FK-13
The 13-residue FKRIVQRIKDFLR central fragment of LL-37 — the smallest LL-37 fragment that retains broad-spectrum antimicrobial activity while losing much of the parent peptide's cytotoxicity toward mammalian cells. Research-only; no clinical development program.
A 13-residue peptide corresponding to LL-37 residues 17–29 (Phe-Lys-Arg-Ile-Val-Gln-Arg-Ile-Lys-Asp-Phe-Leu-Arg). Characterized as a minimal fragment retaining the core antimicrobial activity of the parent cathelicidin LL-37 while substantially reducing cytotoxicity toward host cells. Widely used as a research tool for dissecting the structure-activity relationship of LL-37 and as a template for developing LL-37-derived antimicrobial peptide therapeutics. Not in clinical development as a therapeutic; not FDA-approved.
Mechanism of action
Forms amphipathic α-helical conformation on membrane contact, with cationic residues (arginines and lysines) binding negatively charged phospholipid headgroups of bacterial membranes and hydrophobic residues partitioning into the lipid bilayer. Causes membrane disruption at high concentrations and likely secondary effects on intracellular targets. The central fragment retains this membrane-targeting mechanism of the parent LL-37 while lacking the N-terminal and C-terminal regions that contribute to LL-37's cytotoxicity on mammalian cells; this shifts the therapeutic index toward bacterial selectivity. Also retains chemotactic activity on neutrophils and monocytes.
Primary uses
- Antimicrobial peptide research (in vitro and in vivo microbiology)
- LL-37 structure-activity research
- Template for AMP drug discovery
Typical dosing
⚠ No human dosing established. Any human use would be entirely unregulated and unsupported by any clinical evidence.
Regulatory status
Not FDA-approved. Not in any registered clinical trial. Research peptide only, available from peptide-synthesis vendors.
Status as of this entry's last review, October 4, 2026. Approvals, compounding rules and sport rules change; the regulatory tracker is checked more often.
What we checked
FK-13 is a 13-residue central fragment of LL-37 that keeps broad antimicrobial activity in laboratory tests. This section records what can be checked.
Identity, computed
| Sequence | Phe-Lys-Arg-Ile-Val-Gln-Arg-Ile-Lys-Asp-Phe-Leu-Arg (FKRIVQRIKDFLR) |
| Formula | C80H135N25O17 |
| Average mass | 1719.12 g/mol |
| Monoisotopic mass | 1718.0468 Da |
| Isoelectric point | 12.24 (net charge +3.98 at pH 7) |
FK-13 is LL-37 residues 17 to 29, Phe-Lys-Arg-Ile-Val-Gln-Arg-Ile-Lys-Asp-Phe-Leu-Arg, one residue longer than KR-12; values below are for the free acid.
Regulatory status, read October 5, 2026
Drugs@FDA: 0 records for “FK-13”. EMA medicines list: 0 records for “FK-13”. Run for semaglutide, the same queries return 6 and 5 records (and the registry search 84 trials), so a zero here is an absence, not a broken search. Not named on the WADA 2026 and 2027 Prohibited Lists. As a substance with no current approval for human therapeutic use, it falls under section S0 (non-approved substances), which is prohibited at all times; the lists were read on September 27, 2026.
What the maker or sellers claim
Claim, from research-chemical sellers: FK-13 is an antimicrobial peptide for human use. It has been studied in the laboratory only.
About LL-37, its parent peptide
The 2006 structural study that mapped LL-37's fragments by NMR, cited in References, is where FK-13 comes from. Our LL-37 and KR-12 entries cover the parent and the shorter fragment.
What we looked for and did not find
No human study, no registry record and no dose from a controlled human study.
| Source | Query | Results, October 5, 2026 | What it was |
|---|---|---|---|
| PubMed | FK-13 LL-37 | 10 | fragment papers |
| ClinicalTrials.gov | FK-13 | 0 | |
| Drugs@FDA (openFDA) | FK-13 | 0 | |
| EMA medicines list | FK-13 | 0 |
References
- [pubmed] Li X, et al. "Solution structures of human LL-37 fragments and NMR-based identification of a minimal membrane-targeting antimicrobial and anticancer region." J Am Chem Soc, 2006;128:5776-5785. PMID: 16637646.
- [pubmed] Wang G. "Structures of human host defense cathelicidin LL-37 and its smallest antimicrobial peptide KR-12 in lipid micelles." J Biol Chem, 2008;283:32637-32643. PMID: 18818205.
- [review] Kościuczuk EM, et al. "Cathelicidins: family of antimicrobial peptides. A review." Mol Biol Rep, 2012;39:10957-10970. PMID: 23065264.
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Guides & tools
This entry is for educational purposes only and does not constitute medical advice. Dosing information reflects published regulatory or research data and is not a recommendation. Many compounds described here are not approved for human use in the United States. Consult a licensed medical professional before considering any peptide therapy.