Biomedical Engineering Reference
In-Depth Information
in many more protons being pumped into the interior of the endosome along with
counterions and water. The increase in solutes and water increases the internal
pressure, leading to destabilization and rupture of the endosome causing its contents
to be released. Another strategy for destabilizing the endosome is to use NP con-
structed from acid-labile polymers, such as dextran, that degrade to monomers inside
the endosome causing an increase in osmotic pressure that likewise destabilizes and
ruptures the endosome [135]. Other strategies make use of pH-sensitive peptides
derived from virus particles that undergo conformational changes upon acidification
that destabilize the membrane or form pores that facilitate release of the contents
such as the gALA peptide [136, 137].
13.4.2
efflux of unbound antisense probes
even if the antisense agent makes it efficiently to the cytoplasm, unbound antisense
agent utilizing an always -on reporter must be able to exit the cell efficiently to
reduce the nonspecific background signal (Fig. 13.6). This would be necessary for
radioactive probes such as PeT imaging agents, and in most cases, this aspect of the
design of the imaging agent is neglected and not even discussed. While small mole-
cule probes can easily diffuse out of cells because of their membrane permeability,
the antisense agents cannot, and so some mechanism must be provided for the exit
of the antisense agent or the reporter itself. It is possible that a cell-penetrating pep-
tide might be able to facilitate exit of the antisense agent from the cell by exocytosis
or that unbound probe is degraded to membrane-permeable fragments at a greater
rate than RNA-bound probes. This aspect has received little to no attention but is
critical to the success of the antisense imaging agent.
13.5 recent examples of nanopartIcle antIsense
ImagIng agents
most of the literature concerning genetically targeted NP deals with siRNA and gene
therapy, and very little deals with antisense imaging. There are a few examples, how-
ever, of NP-based antisense imaging systems that have been developed that illustrate
some of the design concepts discussed previously. These NP systems are based on
turn-on optical probes, which avoid the requirement for always -on probes that excess
unbound antisense probe be able to diffuse easily out of the cell.
13.5.1 multiplexed gold nanoparticle-Based strand displacement
probes or nanoflares
An mRNA turn-on antisense gold NP probe system was developed that was based on
quenched strand displacement probe technology (Fig. 13.9) [138-140]. The idea was
to make use of a well-known property of gold as a fluorescent quencher to create a
gold NP with numerous antisense oligodeoxynucleotides that were covalently linked
to the gold surface via a thiol group. These antisense strands were then hybridized to
Search WWH ::
Custom Search