Description:
<jats:title>Abstract</jats:title><jats:p>We report a comprehensive study on novel, highly efficient, and biodegradable hybrid molecular transporters. To this end, we designed a series of cell‐penetrating, cube‐octameric silsesquioxanes (COSS), and investigated cellular uptake by confocal microscopy and flow cytometry. A COSS with dense spatial arrangement of guanidinium groups displayed fast uptake kinetics and cell permeation at nanomolar concentrations in living HeLa cells. Efficient uptake was also observed in bacteria, yeasts, and archaea. The COSS‐based carrier was significantly more potent than cell‐penetrating peptides (CPPs) and displayed low toxicity. It efficiently delivered a covalently attached cytotoxic drug, doxorubicin, to living tumor cells. As the uptake of fluorescently labeled carrier remained in the presence of serum, the system could be considered particularly attractive for the in vivo delivery of therapeutics.</jats:p>