In vivo tumor angiogenesis imaging with site-specific labeled 99mTc-HYNIC-VEGF

Comparative Preclinical Evaluation of HYNIC-Modified Designed Ankyrin Repeat Proteins G3 for the 99mTc-Based Imaging of HER2-Expressing Malignant Tumors

HER2 status determination is a necessary step for the proper choice of therapy and selection of patients for the targeted treatment of cancer. Targeted radiotracers such as radiolabeled DARPins provide a noninvasive and effective way for the molecular imaging of HER2 expression. This study aimed to evaluate tumor-targeting properties of three 99mTc-labeled DARPin G3 variants containing Gly-Gly-Gly-Cys (G3C), (Gly-Gly-Gly-Ser)3-Cys ((G3S)3C), or Glu-Glu-Glu-Cys (E3C) amino acid linkers at the C-terminus and conjugated to the HYNIC chelating agent, as well as to compare them with the clinically evaluated DARPin G3 labeled with 99mTc(CO)3 using the (HE)3-tag at the N-terminus. The labeling of DARPin G3-HYNIC variants provided radiochemical yields in the range of 50-80%. Labeled variants bound specifically to human HER2-expressing cancer cell lines with affinities in the range of 0.5-3 nM. There was no substantial influence of the linker and HYNIC chelator on the binding of 99mTc-labeled DARPin G3 variants to HER2 in vitro; however, [99mTc]Tc-G3-(G3S)3C-HYNIC had the highest affinity. Comparative biodistribution of [99mTc]Tc-G3-G3C-HYNIC, [99mTc]Tc-G3-(G3S)3C-HYNIC, [99mTc]Tc-G3-E3C-HYNIC, and [99mTc]Tc-(HE)3-G3 in healthy CD1 mice showed that there was a strong influence of the linkers on uptake in normal tissues. [99mTc]Tc-G3-E3C-HYNIC had an increased retention of activity in the liver and the majority of other organs compared to the other conjugates. The tumor uptake of [99mTc]Tc-G3-(G3S)3C-HYNIC and [99mTc]Tc-(HE)3-G3 in Nu/j mice bearing SKOV-3 xenografts was similar. The specificity of tumor targeting in vivo was demonstrated for both tracers. [99mTc]Tc-G3-(G3S)3C-HYNIC provided comparable, although slightly lower tumor-to-lung, tumor-to spleen and tumor-to-liver ratios than [99mTc]Tc-(HE)3-G3. Radiolabeling of DARPin G3-HYNIC conjugates with 99mTc provided the advantage of a single-step radiolabeling procedure; however, the studied HYNIC conjugates did not improve imaging contrast compared to the 99mTc-tricarbonyl-labeled DARPin G3. At this stage, [99mTc]Tc-(HE)3-G3 remains the most promising candidate for the clinical imaging of HER2-overexpressing cancers.

177Lu-Labeled Iron Oxide Nanoparticles Functionalized with Doxorubicin and Bevacizumab as Nanobrachytherapy Agents against Breast Cancer

The use of conventional methods for the treatment of cancer, such as chemotherapy or radiotherapy, and approaches such as brachytherapy in conjunction with the unique properties of nanoparticles could enable the development of novel theranostic agents. The aim of our current study was to evaluate the potential of iron oxide nanoparticles, coated with alginic acid and polyethylene glycol, functionalized with the chemotherapeutic agent doxorubicin and the monoclonal antibody bevacizumab, to serve as a nanoradiopharmaceutical agent against breast cancer. Direct radiolabeling with the therapeutic isotope Lutetium-177 (177Lu) resulted in an additional therapeutic effect. Functionalization was accomplished at high percentages and radiolabeling was robust. The high cytotoxic effect of our radiolabeled and non-radiolabeled nanostructures was proven in vitro against five different breast cancer cell lines. The ex vivo biodistribution in tumor-bearing mice was investigated with three different ways of administration. The intratumoral administration of our functionalized radionanoconjugates showed high tumor accumulation and retention at the tumor site. Finally, our therapeutic efficacy study performed over a 50-day period against an aggressive triple-negative breast cancer cell line (4T1) demonstrated enhanced tumor growth retention, thus identifying the developed nanoparticles as a promising nanobrachytherapy agent against breast cancer.

Positron emission tomography molecular imaging-based cancer phenotyping

During the past several decades, numerous studies have provided insights into biological characteristics of cancer cells and identified various hallmarks of cancer acquired in the tumorigenic processes. However, it is still challenging to image these distinctive traits of cancer to facilitate the management of patients in clinical settings. The rapidly evolving field of positron emission tomography (PET) imaging has provided opportunities to investigate cancer's biological characteristics in vivo. This article reviews the current status of PET imaging on characterizing hallmarks of cancer and discusses the future directions of PET imaging strategies facilitating in vivo cancer phenotyping.

Various direct and indirect imaging strategies have been developed in positron emission tomography. Positron emission tomography has shown great potential in characterizing cancer hallmarks in vivo.

Synthetic Pept-Ins as a Generic Amyloid-Like Aggregation-Based Platform for In Vivo PET Imaging of Intracellular Targets

Amyloid-like aggregation of proteins is induced by short amyloidogenic sequence segments within a specific protein sequence resulting in self-assembly into β-sheets. We recently validated a technology platform in which synthetic amyloid peptides ("Pept-ins") containing a specific aggregation-prone region (APR) are used to induce specific functional knockdown of the target protein from which the APR was derived, including bacterial, viral, and mammalian cell proteins. In this work, we investigated if Pept-ins can be used as vector probes for in vivo Positron Emission Tomography (PET) imaging of intracellular targets. The radiolabeled Pept-ins [68Ga]Ga-NODAGA-PEG4-vascin (targeting VEGFR2) and [68Ga]Ga-NODAGA-PEG2-P2 (targeting E. coli) were evaluated as PET probes. The Pept-in based radiotracers were cross-validated in a murine tumor and muscle infection model, respectively, and were found to combine target specificity with favorable in vivo pharmacokinetics. When the amyloidogenicity of the interacting region of the peptide is suppressed by mutation, cellular uptake and in vivo accumulation are abolished, highlighting the importance of the specific design of synthetic Pept-ins. The ubiquity of target-specific amyloidogenic sequence segments in natural proteins, the straightforward sequence-based design of the Pept-in probes, and their spontaneous internalization by cells suggest that Pept-ins may constitute a generic platform for in vivo PET imaging of intracellular targets.

