Positron emission tomography imaging of tumor angiogenesis with a (61/64)Cu-labeled F(ab')(2) antibody fragment

Side-by-Side Comparison of uPAR-Targeting Optical Imaging Antibodies and Antibody Fragments for Fluorescence-Guided Surgery of Solid Tumors

PURPOSE

Radical resection is paramount for curative oncological surgery. Fluorescence-guided surgery (FGS) aids in intraoperative identification of tumor-positive resection margins. This study aims to assess the feasibility of urokinase plasminogen activator receptor (uPAR) targeting antibody fragments for FGS in a direct comparison with their parent IgG in various relevant in vivo models.

PROCEDURES

Humanized anti-uPAR monoclonal antibody MNPR-101 (uIgG) was proteolytically digested into F(ab')2 and Fab fragments named uFab2 and uFab. Surface plasmon resonance (SPR) and cell assays were used to determine in vitro binding before and after fluorescent labeling with IRDye800CW. Mice bearing subcutaneous HT-29 human colonic cancer cells were imaged serially for up to 120 h after fluorescent tracer administration. Imaging characteristics and ex vivo organ biodistribution were further compared in orthotopic pancreatic ductal adenocarcinoma (BxPc-3-luc2), head-and-neck squamous cell carcinoma (OSC-19-luc2-GFP), and peritoneal carcinomatosis (HT29-luc2) models using the clinical Artemis fluorescence imaging system.

RESULTS

Unconjugated and conjugated uIgG, uFab2, and uFab specifically recognized uPAR in the nanomolar range as determined by SPR and cell assays. Subcutaneous tumors were clearly identifiable with tumor-to-background ratios (TBRs) > 2 after 72 h for uIgG-800F and 24 h for uFab2-800F and uFab-800F. For the latter two, mean fluorescence intensities (MFIs) dipped below predetermined threshold after 72 h and 36 h, respectively. Tumors were easily identified in the orthotopic models with uIgG-800F consistently having the highest MFIs and uFab2-800F and uFab-800F having similar values. In biodistribution studies, kidney and liver fluorescence approached tumor fluorescence after uIgG-800F administration and surpassed tumor fluorescence after uFab2-800F or uFab-800F administration, resulting in interference in the abdominal orthotopic mouse models.

CONCLUSIONS

In a side-by-side comparison, FGS with uPAR-targeting antibody fragments compared with the parent IgG resulted in earlier tumor visualization at the expense of peak fluorescence intensity.

Noninvasive Evaluation of EGFR Expression of Digestive Tumors Using 99mTc-MAG3-Cet-F(ab ′ )2-Based SPECT/CT Imaging

Purpose. This study is aimed at investigating the feasibility of cetuximab (Cet) F(ab ${}^{\prime }$ )2 fragment- (Cet-F(ab ${}^{\prime }$ )2-) based single photon emission tomography/computed tomography (SPECT/CT) for assessing the epidermal growth factor receptor (EGFR) expression in digestive tumor mouse models. Methods. Cet-F(ab ${}^{\prime }$ )2 was synthesized using immunoglobulin G-degrading enzyme of Streptococcus pyogenes (IdeS) protease and purified with protein A beads. The product and its in vitro stability in normal saline and 1% bovine serum albumin were analyzed with sodium dodecyl sulfate–polyacrylamide gel electrophoresis. The EGFR expression in the human colon tumor cell line HT29 and the human stomach tumor cell line MGC803 were verified using western blotting and immunocytochemistry. Cet-F(ab ${}^{\prime }$ )2 was conjugated with 5(6)-carboxytetramethylrhodamine succinimidyl ester to demonstrate its binding ability to the MGC803 and HT29 cells. Cet-F(ab ${}^{\prime }$ )2 was conjugated with NHS-MAG3 for 99mTc radiolabeling. The best imaging time was determined using a biodistribution assay at 1, 4, 16, and 24 h after injection of the 99mTc-MAG3-Cet-F(ab ${}^{\prime }$ )2 tracer. Furthermore, 99mTc-MAG3-Cet-F(ab ${}^{\prime }$ )2 SPECT/CT was performed on MGC803 and HT29 tumor-bearing nude mice. Results. HT29 cells had low EGFR expression while MGC803 cell exhibited the high EGFR expression. Cet-F(ab ${}^{\prime }$ )2 and intact cetuximab showed similar high binding ability to MGC803 cells but not to HT29 cells. Cet-F(ab ${}^{\prime }$ )2 and 99mTc-MAG3-Cet-F(ab ${}^{\prime }$ )2 showed excellent in vitro stability. The biodistribution assay showed that the target to nontarget ratio was the highest at 16 h ( $17.29\pm 5.72$ , $n=4$ ) after tracer injection. The 99mTc-MAG3-Cet-F(ab ${}^{\prime }$ )2-based SPECT/CT imaging revealed rapid and sustained tracer uptake in MGC803 tumors rather than in HT29 tumors with high image contrast, which was consistent with the results in vitro. Conclusion. SPECT/CT imaging using 99mTc-MAG3-Cet-F(ab ${}^{\prime }$ )2 enables the evaluation of the EGFR expression in murine EGFR-positive tumors, indicating the potential utility for noninvasive evaluation of the EGFR expression in tumors.

