Immune system
Your body's built-in antibiotic protein and its lab-made upgrades — studied for killing drug-resistant bacteria, including MRSA, that standard antibiotics can't touch.
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Cathelicidin is a germ-fighting peptide the human immune system makes itself, stored as a precursor protein called hCAP-18. Lab-built derivatives of it, such as SAAP-148, are being studied as new antibiotics against drug-resistant bacteria — including MRSA and the stubborn bacterial films (biofilms) that resist standard drugs. These derivatives are research compounds, not approved medicines, though a related peptide has reached early human trials for ear infections.
Cathelicidin (LL-37) is a natural antimicrobial peptide your immune cells produce to kill bacteria, viruses, and fungi. It also reduces inflammation and promotes wound healing, making it a multi-functional defender of your health.
Cathelicidin is a 37-amino acid peptide (a small chain of building blocks) that carries a positive charge and fights germs. Neutrophils (immune cells), cells lining body surfaces and other immune cells make it by cutting it from a larger precursor, hCAP18. It kills a wide range of germs in several ways. It forms spiral (α-helical) shapes that push into bacterial and fungal membranes. It sticks to lipopolysaccharides and lipoteichoic acids (parts of bacterial walls). It also gets inside germs and targets their genetic material (nucleic acids). Beyond killing germs, it tunes the innate immune system (the body's first-line defense). It works through TLR9 and G-protein coupled receptors (cell-surface sensors), which draws immune cells to the spot (chemotaxis), helps them swallow germs (phagocytosis), and lowers inflammatory chemicals (cytokines (immune signaling molecules)) by suppressing the NF-κB pathway (a main inflammation switch).
Evidence check
Some human studies
It has been tested in people, but only in small or early-stage studies. Promising, but not yet proven the way an approved medicine is.
Every numbered marker on this page links to one of the 5 sources at the bottom.
Antibiotic-Resistant Infections
Synthetic versions like SAAP-148 kill dangerous superbugs in lab tests, including MRSA and other bacteria that resist normal antibiotics.
Biofilm (slimy germ colony) destruction
These peptides break through sticky bacterial shields called biofilms that protect infections on wounds and medical devices from treatment.
Immune System Support
Your body naturally makes cathelicidin when vitamin D levels are good, helping white blood cells find and destroy harmful germs faster.
The most-asked question
The research-reported range is 0.5 mg/mL, once daily.
Chronic ear infection, Local antibacterial use, Biofilm infections
0.5 mg/mL drops (about 100 microliters per application)
In a human phase 2a ear study, OP-145 drops at 0.5 mg per mL were applied to the affected ear twice daily for 2 weeks and were safe and well tolerated 4.
The ceiling: 5 (as determined by your healthcare provider based on individual response and tolerance)
Reconstitution, in plain words
Peptides usually arrive as a freeze-dried powder in a small vial. “Reconstitution” just means adding sterile water back in so you can measure a dose. Here's how it works for Cathelicidin (hCAP-18 / Synthetic Derivatives):
Cathelicidin (hCAP-18 / Synthetic Derivatives) comes in pre-measured doses or forms. Follow the exact dosing instructions on your prescription label. No reconstitution or mixing is typically required for this formulation.
Use Cathelicidin (hCAP-18 / Synthetic Derivatives) exactly as prescribed. Each unit contains the labeled amount. Your healthcare provider will determine the appropriate dose based on your individual needs and response.
Three quick questions — your vial, your water, your dose — and it tells you the exact syringe mark. “Find dose amount” is preloaded with Cathelicidin (hCAP-18 / Synthetic Derivatives)'s recorded schedule.
Injection sites & technique
Cathelicidin (hCAP-18 / Synthetic Derivatives) is applied to the skin (the main route used in research)—no injection required
Timing
Apply Cathelicidin (hCAP-18 / Synthetic Derivatives) to clean, dry skin. For best results, use consistently at the same time(s) each day. Evening application is often preferred to allow overnight absorption, unless otherwise directed.
As a topical product, Cathelicidin (hCAP-18 / Synthetic Derivatives) is not affected by food intake. Apply to clean skin and allow adequate absorption time before covering the area.
Cathelicidin (hCAP-18 / Synthetic Derivatives) should be used as directed by your healthcare provider. If combining with other medications or supplements, discuss potential interactions with your provider. Avoid combining with compounds that have overlapping mechanisms unless specifically guided by a medical professional.
The honest list
These show up most often. They usually fade as your body adjusts.
Mild redness, warmth, and stinging at the topical application site. Reflects the peptide's interaction with skin cells and local immune activation.
What to do Use appropriate formulation vehicle (hypromellose ointment). Apply to wound bed rather than intact skin. Monitor and reduce concentration if irritation is excessive.
A short-lived rise in local inflammation, including mild swelling and redness around the treated area, as immune cells are drawn in through a cell-surface receptor called FPR2/ALX.
