Antimicrobial Peptides (AMPs)

Antimicrobial peptides (AMPs) are short, typically cationic (positively charged) peptides that form part of the innate immune system in virtually all multicellular organisms — from insects to humans. They represent one of the oldest defense mechanisms in evolutionary history, predating the adaptive immune system by hundreds of millions of years.

Why antimicrobial peptides matter now

The antibiotic resistance crisis has renewed interest in AMPs as potential alternatives or supplements to conventional antibiotics. The key advantage: AMPs primarily kill microbes by physically disrupting their cell membranes. This mechanism is far harder for bacteria to develop resistance against than the enzyme-targeting approach of most conventional antibiotics, because resistance would require fundamental changes to membrane lipid composition — a much higher evolutionary cost.

Structure and general properties

Most AMPs share several structural features:

• Short — typically 12–50 amino acids
• Cationic — net positive charge (+2 to +9) due to arginine and lysine residues
• Amphipathic — one face is hydrophobic (interacts with lipid membranes), the other is hydrophilic (faces the aqueous environment)
• Structurally flexible — many are disordered in solution but adopt defined structures (alpha-helix, beta-sheet) upon contact with membranes

This combination of properties allows AMPs to selectively target microbial membranes. Bacterial membranes are rich in anionic (negatively charged) phospholipids (phosphatidylglycerol, cardiolipin), which electrostatically attract the cationic peptides. Mammalian cell membranes are primarily composed of zwitterionic phospholipids (phosphatidylcholine, sphingomyelin) and contain cholesterol, which reduces AMP binding. This selectivity provides a therapeutic window.

Major human AMP families

Cathelicidins

Humans produce one cathelicidin: hCAP18/LL-37. The precursor protein hCAP18 is cleaved by proteinase 3 to release the active 37-amino-acid peptide LL-37. It is produced by neutrophils, macrophages, and epithelial cells in the skin, gut, and respiratory tract.

LL-37 has been the most clinically advanced human AMP:

• Broad-spectrum activity against Gram-positive and Gram-negative bacteria, fungi, and enveloped viruses
• Disrupts established bacterial biofilms
• Immunomodulatory — chemottracts immune cells and promotes wound healing
• Expression is regulated by vitamin D (vitamin D deficiency = low LL-37)

Defensins

Defensins are the largest family of human AMPs, divided into alpha-defensins and beta-defensins:

Alpha-defensins (HNP 1-4, HD-5, HD-6) — produced by neutrophils (HNP) and Paneth cells in the small intestine (HD). They form a first-line defense against ingested pathogens and are a major component of neutrophil antimicrobial granules.

Beta-defensins (hBD-1, hBD-2, hBD-3, hBD-4) — produced by epithelial cells in the skin, respiratory tract, and urogenital tract. hBD-1 is constitutively expressed; hBD-2 and hBD-3 are induced by bacterial contact or pro-inflammatory cytokines. hBD-3 has the broadest antimicrobial spectrum, including activity against MRSA.

Histatins

Histatins are a family of histidine-rich peptides found in human saliva. Histatin 5 is the most potent — it has strong antifungal activity against Candida albicans, which partly explains why oral candidiasis is relatively uncommon in healthy individuals despite constant fungal colonization.

Mechanisms of membrane disruption

AMPs kill microbes primarily by disrupting cell membrane integrity. Several models describe how this works:

Barrel-stave model. Peptides insert perpendicular to the membrane, forming a barrel-shaped pore with the hydrophobic face contacting the lipid bilayer and the hydrophilic face lining the aqueous channel. This allows uncontrolled ion flux, dissipating the membrane potential.

Toroidal pore model. Similar to barrel-stave, but the peptides induce the lipid bilayer to bend inward, creating a pore lined by both peptide and lipid head groups. LL-37 is thought to act primarily through this mechanism.

Carpet model. At sub-lytic concentrations, peptides accumulate on the membrane surface like a carpet. Above a threshold concentration, they collectively disrupt the membrane through a detergent-like mechanism, fragmenting it into micelles.

In reality, a single AMP may use multiple mechanisms depending on concentration, membrane composition, and environmental conditions.

Beyond membrane disruption

Many AMPs have intracellular targets in addition to membrane disruption:

• DNA/RNA binding — some AMPs penetrate the membrane and bind nucleic acids, inhibiting replication and transcription
• Protein synthesis inhibition — proline-rich AMPs (in insects) enter cells and bind ribosomes
• Cell wall synthesis — some AMPs interfere with cell wall precursor processing (similar to vancomycin)
• Enzyme inhibition — certain AMPs inhibit proteases and other essential bacterial enzymes

Therapeutic development challenges

Despite their promise, AMP drug development faces several hurdles:

Serum stability. Most natural AMPs are rapidly degraded by proteases in blood. LL-37's half-life in serum is approximately 15 minutes. Solutions include D-amino acid substitution (resistant to L-specific proteases), peptide stapling, cyclization, and PEGylation.

Manufacturing cost. Solid-phase peptide synthesis becomes increasingly expensive with peptide length. A 37-residue peptide like LL-37 is substantially more expensive to produce than a small-molecule antibiotic.

Hemolytic activity. Some AMPs, particularly at higher concentrations, damage red blood cells. Therapeutic development requires optimizing the selectivity index — the ratio of hemolytic concentration to antimicrobial concentration.

Systemic vs. topical. Most clinical development has focused on topical applications (wound infections, skin infections) where local concentrations can be achieved without systemic exposure. Systemic AMP therapy remains technically challenging.

Current clinical landscape

AMPs in clinical development as of 2026:

• LL-37 — Phase I/IIa completed for chronic venous leg ulcers (topical); showed safety and efficacy signals
• Pexiganan (MSI-78) — synthetic magainin analog; reached Phase 3 for diabetic foot infections (topical) but failed to show superiority over standard antibiotics
• Omiganan (MBI-226) — synthetic indolicidin analog; Phase 3 for catheter infections and rosacea
• Brilacidin — defensin mimetic (small molecule); Phase 2 for acute skin infections

The pattern: topical applications are closer to market; systemic use remains in earlier stages.