No regulatory authority has approved LL-37, and no human dosing protocol has been validated for any route, so the entire dosing picture is assembled from cell culture, animal models, and a handful of early-phase trials. The numbers that circulate are laboratory concentrations and topical trial concentrations, not clinical doses, and the difference is not cosmetic: a figure generated in dilute buffer describes a property of the assay as much as a property of the peptide.
LL-37 has no validated human dosing protocol, no published human pharmacokinetic profile for any route, and no regulatory approval, and the published record consists of in vitro concentrations of roughly 0.5 to 64 micrograms per milliliter alongside topical human trial concentrations of 0.5 to 3.2 milligrams per milliliter.
Published in vitro work splits into two concentration bands an order of magnitude apart, and conflating them is the most common misreading of this literature. One band measures direct bacterial killing. The other measures immune signaling at levels close to what human plasma and airway fluid actually contain, which is the whole basis for arguing those experiments describe something the body does rather than something a pipette does.
Direct killing by LL-37 is measured at roughly 1 to 64 micrograms per milliliter while immunomodulatory effects are measured at roughly 0.5 to 5 micrograms per milliliter, a tenfold separation that buffer choice, peptide purity, residual counterion, plasticware, and inoculum density can each shift on their own.
Ionic strength, not the peptide, is the largest single variable behind any LL-37 number. The molecule carries a net charge near +6 at neutral pH and finds its target by electrostatic attraction to anionic lipopolysaccharide, lipoteichoic acid, and phosphatidylglycerol headgroups, so anything that screens that charge or binds the peptide first raises the concentration an effect requires. Mild acidity runs the other way and can preserve or improve activity, which matters given that many wounds and abscesses are acidic.
Raising sodium chloride to the physiological 150 millimolar can weaken or abolish killing that was unambiguous in 10 millimolar phosphate buffer, which is why endogenous LL-37 is abundant in cystic fibrosis sputum yet performs poorly there, and why a concentration quoted without its salt, its protein content, and its pH is close to uninterpretable.
Animal work spans several orders of magnitude and never converges on a dose, because the doses were never derived from a pharmacokinetic model. Each was anchored to whatever concentration worked in the matching in vitro assay and capped by whatever the animal tolerated, which is a defensible way to run an experiment and a poor foundation for a human quantity.
| Criteria | Local delivery models | Systemic delivery models |
|---|---|---|
| Routes reported | Topical wound, intratracheal, intratumoral | Intraperitoneal, intravenous |
| Dose units | Micrograms per wound or per animal; gels at 0.5 to 5 mg/mL | Milligrams per kilogram, often low single digits |
| Basis for the dose | Matching in vitro concentration | In vitro concentration plus tolerability ceiling |
| Route to a human dose | Not convertible by inspection | Allometric scaling does not transfer cleanly |
Mice do not make LL-37, they make CRAMP, so rodent cathelicidin experiments describe the class rather than this specific human peptide, and doses reported as micrograms per wound, micrograms per animal, and milligrams per kilogram cannot be lined up against one another at all.
Human exposure to LL-37 has been small, deliberate, and almost entirely local. The trial record is short enough to list in full, and it argues against a dosing chart rather than supplying one: the tested concentrations did not produce a dose-proportional response, and the largest controlled test came back negative across its full population.
The largest controlled human test of LL-37, a Phase IIb trial treating 148 patients with 0.5 or 1.6 milligrams per milliliter topically, found no significant healing benefit over placebo across its full study population, so controlled human evidence does not currently establish efficacy at any tested concentration.
The sequence itself accounts for most of the constraint. LL-37 is a single linear chain of thirty-seven residues at about 4.5 kilodaltons carrying roughly six lysines and five arginines, which are exactly the residues trypsin cleaves after. Even a hypothetically protease-proof version hits a second barrier, since a hydrophilic, highly cationic peptide of that size shows negligible paracellular flux across an intact intestinal epithelium and no transporter carries it.