Achievement in active agent structures as a power tools in tumor angiogenesis imaging

According to World Health Organization (WHO) cancer is the second most important cause of death globally. Because angiogenesis is considered as an essential process of growth, proliferation and tumor progression, within this review we decided to shade light on recent development of chemical compounds which play a significant role in its imaging and monitoring. Indeed, the review gives insight about the current achievements of active agents structures involved in imaging techniques such as: positron emission computed tomography (PET), magnetic resonance imaging (MRI) and single photon emission computed tomography (SPECT), as well as combination PET/MRI and PET/CT. The review aims to provide the journal audience with a comprehensive and in-deep understanding of chemistry policy in tumor angiogenesis imaging.

The Role of VEGF Receptors as Molecular Target in Nuclear Medicine for Cancer Diagnosis and Combination Therapy

Simple Summary

The rapid development of diagnostic and therapeutic methods of the cancer treatment causes that these diseases are becoming better known and the fight against them is more and more effective. Substantial contribution in this development has nuclear medicine that enables very early cancer diagnosis and early start of the so-called targeted therapy. This therapeutic concept compared to the currently used chemotherapy, causes much fewer undesirable side effects, due to targeting a specific lesion in the body. This review article discusses the possible applications of radionuclide-labelled tracers (peptides, antibodies or synthetic organic molecules) that can visualise cancer cells through pathological blood vessel system in close tumour microenvironment. Hence, at a very early step of oncological disease, targeted therapy can involve in tumour formation and growth.

Abstract

One approach to anticancer treatment is targeted anti-angiogenic therapy (AAT) based on prevention of blood vessel formation around the developing cancer cells. It is known that vascular endothelial growth factor (VEGF) and vascular endothelial growth factor receptors (VEGFRs) play a pivotal role in angiogenesis process; hence, application of angiogenesis inhibitors can be an effective approach in anticancer combination therapeutic strategies. Currently, several types of molecules have been utilised in targeted VEGF/VEGFR anticancer therapy, including human VEGF ligands themselves and their derivatives, anti-VEGF or anti-VEGFR monoclonal antibodies, VEGF binding peptides and small molecular inhibitors of VEGFR tyrosine kinases. These molecules labelled with diagnostic or therapeutic radionuclides can become, respectively, diagnostic or therapeutic receptor radiopharmaceuticals. In targeted anti-angiogenic therapy, diagnostic radioagents play a unique role, allowing the determination of the emerging tumour, to monitor the course of treatment, to predict the treatment outcomes and, first of all, to refer patients for AAT. This review provides an overview of design, synthesis and study of radiolabelled VEGF/VEGFR targeting and imaging agents to date. Additionally, we will briefly discuss their physicochemical properties and possible application in combination targeted radionuclide tumour therapy.

Multimodality Imaging in Tumor Angiogenesis: Present Status and Perspectives

Angiogenesis is a complex biological process that plays a central role in progression of tumor growth and metastasis. It led to a search for antiangiogenic molecules, and to design antiangiogenic strategies for cancer treatment. Noninvasive molecular imaging, such as positron emission tomography (PET) and single photon emission computed tomography (SPECT), could be useful for lesion detection, to select patients likely to respond to antiangiogenic therapies, to confirm successful targeting, and dose optimization. Additionally, nuclear imaging techniques could also aid in the development of new angiogenesis-targeted drugs and their validation. Angiogenesis imaging can be categorized as targeted at three major cell types: (I) non-endothelial cell targets, (II) endothelial cell targets, and (III) extracellular matrix proteins and matrix proteases. Even if radiopharmaceuticals studying the metabolism and hypoxia can be also used for the study of angiogenesis, many of the agents used in nuclear imaging for this purpose are yet to be investigated. The purpose of this review is to describe the role of molecular imaging in tumor angiogenesis, highlighting the advances in this field.

Radiolabeling of VEGF165 with 99mTc to evaluate VEGFR expression in tumor angiogenesis

Angiogenesis is the main process responsible for tumor growth and metastatization. The principal effector of such mechanism is the vascular endothelial growth factor (VEGF) secreted by cancer cells and other components of tumor microenvironment. Radiolabeled VEGF analogues may provide a useful tool to noninvasively image tumor lesions and evaluate the efficacy of anti-angiogenic drugs that block the VEGFR pathway. Aim of the present study was to radiolabel the human VEGF165 analogue with 99mTechnetium (99mTc) and to evaluate the expression of VEGFR in both cancer and endothelial cells in the tumor microenvironment. 99mTc-VEGF showed in vitro binding to HUVEC cells and in vivo to xenograft tumors in mice (ARO, K1 and HT29). By comparing in vivo data with immunohistochemical analysis of excised tumors we found an inverse correlation between 99mTc-VEGF165 uptake and VEGF histologically detected, but a positive correlation with VEGF receptor expression (VEGFR1). Results of our studies indicate that endogenous VEGF production by cancer cells and other cells of tumor microenvironment should be taken in consideration when performing scintigraphy with radiolabeled VEGF, because of possible false negative results due to saturation of VEGFRs.

Vascular targeting of nanoparticles for molecular imaging of diseased endothelium

This review seeks to highlight the enormous potential of targeted nanoparticles for molecular imaging applications. Being the closest point-of-contact, circulating nanoparticles can gain direct access to targetable molecular markers of disease that appear on the endothelium. Further, nanoparticles are ideally suitable to vascular targeting due to geometrically enhanced multivalent attachment on the vascular target. This natural synergy between nanoparticles, vascular targeting and molecular imaging can provide new avenues for diagnosis and prognosis of disease with quantitative precision. In addition to the obvious applications of targeting molecular signatures of vascular diseases (e.g., atherosclerosis), deep-tissue diseases often manifest themselves by continuously altering and remodeling their neighboring blood vessels (e.g., cancer). Thus, the remodeled endothelium provides a wide range of targets for nanoparticles and molecular imaging. To demonstrate the potential of molecular imaging, we present a variety of nanoparticles designed for molecular imaging of cancer or atherosclerosis using different imaging modalities.

Preparation of tetrazine-containing [2 + 1] complexes of 99mTc and in vivo targeting using bioorthogonal inverse electron demand Diels–Alder chemistry

A new ^99mTc-labelled tetrazine for targeted imaging using bioorthogonal chemistry was developed and evaluated in vivo using a trans-cyclooctene derived bisphosphonate targeting regions of high bone turnover and bone lesions.