64Cu-labeled daratumumab F(ab')2 fragment enables early visualization of CD38-positive lymphoma

PURPOSE

Abnormal CD38 expression in some hematologic malignancies, including lymphoma, has made it a biomarker for targeted therapies. Daratumumab (Dara) is the first FDA-approved CD38-specific monoclonal antibody, enabling successfully immunoPET imaging over the past years. Radiolabeled Dara however has a long blood circulation and delayed tumor uptake which can limit its applications. The focus of this study is to develop ⁶⁴Cu-labeled Dara-F(ab')₂ for the visualization of CD38 in lymphoma models.

METHODS

F(ab')₂ fragment was prepared from Dara using an IdeS enzyme and purified with Protein A beads. Western blotting, flow cytometry, and surface plasmon resonance (SPR) were performed for in vitro assay. Probes were labeled with ⁶⁴Cu after the chelation of 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA). Small animal PET imaging and quantitative analysis were performed after injection of ⁶⁴Cu-labeled Dara-F(ab')₂, IgG-F(ab')₂, and Dara for evaluation in lymphoma models.

RESULTS

Flow cytometry and SPR assay proved the specific binding ability of Dara-F(ab')₂ and NOTA-Dara-F(ab')₂ in vitro. Radiolabeling yield of [⁶⁴Cu]Cu-NOTA-Dara-F(ab')₂ was over 90% and with a specific activity of 4.0 ± 0.6 × 10³ MBq/μmol (n = 5). PET imaging showed [⁶⁴Cu]Cu-NOTA-Dara-F(ab')₂ had a rapid and high tumor uptake as early as 2 h (6.9 ± 1.2%ID/g) and peaked (9.5 ± 0.7%ID/g) at 12 h, whereas [⁶⁴Cu]Cu-NOTA-Dara reached its tumor uptake peaked at 48 h (8.3 ± 1.4%ID/g, n = 4). In comparison, IgG-F(ab')₂ and HBL-1 control groups found no noticeable tumor uptake. [⁶⁴Cu]Cu-NOTA-Dara-F(ab')₂ had significantly lower uptake in blood pool, bone, and muscle than [⁶⁴Cu]Cu-NOTA-Dara and its tumor-to-blood and tumor-to-muscle ratios were significantly higher than controls.

CONCLUSIONS

[⁶⁴Cu]Cu-NOTA-Dara-F(ab')₂ showed a rapid and high tumor uptake in CD38-positive lymphoma models with favorable imaging contrast, showing its promise as a potential PET imaging agent for future clinical applications.

Two-pore physiologically based pharmacokinetic model validation using whole-body biodistribution of trastuzumab and different-size fragments in mice

In the past, our lab proposed a two-pore PBPK model for different-size protein therapeutics using de novo derived parameters and the model was validated using plasma PK data of different-size antibody fragments digitized from the literature (Li Z, Shah DK, J Pharmacokinet Pharmacodynam 46(3):305-318, 2009). To further validate the model using tissue distribution data, whole-body biodistribution study of 6 different-size proteins in mice were conducted. Studied molecules covered a wide MW range (13-150 kDa). Plasma PK and tissue distribution profiles is 9 tissues were measured, including heart, lung, liver, spleen, kidney, skin, muscle, small intestine, large intestine. Tumor exposure of different-size proteins were also evaluated. The PBPK model was validated by comparing percentage predictive errors (%PE) between observed and model predicted results for each type of molecule in each tissue. Model validation showed that the two-pore PBPK model was able to predict plasma, tissues and tumor PK of all studied molecules relatively well. This model could serve as a platform for developing a generic PBPK model for protein therapeutics in the future.