Oral and sublingual LL-37 products cannot deliver intact active peptide into circulation, because a thirty-seven residue linear chain carrying roughly six lysines and five arginines is cleaved by pepsin, trypsin, and chymotrypsin, and a 4.5 kilodalton cationic peptide does not cross an intact intestinal epithelium in meaningful quantity.
Handling is where nominal dose and delivered dose come apart. Two laboratories can report different results while believing they used the same dose, and the gap usually opens in the vial, on the tube wall, and in the counterion rather than in the biology.
A vial of synthetic LL-37 labeled one milligram typically contains meaningfully less than one milligram of actual peptide, with counterion and bound water making up the balance, so a concentration taken from the label can overstate the true concentration by a quarter or more.
Persistence is short, and that fact quietly shapes every dosing decision in the field. No validated human pharmacokinetic profile has been published for any route, but the degradation mechanisms are characterized well enough to explain why a single application never produces sustained exposure.
A single application of LL-37 does not create sustained exposure, which is why the studies that see effects either dose repeatedly, as the topical leg ulcer work did twice weekly, or embed the peptide in a matrix that keeps releasing it.
This is the number that governs the whole field, and the floor sits lower than is often assumed. Selectivity is a matter of degree rather than absolute protection, since the mechanism that kills bacteria, an amphipathic helix inserting into a lipid bilayer and disrupting it, is the same one that damages host cells; bacterial membranes are simply anionic while mammalian outer leaflets are largely zwitterionic and cholesterol-rich. A slower kind of harm sits behind the acute numbers, with complexes of LL-37 and self DNA activating plasmacytoid dendritic cells through Toll-like receptor 9 in psoriasis and aberrant cathelicidin processing featuring in rosacea.
| Condition | Dilute low-salt buffer | Physiological salt plus serum |
|---|---|---|
| Bacterial killing | A few micromolar | Climbs toward the host-damage range |
| Host-cell DNA fragmentation | Reported at 6 and 20 micromolar, sixteen hours | Moves with cell type, time, serum content |
| Hemolysis of red blood cells | Concentration-dependent, low micromolar | No single switch-on threshold |
| Window between the two | Thinner than commonly quoted | Narrow, sometimes closed entirely |
Significant DNA fragmentation in human vascular smooth muscle cells has been reported at 6 and 20 micromolar over a sixteen-hour exposure, and adding physiological salt and serum drives the concentration needed for bacterial killing toward that same range, which narrows the window between activity and host damage and under some conditions closes it.
Most of the engineering effort aims at two things: keeping the peptide where it was put, and shielding it from proteases long enough to act. None of it has produced an approved product, and the fair summary is that formulation science currently runs well ahead of the clinical evidence.
The only LL-37 delivery approach carrying controlled human data is a viscous topical vehicle, the 10.5 percent polyvinyl alcohol solution used in the Phase IIb leg ulcer trial, and every carrier and analog beyond it remains preclinical with no approved product to date.
A validated dosing protocol is a specific set of things: a dose, a route, a schedule, and a duration, established in adequately powered controlled trials, supported by pharmacokinetic and safety data, and accepted by a regulator for a named indication in a named population. LL-37 has none of those components in place, and the controlled results that do exist argue against extrapolation rather than enabling it. There is a route problem underneath that tends to get skipped, since a concentration tolerated and useful as a gel spread on an ulcer carries no information about an injected quantity when the exposure, the clearance, and the tissue at risk are all different.
Human dosing for LL-37 is undefined, and closing that gap would take dose-ranging trials with real pharmacokinetic measurement, a defined and stable formulation, an indication chosen to suit local delivery, and enough participants to characterize both the shape of the response curve and the safety profile.
Educational use only. This article describes what the published scientific and clinical literature reports about LL-37. It is not medical advice, and it does not recommend, prescribe, or tell anyone to use anything described here. The regulatory status shown at the top of this page reflects what the record showed on the date given there and can change. mdpep.com does not sell any substance described here, does not endorse human use of it, and does not direct anyone to obtain it.
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