VEGF in nuclear medicine: Clinical application in cancer and future perspectives (Review)

Clinical trials using antiangiogenic drugs revealed their potential against cancer. Unfortunately, a large percentage of patients does not yet benefit from this therapeutic approach highlighting the need of diagnostic tools to non-invasively evaluate and monitor response to therapy. It would also allow to predict which kind of patient will likely benefit of antiangiogenic therapy. Reasons for treatment failure might be due to a low expression of the drug targets or prevalence of other pathways. Molecular imaging has been therefore explored as a diagnostic technique of choice. Since the vascular endothelial growth factor (VEGF/VEGFR) pathway is the main responsible of tumor angiogenesis, several new drugs targeting either the soluble ligand or its receptor to inhibit signaling leading to tumor regression could be involved. Up today, it is difficult to determine VEGF or VEGFR local levels and their non-invasive measurement in tumors might give insight into the available target for VEGF/VEGFR-dependent antiangiogenic therapies, allowing therapy decision making and monitoring of response.

Modulation, bioinformatic screening, and assessment of small molecular peptides targeting the vascular endothelial growth factor receptor

Vascular endothelial growth factor (VEGF) and VEGF receptor (VEGFR) are important factors in tumor growth and metastasis. Molecular probes or drugs designed to target VEGF/VEGFR interactions are crucial in tumor molecular imaging and targeted therapy. Bioinformatic methods enable molecular design based on the structure of bio-macromolecules and their interactions. This study was aimed to identify tumor-targeting small-molecule peptides with high affinity for VEGFR using bioinformatics screening. The VEGFR extracellular immunoglobulin-like modules Ig1–Ig3 were used as the target to systematically alter the primary peptide sequence of VEGF_125–136. Molecular docking and surface functional group interaction methods were combined in an in silico screen for polypeptides, which in theory, would have higher affinities for VEGFR. In vitro receptor competition binding assays were used to assess the affinity of the putative VEGFR-binding polypeptides. Rhodamine-conjugated peptides were used to label and visualize peptide-binding sites on A549 cells. Using bioinformatic screening, we identified 20 polypeptides with potentially higher affinity for VEGFR. The polypeptides were capable of inhibiting the binding of ¹²⁵I-VEGF to VEGFR in a dose-dependent manner. The IC₅₀ values of QKRKRKKSRKKH and RKRKRKKSRYIVLS (80 and 185 nmol/L, respectively) were significantly lower than that of VEGF_125–136 (464 nmol/L); thus, the affinity of these peptides for VEGFR was 6- and 2.5-fold higher, respectively, than that of VEGF_125–136. Rhodamine labeling of A549 cells revealed peptide binding mainly on the plasma membrane and in the cytoplasm. Bioinformatic approaches hold promise for the development of molecular imaging probes. Using this approach, we designed two peptides that showed higher affinity toward VEGFR. These polypeptides may be used as molecular probes or drugs targeting VEGFR, which can be utilized in molecular imaging and targeted therapy of certain tumors.

Site-specific protein labeling in the pharmaceutical industry: experiences from novartis drug discovery

Chemically modified proteins play an important role in several fields of pharmaceutical R&D, starting from various activities in drug discovery all the way down to biopharmaceuticals with improved properties such as antibody-drug conjugates. In the first part of the present chapter the significance and use of labeled proteins in biophysical methods, biochemical and cellular assays, in vivo imaging, and biopharmaceuticals is reviewed in general. In this context, the most relevant methods for site-specific modification of proteins and their application are also described. In the second part of the chapter, in-house (Novartis) results and experience with different techniques for selective protein labeling are discussed, with a focus on chemical or enzymatic (Avi-tag) biotinylation of proteins and their application in biophysical and biochemical assays. It can be concluded that while modern methods of site-specific protein labeling offer new possibilities for pharmaceutical R&D, classical methods are still the mainstay mainly due to being well established. However, site-specific protein labeling is expected to increase in importance, in particular for antibody-drug conjugates and other chemically modified biopharmaceuticals.

HET0016, a selective inhibitor of 20-HETE synthesis, decreases pro-angiogenic factors and inhibits growth of triple negative breast cancer in mice

A selective inhibitor of 20-HETE synthesis, HET0016, has been reported to inhibit angiogenesis. 20-HETE has been known as a second mitogenic messenger of angiogenesis inducing growth factors. HET0016 effects were analyzed on MDA-MB-231 derived breast cancer in mouse and invitro cell line. MDA-MB-231 tumor cells were implanted in animals’ right flank and randomly assigned to early (1 and 2), starting treatments on day 0, or delayed groups (3 and 4) on day 8 after implantation of tumor. Animals received HET0016 (10 mg/kg) treatment via intraperitoneal injection for 5 days/week for either 3 or 4 weeks. Control group received vehicle treatment. Tumor sizes were measured on days 7, 14, 21, and 28 and the animals were euthanized on day 22 and 29. Proteins were extracted from the whole tumor and from cells treated with 10 µM HET0016 for 4 and 24 hrs. Protein array kits of 20 different cytokines/factors were used. ELISA was performed to observe the HIF-1α and MMP-2 protein expression. Other markers were confirmed by IHC. HET0016 significantly inhibited tumor growth in all treatment groups at all-time points compared to control (p<0.05). Tumor growth was completely inhibited on three of ten animals on early treatment group. Treatment groups showed significantly lower expression of pro-angiogenic factors compared to control at 21 days; however, there was no significant difference in HIF-1α expression after treatments. Similar results were found invitro at 24 hrs of HET0016 treatment. After 28 days, significant increase of angiogenin, angiopoietin-1/2, EGF-R and IGF-1 pro-angiogenic factors were found (p<0.05) compared to control, as well as an higher intensity of all factors were found when compared to that of 21 day’s data, suggesting a treatment resistance. HET0016 inhibited tumor growth by reducing expression of different set of pro-angiogenic factors; however, a resistance to treatment seemed to happen after 21 days.

