Crossing the cellular membrane and delivering active pharmaceuticals or biologicals into the cytosol of cells is an essential step in the development of nanomedicines. One of the most important intracellular processes regarding the cellular uptake of biologicals is the endolysosomal pathway. Sophisticated nanocarriers are developed to overcome a major hurdle, the endosomal entrapment, and delivering their cargo to the required site of action. In parallel, in vitro assays are established analyzing the performance of these nanocarriers. Among them, the release of the membrane‐impermeable dye calcein has become a popular and straightforward method. It is accessible for most researchers worldwide, allows for rapid conclusions about the release potential, and enables the study of release mechanisms. This review is intended to provide an overview and guidance for scientists applying the calcein release assay. It comprises a survey of several applications in the study of endosomal escape, considerations of potential pitfalls, challenges, and limitations of the assay, and a brief summary of complementary methods. Based on this review, it is hoped to encourage further research groups to take advantage of the calcein release assay for their own purposes and help to create a database for more efficient cross‐correlations between nanocarriers.

Front Cover: In article number 2200167, Johannes C. Brendel, Anja Traeger, and co‐workers discuss the release of the green fluorescent dye calcein from endosomes, used to study one of the most important intracellular barriers for the uptake of biological substances like genetic material. It includes an overview of various applications as well as the challenges of readily available and commonly used methods.

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Cationic pH‐responsive polymers promise to overcome critical challenges in cellular delivery. Ideally, the polymers become selectively charged along the endosomal pathway disturbing only the local membrane and avoiding unintended interactions or cytotoxic side effects at physiological conditions. Polypiperazines represent a novel, hydrophilic class of pH‐responsive polymers whose response can be tuned within the relevant pH range (5–7.4). The authors discovered that the polypiperazines are effectively binding plasmid DNA (pDNA) and demonstrate high efficiency in transfection. By design of experiments (DoE), a wide parameter space (pDNA and polymer concentration) is screened to identify the range of effective concentrations for transfection. An isopropyl modified polypiperazine is highly efficient over a wide range of concentrations outperforming linear polyethylenimine (l‐PEI, 25 kDa) in regions of low N*/P ratios. A quantitative polymerase chain reaction (qPCR) surprisingly revealed that the pDNA within the piperazine‐based polyplexes can be amplified in contrast to polyplexes based on l‐PEI. The pDNA must therefore be more accessible and bound differently than for other known transfection polymers. Considering the various opportunities to further optimize their structure, polypiperazines represent a promising platform for designing effective soluble polymeric vectors, which are charge‐neutral at physiological conditions.