When lysosomes persist: resolving the proton-sponge paradox in nanoparticle-based intracellular delivery

Proton-sponge-active polymers are widely used in nanomedicine to enhance intracellular delivery, yet the mechanism by which they promote cytosolic release of therapeutic cargo remains under debate. Whether these materials drive complete endolysosomal escape or instead alter lysosomal integrity without full nanoparticle release remains unclear. Here we show that polyethylene imine (PEI), a prototypical proton sponge active polymer, induces lysosomal membrane destabilization rather than full nanoparticle escape. Using PEI-coated mesoporous silica nanoparticles as a model delivery system, we show that PEI promotes cytosolic release of small-molecule cargo while nanoparticles remain confined within membrane-enclosed LAMP1-positive compartments. This behaviour arises from the combination of partial lysosomal membrane permeabilization and lysosomal deacidification, which together enable cargo leakage while impairing detection of lysosomes by pH-dependent probes. Our results resolve a long-standing ambiguity in the nanomedicine field and provide a revised mechanistic framework for interpreting endolysosomal escape in intracellular delivery. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=139 SRC="FIGDIR/small/721565v2_ufig1.gif" ALT="Figure 1"> View larger version (42K): org.highwire.dtl.DTLVardef@74b98org.highwire.dtl.DTLVardef@f405eborg.highwire.dtl.DTLVardef@b0a276org.highwire.dtl.DTLVardef@79f154_HPS_FORMAT_FIGEXP M_FIG C_FIG