Effect of melatonin on tumor growth and angiogenesis in xenograft model of breast cancer

As neovascularization is essential for tumor growth and metastasis, controlling angiogenesis is a promising tactic in limiting cancer progression. Melatonin has been studied for their inhibitory properties on angiogenesis in cancer. We performed an in vivo study to evaluate the effects of melatonin treatment on angiogenesis in breast cancer. Cell viability was measured by MTT assay after melatonin treatment in triple-negative breast cancer cells (MDA-MB-231). After, cells were implanted in athymic nude mice and treated with melatonin or vehicle daily, administered intraperitoneally 1 hour before turning the room light off. Volume of the tumors was measured weekly with a digital caliper and at the end of treatments animals underwent single photon emission computed tomography (SPECT) with Technetium-99m tagged vascular endothelial growth factor (VEGF) C to detect in vivo angiogenesis. In addition, expression of pro-angiogenic/growth factors in the tumor extracts was evaluated by membrane antibody array and collected tumor tissues were analyzed with histochemical staining. Melatonin in vitro treatment (1 mM) decreased cell viability (p<0.05). The breast cancer xenografts nude mice treated with melatonin showed reduced tumor size and cell proliferation (Ki-67) compared to control animals after 21 days of treatment (p<0.05). Expression of VEGF receptor 2 decreased significantly in the treated animals compared to that of control when determined by immunohistochemistry (p<0.05) but the changes were not significant on SPECT (p>0.05) images. In addition, there was a decrease of micro-vessel density (Von Willebrand Factor) in melatonin treated mice (p<0.05). However, semiquantitative densitometry analysis of membrane array indicated increased expression of epidermal growth factor receptor and insulin-like growth factor 1 in treated tumors compared to vehicle treated tumors (p<0.05). In conclusion, melatonin treatment showed effectiveness in reducing tumor growth and cell proliferation, as well as in the inhibition of angiogenesis.

Future Perspectives for Diagnostic Imaging in Urology: From Anatomic and Functional to Molecular Imaging

The future approach of diagnostic imaging in urology follows the technological progress, which made the visualization of in vivo molecular processes possible.

From anatomo-morphological diagnostic imaging and through functional imaging molecular radiology is reached.

Based on molecular probes, imaging is aimed at assessing the in vivo molecular processes, their physiology and function at cellular level.

The future imaging will investigate the complex tumor functioning as metabolism, aerobic glycolysis in particular, angiogenesis, cell proliferation, metastatic potential, hypoxia, apoptosis and receptors expressed by neoplastic cells.

Methods for performing molecular radiology are CT, MRI, PET-CT, PET-MRI, SPECT and optical imaging. Molecular ultrasound combines technological advancement with targeted contrast media based on microbubbles, this allowing the selective registration of microbubble signal while that of stationary tissues is suppressed.

An experimental study was carried out where the ultrasound molecular probe BR55 strictly bound to prostate tumor results in strong enhancement in the early phase after contrast, this contrast being maintained in the late phase. This late enhancement is markedly significant for the detection of prostatic cancer foci and to guide the biopsy sampling.

The 124I-cG250 molecular antibody which is strictly linked to cellular carbonic anhydrase IX of clear cell renal carcinoma, allows the acquisition of diagnostic PET images of clear cell renal carcinoma without biopsy. This WG-250 (RENCAREX) antibody was used as a therapy in metastatic clear cell renal carcinoma. Future advancements and applications will result in early cancer diagnosis, personalized therapy that will be specific according to the molecular features of cancer and leading to the development of catheter–based multichannel molecular imaging devices for cystoscopy-based molecular imaging diagnosis and intervention.

Positron emission tomography imaging of vascular endothelial growth factor receptor expression with (61)Cu-labeled lysine-tagged VEGF121

Overexpression of vascular endothelial growth factor (VEGF) and VEGF receptors (VEGFRs) indicates poor prognosis for cancer patients in a variety of clinical studies. Our goal is to develop a tracer for positron emission tomography (PET) imaging of VEGFR expression using recombinant human VEGF121 with three lysine residues fused to the N-terminus (denoted as K3-VEGF121), which can facilitate radiolabeling without affecting its VEGFR binding affinity. K3-VEGF121 was conjugated with 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) and labeled with (61)Cu (t1/2: 3.3 h; 62% β(+)). The IC50 value of NOTA-K3-VEGF121 for VEGFR-2 was comparable to that of K3-VEGF121 (1.50 and 0.65 nM, respectively) based on a cell binding assay. (61)Cu labeling was achieved with good yield (55 ± 10%) and specific activity (4.2 GBq/mg). Serial PET imaging showed that the 4T1 tumor uptake of (61)Cu-NOTA-K3-VEGF121 was 3.4 ± 0.5, 4.9 ± 1.0, 5.2 ± 1.0, and 4.8 ± 0.8%ID/g (n = 4) at 0.5, 2, 4, and 8 h postinjection, respectively, which was consistent with biodistribution data measured by γ counting. Blocking experiments and ex vivo histology confirmed the VEGFR specificity of (61)Cu-NOTA-K3-VEGF121. Extrapolated human dosimetry calculation showed that liver was the organ with the highest radiation dose. The use of (61)Cu as the radiolabel is desirable for small proteins such as K3-VEGF121, which has a much higher β(+) branching ratio than the commonly used (64)Cu (62% vs 17%), thereby offering stronger signal intensity and lower tracer dose for PET imaging.

Clinical imaging of tumor angiogenesis

Angiogenesis is an integral part of tumor growth and invasion. This has led to the emergence of several antiangiogenic therapies and stimulated efforts to accurately evaluate the extent of angiogenesis before and in response to anticancer treatment. The most commonly used approach has been the assessment of new vessel formation in histological samples. However, it is becoming apparent that this is insufficient for a full understanding of tumor physiology and for in vivo guidance of cancer management. Imaging has the potential to provide noninvasive and repeatable assessment of the angiogenic process. Imaging approaches use a variety of modalities and are aimed at either assessment of the functional integrity of tumor vasculature or assessment of its molecular status. This review summarizes the aims and methods of clinical tumor angiogenesis imaging, including present technologies and ones that will be developed within the next 5-10 years.

Intravascular targets for molecular contrast-enhanced ultrasound imaging

Molecular targeting of contrast agents for ultrasound imaging is emerging as a new medical imaging modality. It combines advances in ultrasound technology with principles of molecular imaging, thereby allowing non-invasive assessment of biological processes in vivo. Preclinical studies have shown that microbubbles, which provide contrast during ultrasound imaging, can be targeted to specific molecular markers. These microbubbles accumulate in tissue with target (over) expression, thereby significantly increasing the ultrasound signal. This concept offers safe and low-cost imaging with high spatial resolution and sensitivity. It is therefore considered to have great potential in cancer imaging, and early-phase clinical trials are ongoing. In this review, we summarize the current literature on targets that have been successfully imaged in preclinical models using molecularly targeted ultrasound contrast agents. Based on preclinical experience, we discuss the potential clinical utility of targeted microbubbles.

