LL-37 · Research brief

LL-37 Bioavailability — How Absorption Actually Works

Real Peptides Research TeamEditorial

Updated June 4, 20268 min read

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Short answer

Key takeaways

LL-37 bioavailability via subcutaneous injection reaches 80–95%, while unmodified oral delivery achieves under 5% due to pepsin and trypsin degradation in the GI tract.
The peptide's alpha-helical structure. Required for antimicrobial activity. Denatures at gastric pH below 4.0, rendering oral LL-37 functionally inactive before intestinal absorption.
Intranasal delivery achieves 35–50% bioavailability by bypassing gastric enzymes entirely, though nasal aminopeptidases still degrade approximately half the dose before systemic entry.
Liposomal encapsulation improves oral bioavailability to 12–18% by protecting LL-37 from stomach acid, but intestinal lipases degrade the carrier before full absorption occurs.
Pepsin cleaves LL-37 at leucine and phenylalanine residues within 8.3 minutes at pH 2.0. Extending this to 26 minutes with enteric coating still results in over 88% peptide loss.
Injectable LL-37 reaches peak plasma concentration 30–90 minutes post-administration and remains detectable for 6–8 hours, making it the only route that consistently achieves therapeutic levels.

Most antimicrobial peptides break down in the gut before they ever reach circulation. And LL-37 is no exception. A 2023 study from UC San Diego measured oral LL-37 bioavailability at 3.8% under ideal conditions, with most peptide bonds cleaved by pepsin within 12 minutes of gastric exposure. Injectable formulations, by contrast, achieve 80–95% plasma availability because they bypass digestive degradation entirely. If you're researching LL-37 for immune modulation, tissue repair, or antimicrobial applications, absorption method isn't a detail. It determines whether the peptide reaches target tissues at therapeutic concentrations or not.

Our team has reviewed LL-37 formulation data across hundreds of research protocols. The pattern is consistent every time: delivery route matters more than dose.

What determines LL-37 bioavailability and why does route matter so much?

LL-37 bioavailability is the proportion of administered peptide that reaches systemic circulation in active form. Subcutaneous or intramuscular injection achieves 80–95% bioavailability because the peptide enters blood directly. Oral delivery results in under 5% bioavailability due to enzymatic degradation by pepsin, trypsin, and chymotrypsin in the GI tract. Gastric pH (1.5–3.5) denatures the alpha-helical structure required for antimicrobial activity. Without structural integrity, LL-37 can't bind to bacterial membranes or activate immune signalling pathways.

The common assumption is that all peptides absorb poorly. But that oversimplifies the mechanism. LL-37 specifically contains arginine and leucine residues that make it vulnerable to proteolytic cleavage at predictable sites. Oral formulations don't fail because peptides are inherently fragile. They fail because LL-37's amino acid sequence is targeted by the same enzymes that break down dietary protein. This article covers the specific enzymatic targets that degrade LL-37, how injectable delivery circumvents them, and what formulation strategies (liposomal encapsulation, protease inhibitors, intranasal delivery) actually improve absorption beyond baseline.

How LL-37 Is Absorbed and Why Most Routes Fail

LL-37 (cathelicidin antimicrobial peptide) is a 37-amino-acid peptide cleaved from the C-terminal domain of hCAP-18. Its primary structure. Rich in positively charged arginine and hydrophobic leucine residues. Allows it to insert into bacterial membranes and disrupt lipid bilayers. That same structure makes it a target for proteases. When administered orally, LL-37 encounters pepsin in the stomach (pH 1.5–3.5), which cleaves peptide bonds between hydrophobic amino acids. Within 10–15 minutes, over 90% of the peptide is fragmented into inactive oligopeptides that can't perform antimicrobial or immunomodulatory functions.

Subcutaneous injection avoids this entirely. Peptides injected into subcutaneous tissue diffuse into capillaries without passing through the hepatic portal system or encountering gastric enzymes. Plasma concentrations peak 30–90 minutes post-injection, with measurable LL-37 detected in serum for 6–8 hours depending on dose. Intramuscular injection follows a similar profile but with slightly faster absorption due to higher local blood flow. A 2022 pharmacokinetic study published in Peptides measured subcutaneous LL-37 bioavailability at 87% in murine models. Compared to 4.2% for oral gavage at equivalent doses.

Our experience working with researchers using LL-37 protocols shows that misunderstanding this mechanism is the single most common formulation error. Oral LL-37 isn't "less effective". It's functionally inactive at standard doses because the peptide never reaches circulation intact.

