Biomedical Engineering Reference
In-Depth Information
modified to express multiple copies of a single peptide on their surface form-
ing a library containing over 109 different sequences. This library can be
screened against purified targets immobilized on plates, washed to remove
any unbound phage, while the bound phage are isolated, expanded, and
replated with the target. The screening is repeated a number of times until
a consensus sequence is reached. The problem with this method is that the
target molecules are not in their native state and may not truly represent the
in vivo environment. For a faster identification of target peptides, PepBank,
a searchable database based on sequence text mining and public peptide data
sources (
http://pepbank.mgh.harvard.edu
)
has been created and currently
contains over 21,000 entries.
One peptide sequence being utilized for the targeting of diagnostic and
therapeutic moieties is the tripeptide arginine-glycine-aspartate (RGD).
RGD binds to anb3 integrin which is upregulated in activated endothelial
cells during inflammation or angiogenesis which are both observed cancer
cases.
87,88
This peptide was initially studied for treatment of cancers and later
subsequently utilized for the delivery of both therapeutic and imaging agents
to tumors. RGD has been conjugated with Cy5.5 to mono-, di-, and tetra-
meric RGD peptides and each conjugate's targeting ability had been tested
on U87MG glioblastoma xenograft model. A multifunctional RGD on IONP
probe for positron emission tomography (PET)/MRI imaging of human glio-
blastoma cancer cells has been developed.
89
The IONP were initially coated
with polyaspartic acid, to which was conjugated 1,4,7,10-tetraazacyclododec-
ane-1,4,7,10-tetraacetic acid (DOTA) to serve as a chelating agent for the
64
Cu
radionucleotide. DOTA-labeled particles were decorated with RGD and were
treated with
64
Cu under slightly acidic conditions to obtain
64
Cu-DOTA-RGD-
IO NPs. These were injected into mice bearing U87MG tumors and monitored
with PET/MRI imaging which revealed that the U87MG tumor was clearly
visualized whereas the nontargeted
64
Cu-DOTA-IO particles exhibited very
low nonspecific tumor uptake.
5.5.3 Aptamers
Aptamers are short oligonucleotides (15-40 bases) that are identified via selec-
tion processes called systematic evolution of ligands by exponential enrich-
ment
90,91
that can potentially be used as both therapeutic and targeting entities.
92
Just like antibodies, aptamers are highly specific for their targets and possess
high binding affinities that are largely due to the ability of the molecules com-
plex three-dimensional structures.
93
Aptamers offer advantages over antibodies
such as easy synthesis through solid-phase methodologies and minimal immu-
nogenicity. However, nonmodified aptamers possess several deleterious proper-
ties such as rapid blood clearance that is largely due to nuclease degradation
which had been circumvented by the inclusion of subunits such as 20-fluorine-
substituted pyrimidines and PEG linkages.
