Polymeric Micelles for siRNA Delivery - RNA Interference from Biology to Therapeutics - page 161

Systemic administration of a nanocarrier system offers great potential due to
dissemination within the bloodstream that allows access to a wide variety of tissues
and cells. Therefore, the first challenge of a system is to be stable in the blood and
protect siRNA from nuclease degradation. In addition, other nonspecific interactions
with serum proteins can induce the nanocarrier dissociation or aggregation. The
interaction with biomacromolecules can induce embolization in microvessels or
capture by the mononuclear phagocyte system (MPS) mainly in the liver and spleen.
The size of the nanocarrier reduces renal filtration that is needed to prolong circula-
tion within the bloodstream. Undesirable biodistribution of nanocarriers may lead to
nonspecific accumulation in healthy tissues and to unwanted systemic effects.
The possible toxicity of a nanocarrier system needs to be addressed before clini-
cal experiments. Complete in vitro assessment allied to further preclinical evalua-
tion of the biodegradability of components is necessary to avoid toxic effects in
humans. Correspondingly, the evaluation of immune stimulation after administra-
tion of siRNA is an important matter. This stimulation is mediated by immune cells,
via the Toll-like receptor pathway and can activate high levels of inflammatory
cytokines such as interleukin-6 (IL-6) and interferon (IFN) [ 16- 18 ] . Another toxi-
cology concern is the oversaturation of RISC in the target cell. Studies suggest that
the exogenous introduction of siRNA may result in the competition with endoge-
nous miRNAs for RISC [ 19, 20 ]. The repression of miRNA-regulated genes leads
to their re-expression, and this can ultimately disturb the normal tissue physiology.
The extravasation from the bloodstream to a desired tissue may occur passively
or actively. The passive accumulation is generally explained by the microvascular
hyperpermeability to circulating macromolecules, whereas the active accumulation
is facilitated by targeting ligands that associate with specific cell-surface receptors
(refer to Sect. 8.3 ).
The extracellular medium is also a challenge to the system. The microenviron-
ment of the tissue presents differences in the pH, enzymes, or ions that can damage
the nanocarrier causing dissociation before cellular entry. Moreover, for an extended
RNAi effect in the tissue, the particle should diffuse in the extracellular matrix to
reach cells located distant from the blood vessels. The siRNA nanocarrier normally
enters cells by the process of endocytosis that facilitates entry into the endocytic
pathway. Following cellular uptake, complexes are mainly transported to endosomes
with decreasing pH. The inability to escape this compartment will lead to the degra-
dation of the nanocarrier within the lysosomes. Thus, escaping the endosomes and
selective disassembly in cytoplasm are the ultimate challenges to be surpassed.
The success of nanoparticle-delivered RNAi therapeutics will depend on the
ability of the nanocarrier to overcome these biological barriers over a specific
period, selectively target the diseased tissue, degrade predictably, be well tolerated
and provide a high therapeutic index [ 21 ]. Thus, the therapeutic application of RNAi
relies on the strategy to safely and efficiently deliver the siRNA to the diseased tissue.
The in vivo application requires the design of formulations that are capable to per-
form several functions in order to overcome these hurdles and reach the cytoplasm
of a diseased cell. As cancer is a potential target disease for RNAi-based therapeu-
tics, this chapter will focus on nanocarrier delivery of siRNA to cancer cells.