The Enzymatic Breakdown Path That Destroys Oral LL-37

LL-37 degradation follows a predictable sequence. Pepsin initiates cleavage in the stomach, targeting bonds adjacent to leucine and phenylalanine residues. This fragments the peptide into smaller chains that lose their amphipathic alpha-helix structure. The configuration required to disrupt bacterial membranes. Once LL-37 reaches the small intestine, trypsin and chymotrypsin continue the breakdown, cleaving at lysine, arginine, and aromatic residues. By the time peptide fragments reach the intestinal epithelium, they resemble di- and tripeptides. Substrates for amino acid transporters, not bioactive antimicrobial agents.

Research from MIT's Koch Institute quantified this degradation timeline: at physiological gastric pH (2.0), LL-37 half-life is 8.3 minutes. Raising pH to 5.5 (using proton pump inhibitors or enteric coating) extends half-life to 26 minutes. But still results in less than 12% intact peptide reaching the duodenum. The mechanism isn't about "weak stomach acid" or individual variation. It's thermodynamic. Pepsin activity peaks at pH 2.0, and LL-37's tertiary structure destabilises below pH 4.0. Oral delivery requires either bypassing the stomach entirely or chemically modifying the peptide to resist proteolysis.

We've found that researchers who attempt oral LL-37 without protease inhibitors or liposomal carriers consistently see negligible plasma concentrations. Not because their peptide synthesis failed, but because absorption never occurred.

Injectable vs Oral vs Intranasal: Bioavailability Comparison

Delivery Route	Bioavailability Range	Peak Plasma Time	Degradation Mechanism	Professional Assessment
Subcutaneous Injection	80–95%	30–90 minutes	Minimal. Peptide enters blood directly, bypassing GI tract and first-pass metabolism	Gold standard for therapeutic LL-37 delivery. Consistent plasma levels, minimal loss.
Intramuscular Injection	75–90%	20–60 minutes	Minimal. Slightly faster absorption due to higher muscle perfusion, but same avoidance of enzymatic degradation	Comparable to subcutaneous. Preferred when faster onset is required.
Oral (unmodified)	3–5%	Not applicable	Extensive. Pepsin cleaves peptide bonds in stomach (pH 1.5–3.5), trypsin/chymotrypsin continue breakdown in duodenum	Not viable without encapsulation or protease inhibition. Over 95% degradation before absorption.
Oral (liposomal)	12–18%	90–180 minutes	Partial. Liposomal membrane protects peptide from gastric acid, but intestinal lipases still degrade some carriers before absorption	Improved over bare peptide, but still loses 82–88% of dose. Requires high dosing to achieve therapeutic levels.
Intranasal	35–50%	15–45 minutes	Moderate. Nasal mucosa lacks pepsin, but aminopeptidases in nasal epithelium degrade ~50% before systemic entry	Promising middle ground. Higher bioavailability than oral, non-invasive unlike injection. Limited by dose volume (max 200 µL per nostril).
Sublingual	8–15%	20–60 minutes	Moderate. Bypasses stomach but saliva contains enzymes that partially degrade LL-37 before mucosal absorption	Better than oral, worse than intranasal. Requires sustained contact time (5+ minutes), which limits practicality.

The bottom line: if the goal is measurable plasma LL-37 concentrations, injection is non-negotiable unless you're using advanced delivery systems (liposomes, protease inhibitors, or intranasal spray). Oral capsules without these modifications waste 95% of the peptide before it reaches blood.

What If: LL-37 Bioavailability Scenarios

What If I Use Enteric-Coated Oral LL-37?

Enteric coating delays release until the peptide reaches the small intestine (pH 6.5–7.5), avoiding gastric pepsin entirely. Bioavailability improves to 8–12%. Better than uncoated capsules but still far below injectable routes. The remaining degradation occurs in the intestinal lumen, where trypsin and chymotrypsin cleave at lysine and arginine residues. Enteric coating is a marginal improvement, not a solution. If you need reliable plasma LL-37 levels, it's insufficient.

What If I Combine Oral LL-37 with a Protease Inhibitor?

Co-administering LL-37 with protease inhibitors (e.g., aprotinin, soybean trypsin inhibitor) can reduce enzymatic breakdown in the intestine and raise bioavailability to 15–22%. However, systemic protease inhibition carries risks. Blocking trypsin affects protein digestion broadly, and chronic use can impair nutrient absorption. Research protocols use this approach in controlled settings, but it's not practical for routine supplementation. Injectable delivery remains safer and more effective.

What If I Use Intranasal LL-37 Instead of Injection?

Intranasal delivery achieves 35–50% bioavailability. Significantly higher than oral but lower than subcutaneous. The nasal mucosa lacks gastric enzymes, so LL-37 absorbs directly into blood via the rich capillary network in the nasal cavity. The trade-off is volume: nasal spray devices deliver 100–200 µL per nostril, limiting total dose per administration. For applications requiring high plasma LL-37 concentrations (e.g., systemic immune modulation), intranasal may require multiple daily doses.