Endoglin/CD105-Based Imaging of Cancer and Cardiovascular Diseases: A Systematic Review

Molecular imaging of pathologic lesions can improve efficient detection of cancer and cardiovascular diseases. A shared pathophysiological feature is angiogenesis, the formation of new blood vessels. Endoglin (CD105) is a coreceptor for ligands of the Transforming Growth Factor-β (TGF-β) family and is highly expressed on angiogenic endothelial cells. Therefore, endoglin-based imaging has been explored to visualize lesions of the aforementioned diseases. This systematic review highlights the progress in endoglin-based imaging of cancer, atherosclerosis, myocardial infarction, and aortic aneurysm, focusing on positron emission tomography (PET), single-photon emission computed tomography (SPECT), magnetic resonance imaging (MRI), near-infrared fluorescence (NIRF) imaging, and ultrasound imaging. PubMed was searched combining the following subjects and their respective synonyms or relevant subterms: "Endoglin", "Imaging/Image-guided surgery". In total, 59 papers were found eligible to be included: 58 reporting about preclinical animal or in vitro models and one ex vivo study in human organs. In addition to exact data extraction of imaging modality type, tumor or cardiovascular disease model, and tracer (class), outcomes were described via a narrative synthesis. Collectively, the data identify endoglin as a suitable target for intraoperative and diagnostic imaging of the neovasculature in tumors, whereas for cardiovascular diseases, the evidence remains scarce but promising.

Engineered Antibody Fragment against the Urokinase Plasminogen Activator for Fast Delineation of Triple-Negative Breast Cancer by Positron Emission Tomography

The urokinase plasminogen activator (uPA) and its cofactors are important regulators of tumor initiation and progression (including metastasis), and its overexpression is associated with unfavorable situations in cancer patients. We have previously used positron emission tomography (PET) imaging with a radiolabeled monoclonal antibody against the uPA (named ATN-291) to detect the uPA signaling activity in various cancer types; however, good tumor contrast can only be observed 24 h postinjection. To shorten the antibody circulation time and decrease interactions of ATN-291 with the mononuclear phagocyte system (MPS), our goal in this study is to develop an engineered antibody fragment (F(ab')₂) from the parent antibody. By pepsin digestion and chromatography purification, ATN-291 F(ab')₂ was obtained and characterized. Subsequently, it was conjugated with NOTA-Bn-NCS or fluorescein isothiocyanate (FITC) for PET imaging and fluorescence-mediated cellular analysis (i.e., flow cytometry or fluorescence microscopy). We confirmed that ATN-291 F(ab')₂ still maintained a good targeting efficacy for the uPA in MDA-MB-231 cells (uPA⁺) and it had a faster blood clearance speed compared with ATN-291, while its interaction with MPS has been significantly decreased. In rodent tumor xenografts, radiolabeled ATN-291 F(ab')₂ had a selective and persistent uptake in MDA-MB-231 tumors, with an early tumor-to-blood ratio of 1.3 ± 0.8 (n = 4) at 2 h postinjection from PET imaging. During our observation, radiolabeled ATN-291 F(ab')₂ was excreted from both renal and hepatobiliary pathways. Radiolabeled ATN-291 F(ab')₂ was also used for detecting uPA fluctuation during the tumor treatment in test animals. We concluded that radiolabeled ATN-291 F(ab')₂ could be used as fast as PET cancer diagnostics with versatile applicability.

Noninvasive Evaluation of CD20 Expression Using 64Cu-Labeled F(ab')2 Fragments of Obinutuzumab in Lymphoma

