DNA release dynamics from bioreducible poly(amido amine) polyplexes.

The DNA release dynamics of bioreducible poly(amido amine) polyplexes were studied in real time by atomic force microscopy (AFM). DNA release is triggered by a depolymerization of high-molecular-weight polycations into low-molecular-weight oligocations that occurs by means of the thiol and disulfide exchange reaction mechanism. AFM images were captured in a simulated physiological reducing environment that used dithiothreitol. Distinctive stages of disassembly are common among various polyplexes that have different disulfide content, molecular weight, and polymer architecture, while the DNA release rate depends upon the disulfide content. In the first stage, polyplexes evolve from metastable structures into the more stable toroid structure upon the depolymerization. In the second stage, toroids either aggregate or fuse into larger toroids. In the last stage, DNA wormlike chains and loops are held by a central compact core. The results confirm the prospect of bioreducible poly(amido amine)s as controlled DNA delivery vectors. The study offers new physical insights into the DNA release pathway including intermediate structures that have a high degree of structural heterogeneity and disassembly induced particle growth. The study identifies disassembly induced colloidal and morphological instability as an important issue to be addressed.