Stability polyplexes

Woodle, M.C. et al. (2001). StericaUy stabilized polyplex Ugand-mediated activity. J Control Release. 74, 309-311. 

Insoluble polyplexes are the result of a two-phase system of supernatant liquid solution (containing either polycation as in Scheme 10, or DNA as in Scheme 11) or colloidal suspension (in the particular case of polymeric micelles in solution as shown in Scheme 11, case 1) from which the polyplexes precipitate. For non-sterically stabilized polyplexes, when the molar mixing ratio approaches unity or a low quantity of salt is added (screening of charges of the polyplex) [74] and therefore the electrostatic stabilization is insufficient, secondary aggregation occurs and the aggregates precipitate (this is also a function of the density of the polyplexes) (Scheme 10) [47]. 

Trubetskoy and colleagues [145] introduced disulfide crosslinks into polylysines by treatment with dimethyl-3-3 -dithiobispropionimidate (DTBP) after complex formation. This step enhances the stability of polylysine polyplexes ( caged DNA polyplexes ), as demonstrated by lack of aggregation at high ionic strength. The bonds stabilize polyplexes in an extracellular medium and are bioreducible in intracellular compartments. It is thought that they should not hinder subsequent intracellular release to facilitate access of DNA to the nuclear transcription apparatus. 

Polyplex surface shielding solves several crucial problems, but may also create new problems. Shielding can strongly reduce the efficiency of subsequent cellular steps of the delivery process [68, 69], and also can negatively alter other polyplex characteristics. For pDNA/PEI polyplexes with optimum medium size of PEI, PEG was found to reduce the polyplex stability in vivo [64, 65, 81]. For a discussion of these aspects see Sect. 3.1. 

A highly stable and shielded polyplex should circulate in the blood stream without undesired interactions until it reaches the target cell. At that location, specific interactions with the cell surface should trigger intracellular uptake. While lipid membrane interaction is undesired at the cell surface, it should happen subsequently within the endosomal vesicle and mediate polyplex delivery into the cytosol. During or after intracellular transport to the site of action, the polyplex stability should be weakened to an extent that the nucleic acid is accessible to exert its function. 

The following sections discuss how polymers and polyplexes can be chemically designed to be bioresponsive in three key delivery functions (1) polyplex surface shielding, (2) interaction with lipid bilayers, and (3) polyplex stability. 

Merkel OM, Librizzi D, Pfestroff A, Schurrat T, Buyens K, Sanders NN, De Smedt SC, Behe M, Kissel T (2009) Stability of siRNA polyplexes from poly(ethylenimine) and poly (ethylenimine)-g-poly(ethylene glycol) under in vivo conditions effects on pharmacokinetics and biodistribution measured by Fluorescence Fluctuation Spectroscopy and Single Photon Emission Computed Tomography (SPECT) imaging. J Control Release 138 148-159... 

Russ V, Frohlich T, Li Y, Halama A, Ogris M, Wagner E (2010) Improved in vivo gene transfer into tumor tissue by stabilization of pseudodendritic oligoethylenimine-based polyplexes. J Gene Med 12 180-193... 

Bello-Roufai, M. and Midoux, P. (2001) Histidylated polylysine as DNA vector elevation of the imidazole protonation and reduced cellular uptake without change in the polyfection efficiency of serum stabilized negative polyplexes. Bioconjug. Chem., 12, 92-99. 

Chitosan is generally considered nontoxic, with the rare reported toxicity explained by Koping-Hoggard et al. as a result of impurities [94]. In their study conducted with ultrapure chitosan, transfection efficiency of 293 cells was shown to be dependent on the polyplex stability, which in mm was dependent on the deacetylation degree of chitosan. A deacetylation degree of at least 65% was... 

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