Abstract: The present invention features compositions and methods for enhancing endosomal escape of therapeutic agents in the cytoplasm of a cell to thereby provide more effective therapy. The endocytic pathway is the major uptake mechanism of cells, and biological agents such as proteins, DNA, and siRNA become entrapped in endosomes and subsequently degraded by lysosomal enzymes. The present invention facilitates endosomal escape by providing fusion proteins with membrane disruptive activity. In some embodiments, the fusion protein has endosomolytic activity that is activated at endosomal or lysosomal pH. In other embodiments, the fusion protein has reversible and pH sensitive membrane disruptive activity so that once internalized into the endosomal or lysosomal compartments, the resulting reduction in pH causes the pH sensitive fusion protein to activate and disrupt the endosomal or lysosomal membrane.

Abstract: The present invention features compositions and methods for delivering a therapeutic agent to the cytoplasm of a cell. We have developed, inter alia, a system in which two or more distinct moieties—at least one therapeutic moiety and at least one potentiating moiety—selectively target and specifically bind cell surface molecules that are then internalized to an intracellular, membrane-bound compartment, such as an endosome. In some embodiments, as discussed further below, a third moiety that induces clustering of the targeted cell surface molecule can also be employed. Regardless of whether the compositions and methods include two or three moieties, the therapeutic agent can be any agent one wishes to deliver to the cytoplasm of a cell, and the potentiating agent can be any agent that destabilizes the intracellular, sub-cellular compartment in which the therapeutic agent is sequestered. The potentiating moiety can include, for example, a lytic agent (i.e.