approved or studied in humans?

Not approved, but tested in humans. Only locally though: applied to hard-to-heal leg ulcers (one trial with 34 and one with 148 participants) and as an injection into melanoma metastases (4 participants). There is no trial with LL-37 injected under the skin. The information here describes the state of research.239

Where does LL-37 come from in the body?

It is cleaved from the precursor hCAP18. Immune cells make it all the time, skin and mucous membranes mainly during inflammation and infection. In wounds the level rises until about 48 hours after the injury and falls again once the wound has closed. Immune cells also make it depending on vitamin D.481

How does LL-37 work?

In two ways. It attaches to the membrane of bacteria and breaks it open. It also attracts immune cells such as neutrophils, monocytes and T cells, via the receptor FPR2 in cell studies. In a skin model it helps wounds close with new skin.154

What did the wound trials show?

In the first trial with 34 participants leg ulcers healed about six and three times faster than with placebo at the two lower concentrations, the mean area fell by 68 and 50 percent. The highest concentration worked no better than placebo. The larger phase 2b trial with 148 participants found no overall difference, only a post hoc analysis of large wounds favoured LL-37.23

Can LL-37 increase inflammation?

Yes, that has been described. In rosacea cathelicidin is strongly raised in the facial skin and cut differently, in mice these fragments triggered inflammation. In psoriasis LL-37 binds the body's own DNA and makes it visible to certain immune cells, which then release interferon. The entry points this out too.76

Which side effects are known?

In both wound trials LL-37 was well tolerated, there were no safety concerns at the wound or in the body. There are no study data for LL-37 injected under the skin. The entry mentions possible reactions at the injection site because LL-37 attacks cell membranes.23

Glossary

Terms explained briefly

Dotted words in the text open these explanations directly.

Antimicrobial peptide
A short protein that attacks bacteria, fungi or viruses directly, usually via their membrane.
Cathelicidin
A family of the body's own defence peptides. Humans have only one member, from which LL-37 is made.
FPR2
Formyl peptide receptor 2 (formerly FPRL1) on immune cells. Through it LL-37 attracts these cells.
hCAP18
The precursor protein from which LL-37 is cleaved. It is found in immune cells and in skin and mucous membranes.
Innate immune defence
The part of defence that reacts to pathogens at once and without prior learning, for example via defence peptides and phagocytes.
Lyophilisate
Freeze-dried powder. Without water a peptide keeps much longer.
Membrane
The envelope of a cell or bacterium. If it is pierced, the pathogen dies.
Placebo
Dummy preparation without active substance. Comparing against it shows what is due to the substance itself.
Rosacea
An inflammatory skin condition of the face with redness, bumps and dilated small vessels.
Subcutaneous
Into the fat layer just under the skin.
Venous leg ulcer
An open, poorly healing wound on the lower leg, usually caused by congested blood flow in the veins.

Evidence

Sources

Show all 10 sourcesHide sources 8 Studies1 Trial registry1 Database checked 26 Oct 2026

Studies

  1. Review of the structure and action of LL-37

    Dürr et al., Biochimica et Biophysica Acta, 2006

    DOI 10.1016/j.bbamem.2006.03.030 PMID 16716248
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  2. First study in humans: applied to leg ulcers (phase 1/2)

    Grönberg et al., Wound Repair and Regeneration, 2014

    DOI 10.1111/wrr.12211 PMID 25041740
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  3. Phase 2b trial with 148 participants, applied to leg ulcers

    Mahlapuu et al., Wound Repair and Regeneration, 2021

    DOI 10.1111/wrr.12977 PMID 34687253
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed, PubMed Central
  4. LL-37 in the closure of skin wounds

    Heilborn et al., Journal of Investigative Dermatology, 2003

    DOI 10.1046/j.1523-1747.2003.12069.x PMID 12603850
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  5. LL-37 attracts immune cells via FPRL1 (FPR2)

    Yang et al., Journal of Experimental Medicine, 2000

    DOI 10.1084/jem.192.7.1069 PMID 11015447
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  6. LL-37 makes the body's own DNA visible in psoriasis

    Lande et al., Nature, 2007

    DOI 10.1038/nature06116 PMID 17873860
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  7. Cathelicidin and inflammation in rosacea

    Yamasaki et al., Nature Medicine, 2007

    DOI 10.1038/nm1616 PMID 17676051
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  8. Vitamin D controls cathelicidin production in immune cells

    Liu et al., Science, 2006

    DOI 10.1126/science.1123933 PMID 16497887
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed

Trial registry

  1. Search on ClinicalTrials.gov

    ClinicalTrials.gov, U.S. National Library of Medicine

    Checked 26 Oct 2026 via ClinicalTrials.gov API v2

Database

  1. Structure of LL-37 in PubChem

    PubChem, U.S. National Library of Medicine (CID 16198951)

    Checked 26 Oct 2026 via PubChem

In the shop

LL-37 in the catalog

Sizes, prices and shipping times are on the product page.

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