Malignant transformation of tumor accompanies profound changes in the normal neighboring tissue, called tumor stroma. The tumor stroma provides an environment favoring local tumor growth, invasion, and metastatic spreading. Nuclear imaging (PET/SPECT) measures biochemical and physiologic functions in the human body. In oncology, PET/SPECT is particularly useful for differentiating tumors from postsurgical changes or radiation necrosis, distinguishing benign from malignant lesions, identifying the optimal site for biopsy, staging cancers, and monitoring the response to therapy. Indeed, PET/SPECT is a powerful, proven diagnostic imaging modality that displays information unobtainable through other anatomical imaging, such as CT or MRI. When combined with coregistered CT data, [18F]fluorodeoxyglucose ([18F]FDG)-PET is particularly useful. However, [18F]FDG is not a target-specific PET tracer. This paper will review the tumor microenvironment targeting oncologic imaging such as angiogenesis, invasion, hypoxia, growth, and homing, and also therapeutic radiopharmaceuticals to provide a roadmap for additional applications of tumor imaging and therapy.
The tumor stroma, consisting of cells, structural proteins, and signaling molecules, which includes fibroblasts/myofibroblasts, glial, epithelial (EC), fat, vascular, smooth muscle, and immune cells along with the extracellular matrix (ECM) and extracellular molecules, is playing a central role in tumor initiation, progression, and metastasis (Figure
The tumor stroma is critical for tumor growth. Malignant transformation is a multistep process involving profound changes in the normal neighboring tissue, also called tumor stroma. The tumor stroma provides an environment favoring local tumor growth, invasion, and metastatic spreading.
Fibroblasts are the main cellular component of tumor stroma, comprising an integral component of the tumor. Fibroblasts are responsible for the deposition of the fibrillar ECM, which is continually remodeled through a dynamic process of ECM protein production and degradation by fibroblast-derived matrix metalloproteinases (MMPs). Tumor hypoxia influences cytokines and growth factors such as Transforming growth factor-beta (TGF-b), stromal cell- derived factor-1 (SDF-1), matrix metalloproteinase (MMP), vascular endothelial growth factor (VEGF), and hypoxia-inducible factor-1 alpha (HIF-1
The potential high sensitivity and specificity of nuclear imaging techniques such as positron emission tomography (PET) and single photon emission computed tomography (SPECT) are an attractive option for medical diagnosis. The high sensitivity of radioisotopes and wide range of biomolecules which are labeled by these radioactive isotopes, such as radioactive halogens (18F, 76Br, 77Br, and 124I), [11C] and radioactive metals (111In, 99mTc, 68Ga, and 64Cu) made these imaging techniques convenient. In general, radioactive halogens and carbon are widely used to label all kinds of radiopharmaceuticals, but mostly for labeling small molecules, while the radioactive metals are mainly used for labeling large molecules such as proteins, peptides, and antibodies by conjugation of metal chelators. All oncologic imaging tracers are molecularly targeted radiopharmaceuticals based on the tumor biochemistry such as increased metabolism, hyperproliferation, angiogenesis, hypoxia, apoptosis, and specific tumor biomarkers including tumor specific antigens and tumor-specific receptors. This paper will review the tumor microenvironment targeting oncologic imaging and therapeutic radiopharmaceuticals. We hope to provide a roadmap for additional applications of tumor imaging and therapeutic agents which could help researchers and clinicians.
Tumor angiogenesis is an essential mechanism for tumor growth and development of metastasis [
Schematic presentation of the RGD or peptide-conjugated radiotracer design. The pharmacokinetic modifying linker (PKM) is used to improve the radiotracer excretion kinetics. For the metal radioisotopes, bifunctional chelators were attached to targeting molecules, and for [18F], molecular synthon is needed to attach biomolecule.
