## 4 Drug Delivery
### Redox-Responsive Drug Delivery
Micelles with the highest FA surface density (those with free PEG arms conjugated to FA) were expected to enable higher cellular uptake. The A\({}_{2}\)(AB)\({}_{2}\) micelles promoted the CPT uptake the most at 25%, and 11 and 15-20% in A(AB)\({}_{3}\) and A\({}_{3}\)(AB) micelles, respectively, suggesting a compromise between FA surface density and CPT-loading. Considering CPT loading, GSH-sensitivity, and cellular uptake, PEG\({}_{2}\)-(PEG-PCL)\({}_{2}\) micelles were the most optimal as anti-tumor agents. AB\({}_{2}\) (A = PEG, B = PLL) type miktoarm stars have been prepared using N-carboxyanhydride (NCA) ROP on a PEG-based macroinitiator, with or without bioreducible disulfide linkages conjugating the polymeric arms [73]. The disulfide linker-containing system employed cystamine, which was conjugated directly to the PEG macroinitiator, and its NH\({}_{2}\) end was employed for ROP. PLL is cationic at physiological pH, and it can electrostatically bind with negatively charged biomolecules, such as plasmid DNA (pDNA). It was demonstrated that a 5:1 wt. ratio of miktoarm polymer to pDNA was sufficient for complete binding. Miktoarm star/pDNA polyplexes were prepared using this ratio, and these formed spherical particles of 57.67 and 142.62 nm diameters, with or without reducible disulfide linkers, respectively. These sizes were constant when incubated with fetal bovine serum for 24 h, indicating good nanoparticle stability and applicability for intravenous administration. This was in contrast to complexes of pDNA with just PLL arms, which grew to about 450 nm after 24 h. The significance of the effect of the linkers or the reasons for the disparate sizes is not fully understood. Unlike the non-bioreducible polyplexes, pDNA bound to miktoarm stars was released in response to DTT, a reducing agent substitute for biological GSH. The _in vitro_ transfection efficiency of miktoarm polyplexes containing disulfide linkers into HeLa cells, was observed to be much higher than that of the non-reducible polyplexes, due to the intracellular reduction of these linkers. A \({}^{1}\)O\({}_{2}\)-responsive miktoarm star of AB\({}_{2}\) (A = PEG, B = PCL) composition has been prepared, using the ROP of caprolactone from a PEG macroinitiator, containing a \(\beta\)-aminoacrylate junction [74]. The latter was found to be 100% in the E-configuration, and its \({}^{1}\)O\({}_{2}\) induced cleavage was confirmed under red light laser irradiation, after exposure to the \({}^{1}\)O\({}_{2}\)-generating photosensitizer, chlorin e6 (Ce6) (Scheme 6). Ce6 and DOX were loaded into the micelles with 78% and 29% efficiencies,  
Figure 9: **(a)** Scheme of A(AB)\({}_{3}\) (P140, P160), A\({}_{2}\)(AB)\({}_{2}\) (P240), and A\({}_{3}\)(AB) (P340) assembly into micelles, their response to GSH, **(b)** their size distributions from DLS, and **(c)** a representative TEM micrograph of A\({}_{2}\)(AB)\({}_{2}\) (P240) micelles. Reprinted with permission from reference [57]. Copyright 2015 Royal Society of Chemistry. respectively. Irradiation of these Ce6/DOX co-loaded samples led to the disruption of micelles into irregular aggregates, which triggered 68% DOX release after 24 h, as opposed to only 26% in the dark. Accordingly, laser irradiation of co-loaded AB\({}_{2}\) micelles resulted in large DOX uptake into the cytoplasmic regions of MDA-MB-231 cells, more so than even free DOX, while micelles kept in the dark maintained good cargo retention.