Modified Cyclic Peptides Can Enter Cells Despite Low Passive Membrane Permeability | Research

The takeaway

Cyclic peptides containing oxadiazole groups showed high cell penetration through a non-passive mechanism, offering a new framework for designing peptide drugs that can reach intracellular targets.

High penetration, low permeability
Oxadiazole cyclic peptides entered cells efficiently but showed poor passive membrane permeability, suggesting active cellular uptake drives their cell-penetrating ability

What the researchers found

Oxadiazole-containing (Odz) cyclic peptides showed high cell penetration that depended on the position of specific side chains and the chloroalkane tag used for detection. NMR analysis revealed these macrocycles adopt a β-turn conformation. Intriguingly, despite high cell penetration in living cells, they showed low passive permeability in artificial membrane assays.

This discrepancy suggests these cyclic peptides enter cells through an active or energy-dependent mechanism rather than simply diffusing through cell membranes — an important finding for understanding and designing cell-penetrant macrocyclic peptides.

Why it matters

Getting peptide drugs inside cells is one of the biggest challenges in drug development. Cyclic peptides are promising drug candidates because of their stability and ability to block protein-protein interactions, but most cannot cross cell membranes. This study shows that incorporating oxadiazole groups into cyclic peptides can enable cell entry, and the finding that entry occurs through a non-passive mechanism opens new strategies for designing membrane-permeable peptide therapeutics.

The numbers in context

β-turn conformation confirmed by NMR · High cell penetration observed · Low passive permeability on artificial membranes · Side chain position-dependent activity

How the study worked

Researchers synthesized oxadiazole-containing cyclic peptides with varying side chain positions and chloroalkane tags. Cell penetration was measured using the Chloroalkane Penetration Assay (CAPA) in living cells. Passive permeability was tested on artificial membranes (PAMPA or similar). Solution NMR spectroscopy was used to determine the three-dimensional conformation of the cyclic peptides.

Who was studied

In vitro cell penetration and artificial membrane permeability studies

What this study cannot tell us

The study focused on a specific structural framework (oxadiazole-containing macrocycles), and results may not generalize to all cyclic peptides. The mechanism of active cell entry was not fully elucidated. No biological activity or target engagement was demonstrated — only cell penetration. The chloroalkane tag required for the assay may influence the peptides' behavior.

How to read the evidence

This is a basic research study characterizing the cell penetration properties of a novel peptide scaffold. It is early-stage work focused on biophysical properties rather than therapeutic application.

When this study was published

Published in 2024, this is very recent work contributing to the active field of cell-penetrant cyclic peptide design. The findings may inform future drug design efforts.

The bigger picture

The pharmaceutical industry is increasingly interested in cyclic peptides as a drug class that bridges the gap between small molecules and biologics. A major hurdle is cell permeability. This study contributes to solving that problem by showing that oxadiazole modifications can enable cell entry and — importantly — that high cell penetration doesn't always require passive membrane permeability, challenging a common assumption in the field.

Questions still open

What is the specific active mechanism by which these oxadiazole cyclic peptides enter cells?
Can this framework be applied to create cell-penetrant cyclic peptides that target specific intracellular proteins?
How does the chloroalkane tag affect the observed cell penetration — would untagged peptides behave similarly?

Common questions

Why is cell penetration so important for peptide drugs?

Many disease-causing proteins are located inside cells, where they are inaccessible to most peptide drugs. Peptides are generally too large and water-loving to cross cell membranes on their own. Finding ways to make cyclic peptides cell-penetrant — as shown with oxadiazole modifications — could unlock a vast number of previously 'undruggable' targets.

What does it mean that these peptides have high cell penetration but low passive permeability?

It means the peptides aren't simply diffusing through cell membranes like small-molecule drugs do. Instead, cells appear to actively transport them inside, possibly through endocytosis or another energy-dependent process. This is surprising because drug designers often optimize for passive permeability, but this study suggests there's an alternative path to cell entry.

Read the original research

Cell penetration of oxadiazole-containing macrocycles.

RSC chemical biology, 5(4), 328-334

View on PubMed ↗Publisher’s paper ↗

Citation

Huh, Sungjoon; Batistatou, Nefeli; Wang, Jing; Saunders, George J; Kritzer, Joshua A; Yudin, Andrei K. (2024). Cell penetration of oxadiazole-containing macrocycles.. RSC chemical biology, 5(4), 328-334. https://doi.org/10.1039/d3cb00201b