LL-37 common uses, half-life, side effects and more

LL-37 is the working end of a human antimicrobial protein that the body makes in skin, gut lining, and airways. Neutrophils release it where there is an injury or an infection. The only human trials applied it as a gel directly onto chronic leg ulcers. People also buy the powder to inject, spray up the nose, or inhale, aiming at infections, biofilm, and gut problems. No human trial has given it by any of those routes. It is not an approved medicine anywhere and the research-chemical powder is not the material the trials used.

• Chronic venous leg ulcers in investigational topical trials
• Diabetic foot ulcer research using a topical cream
• Biofilm and stubborn infection claims drawn from laboratory work
• Viral and respiratory illness claims from community use
• Gut dysbiosis and inflammatory bowel claims

Topical trials dosed twice weekly and community injection protocols run daily

Topical trials applied 0.5 mg/mL or 1.6 mg/mL to the ulcer bed twice weekly at 25 microliters per square centimeter of wound. That delivers 12.5 or 40 micrograms per square centimeter. A 3.2 mg/mL arm was also tested and it produced more frequent and more severe local reactions without helping. That arm is not a usable ceiling. A diabetic foot ulcer trial used a 0.5 mg/g cream twice weekly for four weeks. Injected doses rest on no trial of any kind. The figures that circulate run from 100 to 300 micrograms under the skin daily. Some vendor pages reach 1 mg daily or higher.

• Trial dosing was set per square centimeter of wound rather than as a fixed amount
• The trial gel was mixed immediately before each application because the diluted peptide is unstable
• Vendors sell 5 mg and 10 mg vials without saying how much water to add
• Published reconstitution recipes differ tenfold in final strength so a dose written in syringe units means nothing on its own

No human half-life exists because no human pharmacokinetic study has ever been run. Plasma does not rapidly destroy LL-37. A blood lipoprotein binds it and switches it off instead. That same binding is why serum blocks its antibacterial action. Mice provide the only living-animal pharmacokinetics.

• Redness, swelling, warmth, scaling, papules, and pustules were common around treated ulcers
• Event counts ran higher on the peptide in the larger trial though the share of patients affected was similar and most of the gap was dosing mistakes
• Mast cells dump their contents on first exposure through a route that needs no prior sensitization
• Higher concentrations damage human cells and rupture red blood cells in laboratory tests
• Injection-site reactions were expected often enough that one trial excluded them from its dose-limiting rules

LL-37 is the tail of a longer protein that neutrophils carry to a wound. An enzyme called proteinase 3 cuts it loose outside the cell. The freed peptide carries a strong positive charge that pulls it onto negatively charged bacterial membranes. It then breaks those membranes open. It blocks biofilm formation in laboratory dishes at far below the killing dose. It also summons neutrophils, monocytes, and T cells through a receptor called FPRL1. It mops up bacterial toxin in some settings and drives inflammation in others. The damaging side appears when it ferries a person's own DNA or RNA into immune sensors. That route underlies psoriasis and part of lupus. Blood plasma largely switches the peptide off and what an injection accomplishes is unknown.

Human evidence covers topical use on wounds and nothing else. A 34-person leg ulcer study found faster healing at the lowest concentration. The 148-person follow-up missed its primary endpoint outright. A subgroup with wounds above 10 square centimeters did better in an analysis run after the results were in. A 25-person diabetic foot ulcer trial reported better granulation without a significant change in wound area. Every human wound trial of this peptide comes from a single company and company employees are among the authors. No independent group has repeated the healing result. A 2011 paper reporting that LL-37 activates the IGF-1 receptor was retracted in 2022. Two of its authors also wrote the wound trials.

• Repeated injection into skin is the standard way researchers give mice rosacea and long-term dosing left lesions that did not fully recover
• The FDA reviewed LL-37 for pharmacy compounding and flagged immunogenicity, impurity, male reproductive harm, and tumor promotion in animal and cell work
• Animal and cell studies show it speeds some tumors and slows others while building artery plaque in mice carrying the human gene
• Psoriasis, rosacea, lupus, and mast cell disorders can all be driven by this peptide's own biology
• Whether injected LL-37 provokes antibodies has never been tested and antibodies against it impair wound healing
• Pregnancy and breastfeeding are unstudied and the peptide kills sperm in laboratory tests