Imaging key biomarkers of tumor angiogenesis

Angiogenesis is a fundamental requirement for tumor growth and therefore it is a primary target for anti-cancer therapy. Molecular imaging of angiogenesis may provide novel opportunities for early diagnostic and for image-guided optimization and management of therapeutic regimens. Here we reviewed the advances in targeted imaging of key biomarkers of tumor angiogenesis, integrins and receptors for vascular endothelial growth factor (VEGF). Tracers for targeted imaging of these biomarkers in different imaging modalities are now reasonably well-developed and PET tracers for integrin imaging are currently in clinical trials. Molecular imaging of longitudinal responses to anti-angiogenic therapy in model tumor systems revealed a complex pattern of changes in targeted tracer accumulation in tumor, which reflects drug-induced tumor regression followed by vascular rebound. Further work will define the competitiveness of targeted imaging of key angiogenesis markers for early diagnostic and image-guided therapy.

Imaging key biomarkers of tumor angiogenesis

Angiogenesis is a fundamental requirement for tumor growth and therefore it is a primary target for anti-cancer therapy. Molecular imaging of angiogenesis may provide novel opportunities for early diagnostic and for image-guided optimization and management of therapeutic regimens. Here we reviewed the advances in targeted imaging of key biomarkers of tumor angiogenesis, integrins and receptors for vascular endothelial growth factor (VEGF). Tracers for targeted imaging of these biomarkers in different imaging modalities are now reasonably well-developed and PET tracers for integrin imaging are currently in clinical trials. Molecular imaging of longitudinal responses to anti-angiogenic therapy in model tumor systems revealed a complex pattern of changes in targeted tracer accumulation in tumor, which reflects drug-induced tumor regression followed by vascular rebound. Further work will define the competitiveness of targeted imaging of key angiogenesis markers for early diagnostic and image-guided therapy.

Molecular imaging with SPECT as a tool for drug development

Molecular imaging techniques are increasingly being used as valuable tools in the drug development process. Radionuclide-based imaging modalities such as single-photon emission computed tomography (SPECT) and positron emission tomography (PET) have proven to be useful in phases ranging from preclinical development to the initial stages of clinical testing. The high sensitivity of these imaging modalities makes them particularly suited for exploratory investigational new drug (IND) studies as they have the potential to characterize in vivo pharmacokinetics and biodistribution of the compounds using only a fraction of the intended therapeutic dose (microdosing). This information obtained at an early stage of clinical testing results in a better selection among promising drug candidates, thereby increasing the success rate of agents entering clinical trials and the overall efficiency of the process. In this article, we will review the potential applications of SPECT imaging in the drug development process with an emphasis on its applications in exploratory IND studies.

scVEGF microbubble ultrasound contrast agents: a novel probe for ultrasound molecular imaging of tumor angiogenesis

OBJECTIVE

To develop a novel microbubble (MB) ultrasound contrast agent covalently coupled to a recombinant single-chain vascular endothelial growth factor construct (scVEGF) through uniform site-specific conjugation for ultrasound imaging of tumor angiogenesis.

METHODS

Ligand conjugation to maleimide-bearing MB by thioether bonding was first validated with a fluorophore (BODIPY-cystine), and covalently bound dye was detected by fluorometry and flow cytometry. MBs were subsequently site-specifically conjugated to cysteine-containing Cys-tag in scVEGF, and bound scVEGF was quantified by enzyme-linked immunosorbent assay. Targeted adhesion of scVEGF-MB was investigated with in vitro parallel plate flow chamber assays with recombinant murine VEGFR-2 substrates and human VEGFR-2-expressing porcine endothelial cells (PAE/KDR). A wall-less ultrasound flow phantom, with flow channels coated with immobilized VEGFR-2, was used to detect adhesion of scVEGF-MB with contrast ultrasound imaging. A murine model of colon adenocarcinoma was used to assess retention of scVEGF-MB with contrast ultrasound imaging during tumor angiogenesis in vivo.

RESULTS

Proof-of-principle of ligand conjugation to maleimide-bearing MB was demonstrated with a BODIPY-cysteine fluorophore. Conjugation of BODIPY to MB saturated at 10-fold molar excess BODIPY relative to maleimide groups on MB surfaces. MB reacted with scVEGF and led to the conjugation of 1.2 × 10(5) molecules scVEGF per MB. Functional adhesion of sc-VEGF-MB was shown in parallel plate flow chamber assays. At a shear stress of 1.0 dynes/cm2, scVEGF-MB exhibited 5-fold higher adhesion to both recombinant VEGFR-2 substrates and VEGFR-2-expressing endothelial cells compared with nontargeted control MB. Additionally, scVEGF-MB targeted to immobilized VEGFR-2 in an ultrasound flow phantom showed an 8-fold increase in mean acoustic signal relative to casein-coated control channels. In an in vivo model of tumor angiogenesis, scVEGF MB showed significantly higher ultrasound contrast signal enhancement in tumors (8.46 ± 1.61 dB) compared with nontargeted control MB (1.58 ± 0.83 dB).

CONCLUSIONS

These results demonstrate the functionality of a novel scVEGF-bearing MB contrast agent, which could be useful for molecular imaging of VEGFR-2 in basic science and drug discovery research.

Optical advances in skeletal imaging applied to bone metastases

Optical Imaging has evolved into one of the standard molecular imaging modalities used in pre-clinical cancer research. Bone research however, strongly depends on other imaging modalities such as SPECT, PET, x-ray and μCT. Each imaging modality has its own specific strengths and weaknesses concerning spatial resolution, sensitivity and the possibility to quantify the signal. An increasing number of bone specific optical imaging models and probes have been developed over the past years. This review gives an overview of optical imaging modalities, models and probes that can be used to study skeletal complications of cancer in small laboratory animals.

Molecular imaging metrics to evaluate response to preclinical therapeutic regimens

Molecular imaging comprises a range of techniques, spanning not only several imaging modalities but also many disease states and organ sites. While advances in new technology platforms have enabled a deeper understanding of the cellular and molecular basis of malignancy, reliable non-invasive imaging metrics remain an important tool for both diagnostics and patient management. Furthermore, the non- invasive nature of molecular imaging can overcome shortcomings associated with traditional biological approaches and provide valuable information relevant to patient care. Integration of information from multiple imaging techniques has the potential to provide a more comprehensive understanding of specific tumor characteristics, tumor status, and treatment response.