CD20-overexpressed non-Hodgkin lymphoma typically indicates progressive malignancy. Obinutuzumab is a next-generation Food and Drug Administration-approved humanized monoclonal antibody that targets CD20. Previous studies with ⁸⁹Zr-labeled obinutuzumab have successfully imaged CD20 in vivo. However, delayed tumor uptake and increased radioactive exposure caused by long blood circulation limit its clinical translation. This study aimed to develop ⁶⁴Cu-labeled F(ab')₂ fragments of obinutuzumab for imaging CD20 in lymphoma xenograft tumor models. Methods: F(ab')₂ fragments were produced from obinutuzumab using an IgG-degrading enzyme of Streptococcus pyogenes (IdeS) enzyme and purified with protein A beads. Sodium dodecyl sulfate polyacrylamide gel electrophoresis and high-performance liquid chromatography were performed to evaluate the products and their stability. F(ab')₂ products were conjugated with p-SCN-Bn-NOTA (NOTA) for ⁶⁴Cu radiolabeling. Western blotting was performed to screen the CD20 expression levels of lymphoma cells. Enzyme-linked immunosorbent assay, flow cytometry, and confocal imaging were used to test the binding affinity in vitro. Serial PET imaging and biodistribution studies in subcutaneous lymphoma-bearing mice were performed using ⁶⁴Cu-NOTA-F(ab')₂-obinutuzumab or ⁶⁴Cu-NOTA-F(ab')₂-IgG. Results: F(ab')₂-obinutuzumab and F(ab')₂-IgG produced by the IdeS digestion system were confirmed with sodium dodecyl sulfate polyacrylamide gel electrophoresis and high-performance liquid chromatography. The radiochemical purity of ⁶⁴Cu-labeled F(ab')₂ fragments was no less than 98%, and the specific activity was 56.3 ± 7.9 MBq/mg (n = 6). Among the 5 lymphoma cell lines, Ramos showed the strongest expression of CD20, and CLL-155 showed the lowest, as confirmed by enzyme-linked immunosorbent assay, flow cytometry, and confocal imaging. PET imaging revealed rapid and sustained tumor uptake of ⁶⁴Cu-NOTA-F(ab')₂-obinutuzumab in Ramos tumor-bearing mice. The peak tumor uptake (9.08 ± 1.67 percentage injected dose per gram of tissue [%ID/g]) in the Ramos model was significantly higher than that in the CCL-155 model (2.78 ± 0.62 %ID/g) or the ⁶⁴Cu-NOTA-F(ab')₂-IgG control (1.93 ± 0.26 %ID/g, n = 4, P < 0.001). The tumor-to-blood and tumor-to-muscle ratios were 7.3 ± 1.6 and 21.9 ± 9.0, respectively, at 48 h after injection in the ⁶⁴Cu-NOTA-F(ab')₂-obinutuzumab group. Of the measured off-target organs, the kidneys showed the highest uptake. Ex vivo immunofluorescent staining verified the differential CD20 expression in the Ramos and CCL-155 tumor models. Conclusion: This study demonstrated that ⁶⁴Cu-NOTA-F(ab')₂-obinutuzumab had a rapid and sustained tumor uptake in CD20-positive lymphoma with high contrast, which could enable noninvasive evaluation of CD20 levels in the clinic.

ImmunoPET: Concept, Design, and Applications

Immuno-positron emission tomography (immunoPET) is a paradigm-shifting molecular imaging modality combining the superior targeting specificity of monoclonal antibody (mAb) and the inherent sensitivity of PET technique. A variety of radionuclides and mAbs have been exploited to develop immunoPET probes, which has been driven by the development and optimization of radiochemistry and conjugation strategies. In addition, tumor-targeting vectors with a short circulation time (e.g., Nanobody) or with an enhanced binding affinity (e.g., bispecific antibody) are being used to design novel immunoPET probes. Accordingly, several immunoPET probes, such as 89Zr-Df-pertuzumab and 89Zr-atezolizumab, have been successfully translated for clinical use. By noninvasively and dynamically revealing the expression of heterogeneous tumor antigens, immunoPET imaging is gradually changing the theranostic landscape of several types of malignancies. ImmunoPET is the method of choice for imaging specific tumor markers, immune cells, immune checkpoints, and inflammatory processes. Furthermore, the integration of immunoPET imaging in antibody drug development is of substantial significance because it provides pivotal information regarding antibody targeting abilities and distribution profiles. Herein, we present the latest immunoPET imaging strategies and their preclinical and clinical applications. We also emphasize current conjugation strategies that can be leveraged to develop next-generation immunoPET probes. Lastly, we discuss practical considerations to tune the development and translation of immunoPET imaging strategies.

