For the week of 28 September 2026, the cyclic-peptide study not used in earlier editions of this series is Dubkara and colleagues’ 22 September 2026 report on lipopeptide–gramicidin S (GS) conjugates. As of a 2 October 2026 literature capture, that abstract gives Gram-negative MIC values, mammalian cytotoxicity and hemolysis ratios, and membrane-permeability experiments as a bactericidal confirmation—not intestinal flux or oral bioavailability. A second September abstract, on the zebrafish phosvitin-derived peptide Pt5-1c, treats permeability as a cGAMP-delivery problem. The two assay systems should not be merged into an uptake-equals-efficacy claim.
Selectivity numbers in the GS conjugate series
The scaffold is GS, a cyclic decapeptide described as acting mainly on Gram-positive bacteria. The abstract notes hemolytic toxicity and a primarily topical use against S. aureus infections of the eye, ear, and throat. The authors state that no clinically significant resistance to GS has been reported to date, which they attribute to a unique mechanism of action (retrieved abstract).
To extend activity toward Gram-negative organisms, the group conjugated cationic amino acid residues and lipopeptides onto the ornithine (Orn) side-chain and prepared 14 peptides. Enhanced activity is reported against A. baumannii, P. aeruginosa and E. coli, with MIC ranging from 2 to 8 μg/mL. Peptides 4, 6, 10 and 11 are described as showing lesser cytotoxicity (CC50/MIC of A. baumannii 5–37) and RBC hemolysis (HC50/MIC of A. baumannii 14–26) than GS. Analogues with improved activity retained activity against twenty-five drug-susceptible and MDR A. baumannii clinical isolates. Bactericidal activity is further supported in the abstract by time-kill kinetics, scanning electron microscopy and membrane permeability experiments (retrieved abstract).
Those ratios therefore compare antibacterial potency with residual mammalian cytotoxicity and hemolysis, using A. baumannii MIC as the denominator. They are not epithelial permeability coefficients. The abstract does not name the membrane-permeability probe, the bacterial strain used in that assay, a time point, or a fold-change versus parent GS, so the experiments cannot be read as Caco-2, PAMPA, or other flux measurements.
“Permeability” in a second September paper means cargo delivery
Zhao et al. start from cGAMP’s high hydrophilicity and polyanionic charge, which they state cause poor membrane permeability and suboptimal bioavailability. They identify Pt5-1c as an immunomodulatory peptide that amplifies cGAMP-mediated signaling by enhancing extracellular availability. Microscale thermophoresis (MST) and LC-MS/MS analyses are reported to show that Pt5-1c directly complexes with cGAMP, facilitating delivery to target cells and triggering a STING-dependent cascade. In in vitro and in vivo models, the authors report a superior interferon response and protection against virus challenges (retrieved abstract).
The abstract does not describe Pt5-1c as cyclic. The permeability claim is facilitated cellular delivery of a dinucleotide second messenger, not passive permeation of a macrocycle. Extracellular complexation is not evidence of oral peptide absorption, and the abstract does not report CC50/MIC or HC50/MIC ratios analogous to the GS series.
What the two abstracts support—and what they do not
| Item | GS conjugates | Pt5-1c plus cGAMP |
|---|---|---|
| First-publication date | 22 September 2026 | 11 September 2026 |
| Architecture in abstract | Cyclic decapeptide conjugates (14 peptides) | Phosvitin-derived antimicrobial peptide; cyclicity not stated |
| Permeability-related method | Membrane permeability experiments (details not in abstract) | MST and LC-MS/MS complexation; cellular delivery |
| Endpoint reported in abstract | MIC 2–8 μg/mL; activity on 25 A. baumannii isolates | STING-dependent interferon response; virus-challenge models |
| Selectivity numbers | CC50/MIC 5–37; HC50/MIC 14–26 for peptides 4, 6, 10, 11 vs A. baumannii | Not reported as CC50/MIC or HC50/MIC |
| Oral or epithelial Papp | Not reported | Not reported |
Illustrative comparison (not a dosing or product claim): a membrane-active cyclic peptide can look “permeable” in a bacterial-killing assay while still showing measurable RBC hemolysis; a peptide that is not described as cyclic can look “permeable” because it shuttles a charged cargo. Those are different physical events.
One specific remaining question is whether the GS membrane-permeability experiments show a quantitative increase in bacterial-membrane disruption relative to parent GS at concentrations below the reported hemolysis window. Without a probe identity, time point, or fold-change in the abstract, the permeability–selectivity trade-off cannot be ranked on a common permeability scale from this record alone.
Checklist for using these abstracts:
- Tie every MIC to the named Gram-negative species and the 2–8 μg/mL range as stated.
- Keep CC50/MIC and HC50/MIC attached to peptides 4, 6, 10 and 11 and to A. baumannii.
- Treat GS membrane-permeability experiments as mechanistic bactericidal evidence, not oral-absorption data.
- Treat Pt5-1c/cGAMP results as complexation-plus-delivery evidence in the reported in vitro and in vivo models.
- Do not infer clinical suitability or human dosing from either paper.
Frequently Asked Questions
Do the GS membrane-permeability experiments measure oral absorption?
No. The 22 September 2026 abstract lists membrane permeability experiments with time-kill kinetics and scanning electron microscopy as support for bactericidal activity. It does not report intestinal flux, Caco-2 or PAMPA values, or oral bioavailability.
Is Pt5-1c described as a cyclic peptide?
The 11 September 2026 retrieved abstract identifies Pt5-1c as a zebrafish phosvitin-derived antimicrobial peptide and does not state that it is cyclic. Its permeability-related claim is complexation and cellular delivery of cGAMP.
Explore Further
Browse our research peptide catalog and review third-party lab reports & COAs for published batches.
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Research use only. The information above is provided for educational and laboratory research purposes only. The compounds discussed are not approved for human or veterinary use, diagnosis, treatment, or the prevention of any disease. Nothing here is medical advice.