What to do This is expected and generally beneficial — indicates immune cell recruitment to the infection site. Monitor for excessive inflammation. Should resolve within 24-48 hours.
Increased wound exudate in the first 24-48 hours of treatment as biofilm disruption releases bacterial contents and immune infiltration increases.
What to do Change wound dressings as needed. Increased exudate initially is expected with biofilm disruption. Should normalize within 2-3 days as bacterial load decreases.
Worth knowing about, but most people never see these.
Mast cell degranulation triggered by high concentrations of cationic peptide, causing localized hives or wheals near the application site.
What to do Reduce concentration. Apply antihistamine cream if needed. Usually self-limiting. Discontinue if reaction spreads beyond the treatment area.
Sharp stinging or burning sensation with subcutaneous (under the skin) injection in research protocols, related to the highly cationic nature of the peptide interacting with tissue.
What to do Slow injection rate. Ensure proper dilution in physiological buffer. Cold compress post-injection.
Rare, but if any of these happen, stop and contact a medical professional.
At concentrations exceeding 50 µM, SAAP-148 demonstrates hemolytic activity against human red blood cells. This is the primary dose-limiting toxicity identified in preclinical (animal or lab studies, not human trials) studies.
What to do Maintain dosing within the established therapeutic window (<25.6 µM for topical, <50 µM absolute maximum). Monitor for signs of hemolysis if systemic (affecting the whole body) exposure occurs. This toxicity is concentration-dependent and avoidable with proper dosing.
Week by week
Is it right for you?
Cathelicidin (hCAP-18 / Synthetic Derivatives) should only be started, adjusted, or discontinued under medical supervision. This information is for educational purposes only and does not replace professional medical advice. Never stop a prescribed treatment without consulting your healthcare provider first, as abrupt discontinuation may have consequences.
Use with caution—discuss with your healthcare provider.
Use with caution—discuss with your healthcare provider.
Use with caution—discuss with your healthcare provider.
May be used together under medical guidance.
May be used together under medical guidance.
May be used together under medical guidance.
Generally safe to take alongside Cathelicidin (hCAP-18 / Synthetic Derivatives). Space doses apart if taking oral formulations to ensure optimal absorption.
Helpful if experiencing any GI side effects that could lead to dehydration. Safe to combine.
Cathelicidin (hCAP-18 / Synthetic Derivatives) is particularly well-suited for individuals focused on research into next-generation antimicrobials against multidrug-resistant infections. Research and clinical experience suggest meaningful benefits in this area when used as part of a comprehensive treatment approach.
Cathelicidin (hCAP-18 / Synthetic Derivatives) is particularly well-suited for individuals focused on topical antimicrobial development for wound infections with biofilm involvement. Research and clinical experience suggest meaningful benefits in this area when used as part of a comprehensive treatment approach.
Cathelicidin (hCAP-18 / Synthetic Derivatives) is particularly well-suited for individuals focused on understanding vitamin d-cathelicidin innate immune axis for immune optimization. Research and clinical experience suggest meaningful benefits in this area when used as part of a comprehensive treatment approach.
Cathelicidin (hCAP-18 / Synthetic Derivatives) is particularly well-suited for individuals focused on development of anti-biofilm therapeutics for chronic device-related and wound infections. Research and clinical experience suggest meaningful benefits in this area when used as part of a comprehensive treatment approach.
Cathelicidin peptides (natural and synthetic) are not FDA-approved. Human data are limited: in a randomized Phase IIa trial, ear drops of the LL-37-derived peptide OP-145 were safe and well tolerated in chronic middle-ear infection 34. Animal studies show broad germ-killing activity without whole-body toxicity at therapeutic doses. However, there is a concern about how the immune system reacts: cathelicidin derivatives can switch on the body's first-line immune defenses and cause inflammation at high concentrations. How the peptide acts on immune cells in humans is not fully understood. Its effects on long-term immune messaging, on tolerance, and on unintended immune activation have not been studied. Bacteria becoming resistant to cathelicidin-based treatments is theoretically possible but has not been documented in people.
Evidence is mostly from lab tests of germ-killing activity (in vitro (in lab dishes, not living bodies) antimicrobial assays, meaning lab tests on cells or microbes), animal infection models, and preclinical (animal or lab studies, not human trials) immune cell studies. 13 Human data are limited to small trials of synthetic versions, such as a dose-finding study and a randomized Phase IIa trial of OP-145 ear drops (34 patients; treatment success 47% vs 6% on placebo) 4. Published research focuses on how the peptide kills germs, not on safety or tolerability in people.
Keeping it good
Store Cathelicidin (hCAP-18 / Synthetic Derivatives) in the refrigerator at 36-46°F (2-8°C) in its original packaging. Protect from light and moisture. Do not freeze. Check the expiration date before use. Some formulations may be stored at room temperature for limited periods—check your specific product labeling.
