Imaging Genetic Information - Nanotechnology for Biomedical Imaging and Diagnostics - page 383

to favor the PNA•mRNA hybrid. The strand displacement cSCK system was able to
detect a fourfold increase in iNOS expression induced by LPS/γ-IFN, though not the
100-fold level expected from the RT-PcR assays. The observed difference in expres-
sion level could be due to the fact that PcR was detecting whole cell RNA, while the
probe might only be detecting cytosolic RNA. It is also possible that the DNA
quenching strand or the quencher was being degraded in the cell. The cationic cScK
NP delivery system was also investigated for its ability to image iNOS mRNA in an
injured lung mouse with a radioactively labeled PNA•ODN duplex [147]. In this
case, the PNA was linked to a TyrArg 9 peptide, the tyrosine of which was used to
introduce radioactive 123 I and the Arg 9 of which was a cell-penetrating peptide used
to facilitate exit of unbound PNA from the cell. A statistically significant difference
was observed in the retention of the targeted PNA probe compared to the untargeted
probe in the injured lungs, but it was not sufficiently different that it could be used
diagnostically.

13.6 conclusIons

The ability to image gene expression with antisense agents would be a very attractive
and general way for noninvasively diagnosing and monitoring disease. Unfortunately,
there are many hurdles yet to be surmounted before such methodology could become
practical. While optical turn -on probes do not have to be able to exit the cytoplasm,
they do not have the sensitivity or detection depth that can be achieved by PeT
imaging agents. What is therefore still needed to achieve clinically useful antisense
imaging is a general, nontoxic method for efficient cytosolic delivery of antisense
PeT agents in humans and a system for enabling unbound probe or the attached
radionuclide reporter to efficiently escape from the cell.

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