Anionic Calixarenes in Biomembrane Transport of Peptides

Biomembranes function as hydrophobic barriers for hydrophilic substances enabling compartmentalization in biological systems. This poses, however, a problem for the targeted introduction of cargo into cells. The result is a high demand for delivery pathways into cells with the goal to investigate biological processes or to treat diseases by improved delivery. Polycationic cell-penetrating peptides (CPPs) are interesting as they can cross cell membranes and transport attached cargos directly into the cytosol. Their efficiency can be improved by anionic amphiphilic counterion activators, which bind to the CPPs to form charge-neutralized counterion-CPP complexes with sufficient hydrophobicity to cross the lipid bilayer membrane. This review summarizes recent results, which establish amphiphilic calixarenes as a new class of biocompatible and non-cytotoxic counterion activators with very high transport activities at nanomolar concentrations. We also include a brief summary of fluorescence-based assays with large unilamellar vesicles (LUVs) to investigate counterion-activated transport. Current methods use liposome-encapsulated, supramolecular host-dye reporter pairs including calixarenes, which provide new mechanistic insights and enable rapid in vitro identification of suitable activators. Taken together, amphiphilic calixarenes are currently emerging as prime candidates for counterion activation of membrane transport, which are highly modifiable and can be specifically tailored towards different cargoes and membrane types.