Once reconstituted, Cathelicidin (hCAP-18 / Synthetic Derivatives) should be kept refrigerated at 36-46°F (2-8°C) and used within the timeframe specified on your product labeling (typically 14-28 days). Label the vial with the reconstitution date. Do not use if the solution appears cloudy, discolored, or contains particles.
Not our opinion — the published record
2017
Cathelicidins kill harmful bacteria in an immunologically quiet way by punching holes in their cell membranes, while simultaneously binding to bacterial toxins to prevent dangerous immune reactions.
Coorens M, Schneider VAF, de Groot AM, et al.
2019
Research (2019) on cathelicidin contributes important scientific knowledge about its biological and pharmacological properties.
Dijksteel GS, Ulrich MMW, Middelkoop E
2023
Clinical trial (2020) provides evidence for cathelicidin's effectiveness in therapeutic management.
Ön A, Vejzovic D, Jennings J, et al.
2020
In a phase IIa trial (a mid-stage trial testing whether a treatment works), P60.4Ac (OP-145) ear drops at 0.5 mg/mL twice daily for 2 weeks gave treatment success in 47% of patients versus 6% with placebo.
Peek NFAW, et al.
2024
Research (2004) demonstrates cathelicidin's potent antimicrobial activity and broad-spectrum effectiveness against pathogenic microorganisms.
Dzurová L, Holásková E, Pospíšilová H, et al.
If this isn't the right fit
Your questions
LL-37 is the 37-amino-acid mature peptide cleaved from the cathelicidin precursor protein hCAP-18 by neutrophil proteinase 3. This cathelicidin entry focuses on the broader cathelicidin system — the hCAP-18 precursor protein, the vitamin D-regulated CAMP gene, and especially the next-generation synthetic derivatives SAAP-148 and OP-145 that were engineered to overcome LL-37's key limitation of losing antimicrobial activity under physiological conditions. SAAP-148 retains full potency in human plasma and physiological salt, while LL-37 does not.
LL-37's major translational barrier is that it loses most of its antimicrobial activity in the presence of physiological salt concentrations (150 mM NaCl), serum proteins, and human plasma. SAAP-148 was specifically engineered at Leiden University Medical Center to solve this problem — through optimization of charge distribution (+11 vs LL-37's +6) and hydrophobic core stability, SAAP-148 maintains full bactericidal and anti-biofilm activity under physiological conditions. This means the concentrations that work in the lab actually work in human wound environments.
The CAMP gene, which makes the cathelicidin precursor hCAP-18, has a vitamin D switch (a vitamin D response element, VDRE) next to it. When 1,25-dihydroxyvitamin D3 (active vitamin D) attaches to the vitamin D receptor, it directly turns up the CAMP gene in neutrophils, macrophages (immune cells that engulf germs), and cells lining the body's surfaces. This means your blood level of 25-hydroxyvitamin D directly influences how much germ-fighting peptide you make — which helps explain why vitamin D deficiency is linked to more infections, particularly tuberculosis and respiratory infections.
Resistance development is extremely unlikely with cathelicidin-derived peptides because they target the fundamental physical structure of bacterial membranes rather than a specific protein or metabolic pathway. The multi-target membrane disruption mechanism — involving electrostatic binding, lipid insertion, pore formation, and membrane depolarization — would require bacteria to completely restructure their membrane architecture, which is incompatible with viability. Serial passage experiments with SAAP-148 show no significant resistance development even after extended exposure.
OP-145 (also known as P60.4Ac), a 24-amino-acid cathelicidin-derived peptide with a +7 net charge, was evaluated in a Phase I/II clinical trial for chronic suppurative otitis media (CSOM) — chronic ear infections involving biofilms. The trial demonstrated that OP-145 ear drops were safe, well-tolerated, and showed efficacy against chronic biofilm infections that had failed conventional antibiotic therapy. This trial represented the first clinical validation of a synthetic cathelicidin derivative for treating human biofilm infections.
Cathelicidin (LL-37 cleaved from hCAP-18) is indeed one of several antimicrobial peptides found in human sweat, alongside dermcidin and human beta-defensins. However, cathelicidin is much more broadly expressed — it is produced by neutrophils (stored in specific granules), macrophages, epithelial (surface-lining tissue) cells of the skin, respiratory tract, urinary tract, and gastrointestinal tract. Its expression in these barrier tissues makes it one of the body's first-line defenders against invading pathogens, well before adaptive immune responses engage.
5 sources
Every numbered marker on this page points to one of these. Same sources, same numbering as the Cathelicidin (hCAP-18 / Synthetic Derivatives) research profile on Peptide Initiative.
Want the deep science?
Every claim on this page traces back to the research database behind Peptide Initiative — studies, citations, clinical trial data, and the technical detail we deliberately left out here.
Open the research profileThis guide is education, not medical advice. Cathelicidin (hCAP-18 / Synthetic Derivatives) is a research compound that is not FDA-approved for human use. Talk to a clinician before taking anything.
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