Chemistry Reference
In-Depth Information
15.2.2
peptide-Based agents
a number of peptides have been used as the targeting ligands for the development of dual-modality PET/optical agents [19].
One of the most intensively studied molecular targets using peptide-based imaging probes is integrin α
v
β
3
, which binds to
arginine-glycine-aspartic acid (RgD)-containing peptides/proteins [20, 21]. as a key protein involved in tumour angiogenesis
and metastasis [21-23], integrin α
v
β
3
is overexpressed in a variety of solid tumour types (e.g., melanoma, late stage glioblas-
toma, breast, prostate, and ovarian cancer) but is not readily detectable in resting endothelial cells or most normal organs [24],
which makes it a universally applicable target for molecular imaging and therapy of cancer. Because one of the key require-
ments during tumour development is angiogenesis [25-27], tremendous effort has been devoted to angiogenesis-related
research over the last decade, particularly those involving noninvasive molecular imaging techniques [28].
an engineered Cystine knot peptide (knottin 2.5D), which contains the RgD motif and exhibits high binding affinity for
integrin α
v
β
3
and α
v
β
5
, has been used to develop a PET/optical imaging agent for noninvasive monitoring of integrin expres-
sion in tumour models (Figure 15.3) [29]. Synthesis of this agent was achieved by stoichiometric conjugation of the NIR
fluorescent (NIRF) dye Cy5.5, DOTa (1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid, for
64
Cu-labelling),
and the N-terminus of knottin 2.5D to a peptide linker. Solid-phase peptide synthesis using appropriate protective groups
was employed to assure consistency in the 3D structure and target binding affinity of the knottin 2.5D peptide. The advantage
of dual-labelled knottin 2.5D over the analogous
64
Cu-labelled single-modality probe was demonstrated by decreased wash-
out and significantly better retention of the PET/optical dual-modality probe in the tumour. good correlation between the
two imaging modalities was achieved, suggesting stability of the dual-modality probe
in vivo
. Future studies with different
combinations of imaging labels may yield additional information through multimodal imaging.
Disrupted expression and activity of matrix metalloproteinases (MMPs) is present in many diseases such as rheumatoid
arthritis, atherosclerosis, heart failure, pulmonary emphysema, and tumour development/metastasis [30-35]. a cyclic pep-
tide that contains a peptide sequence cleavable by MMP-2/9 (i.e., HWgF) was dual-labelled with
68
ga and the NIRF dye
IRDye800CW [36]. Solid-phase synthesis of the peptide KKaHWgFTlD, followed by DOTa conjugation, yielded the
DOTa-KKaHWgFTlD conjugate. 800CW was then attached to the lysine residue to render the dual-modality probe.
although fluorescence properties of the dye survived the harsh conditions associated with
68
ga-labelling, which was carried
out at 95°C and pH 4, the binding affinity of the peptide to MMP-2/9 was significantly compromised. The use of other PET
isotopes is warranted in future investigation of this class of imaging probes. For example,
64
Cu can be labelled with DOTa
at mild conditions (37°C) in close to neutral pH values [37-39].
-
-
SO
3
SO
3
Cy5.5
-
-
O
3
S
SO
3
N
N
O
NH
O
O
N
N
Knottin 2.5D
HN
O
OH
O
H
NH
O
O
N
N
OH
DOTA
N
N
O
OHHO
O
FIgure 15.3
a knottin-based probe for
in vivo
NIRF and PET imaging in mouse tumour models.
