20The mechanism of cellular uptake by endocytosis and subsequent oxidative stress has been 21 identified as the paradigm for the toxic response of cationically surface charged nanoparticles. In 22 an attempt to circumvent the process, the effect of increased cellular membrane permeability on 23 the uptake mechanisms of poly (amidoamine) dendrimers generation 4 (G4) and 6 (G6) in vitro 24 was investigated. Immortalised, non-cancerous human keratinocyte (HaCaT) cells were treated 25 with DL-Buthionine-(S,R)-sulfoximine (BSO). Active uptake of the particles was monitored 26 using fluorescence microscopy to identify and quantify endosomal activity and resultant 27 oxidative stress, manifest as increased levels of reactive oxygen species, monitored using the 28 carboxy-H 2 DCFDA dye. Dose dependent cytotoxicity for G4 and G6 exposure was registered 29 using the cytotoxicity assays Alamar Blue and MTT, from 6 to 72 hours. 30Reduced uptake by endocytosis is observed for both dendrimer species. A dramatic change, 31 compared to untreated cells, is observed in the cytotoxic and oxidative stress response of the 32 BSO treated cells. The significantly increased mitochondrial activity, dose dependent anti-33 oxidant behaviour and reduced degree of endocytosis for both dendrimer generations, in BSO 34 treated cells, indicates enhanced permeability of the cell membrane, resulting in the passive, 35 diffusive uptake of dendrimers, replacing endocytosis as the primary uptake mechanism. The 36 complex MTT response reflects the importance of glutathione in maintaining redox balance 37 within the mitochondria. The study highlights the importance of regulation of this redox balance 38 for cell metabolism, but also points to the potential of controlling the nanoparticle uptake 39 mechanisms, and resultant cytotoxicity, with implications for nanomedicine. 40 41 3