Molecular imaging of angiogenesis with SPECT

Single-photon emission computed tomography (SPECT) and position emission tomography (PET) are the two main imaging modalities in nuclear medicine. SPECT imaging is more widely available than PET imaging and the radionuclides used for SPECT are easier to prepare and usually have a longer half-life than those used for PET. In addition, SPECT is a less expensive technique than PET. Commonly used gamma emitters are: (99m)Tc (E(max) 141 keV, T (1/2) 6.02 h), (123)I (E(max) 529 keV, T (1/2) 13.0 h) and (111)In (E(max) 245 keV, T (1/2) 67.2 h). Compared to clinical SPECT, PET has a higher spatial resolution and the possibility to more accurately estimate the in vivo concentration of a tracer. In preclinical imaging, the situation is quite different. The resolution of microSPECT cameras (<0.5 mm) is higher than that of microPET cameras (>1.5 mm). In this report, studies on new radiolabelled tracers for SPECT imaging of angiogenesis in tumours are reviewed.

Positron emission tomography tracers for imaging angiogenesis

Position emission tomography imaging of angiogenesis may provide non-invasive insights into the corresponding molecular processes and may be applied for individualized treatment planning of antiangiogenic therapies. At the moment, most strategies are focusing on the development of radiolabelled proteins and antibody formats targeting VEGF and its receptor or the ED-B domain of a fibronectin isoform as well as radiolabelled matrix metalloproteinase inhibitors or alpha(v)beta(3) integrin antagonists. Great efforts are being made to develop suitable tracers for different target structures. All of the major strategies focusing on the development of radiolabelled compounds for use with positron emission tomography are summarized in this review. However, because the most intensive work is concentrated on the development of radiolabelled RGD peptides for imaging alpha(v)beta(3) expression, which has successfully made its way from bench to bedside, these developments are especially emphasized.

Bioinorganic aspects of angiogenesis

Angiogenesis is a physiologic process characterized by the sprouting of a new blood vessel from a pre-existing one. In mammalians the angiogenesis process is dormant, except for few physiological conditions such as wound healing and ovulation. In healthy individuals angiogenesis is finely tuned by pro- and anti-angiogenic factors. The shift from this equilibrium, under pathological conditions (pathological angiogenesis) is associated with several human diseases of high social impact. An efficient angiogenesis also requires that angiogenic factors cooperate with microenvironment derived co-factors, including metals. In this Perspective we describe the bioinorganic aspects of angiogenesis which contribute to a better understanding of the molecular mechanisms and regulation of angiogenesis. In particular, the role of metals, especially copper, metalloproteinases, and the current status on the imaging of angiogenesis targeting VEGF or VEGF receptors will be discussed.

'In vivo' optical approaches to angiogenesis imaging

In recent years, molecular imaging gained significant importance in biomedical research. Optical imaging developed into a modality which enables the visualization and quantification of all kinds of cellular processes and cancerous cell growth in small animals. Novel gene reporter mice and cell lines and the development of targeted and cleavable fluorescent "smart" probes form a powerful imaging toolbox. The development of systems collecting tomographic bioluminescence and fluorescence data enabled even more spatial accuracy and more quantitative measurements. Here we describe various bioluminescent and fluorescent gene reporter models and probes that can be used to specifically image and quantify neovascularization or the angiogenic process itself.

ScVEGF-PEG-HBED-CC and scVEGF-PEG-NOTA conjugates: comparison of easy-to-label recombinant proteins for [68Ga]PET imaging of VEGF receptors in angiogenic vasculature

INTRODUCTION

VEGF receptors play a key role in angiogenesis and are important targets for several approved and many experimental drugs. Imaging of VEGF receptor expression in malignant tumors would provide important information, which can influence patient management. The aim of this study was the development of an easy-to-label positron-emitting tracer for imaging VEGF receptors. The tracer is based on engineered single-chain VEGF (scVEGF), expressed with cysteine-containing fusion tag (Cys-tag) for site-specific conjugation of PEGylated bifunctional chelating agents, HBED-CC or NOTA, suitable for labeling with (68)Ga at ambient temperature.

METHODS

scVEGF-PEG-HBED-CC was synthesized by activating a single carboxyl group of the [Fe(HBED-CC)](-) complex with N-hydroxysuccinimide. Reaction of the activated complex with NH(2)-PEG-maleimide was followed by site-specific conjugation of PEGylated chelator to a thiol group in Cys-tag of scVEGF. The scVEGF-PEG-NOTA conjugate was synthesized using NHS-PEG-maleimide and p-NH(2)-Bn-NOTA. (68)Ga complexation was performed in HEPES buffer (pH 4.2) at room temperature. The functional activity after labeling was tested by radioligand cell binding assays. Biodistribution and PET studies in tumor-bearing mice were performed after 1, 2, 3 and 4 h postinjection.

RESULTS

The radiolabeling of scVEGF-PEG-HBED-CC proved more efficient than scVEGF-PEG-NOTA allowing to stop the reaction after 4 min (>97% radiochemical yield). Radioligand cell binding assays performed on HEK-293 cells overexpressing VEGFR-2 revealed no change in the binding properties of (68)Ga-radiolabeled scVEGF relative to other scVEGF-based tracers. Both tracers showed comparable results in biodistribution, such as tumor accumulation and low liver uptake. The tracers were stable in 50% human serum for at least 72 h.

CONCLUSIONS

The conjugates scVEGF-PEG-HBED-CC and scVEGF-PEG-NOTA revealed comparable in vivo characteristics and allowed easy-to-perform labeling with high stability for fast [(68)Ga]PET imaging of VEGF receptors in angiogenic vasculature.

Changes in vascular permeability and expression of different angiogenic factors following anti-angiogenic treatment in rat glioma

BACKGROUND

Anti-angiogenic treatments of malignant tumors targeting vascular endothelial growth factor receptors (VEGFR) tyrosine kinase are being used in different early stages of clinical trials. Very recently, VEGFR tyrosine kinase inhibitor (Vetanalib, PTK787) was used in glioma patient in conjunction with chemotherapy and radiotherapy. However, changes in the tumor size, tumor vascular permeability, vascular density, expression of VEGFR2 and other angiogenic factors in response to PTK787 are not well documented. This study was to determine the changes in tumor size, vascular permeability, fractional plasma volume and expression of VEGFR2 in PTK787 treated U-251 glioma rat model by in vivo magnetic resonance imaging (MRI) and single photon emission computed tomography (SPECT). The findings were validated with histochemical and western blot studies.

