Biology Reference
In-Depth Information
macrophages, and dendritic cells
109
). In these cells, cysteine proteases
(possibly cathepsins B, H, or L) are believed to be responsible for the
activation of the PGC probe. Such PGC-based sensors have been
designed to target a number of proteases including those expressing
cathepsin D,
69,74
cathepsin B,
72
MMP,
70,81,110
urokinase plasminogen
activator,
84,111
caspase-1,
79
and coagulation enzymes.
112
Both commercially
available (ProSense
TM
, MMPSense
TM
) and laboratory-synthesized PGC-
based fluorescent probes have demonstrated the ability to image a variety of
cancers in animal models including ovarian,
94
colon,
88
pancreatic,
92,96
lung,
83
as well as fibrosarcoma,
84
glioma,
77
and metastatic disease.
76,113
High-molecular-weight enzyme-activatable sensors should have a number
of advantages in the detection of tumors accessible to potential clinical
optical imaging applications (i.e., those within 7-10 mm of the skin or
endoluminal surface such as breast, prostate, and colon cancers), especially
given the demonstrated sensitivity of these sensors, that is, in detecting
subnanomole levels of tumor-associated enzymes. They may also prove
valuable for the differentiation and staging of certain cancers. For example,
a well-differentiated human breast cancer (BT20) and a highly invasive
metastatic human breast cancer (DU4475) selectively express cathepsin B,
which can be readily detected
in vivo
with PGC-based fluorescent sensors.
75
Moreover, in tumors of equal size, there was a 1.5-fold higher fluorescence
signal in the highly invasive breast cancer, which correlated with a higher
cathepsin B protein content. PGC-based metalloproteinase-3 sensing
probes have also been used to assess therapeutic efficacy after treatment of
tumors with oncolytic adenovirus.
81
A summary of the main examples of
PGC-assisted imaging in animal models, including models of cancer,
is
given in
Table 9.1
.
8. ALTERNATIVE ENZYME-TARGETED STRATEGIES
IN IMAGING CANCER
Although results from
in vitro
and
in vivo
testing of PGC-based probes
have been overall highly promising, some limitations do remain. One is that
enzymes that are released and act in the extracellular space (such as MMPs,
uPA, and thrombin) will produce fluorescent cleaved fragments that are sus-
ceptible to rapid diffusion and elimination from the target tissue. To address
this issue, Tsien and colleagues
114
used nonquenched NIRF (Cy5.5)-tagged
polyarginine cell-penetrating peptides (CPPs) that bear a positive charge and
linked them through a protease-cleavable linker to a negatively charged
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