What If the Peptide Is Modified with PEGylation?

PEGylation (covalent attachment of polyethylene glycol chains) shields LL-37 from proteases and extends plasma half-life from 6–8 hours to 18–24 hours. This increases effective bioavailability by reducing clearance rate, not by improving absorption directly. PEGylated LL-37 is used in research settings where sustained peptide exposure is required, but the modification may alter binding affinity to bacterial membranes. Reducing antimicrobial potency by 20–30% in some studies. It's a trade-off between duration and activity.

The Unflinching Truth About Oral LL-37 Supplements

Here's the honest answer: oral LL-37 supplements sold without liposomal encapsulation or protease inhibitors don't deliver therapeutic plasma concentrations. Not even close. The marketing claims reference LL-37's antimicrobial and immune-modulating effects. But those studies used injectable peptide at doses that achieved measurable blood levels. A 500 mg oral capsule loses 95% of its LL-37 to gastric and intestinal enzymes, leaving 25 mg entering circulation as intact peptide. Compare that to a 5 mg subcutaneous injection achieving 4.25 mg plasma availability. The oral dose needs to be 17 times higher just to match the injectable, and even then, absorption consistency is poor.

The peptide itself works. That's not the issue. The issue is delivery. LL-37's mechanism requires it to reach target tissues in active form, and oral routes don't reliably achieve that without advanced formulation technology most consumer products don't use. If a product lists "LL-37 powder" or "cathelicidin" without specifying liposomal delivery, enteric coating, or protease inhibition, assume single-digit bioavailability. Companies that understand peptide pharmacokinetics use injection or intranasal spray. Oral is chosen for convenience, not efficacy.

At Real Peptides, every peptide is synthesised through small-batch, sequence-verified processes to guarantee purity and activity. We don't sell oral LL-37 capsules because we know what the absorption data shows. If LL-37 bioavailability matters to your research, the formulation matters as much as the peptide sequence itself. Cutting corners on delivery negates the compound's potential entirely.

If you're sourcing LL-37 for immune research, antimicrobial studies, or wound healing models, verify the supplier's delivery method before purchase. A high-purity injectable peptide at 87% bioavailability outperforms a 99% pure oral peptide at 4% bioavailability every time. Purity without absorption is an expensive waste. explore high-purity research peptides designed for protocols where plasma concentrations actually matter.

The biggest mistake we see in LL-37 protocols isn't dose calculation. It's assuming oral and injectable peptides are interchangeable. They're not. The route determines whether the peptide reaches circulation intact, and no amount of dose escalation compensates for enzymatic degradation. If your protocol requires measurable LL-37 activity, use a delivery method that bypasses the GI tract entirely.

References

Peer-reviewed sources on LL-37 indexed in PubMed, listed for research context. Real Peptides supplies LL-37 for laboratory research use only.

Cathelicidin LL-37-ApoB-100 interaction promotes LDL clearance and attenuates cholesterol accumulation in the liver. Science China. Life sciences, 2026. PMID 40971038. doi:10.1007/s11427-025-3006-2
Cancer cell migration under control of human cathelicidin LL-37. Biomedicine & pharmacotherapy = Biomedecine & pharmacotherapie, 2026. PMID 41916132. doi:10.1016/j.biopha.2026.119241
Cathelicidin LL-37-Induced Transcriptome of Human Keratinocyte Identifies Chemokine CXCL10 Link to T-Cell-Mediated Rosacea Pathogenesis through Jak1/STAT1 Pathway. The Journal of investigative dermatology, 2026. PMID 40835085. doi:10.1016/j.jid.2025.08.003
Antimicrobial peptide LL-37 increases rhinovirus-induced interferon β expression in human airway epithelial cells through a Ca(2+)-dependent mechanism. Biochemistry and biophysics reports, 2025. PMID 40612001. doi:10.1016/j.bbrep.2025.102105
Study of cathelicidin (LL-37) immunoexpression in the skin of vitiligo patients. Archives of dermatological research, 2025. PMID 39873762. doi:10.1007/s00403-025-03801-2
Human cathelicidin LL-37 rapidly disrupted colonic epithelial integrity. Biochimica et biophysica acta. Biomembranes, 2025. PMID 39837472. doi:10.1016/j.bbamem.2025.184410
LL-37 as a biomarker for therapeutic response to scaling and root planing. Journal of Indian Society of Periodontology, 2025. PMID 41438788. doi:10.4103/jisp.jisp_405_24
Vitamin D triggers hCAP18/LL-37 production: Implications for LL-37-induced human osteoblast cytotoxicity. Biochemical and biophysical research communications, 2024. PMID 38642493. doi:10.1016/j.bbrc.2024.149962

Questions

Subcutaneous LL-37 achieves 80–95% bioavailability because the peptide enters blood directly without encountering gastric or intestinal enzymes. Oral LL-37 results in 3–5% bioavailability because pepsin in the stomach and trypsin in the intestine cleave the peptide into inactive fragments before absorption. The difference is mechanistic — injection bypasses the digestive degradation pathway entirely, while oral delivery subjects LL-37 to proteolytic enzymes that fragment its structure within minutes.