METHODOLOGIES AND PRINCIPAL FINDINGS

Seven days after implantation of U251 glioma cells, animals were treated with either PTK787 or vehicle-only for two weeks, and then tumor size, tumor vascular permeability transfer constant (K(trans)), fractional plasma volume (fPV) and expression of VEGFR2 and other relevant angiogenic factors were assessed by in vivo MRI and SPECT (Tc-99-HYNIC-VEGF), and by immunohistochemistry and western blot analysis. Dynamic contrast-enhanced MRI (DCE-MRI) using a high molecular weight contrast agent albumin-(GdDTPA) showed significantly increased K(trans) at the rim of the treated tumors compared to that of the central part of the treated as well as the untreated (vehicle treated) tumors. Size of the tumors was also increased in the treated group. Expression of VEGFR2 detected by Tc-99m-HYNIC-VEGF SPECT also showed significantly increased activity in the treated tumors. In PTK787-treated tumors, histological staining revealed increase in microvessel density in the close proximity to the tumor border. Western blot analysis indicated increased expression of VEGF, SDF-1, HIF-1alpha, VEGFR2, VEGFR3 and EGFR at the peripheral part of the treated tumors compared to that of central part of the treated tumors. Similar expression patters were not observed in vehicle treated tumors.

CONCLUSION

These findings indicate that PTK787 treatment induced over expression of VEGF as well as the Flk-1/VEGFR2 receptor tyrosine kinase, especially at the rim of the tumor, as proven by DCE-MRI, SPECT imaging, immunohistochemistry and western blot.

Advanced carotid plaque imaging

Treatment of carotid artery stenosis by endarterectomy or stenting can significantly reduce stroke risk. In clinical practice, indication for surgery or stenting is primarily based on the degree of stenosis, but there is growing awareness that pathophysiological features within a vulnerable plaque play a key role in predicting stroke risk. Important molecular processes associated with plaque vulnerability are inflammation, lipid accumulation, proteolysis, apoptosis, angiogenesis and thrombosis. The rapidly emerging field of molecular and functional imaging strategies allows identification of pathophysiological processes in carotid artery stenosis. We aimed to review the literature regarding the current most promising advanced imaging techniques in carotid artery disease. Various advanced imaging methods are available, such as high-resolution magnetic resonance imaging (HR-MRI), single photon emission computed tomography (SPECT), positron emission tomography (PET) and near-infrared fluorescence (NIRF). Radionuclide and fluorescent tracers that identify inflammation, apoptosis and proteolysis, such as FDG, MMP probes and Annexin A5, are promising. A combination of activity of molecular processes and detailed anatomic information can be obtained, providing a powerful tool in the identification of the vulnerable plaque. With these developments, we are entering a new era of imaging techniques in the selection of patients for carotid surgery.

Molecular imaging in clinical trials

Imaging of biological processes using specific molecular probes allows exploration of the mechanism of action and efficacy for new therapies. This molecular imaging has made use of modalities including single photon emission computed tomography (SPECT), positron emission tomography (PET), magnetic resonance imaging (MRI), and optical techniques. Molecular imaging can be used to explore many of the hallmarks of cancer biology, including angiogenesis, proliferation, tissue invasion, evasion of apoptosis, and self-sufficiency in growth signals. Since many of these aspects of cancer biology are in turn the targets of novel therapies in development, molecular imaging techniques have great potential to inform trials of these new agents. The high cost of clinical drug development mandates the optimisation of early phase trial design to maximise the collection of evidence for efficacy and proof of mechanism, endpoints which have, in a number of examples, already been provided by molecular imaging. The variety provided by novel chemistry, and the availability of isotopes with varying physical properties, particularly suits PET imaging as a functional modality for application in clinical trials.

Molecular imaging: 18F-FDG PET and a whole lot more

The intention of this review is to provide information about the rapidly evolving field of molecular imaging and its potential impact on the clinical practice of nuclear medicine. On completing this article the reader should be able to define molecular imaging, describe the ways in which molecular imaging can be used, identify some of the biologic processes that can be targeted with molecular imaging agents, and list the modalities that can be used for molecular imaging, along with the strengths and weaknesses of each.

Site-specifically biotinylated VEGF(121) for near-infrared fluorescence imaging of tumor angiogenesis

The vascular endothelial growth factor (VEGF)/VEGF receptor (VEGFR) pathway is considered to be one of the most important regulators of angiogenesis and a key target in anticancer treatment. Imaging VEGFR expression can serve as a new paradigm for assessing the efficacy of antiangiogenic cancer therapy, improving cancer management, and elucidating the role and modulation of VEGF/VEGFR signaling during cancer development and intervention. In this study we developed an Avi-tagged VEGF(121) protein, which is site-specifically biotinylated in the presence of bacterial BirA biotin ligase. BirA biotinylated VEGF(121)-Avi (VEGF(121)-Avib) forms a stable complex with streptavidin-IRDye800 (SA800) that retains high affinity for VEGFR in vitro and allows receptor specific targeting in vivo in a 67NR murine xenograft model. In contrast, chemical coupling of IRDye800 abrogated the VEGFR binding ability of the modified protein both in vitro and in vivo. The VEGF(121)-Avib/SA800 complex (VEGF-Avib/SA800) may be used for quantitative and repetitive near-infrared fluorescence imaging of VEGFR expression and translated into clinic for evaluating cancer and other angiogenesis related diseases.

Advanced magnetic resonance imaging of cerebral cavernous malformations: part II. Imaging of lesions in murine models

OBJECTIVE

We sought to assess the appearance of cerebral cavernous malformations (CCM) on magnetic resonance imaging (MRI) scans in murine Ccm1 and Ccm2 gene knockout models and to develop a technique of lesion localization for correlative pathobiological studies

METHODS

Brains from 18 CCM mutant mice (Ccm1 Trp53 and Ccm2 Trp53) and 28 control animals were imaged by gradient recalled echo (T2*)-weighted MRI scans at 4.7- and 14.1-T in vivo and/or ex vivo. After MRI scanning, the brains were removed and stained with hematoxylin and eosin, and cells were laser-microdissected for molecular biological studies.