89Zr-Labeled Anti-PD-L1 Antibody Fragment for Evaluating In Vivo PD-L1 Levels in Melanoma Mouse Model

The rise of programmed death-1 (PD-1)/PD-L1 immune checkpoint inhibitor therapy has been one of the most promising developments in melanoma research. However, not all the melanoma patients respond to such immune checkpoint blockade. There is a great need of biomarkers for appropriate melanoma patient selection and therapeutic efficacy monitoring. The objective of this study is to develop a novel radiolabeled anti-PD-L1 antibody fragment, as an imaging biomarker, for evaluating the in vivo PD-L1 levels in melanoma. The Df-conjugated F(ab')₂ fragment of the anti-mouse PD-L1 antibody was successfully synthesized and radiolabeled with ⁸⁹Zr. Both Df-F(ab')₂ and ⁸⁹Zr-Df-F(ab')₂ maintained the nano-molar murine PD-L1 targeting specificity and affinity. ⁸⁹Zr-Df-F(ab')₂ showed less uptake in normal liver tissue in mice compared with its full antibody counterpart ⁸⁹Zr-Df-anti-PD-L1. Positron emission tomography (PET)/computed tomography images clearly showed that ⁸⁹Zr-Df-F(ab')₂ possessed superior pharmacokinetics and imaging contrast over the radiolabeled full antibody, with much earlier and higher tumor uptake (5.5 times more at 2 h post injection) and much lower liver background (51% reduction at 2 h post injection). The specific and high murine PD-L1-targeting uptake at tumor foci coupled with fast clearance of ⁸⁹Zr-Df-F(ab')₂ highlighted its potential for in vivo PET imaging of murine PD-L1 levels and future development of radiolabeled anti-human PD-L1 fragment for potential application in melanoma patients.

Preparation and preliminary bioevaluation studies of 68 Ga-NOTA-rituximab fragments as radioimmunoscintigraphic agents for non-Hodgkin lymphoma

Rituximab is used for the treatment of non-Hodgkin lymphoma (NHL). This study focuses on development of ⁶⁸ Ga-labeled rituximab fragments, (⁶⁸ Ga-NOTA-F (ab')-rituximab and ⁶⁸ Ga-NOTA-F (ab')₂ -rituximab, as PET-imaging agents for NHL. Rituximab was digested with immobilized pepsin and papain to yield F (ab')₂ and Fab fragments respectively that were characterized by size exclusion HPLC (SE-HPLC) and SDS-PAGE. They were conjugated with p-SCN-Bn-NOTA, labeled with ⁶⁸ Ga and characterized by SE-HPLC. Intact rituximab was labeled with gallium-68 for comparison. Specificity of ⁶⁸ Ga-labeled immunoconjugates was ascertained by immunoreactivity and cell binding studies in Raji cells, while biodistribution studies were performed in normal Swiss mice. Gradient SDS-PAGE under nonreducing condition showed molecular weights of F (ab')₂ -rituximab and F (ab')-rituximab as approximately 100 and 40 Kd, respectively. Radiochemical purity (RCP) of ⁶⁸ Ga-NOTA-F (ab')₂ -rituximab and ⁶⁸ Ga-NOTA-F (ab')-rituximab were 98.2 ± 0.5% and 98.8 ± 0.2% respectively with retention times of 17.1 ± 0.1 min and 19.3 ± 0.1 min in SE-HPLC. ⁶⁸ Ga-labeled rituximab fragments were stable in saline and serum up to 2-hour post preparation and exhibited specificity to CD20 antigen. Immunoreactivity of ⁶⁸ Ga-labeled immunoconjugates was greater than 80%. Clearance of the fragmented radioimmunoconjugates was predominantly through renal route. Preliminary results from this study demonstrate the potential of ⁶⁸ Ga- NOTA-F (ab')₂ -rituximab and ⁶⁸ Ga-NOTA-F (ab')-rituximab as PET imaging agents for NHL.

Antibody-Based Tracers for PET/SPECT Imaging of Chronic Inflammatory Diseases

Chronic inflammatory diseases are often progressive, resulting not only in physical damage to patients but also social and economic burdens, making early diagnosis of them critical. Nuclear medicine techniques can enhance the detection of inflammation by providing functional as well as anatomical information when combined with other modalities such as magnetic resonance imaging, computed tomography or ultrasonography. Although small molecules and peptides were mainly used for the treatment and imaging of chronic inflammatory diseases in the past, antibodies and their fragments have also been emerging for chronic inflammatory diseases as they show high specificity to their targets and can have various biological half-lives depending on how they are engineered. In addition, imaging with antibodies or their fragments can visualize the in vivo biodistribution of the probes or help monitor therapeutic responses, thereby providing physicians with a greater understanding of drug behavior in vivo and another means of monitoring their patients. In this review, we introduce various targets and radiolabeled antibody-based probes for the molecular imaging of chronic inflammatory diseases in preclinical and clinical studies. Targets can be classified into three different categories: 1) cell-adhesion molecules, 2) surface markers on immune cells, and 3) cytokines or enzymes. The limitations and future directions of using radiolabeled antibodies for imaging inflammatory diseases are also discussed.