Chemical structures of RGD tracers. 18F-labeled galacto-RGD [
The iodinated derivatives of RGD peptide showed receptor-specific tumor accumulation, along with high activity in the liver due to predominant hepatobiliary excretion [
There have also been several efforts to develop radiometalated analogues of RGD peptides. These peptides were conjugated with metal chelator at lysine moiety of peptide and the resulting compounds were labeled with 99mTc, 188Re, 90Y, and recently with 68Ga [
Coronal microPET images of 68Ga-NOTA-RGD in mice bearing SNU-C4 xenografts with and without cold c(RGDyK) (60 mg). Arrows indicate tumor positions. Acquisition time was 20 min [
To improve the binding affinity, multimeric RGD peptides have been developed. First, cyclic RGD dimers, such as E [c(RGDfK)]2 were developed as diagnostic (99mTc) and therapeutic (90Y and 64Cu) radiotracers [
A quantum-dot- (QD-) based probe was reported recently for both near-infrared fluorescence (NIRF) and PET imaging of integrin
For dual-modality imaging with SPECT and fluorescence, RGD peptide was labeled with 111In and IRDye800 (LI-COR Biosciences) and evaluated in integrin
VEGF is considered a major angiogenic factor responsible for the development of the tumor vasculature network. VEGF-A is the best-characterized member of the VEGF family and is thought to be the most critical regulator of the development of the vascular system in various tumors [
Currently, the most widely used drug in clinic is bevacizumab (Genentech), a humanized monoclonal antibody, which binds to all VEGF isoforms and thereby blocks the VEGF-induced endothelial cell proliferation, permeability, survival, and growth [
In contrast to the result observed for labeled antibody VG76e, 89Zr-bevacizumab has not shown tumor clearance up to 168 h after injection, which indicates the slow clearance of bevacizumab, thus inappropriate as a diagnostic imaging probe. In an alternative approach, Cai et al. reported VEGF-receptor imaging in a human glioma bearing mouse model with 64Cu-DOTA-VEGF121 [
Integrin
Coronal microPET images and radioactivity accumulation quantification of nude mice bearing U87MG tumors (treated with [90Y]-Abegrin, [90Y]-IgG, Abegrin, or saline) after i.v. injection of [18F]FDG (a) and [18F]fluoro-L-thymidine (b).
Low-molecular-weight peptides show fast blood clearance and rapid tumor penetration in contrast to monoclonal antibodies. Even though in recent years structurally modified RGD peptides and analogues were used as the integrin
Integrin-targeted radiotherapy by nonpeptide antagonists has also been reported. DOTA-conjugated nonpeptide integrin
Theranostics for cancer therapy using radiolabeled peptide derivatives has been attempted recently [
The matrix metalloproteinase family shares specific functional and structural components necessary for extracellular secretion and activation of the enzyme. MMP family members are classified on the basis of additional protein domains such as hemopexin or a fibronectin-like region that contribute to their individual characteristics [
Initial attempts to develop radiolabeled MMPis have shown unsuccessful
Recently there was a report on Marimastat, a noncovalent MMPi labeled with 18F. In the reported method, shelf-stable arylboronic esters conjugate was used as a captor for aqueous 18F fluoride in a novel method. Developed tracer was localized to the tumors [
Urokinase-type plasminogen activator (uPA) and its cell-surface receptor (uPAR) are central molecules for cell surfaces-associated plasminogen activation [
Li et al. demonstrated the noninvasive imaging of uPAR expression in a living subject for the first time by using a small linear peptide (D-Cha-F-s-r-Y-L-W-S) (AE105) with high affinity for human uPAR in uPAR xenotransplanted mouse tumor models after labeling with the positron emitter 64Cu [
In a subsequent study, Liu et al. synthesized and characterized a small peptide inhibitor of the uPA-uPAR interaction and modified it to contain a C-terminal DOTA chelating moiety for labeling with 111In to obtain (NAc-dD-CHA-F-dS-dR-Y-L-W-S-Ala)2-K-K([111In]-DOTA) [
Hypoxia can occur due to structural abnormalities of microvessels and the limited diffusion distance (<70 mm) of oxygen within the tumor. Increase in tumor aggressiveness and metastatic potential of solid tumors is believed to be highly associated with the presence of hypoxia within the cancer [
For the hypoxia detection, the tracer should be specific for hypoxia, its uptake should reflect clinically relevant cellular
Schematic representation of the proposed mechanism for the binding of nitroimidazole radiotracers in hypoxic environment.
