Bioresponsive Nanoparticles for the Intracellular Delivery of RNAi Therapeutics - Nanotechnology for the Delivery of Therapeutic Nucleic Acids - page 133

between the anionic phosphate backbone in the siRNA and polymer-
bearing cationic amines (referred to as the N : P ratio). Synthetic
(e.g. polyethylenimine (PEI) [36], poly(amido amine) [37]) and
natural (e.g. chitosan [38-40], atelocollagen [41, 42]) polycations
that can influence the biological activity of the nanoparticles have
been used. For example, PEI exhibits endosomolytic properties that
facilitate cytosolic delivery [43] whilst chitosan is mucoadehesive
that potentiates delivery across mucosal surfaces [44]. The high
N : P ratio (NP) and use of high molecular weight polymers normally
a necessity for stable nanoparticles results in a net positive surface
charge that facilitates cellular interactions and cell uptake. Inclusion
of high molecular weight polymers shown to improve delivery,
however, can result in insufficient release of the nucleic acid
cargo [45] and toxicity due to cellular membrane disruption and
intracellular interactions [46].

6.3.2 Bioresponsive Systems

The incorporation of functional components during polymer
synthesis or by post-modification can be used to control
nanoparticle stability, cellular uptake, intracellular trafficking, and
polymer toxicity [11, 47]. Attention has been focused on inclusion of
bioresponsive components and linkages whose activity is triggered
by the intracellular conditions such as endosomal pH [48-50] or the
cellular redox gradient [51-53] to facilitate, for example, cytosolic
release of nanoparticle siRNA. These next-generation bioresponsive
systems aim to fulfil the intracellular delivery requirements of RNAi
triggers, namely cellular uptake, endosomal escape, targeting to
specific subcellular compartments and siRNA release. Development
of a non-toxic polyplex system capable of modulating cytoplasmic
and nuclear trafficking and facilitating siRNA release should improve
the therapeutic potential of RNAi therapeutics.
In the following sections, we will focus on two different
bioresponsive nanoparticle delivery systems, the “reducible co-
polypeptide (rCPP)” [54] and “reducible hyperbranched (rHB)” [55]
systems, for different RNAi triggers developed in our laboratory at
the Interdisciplinary Nanoscience Center (iNANO) at the University
of Aarhus Denmark (in collaboration with the Oupicky Lab, Wayne
State University, USA) (Fig. 6.1). The rational for their use is based
on controlling (1) the intracellular trafficking required for RNAi