Imaging Tiny Hepatic Tumor Xenografts via Endoglin-Targeted Paramagnetic/Optical Nanoprobe

Surgery is the mainstay for treating hepatocellular carcinoma (HCC). However, it is a great challenge for surgeons to identify HCC in its early developmental stage. The diagnostic sensitivity for a tiny HCC with a diameter less than 1.0 cm is usually as low as 10-33% for computed tomography (CT) and 29-43% for magnetic resonance imaging (MRI). Although MRI is the preferred imaging modality for detecting HCC, with its unparalleled spatial resolution for soft tissue, the commercially available contrast agent, such as Gd³⁺-DTPA, cannot accurately define HCC because of its short circulation lifetime and lack of tumor-targeting specificity. Endoglin (CD105), a type I membrane glycoprotein, is highly expressed both in HCC cells and in the endothelial cells of neovasculature, which are abundant at the tumor periphery. In this work, a novel single-stranded DNA oligonucleotide-based aptamer was screened by systematic evolution of ligands in an exponential enrichment assay and showed a high binding affinity ( K_D = 98 pmol/L) to endoglin. Conjugating the aptamers and imaging reporters on a G5 dendrimer created an HCC-targeting nanoprobe that allowed the successful visualization of orthotopic HCC xenografts with diameters as small as 1-4 mm. Significantly, the invasive tumor margin was clearly delineated, with a tumor to normal ratio of 2.7 by near-infrared (NIR) fluorescence imaging and 2.1 by T₁-weighted MRI. This multimodal nanoprobe holds promise not only for noninvasively defining tiny HCC by preoperative MRI but also for guiding tumor excision via intraoperative NIR fluorescence imaging, which will probably gain benefit for the patient's therapeutic response and improve the survival rate.

Intrabilayer 64Cu Labeling of Photoactivatable, Doxorubicin-Loaded Stealth Liposomes

Doxorubicin (Dox)-loaded stealth liposomes (similar to those in clinical use) can incorporate small amounts of porphyrin-phospholipid (PoP) to enable chemophototherapy (CPT). PoP is also an intrinsic and intrabilayer ⁶⁴Cu chelator, although how radiolabeling impacts drug delivery has not yet been assessed. Here, we show that ⁶⁴Cu can radiolabel the stable bilayer of preformed Dox-loaded PoP liposomes with inclusion of 1% ethanol without inducing drug leakage. Dox-PoP liposomes labeled with intrabilayer copper behaved nearly identically to unlabeled ones in vitro and in vivo with respect to physical parameters, pharmacokinetics, and CPT efficacy. Positron emission tomography and near-infrared fluorescence imaging visualized orthotopic mammary tumors in mice with passive liposome accumulation following administration. A single CPT treatment with 665 nm light (200 J/cm²) strongly inhibited primary tumor growth. Liposomes accumulated in lung metastases, based on NIR imaging. These results establish the feasibility of CPT interventions guided by intrinsic multimodal imaging of Dox-loaded stealth PoP liposomes.

Selecting Targets for Tumor Imaging: An Overview of Cancer-Associated Membrane Proteins

Tumor targeting is a booming business: The global therapeutic monoclonal antibody market accounted for more than $78 billion in 2012 and is expanding exponentially. Tumors can be targeted with an extensive arsenal of monoclonal antibodies, ligand proteins, peptides, RNAs, and small molecules. In addition to therapeutic targeting, some of these compounds can also be applied for tumor visualization before or during surgery, after conjugation with radionuclides and/or near-infrared fluorescent dyes. The majority of these tumor-targeting compounds are directed against cell membrane-bound proteins. Various categories of targetable membrane-bound proteins, such as anchoring proteins, receptors, enzymes, and transporter proteins, exist. The functions and biological characteristics of these proteins determine their location and distribution on the cell membrane, making them more, or less, accessible, and therefore, it is important to understand these features. In this review, we evaluate the characteristics of cancer-associated membrane proteins and discuss their overall usability for cancer targeting, especially focusing on imaging applications.