RESULTS

T2*-weighted MRI scans of brains in vivo and ex vivo revealed lesions similar to human CCMs in mutant mice, but not in control animals. Stereotactic localization and hematoxylin and eosin staining of correlative tissue sections confirmed lesion histology and revealed other areas of dilated capillaries in the same brains. Some lesions were identified by MRI scans at 14.1-T, but not at 4.7-T. Polymerase chain reaction amplification from Ccm1 and beta-actin genes was demonstrated from nucleic acids extracted from laser microdissected lesional and perilesional cells.

CONCLUSION

The high-field MRI techniques offer new opportunities for further investigation of disease pathogenesis in vivo, and the localization, staging, and histobiological dissection of lesions, including the presumed earliest stages of CCM lesion development.

A molecular imaging paradigm to rapidly profile response to angiogenesis-directed therapy in small animals

PURPOSE

The development of novel angiogenesis-directed therapeutics is hampered by the lack of non-invasive imaging metrics capable of assessing treatment response. We report the development and validation of a novel molecular imaging paradigm to rapidly assess response to angiogenesis-directed therapeutics in preclinical animal models.

PROCEDURES

A monoclonal antibody-based optical imaging probe targeting vascular endothelial growth factor receptor-2 (VEGFR2) expression was synthesized and evaluated in vitro and in vivo via multispectral fluorescence imaging.

RESULTS

The optical imaging agent demonstrated specificity for the target receptor in cultured endothelial cells and in vivo. The agent exhibited significant accumulation within 4T1 xenograft tumors. Mice bearing 4T1 xenografts and treated with sunitinib exhibited both tumor growth arrest and decreased accumulation of NIR800-alphaVEGFR2ab compared to untreated cohorts (p = 0.0021).

CONCLUSIONS

Molecular imaging of VEGFR2 expression is a promising non-invasive biomarker for assessing angiogenesis and evaluating the efficacy of angiogenesis-directed therapies.

Multimodality molecular imaging of tumor angiogenesis

Molecular imaging is a key component of 21st-century cancer management. The vascular endothelial growth factor (VEGF)/VEGF receptor signaling pathway and integrin alpha v beta 3, a cell adhesion molecule, play pivotal roles in regulating tumor angiogenesis, the growth of new blood vessels. This review summarizes the current status of tumor angiogenesis imaging with SPECT, PET, molecular MRI, targeted ultrasound, and optical techniques. For integrin alpha v beta 3 imaging, only nanoparticle-based probes, which truly target the tumor vasculature rather than tumor cells because of poor extravasation, are discussed. Once improvements in the in vivo stability, tumor-targeting efficacy, and pharmacokinetics of tumor angiogenesis imaging probes are made, translation to clinical applications will be critical for the maximum benefit of these novel agents. The future of tumor angiogenesis imaging lies in multimodality and nanoparticle-based approaches, imaging of protein-protein interactions, and quantitative molecular imaging. Combinations of multiple modalities can yield complementary information and offer synergistic advantages over any modality alone. Nanoparticles, possessing multifunctionality and enormous flexibility, can allow for the integration of therapeutic components, targeting ligands, and multimodality imaging labels into one entity, termed "nanomedicine," for which the ideal target is tumor neovasculature. Quantitative imaging of tumor angiogenesis and protein-protein interactions that modulate angiogenesis will lead to more robust and effective monitoring of personalized molecular cancer therapy. Multidisciplinary approaches and cooperative efforts from many individuals, institutions, industries, and organizations are needed to quickly translate multimodality tumor angiogenesis imaging into multiple facets of cancer management. Not limited to cancer, these novel agents can also have broad applications for many other angiogenesis-related diseases.

Molecular imaging in drug development

Molecular imaging can allow the non-invasive assessment of biological and biochemical processes in living subjects. Such technologies therefore have the potential to enhance our understanding of disease and drug activity during preclinical and clinical drug development, which could aid decisions to select candidates that seem most likely to be successful or to halt the development of drugs that seem likely to ultimately fail. Here, with an emphasis on oncology, we review the applications of molecular imaging in drug development, highlighting successes and identifying key challenges that need to be addressed for successful integration of molecular imaging into the drug development process.

Targeted molecular imaging in oncology: focus on radiation therapy

Anatomically based technologies (computed tomography scans, magnetic resonance imaging, and so on) are in routine use in radiotherapy for planning and assessment purposes. Even with improvements in imaging, however, radiotherapy is still limited in efficacy and toxicity in certain applications. Further advances may be provided by technologies that image the molecular activities of tumors and normal tissues. Possible uses for molecular imaging include better localization of tumor regions and early assay for the radiation response of tumors and normal tissues. Critical to the success of this approach is the identification and validation of molecular probes that are suitable in the radiotherapy context. Recent developments in molecular-imaging probes and integration of functional imaging with radiotherapy are promising. This review focuses on recent advances in molecular imaging strategies and probes that may aid in improving the efficacy of radiotherapy.

Angiogenesis and gliomas: current issues and development of surrogate markers

Despite significant improvements, current therapies have yet to cure infiltrative gliomas. Glioma progression is strongly dependent on the development of a new vascular network that occurs primarily by angiogenesis. Hypoxia and genetic anomalies within a glioma trigger the angiogenic switch, thus upregulating angiogenic factors and downregulating antiangiogenic factors. The main factors indicative of angiogenesis are now well known, and more recently, differences based on grade and subtype have been reported. New data also indicate a potential role for postnatal vasculogenesis with bone marrow endothelial progenitors in addition to angiogenesis in tumor vascular development. All of these factors may have therapeutic implications. Antiangiogenic therapies are presently being developed; more than 80 trials are ongoing. Initial results indicate that epidermal growth factor receptor inhibitors, anti-metalloproteases, and thalidomide do not demonstrate strong anti-tumor activity. Thus, antiangiogenic agents combined with conventional therapies and second-generation antiangiogenic drugs for targeting multiple molecular pathways are presently being tested. Clinical experience also demonstrates the failure of conventional imaging to monitor these new approaches accurately. New advances in the design of surrogate markers for angiogenesis have been reported for both magnetic resonance and molecular imaging techniques. This article summarizes the mechanisms of the angiogenic switch based on tumor grade and subtype, reviews completed and ongoing clinical trials, and details the present and the future of surrogate markers for angiogenesis in gliomas.