Chemistry Reference
In-Depth Information
Somatostatin receptors, in particular subtype 2 (sst2), are usually upregulated in endocrine tumours [40, 41]. analogues
of somatostatin such as the octreotide and octreotate peptides, which have improved biological half-lives over somatostatin
itself [42, 43], have been extensively studied for cancer imaging in both animal models and clinical settings [44, 45]. In one
report, the peptide octreotate was conjugated to both DOTa and the NIRF dye Cypate [46]. In addition to optical methods,
radiolabelling with
64
Cu and
177
lu enables
in vivo
imaging with PET and SPECT respectively. However, although target
specificity of the agents was demonstrated
in vitro
, tumour uptake of the tracer was very low in aR42J tumour-bearing rats,
which exhibited predominantly hepatobiliary clearance.
Bombesin, a peptide with 14 amino acid residues that binds to the gastrin-releasing peptide receptor (gRPR) [47], has
also been dual-labelled with the dye FITC (fluorescein isothiocyanate) and DTPa (diethylenetriaminepentaacetic acid), or
DOTa, by the same research group [48]. It was found that a spacer between the peptide and the two labels was necessary to
maintain the target binding affinity of the probe. Furthermore, positioning of the label also had a significant impact on the
target-binding properties. However, no PET studies were carried out on these potential PET/optical dual-modality agents.
Different from the direct targeting strategies mentioned above, an activatable multimodality probe lS498 has been con-
structed [49]. This agent contains DOTa (for
64
Cu-labelling), a NIRF dye lS-276, a fluorescence-quenching molecule QC-1,
as well as a peptide sequence asp-glu-Val-asp (DEVD) that can be cleaved by Caspase-3 [50], an enzyme upregulated in
apoptotic cells [51]. Enzyme kinetics assay showed that lS498 could be readily cleaved by Caspase-3, which was also suc-
cessfully tested in living mice using fluorescence imaging (Figure 15.4). It was suggested that the 'always on' nuclear signal
can be useful for quantifying and localising the distribution of the probe, while optical imaging can report the functional
status of a molecular event (e.g., enzymatic activity).
15.2.3
Macromolecule-Based agents
Dual-modality imaging using a small molecule- or peptide-based probe is very challenging due to the limited number of
attachment points and the potential interference with its receptor binding affinity. Therefore, various nanoparticles [6, 7] and
macromolecules (e.g., antibodies and polymers), which have more functional groups available to attach the image labels for
dual-modality molecular imaging, have also been investigated. a dual-modality probe for PET and NIRF imaging has been
reported that employs a monoclonal antibody (mab) that binds to CD20 [52]. The mab NuB2 was conjugated to DOTa and
an alexaFluor 750 dye, where no perturbation of the absorption and fluorescence spectra of the dye was observed upon con-
jugation to the mab. The imaging characteristics of
64
Cu-DOTa-NuB2-alexaFluor 750 were assessed in a CD20-positive
Raji lymphoma-bearing mouse, which had much higher uptake than the nonconjugated
64
Cu-DOTa-alexaFluor 750.
Furthermore, robust correlation between the biodistribution ratios obtained by radioactivity and fluorescence measurement
was achieved, demonstrating the effectiveness of this dual-modality agent in imaging CD20-expressing lymphomas.
The pivotal role of vascular endothelial growth factor (VEgF) in cancer is underscored by the approval of bevacizumab
(Bev, a humanised anti-VEgF mab) as first-line treatment of cancer patients [53]. We recently reported dual-labelled Bev
(with 800 CW and
64
Cu) for both PET and NIRF imaging of VEgF [54]. Because it has been generally recognised that NOTa
(1,4,7-triazacyclononane-1,4,7-triacetic acid) is one of the most suitable chelators for
64
Cu-labelling [55], NOTa was used as
the chelator for
64
Cu in this study instead of DOTa. Flow cytometry analysis of u87Mg human glioblastoma cells revealed no
difference in VEgF binding affinity/specificity between Bev and NOTa-Bev-800CW. Serial PET imaging of u87Mg tumour-
bearing female nude mice revealed excellent tumour contrast and high tumour uptake of
64
Cu-NOTa-Bev-800CW, corrobo-
rated by
in vivo
/
ex vivo
NIRF imaging and biodistribution studies (Figure 15.5). Furthermore, tumour uptake as measured by
ex vivo
NIRF imaging had a good linear correlation with the percentage injected dose per gram of tissue (%ID/g) values
obtained from PET, which warrants further investigation and future clinical translation of such Bev-based imaging agents for
many applications such as disease diagnosis, patient stratification, treatment monitoring, and image-guided surgery.
In addition to the integrin α
v
β
3
and VEgF/VEgF receptors (VEgFRs) mentioned above, for which several tracers have
entered clinical investigation [28, 56-59], another attractive target related to tumour angiogenesis that deserves more inves-
tigation is CD105 (also called endoglin), a 180 kDa disulphide-linked homodimeric transmembrane protein [60-62].
Immunohistochemistry of CD105 on tumour tissue is now the accepted standard approach for determining tumour microves-
sel density (MVD), a quantitative measure of tumour angiogenesis and an independent prognostic factor for survival in
patients with many types of solid tumours [61, 62]. Noninvasive imaging of CD105 expression (essentially noninvasive
measurement of whole-body MVD) has promising clinical potential in cancer diagnosis, anti-angiogenic drug development,
personalised medicine, and ultimately the day-to-day cancer patient management.
after demonstrating the feasibility of PET or NIRF imaging of CD105 using an anti-CD105 mab called TRC105 [39,
63-65], we are currently optimising the development of dual-modality PET/NIRF agents for noninvasive imaging of CD105
[66, 67]. In both studies, good correlation of tracer uptake based on PET and NIRF measurements was achieved, confirming that
imaging in the NIR window in small animal models can give semi-quantitative to quantitative information in superficial (tumour)