Molecular Imaging of Angiogenesis in Cardiac Regeneration

PURPOSE OF REVIEW

Myocardial infarction (MI) leading to heart failure displays an important cause of death worldwide. Adequate restoration of blood flow to prevent this transition is a crucial factor to improve long-term morbidity and mortality. Novel regenerative therapies have been thoroughly investigated within the past decades.

RECENT FINDINGS

Increased angiogenesis in infarcted myocardium has shown beneficial effects on the prognosis of MI; therefore, the proangiogenic capacity of currently tested treatments is of specific interest. Molecular imaging to visualize formation of new blood vessels in vivo displays a promising option to monitor proangiogenic effects of regenerative substances.

SUMMARY

Based on encouraging results in preclinical models, molecular angiogenesis imaging has recently been applied in a small set of patients. This article reviews recent literature on noninvasive in vivo molecular imaging of angiogenesis after MI as an integral part of cardiac regeneration.

SPECT and PET imaging of angiogenesis and arteriogenesis in pre-clinical models of myocardial ischemia and peripheral vascular disease

Purpose

The extent of neovascularization determines the clinical outcome of coronary artery disease and other occlusive cardiovascular disorders. Monitoring of neovascularization is therefore highly important. This review article will elaborately discuss preclinical studies aimed at validating new nuclear angiogenesis and arteriogenesis tracers. Additionally, we will briefly address possible obstacles that should be considered when designing an arteriogenesis radiotracer.

Methods

A structured medline search was the base of this review, which gives an overview on different radiopharmaceuticals that have been evaluated in preclinical models.

Results

Neovascularization is a collective term used to indicate different processes such as angiogenesis and arteriogenesis. However, while it is assumed that sensitive detection through nuclear imaging will facilitate translation of successful therapeutic interventions in preclinical models to the bedside, we still lack specific tracers for neovascularization imaging. Most nuclear imaging research to date has focused on angiogenesis, leaving nuclear arteriogenesis imaging largely overlooked.

Conclusion

Although angiogenesis is the process which is best understood, there is no scarcity in theoretical targets for arteriogenesis imaging.

In vivo targeting of metastatic breast cancer via tumor vasculature-specific nano-graphene oxide

Angiogenesis, i.e. the formation of neovasculatures, is a critical process during cancer initiation, progression, and metastasis. Targeting of angiogenic markers on the tumor vasculature can result in more efficient delivery of nanomaterials into tumor since no extravasation is required. Herein we demonstrated efficient targeting of breast cancer metastasis in an experimental murine model with nano-graphene oxide (GO), which was conjugated to a monoclonal antibody (mAb) against follicle-stimulating hormone receptor (FSHR). FSHR has been confirmed to be a highly selective tumor vasculature marker, which is abundant in both primary and metastatic tumors. These functionalized GO nano-conjugates had diameters of ∼120 nm based on atomic force microscopy (AFM), TEM, and dynamic laser scattering (DLS) measurement. (64)Cu was incorporated as a radiolabel which enabled the visualization of these GO conjugates by positron emission tomography (PET) imaging. Breast cancer lung metastasis model was established by intravenous injection of click beetle green luciferase-transfected MDA-MB-231 (denoted as cbgLuc-MDA-MB-231) breast cancer cells into female nude mice and the tumor growth was monitored by bioluminescence imaging (BLI). Systematic in vitro and in vivo studies have been performed to investigate the stability, targeting efficacy and specificity, and tissue distribution of GO conjugates. Flow cytometry and fluorescence microscopy examination confirmed the targeting specificity of FSHR-mAb attached GO conjugates against cellular FSHR. More potent and persistent uptake of (64)Cu-NOTA-GO-FSHR-mAb in cbgLuc-MDA-MB-231 nodules inside the lung was witnessed when compared with that of non-targeted GO conjugates ((64)Cu-NOTA-GO). Histology evaluation also confirmed the vasculature accumulation of GO-FSHR-mAb conjugates in tumor at early time points while they were non-specifically captured in liver and spleen. In addition, these GO conjugates can serve as good drug carriers with satisfactory drug loading capacity (e.g. for doxorubicin [DOX], 756 mg/g). Enhanced drug delivery efficiency in cbgLuc-MDA-MB-231 metastatic sites was demonstrated in DOX-loaded GO-FSHR-mAb by fluorescence imaging. This FSHR-targeted, GO-based nanoplatform can serve as a useful tool for early metastasis detection and targeted delivery of therapeutics.

