The potential of reconstituted Sendai viral envelopes containing only the fusion protein (F‐virosomes) was evaluated for a targeted cytosolic delivery of lysozyme to human hepatoblastoma cells (HepG2) in culture. 125I‐Lysozyme loaded into F‐virosomes was used to monitor its fusion‐mediated transfer to the HepG2 cells. Using fusion assay based on the transfer of water soluble probe, we have demonstrated the existence of aqueous connection between F‐virosomes and target cells. Target specificity of the F‐virosomes was ensured by the strong interaction between terminal β‐galactose moiety of F protein and the asialoglycoprotein receptor on the membrane of HepG2 cells. Incubation of the loaded F‐virosomes with cells resulted in fusion‐mediated injection, as inferred from the ability of cells to internalize lysozyme in the presence of azide (an inhibitor of the endocytotic process). Binding as well as fusion of the F‐virosomes to HepG2 cells was solely mediated by the F protein. Introduction of 125I‐lysozyme into the HepG2 cells was confirmed by selective accumulation of acid and anti‐body‐precipitable radioactivity in the cytosolic compartment. The structural integrity of the internalized lysozyme was also assessed. The potential usefulness of F‐virosomes with defined specificities as biological carrier for both in vitro and in vivo cytosolic delivery of macromolecules and drugs has been established.
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