Smart drug delivery systems for potential targeted cancer therapy: Exploiting increased glutathione levels in tumor microenvironments Over the past decades, nanoparticulate drug carrier systems have emerged as promising tools in medicine. A persistent challenge in current pharmacotherapy is the limited selectivity of active pharmaceutical ingredients, resulting in undesirable side effects. Smart drug delivery systems, which release encapsulated active pharmaceutical ingredients in response to specific stimuli, offer a potential solution by enabling controlled drug release. This approach can be particularly relevant for exploiting biochemical differences between extracellular and intracellular environments. In this study, self-immolative polydisulfide based polymers manufactured from dithiothreitol were processed into nanoparticle formulations to respond preferentially to elevated glutathione levels, which are characteristic of intracellular environments and are often increased in tumor cells. The influence of polymer chain length on the physicochemical properties of the resulting nanoparticles was investigated. Lumogen® Red was incorporated as a model substance to determine the loading capacity of the carrier system. Degradation was characterized using dynamic light scattering and asymmetric flow field-flow fractionation, as well as by imaging techniques such as atomic force microscopy. Selective release of the embedded substance was demonstrated at elevated glutathione concentrations, while no significant release was observed at extracellularly relevant levels (10 µM glutathione), where the behavior was comparable to the buffer control. Increased release was observed under intracellularly relevant conditions (2 – 10 mM glutathione). These findings support a redox-responsive behavior under intracellular-like conditions. The latter was proven for primary fibroblasts and the cancer cell lines BT-474, MCF-7 and SK-BR-3 by quantification of intracellular low molecular weight thiols. The nanoparticle uptake was confirmed in the investigated cell lines by visualization via confocal laser scanning microscopy. Via lysosomal staining it was shown that nanoparticles accumulate in lysosomes. Furthermore, the carrier system itself showed no cytotoxic properties in cell culture studies against the four different cell types. The developed system is a suitable and very promising smart drug delivery system in the context of controlled drug release. 9 Elsevier BV