LL-37 vs VIP.

LL-37 is the only human cathelicidin-derived antimicrobial peptide, a 37-residue cationic, amphipathic alpha-helical peptide (named for its two leading leucine residues) cleaved from the C-terminus of the precursor protein hCAP18 (gene CAMP). It is an endogenous component of innate immunity, produced by neutrophils, epithelial cells, keratinocytes, and other tissues, where it acts as both a broad-spectrum antimicrobial and a host-defense signaling molecule.

VIP (vasoactive intestinal peptide, also called vasoactive intestinal polypeptide) is a 28-amino-acid neuropeptide first isolated from porcine intestine in the early 1970s. It is highly conserved across mammals and belongs to the secretin/glucagon peptide superfamily, sharing close homology with PACAP (pituitary adenylate cyclase-activating polypeptide). It is widely distributed in the central and peripheral nervous systems and in the gut, and is notably enriched in lung tissue. The synthetic form used in human trials is known as aviptadil.

As a cationic amphipathic peptide, LL-37 binds anionic microbial membranes and disrupts them (a detergent-like/toroidal-pore mechanism), giving broad activity against Gram-positive and Gram-negative bacteria, some fungi, and enveloped viruses, plus the ability to neutralize LPS and disrupt biofilms. Beyond direct killing, it is strongly immunomodulatory: it is chemotactic for neutrophils, monocytes, and T cells (acting partly via the FPR2/FPRL1 receptor), promotes angiogenesis and keratinocyte migration and proliferation (relevant to wound re-epithelialization), and modulates Toll-like-receptor and inflammatory signaling. A double-edged feature is that LL-37 can bind self-DNA/RNA and convert it into a potent activator of plasmacytoid dendritic cells via TLR9/TLR7, a pathway implicated in psoriasis and other autoimmunity.

VIP signals through two class B G-protein-coupled receptors, VPAC1 and VPAC2, both of which bind VIP and PACAP with high affinity (a related receptor, PAC1, is PACAP-selective). Receptor activation couples primarily to adenylate cyclase, raising intracellular cAMP, with secondary coupling to phospholipase C in some contexts. Downstream effects include smooth-muscle relaxation and vasodilation, bronchodilation, stimulation of exocrine and electrolyte secretion, glucose-dependent insulin secretion (largely via VPAC2), and broad immunomodulatory/anti-inflammatory actions that shift cytokine balance away from pro-inflammatory mediators. In the lung, VIP receptors are concentrated on alveolar type II cells, which is part of the rationale for studying it in acute lung injury.

Direct human interventional evidence is limited and centers on chronic wounds. A multicentric, prospective, randomized, placebo-controlled trial evaluated topical LL-37 in hard-to-heal venous leg ulcers (Gronberg et al., Wound Repair Regen, 2021, PMID 34687253), building on an earlier safety/efficacy study reporting improved healing of venous leg ulcers (PMID 25041740); these are small, wound-specific studies rather than large confirmatory trials. The great majority of the LL-37 literature is mechanistic, in vitro, or animal-based: for example, cathelicidin's protective role against urinary-tract infection was shown in mice and human cells (Chromek et al., Nat Med, 2006, PMID 16751768). Much of the peptide's purported breadth (antiviral, anticancer, antibiofilm, metabolic effects) remains preclinical, and endogenous LL-37 biology should not be conflated with proven benefit from administering exogenous LL-37 in humans. Honest summary: human efficacy data exist mainly for topical chronic-wound healing and remain early-stage; systemic therapeutic use is not established.

Human evidence is mixed and, for most systemic indications, limited. The strongest, most rigorous human data come from TESICO (ACTIV-3b), a randomized, placebo-controlled trial of intravenous aviptadil in COVID-19-associated hypoxaemic respiratory failure published in Lancet Respiratory Medicine (2023); it enrolled ~471 randomized participants and was stopped for futility, finding no improvement in clinical outcomes versus placebo. The earlier enthusiasm rested largely on small open-label series and preclinical/animal models of acute lung injury and cytokine suppression, which did not translate into proven benefit in the controlled setting. The best-established human use is local, not systemic: aviptadil combined with phentolamine (Invicorp) is an approved intracavernosal injection for erectile dysfunction in several countries. VIP has also held orphan-drug designations for pulmonary hypertension and sarcoidosis, but those indications lack confirmatory pivotal trial evidence.

LL-37 has a genuinely double-edged profile: the same self-nucleic-acid-binding and dendritic-cell-activating activity that aids host defense is mechanistically implicated in autoimmune and inflammatory disease, and LL-37 is recognized as an autoantigen targeted by T cells in psoriasis. Elevated or dysregulated cathelicidin has also been linked to rosacea, lupus, atherosclerosis, and a context-dependent (pro- or anti-) role in cancer, so effects are highly tissue- and concentration-dependent. At higher concentrations the peptide can be cytotoxic and hemolytic to host cells, and it can be degraded or inactivated by serum and proteases, complicating systemic delivery. Human safety data are essentially confined to localized topical wound use; the safety of exogenous systemic administration in humans is not established.

As an endogenous vasodilator, systemic VIP/aviptadil can cause hypotension, flushing, and diarrhea, and intravenous infusion has been associated with these effects in trials. Excess endogenous VIP, as seen in VIPoma tumors, produces severe secretory (watery) diarrhea, hypokalemia, and dehydration, illustrating its potent secretory pharmacology. For intracavernosal use, the combination product is reported to carry low rates of penile pain and priapism relative to some other injectables. Long-term safety of systemic administration is not well characterized, and the controlled COVID-19 data did not establish a net clinical benefit; safety beyond the studied settings remains uncertain.

LL-37 is not an FDA-approved drug; it is an endogenous human peptide studied as an investigational and research-use agent. Clinical work has been early-phase and indication-specific (notably topical chronic-wound trials), and no LL-37 product holds general marketing approval.

VIP itself is not an FDA-approved drug; intravenous aviptadil is investigational in the United States and was studied under FDA mechanisms including orphan-drug designations (pulmonary hypertension, sarcoidosis) and emergency COVID-19 trials. The aviptadil/phentolamine combination (Invicorp) is approved for erectile dysfunction in several countries including the UK, Denmark, and New Zealand, but not broadly in the US.