Cancer Genetics and Epigenetics 2017, Vol.5, No.7, 33-38
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response (Maute et al., 2015). Therefore, these molecular imaging methods are able to establish a basis for the
non-invasive quantification of temporal or spatial features of immune checkpoints.
Whole mAbs are relatively large, have poor penetrative abilities, slow metabolic turnover, and may carry the risk
of false-positive results due to the activity of metabolic residues in the blood pool. This results in low imaging
efficiency, especially in target regions where the expression of PD-L1 is relatively low. To increase imaging
efficiency, researchers need to raise effective antigen binding fragments. Nanobodies (Nbs) are single-domain
antibodies with a variable heavy chain first identified in serum taken from camels, and are currently the smallest
possible antigen-binding fragments that can be obtained, making them the most promising alternative to mAbs.
Being relatively stable, soluble, and high-affinity, Nbs can quickly enter tissues, specifically bind antigens;
unbound Nbs can be rapidly cleared by the kidneys (Muyldermans, 2013). Dr. Broos K and his colleagues (Broos
et al., 2017) used 99mTc markers to tag 4 types of high-affinity PD-L1-specific nanobodies (Nbs) (C3, C7, E2,
and E4, respectively), and used SPECT/CT imaging to assess the expression of PD-L1 on lung epithelial cells in
murine TC-1 cancer models. The results showed that 99mTc-tagged NbsC3 and E2 had good antigen binding
ability, and therefore, SPECT/CT imaging of 99mTc-tagged Nbs C3 and E2 may serve as a non-invasive method
to assess tumor PD-L1 expression levels, with signal intensities that correlate with PD-L1 expression levels. At
Johns Hopkins University, Professor S. Chatterjee and his colleagues (Chatterjee et al., 2016) made use of
MPDL3280A, a mouse and human cross-reactive PD-L1 antibody, to develop a new radiotracer,
111In-PD-L1-mAb. Consequent SPECT imaging of various cell lines in murine tumor models with different
PD-L1 expression levels demonstrate the specificity of 111In-PD-L1-mAb in identify PD-L1 expression. Results
show that in murine models of PD-L1-positive breast cancer (MDAMB231) and non-small cell lung cancer in
particular, 111In-PD-L1-mAb shows higher specificity and sustained high uptake levels, which provethat
111In-PD-L1-mAb can be used to evaluate the expression levels of PD-L1 in tumor tissues.
The analysis of fluorescent protein markers has the advantages of high-sensitivity, broad selection, high affinity,
dynamic imaging, and higher semblance to physiological conditions. Near-infrared spectroscopy (NIR)
technology has been used in the detection of tumor border zones in preoperative staging processes in ovarian
cancer and breast cancer. In addition, targeting MPDL3280A, the mouse and human cross-reactive PD-L1
antibody, S. Chatterjee et al. simultaneously (Chatterjee et al., 2016) developed a Near-Infrared Fluorescent dye
(NIR) to mark PD-L1 mAbs, and assess the expression of PD-L1 in tumor tissues. Optical imaging results show
that NIR-PD-L1-mAb tracer have high specific uptake in murine models of PD-L1 positive breast cancer
(MDAMB231) and non-small cell lung cancer (H2444). This also shows that optical imaging methods can be used
to non-invasively assess PD-L1 expression in tumors, to guide further clinical advances.
3.4 Issues faced by molecular imaging of immune checkpoints
As Immuno-PET has a few clinical shortcomings, including relatively long clearance time and high background
signal, and the fact that patients are asked to return to the hospital for follow-up imaging a few days after the
injection of tracers. Recent studies have also shown that whole mAbs have limited penetrability (Wittrup et al.,
2012). In addition, the optimization of small-molecule high-affinity protein binding agents has become a major
challenge in Immuno-PET imaging (Hettich et al., 2016). We should attempt to continuously improve imaging
techniques of mAbs, allowing for improved clinical applications.
3.5 Future prospects and outlook
Following the continuous development of novel immune checkpoint drugs, and the validation of tracers marking
different mAbs, the visualization of primary tumors and metastatic tumor targets has become a dire need of
doctors from different clinical departments. The evaluation of molecular imaging for preclinical studies, especially
using Immuno-PET, allows for non-invasive, dynamic live imaging, that provides a comprehensive and accurate
assessment of immune checkpoints expression in tumor tissue. Thus, it aids preclinical evaluation prior to the
administration of immune checkpoint therapy, as well as patient selection. If molecular imaging can indeed be
used in a clinical setting in the future and provide accurate patient selection and treatment tracking in targeted
immunotherapies, it will serve as a beacon of hope for patients with cancer.