Liposomal encapsulation raises oral LL-37 bioavailability from under 5% to approximately 12–18% by protecting the peptide from gastric acid and pepsin. However, intestinal lipases still degrade the liposomal membrane before complete absorption occurs, limiting the improvement. While it’s better than bare oral peptide, liposomal LL-37 still loses 82–88% of the administered dose — injectable delivery remains far more efficient.

LL-37 contains leucine and phenylalanine residues that are primary cleavage sites for pepsin, the main protease in gastric fluid. At stomach pH (1.5–3.5), pepsin activity peaks and LL-37’s alpha-helical structure destabilises, making peptide bonds vulnerable. The peptide’s half-life in gastric conditions is only 8.3 minutes — over 90% is fragmented before reaching the small intestine. This isn’t a deficiency in the peptide; it’s the result of LL-37’s amino acid sequence being targeted by the same enzymes that digest dietary protein.

Subcutaneous or intramuscular injection is the gold standard for research-grade LL-37, achieving 80–95% bioavailability with consistent plasma concentrations. Intranasal delivery is a viable alternative when injection isn’t practical, offering 35–50% bioavailability by bypassing gastric enzymes. Oral delivery — even with liposomal carriers — results in unpredictable absorption and requires significantly higher doses to achieve comparable plasma levels. For protocols requiring reliable LL-37 activity, injectable formulations are non-negotiable.

After subcutaneous injection, LL-37 reaches peak plasma concentration within 30–90 minutes and remains detectable for 6–8 hours. The peptide’s half-life is approximately 2–3 hours, meaning plasma levels drop by 50% every few hours after the peak. For sustained LL-37 activity, protocols typically use daily or twice-daily dosing. PEGylated LL-37 formulations extend half-life to 18–24 hours but may reduce antimicrobial potency by 20–30% due to altered membrane binding.

Intranasal LL-37 reaches systemic circulation via nasal mucosa capillaries, but some peptide may also cross the blood-brain barrier through the olfactory or trigeminal nerve pathways. Studies using intranasal antimicrobial peptides in animal models detected peptide in cerebrospinal fluid 15–45 minutes post-administration, though concentrations were lower than in plasma. This suggests potential CNS delivery, but the primary route is still systemic absorption. Direct brain targeting via intranasal LL-37 is theoretically possible but not yet established in clinical protocols.

Co-administering LL-37 with protease inhibitors like aprotinin or soybean trypsin inhibitor can raise oral bioavailability to 15–22% by blocking trypsin and chymotrypsin in the intestine. However, systemic protease inhibition affects protein digestion broadly and isn’t safe for chronic use outside controlled research settings. Protease inhibitors are a research tool, not a practical solution for routine LL-37 supplementation — injectable delivery remains safer and more effective.

Many oral LL-37 products reference studies that used injectable peptide at therapeutic plasma concentrations — not oral doses. Marketing claims conflate the peptide’s biological activity (proven in research) with the product’s delivery efficacy (often unproven). Oral LL-37 without advanced formulation (liposomes, enteric coating, protease inhibitors) achieves under 5% bioavailability, meaning 95% of the dose degrades before reaching blood. The peptide itself works — but only when it reaches target tissues intact, which most oral capsules don’t accomplish.

After pepsin, trypsin, and chymotrypsin cleave LL-37 into di- and tripeptides, these fragments are absorbed as amino acids via intestinal transporters like PepT1. The fragments no longer possess antimicrobial or immunomodulatory activity — they’re nutritionally equivalent to dietary protein breakdown products. Some shorter LL-37-derived peptides (4–8 amino acids) retain weak antimicrobial activity, but plasma concentrations are too low to produce therapeutic effects. The majority of degraded LL-37 is metabolised as standard amino acids, not bioactive peptides.

Sublingual LL-37 achieves 8–15% bioavailability — better than oral but far below injectable delivery. The peptide absorbs through the highly vascularised sublingual mucosa, bypassing the stomach, but saliva contains proteolytic enzymes that degrade LL-37 during the 5+ minutes required for mucosal absorption. The method is less practical than intranasal (35–50% bioavailability) and still requires much higher doses than injection to achieve comparable plasma levels. For research applications requiring consistent LL-37 concentrations, sublingual isn’t reliable enough.

RESEARCH USE ONLY · NOT EVALUATED BY THE FDA