Delivery of Single siRNA Molecules - Nanotechnology for the Delivery of Therapeutic Nucleic Acids - page 101

as the nanoparticle is produced by complexing the cationic residues
of the PTD to the anionic siRNA, electrostatically. Furthermore,
some unwanted characteristics of siRNA, such as RNAse-mediated
degradation and TLR-mediated innate immune responses,
are overcome by encapsulating the siRNA molecule in nanoparticle
formulations. The negatively charged phosphodiester backbone
of the siRNA easily binds cationic lipids, peptides, and polymers
electrostatically, resulting in nanoparticles of varying sizes of around
1-1000 nm. After the siRNA has been administered systemically,
it needs to bind and cross the vascular endothelial barrier during
the bloodstream circulation. Most molecules larger than 5 nm will
remain in circulation until they are cleared from the body and do
not generally cross the capillary endothelium (Whitehead

et al.,

2009). However, some tissues, such as liver, spleen, and certain
tumors, allow passage of larger molecules, whereas molecules
less than 50 kDa in size are usually cleared through the kidneys.
Indeed, phosphorothioate oligonucleotides have been shown to
have a plasma clearance half-life of only 5 minutes (Rappaport
et al.,

1995).
Intravenously injected particles with a diameter bigger than
100 nm have a high probability of becoming trapped in the
reticuloendothelial system in the liver, spleen, lung, and bone marrow,
followed by macrophage and monocyte degradation (Pecot et al.,

2011). At the same time, a large-sized nanoparticle will, due to the
size restriction of kidney filtration, have an increased circulation time
following systemic administration that will increase the chance of
engaging the desired target to further benefit their pharmacokinetic
properties. However, the physical and chemical properties of the
surface can lead to hemolysis, thrombogenicity, and complement
activation that can cause alterations in biodistribution and potential
toxicity (Dobrovolskaia

2008). Furthermore, both cationic
lipids and polymers known to be toxic, due to the cationic charge
ratio that is required for packaging and the unnatural composition
of the polymer or lipid, may very well engage the lipids in the plasma
membrane of the cell, resulting in cellular toxicity.
In addition to the plasma membrane, there are other extracellular
factors in the microenvironment that may need to be overcome.
For instance, the siRNA could be prematurely released from the
nanoparticle because of the chemical and physical properties of the
extracellular matrix (Burke and Pun, 2008). Moreover, the diffusion
constant of nanoparticles through the interstitial spaces of tissues is

et al.,