Detection of tumor hypoxia with radionuclides was first demonstrated with [14C]-misonidazole by autoradiography [
Chemical structures of radiolabeled hypoxia imaging agents. 18F-fluoroazomycin arabinoside ([18F]FAZA) [
18F-misonidazole ([18F]FMISO) has been used for PET imaging quantification of hypoxia in a variety of tumors, including head and neck cancer, non-small-cell lung cancer, breast cancer, and brain tumors [
64Cu-ATSM is a nonnitroimidazole compound developed for hypoxia imaging [
However, the production of these radiotracers is limited to cyclotron systems, which are expensive as well as difficult to handle. Other than the cyclotron produced radioisotopes, an alternative method to label biomolecules is the use of 68Ga, which can be obtained from a commercially available radionuclide generator system [
68Ga-NOTA-NI microPET images of mice bearing a CT-26 tumors after intravenous administration of 68Ga-NOTA-NI (13.3 MBq/0.1 mL) (30 and 60 min after injection). Arrows indicate tumor positions [
Because of the importance of the epidermal growth factor receptor (EGFR) signaling pathway in malignant progression of various types of tumors [
Chemical structures of EGFR inhibitors used for radiolabeling. Labeled EGFR reversible inhibitors (PD153035 analogs and [18F]ML01) [
Inhibitors like [4-(3-iodoanilino)-quinazolin-6-yl]-amide-(3-morpholin-4-yl-propyl)-amide (ML04) [
Levashova et al. reported dEGF (a Cys-tagged dimeric EGF)-based SPECT tracer prepared by direct radiolabeling of Cys-tag with 99mTc, while the corresponding PET tracers were prepared by conjugating a PEGylated DOTA chelator to Cys-tag, followed by radiolabeling with 64Cu [
Somatostatin is a regulatory peptide and its action is mediated by membrane-bound receptors (SSTRs), G-protein-coupled receptors, that are highly expressed in many different types of human tumors, notably neuroendocrine tumors (NET) [
68Ga-based PET tracers (68Ga-DOTATOC, 68Ga-DOTATATE, and 68Ga-DOTANOC) are widely used, in which 68Ga-DOTANOC has affinity for sst2, sst3, and sst5, resulting in a better diagnostic sensitivity than 68Ga-DOTATATE, which has the highest sst2 affinity [
The chemokine receptors, which belong to a family of seven transmembrane domain G-protein-coupled receptors, consist of 18 members [
Recently, Nimmagadda et al. imaged CXCR4 expression in two human breast cancer cell lines with 64Cu-labeled CXCR4 inhibitor AMD3100 (Figure
Structures of radiotracers developed for imaging CXCR4 expression in tumors. 64Cu-labeled small molecule CXCR4 inhibitors (64Cu-AMD3100 and 64Cu-AMD3465) [
There are also recent reports on labeling of CXCR4 peptide antagonist T140 (Figure
Molecular imaging will play a key role in shaping 21st century cancer management. Cooperative efforts are needed from biologists, chemists, engineers, medical physicists, and mathematicians in identifying, synthesizing, and characterizing excellent imaging probes and in developing high sensitivity imaging instruments. The probes developed for tumor angiogenesis imaging can also have broad applications for other angiogenesis-related diseases, such as myocardial infarction, stroke, atherosclerosis, chronic inflammation, and others. Even though there is still room for improvement, the diagnostic and radiotherapeutic targeting of neuroendocrine tumors with peptide-based nuclear probes has been proven very useful. The development of new radiopeptides with improved pharmacokinetics could help to explore the full potential of targeted radiation therapy. The investigation of integrin
This work was supported by NIH P50 CA 128301-0002 (Shim, H.). The authors are grateful to Ms. Jessica Paulishen for careful reading of the paper and helpful remarks.