# Cytosolic Delivery of Bioactive Cyclic Peptide Cargo by Spontaneous Membrane Translocating Peptides

## TL;DR

Scientists tested special peptides that can carry cyclic peptides into cells, showing they can deliver a bioactive cyclic peptide to the cytosol, which could help develop new drugs.

## Contribution

The study demonstrates that synthetically evolved peptides can deliver cyclic peptides into the cytosol, opening new possibilities for drug development.

## Key findings

- Three SMTPs successfully delivered phalloidin to the cytosol of human cells.
- One SMTP achieved delivery at concentrations as low as 3 μM.
- SMTPs were originally selected for membrane translocation with a fluorescent dye, not cyclic peptides.

## Abstract

Cyclic peptides that
inhibit protein–protein interactions
have significant advantages over linear peptides and small molecules
for modulating cellular signaling networks in cancer and other diseases.
However, the permeability barrier of the plasma membrane remains a
formidable obstacle to the development of cyclic peptides into applicable
drugs. Here, we test the ability of a family of synthetically evolved
spontaneous membrane translocating peptides (SMTPs) to deliver phalloidin,
a representative bioactive cyclic peptide, to the cytosol of human
cells in culture. Phalloidin does not enter cells spontaneously, but
if delivered to the cytosol, it inhibits actin depolymerization. We
thus use a wound-healing cell mobility assay to assess the biological
activity of phalloidin conjugated to three SMTPs that we previously
discovered. All three SMTPs can deliver phalloidin to the cell cytosol,
and one does so at concentrations as low as 3 μM. Delivery occurs
despite the fact that the SMTPs were originally selected based on
membrane translocation with no cargo other than a small fluorescent
dye. These results show that SMTPs are viable delivery vehicles for
cyclic peptides, although their efficiency is moderate. Further, these
results suggest that one additional generation of synthetic molecular
evolution could be used to optimize SMTPs for the efficient delivery
of any bioactive cyclic peptide into cells.