Successful cytosolic delivery of nanomaterials is becoming more and more important, given the increase in intracellular applications of quantum dots, gold nanoparticles, liposomal drug formulations and polymeric gene delivery vectors. Most nanomaterials are taken up by the cell via endocytosis, yet endosomal escape has long been recognized as a major bottleneck in cytosolic delivery. Although it is essential to detect and reliably quantify endosomal escape, no consensus has been reached so far on the methods to do so. This review will summarize and discuss for the first time the different assays used to investigate this elusive step to date.

To gain a better understanding of intracellular processing of nanomedicines, we employed quantitative live-cell fluorescence colocalization microscopy to study endosomal trafficking of polyplexes in retinal pigment epithelium cells. A new, dynamic colocalization algorithm was developed, based on particle tracking and trajectory correlation, allowing for spatiotemporal characterization of internalized polyplexes in comparison with endosomal compartments labeled with EGFP constructs. This revealed early trafficking of the polyplexes specifically to Rab5- and flotillin-2-positive vesicles and subsequent delivery to Rab7 and LAMP1-labeled late endolysosomes where the major fraction of the polyplexes remains entrapped for days, suggesting the functional loss of these nanomedicines. Colocalization of polyplexes with the autophagy marker LC3 suggests for the first time that the process of xenophagy could play an important role in the persistent endosomal entrapment of nanomedicines.

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