Imaging Genetic Information - Nanotechnology for Biomedical Imaging and Diagnostics - page 365

DNA

Protein

RNA

More agent in
diseased/stressed cell

Less agent in
normal cell

fIgure 13.1 Idealized picture of gene expression imaging in which antisense probes accu-
mulate in a cell expressing a unique or overexpressed mRNA sequence. If an always -on probe
is used, such as a radiolabeled antisense agent, the probe must be able to enter the cytoplasm,
and any unbound probe or reporter must be able to efficiently exit the cytoplasm. Turn-on
probes only need to enter efficiently. The problem is that antisense agents themselves are
membrane impermeable and thus must be attached to cell-penetrating carriers. Another
problem is that mRNA is folded and bound to proteins, making many sites inaccessible to a
probe in its folded state. ( See insert for color representation of the figure.)

membrane permeable so that they can readily diffuse in and out of the cell and
concentrate in the target cells [11], but there are still many challenges with imaging
intracellular targets [12]. Attempting to detect a target protein within a cell by an anti-
body or aptamer is even more problematic, however, as such agents are not membrane
permeable and would require some sort of auxiliary mechanism to facilitate cell entry
and exit. If entering by an endocytosis mechanism, the imaging agent would still have
to be able to exit the endosome to access the target biomolecule within the cytoplasm,
which would require additional auxiliaries. Once the imaging agent is in the cyto-
plasm, unbound imaging agent or the reporting group would also have to be able to
rapidly exit the cytoplasm so that a target-specific signal can be generated, which is
likely to be problematic since such agents are not cell permeable. To circumvent the
latter problem, researchers have been designing imaging agents that only turn-on upon
binding to the target protein [13-15]. While a strategy based on targeting proteins
affords many opportunities for the design of target-specific agents, a specific ligand
or binding agent with the appropriate pharmacokinetics would need to be developed
for each target, which would be a very time-consuming and costly process.

13.2 nucleIc acIds as BIomarkers for dIsease

A much more attractive and universal intracellular target from a design point of view
would be to directly target genetic information associated with a disease state because
information in the form of DNA or RNA can in principle be easily recognized by