Cathelicidin (hCAP-18 / Synthetic Derivatives)

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 that encodes the cathelicidin precursor hCAP-18 has a vitamin D response element (VDRE) in its promoter region. When 1,25-dihydroxyvitamin D3 (active vitamin D) binds the vitamin D receptor, it directly upregulates CAMP gene transcription in neutrophils, macrophages, and epithelial cells. This means circulating 25-hydroxyvitamin D levels directly influence antimicrobial peptide production — explaining why vitamin D deficiency is associated with increased susceptibility to 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 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.