Stromal Targets for Fluorescent-Guided Oncologic Surgery

Pre-operative imaging techniques are essential for tumor detection and diagnosis, but offer limited help during surgery. Recently, the applicability of imaging during oncologic surgery has been recognized, using near-infrared fluorescent dyes conjugated to targeting antibodies, peptides, or other vehicles. Image-guided oncologic surgery (IGOS) assists the surgeFon to distinguish tumor from normal tissue during operation, and can aid in recognizing vital structures. IGOS relies on an optimized combination of a dedicated fluorescent camera system and specific probes for targeting. IGOS probes for clinical use are not widely available yet, but numerous pre-clinical studies have been published and clinical trials are being established or prepared. Most of the investigated probes are based on antibodies or peptides against proteins on the membranes of malignant cells, whereas others are directed against stromal cells. Targeting stroma cells for IGOS has several advantages. Besides the high stromal content in more aggressive tumor types, the stroma is often primarily located at the periphery/invasive front of the tumor, which makes stromal targets particularly suited for imaging purposes. Moreover, because stroma up-regulation is a physiological reaction, most proteins to be targeted on these cells are “universal” and not derived from a specific genetic variation, as is the case with many upregulated proteins on malignant cancer cells.

Influence of molecular size on tissue distribution of antibody fragments

Biodistribution coefficients (BC) allow estimation of the tissue concentrations of proteins based on the plasma pharmacokinetics. We have previously established the BC values for monoclonal antibodies. Here, this concept is extended by development of a relationship between protein size and BC values. The relationship was built by deriving the BC values for various antibody fragments of known molecular weight from published biodistribution studies. We found that there exists a simple exponential relationship between molecular weight and BC values that allows the prediction of tissue distribution of proteins based on molecular weight alone. The relationship was validated by a priori predicting BC values of 4 antibody fragments that were not used in building the relationship. The relationship was also used to derive BC50 values for all the tissues, which is the molecular weight increase that would result in 50% reduction in tissue uptake of a protein. The BC50 values for most tissues were found to be ~35 kDa. An ability to estimate tissue distribution of antibody fragments based on the BC vs. molecular size relationship established here may allow better understanding of the biologics concentrations in tissues responsible for efficacy or toxicity. This relationship can also be applied for rational development of new biotherapeutic modalities with optimal biodistribution properties to target (or avoid) specific tissues.

Noninvasive brain cancer imaging with a bispecific antibody fragment, generated via click chemistry

Early diagnosis remains a task of upmost importance for reducing cancer morbidity and mortality. Successful development of highly specific companion diagnostics targeting aberrant molecular pathways of cancer is needed for sensitive detection, accurate diagnosis, and opportune therapeutic intervention. Herein, we generated a bispecific immunoconjugate [denoted as Bs-F(ab)2] by linking two antibody Fab fragments, an anti-epidermal growth factor receptor (EGFR) Fab and an anti-CD105 Fab, via bioorthogonal "click" ligation of trans-cyclooctene and tetrazine. PET imaging of mice bearing U87MG (EGFR/CD105(+/+)) tumors with (64)Cu-labeled Bs-F(ab)2 revealed a significantly enhanced tumor uptake [42.9 ± 9.5 percentage injected dose per gram (%ID/g); n = 4] and tumor-to-background ratio (tumor/muscle ratio of 120.2 ± 44.4 at 36 h postinjection; n = 4) compared with each monospecific Fab tracer. Thus, we demonstrated that dual targeting of EGFR and CD105 provides a synergistic improvement on both affinity and specificity of (64)Cu-NOTA-Bs-F(ab)2. (64)Cu-NOTA-Bs-F(ab)2 was able to visualize small U87MG tumor nodules (<5 mm in diameter), owing to high tumor uptake (31.4 ± 10.8%ID/g at 36 h postinjection) and a tumor/muscle ratio of 76.4 ± 52.3, which provided excellent sensitivity for early detection. Finally, we successfully confirmed the feasibility of a ZW800-1-labeled Bs-F(ab)2 for near-infrared fluorescence imaging and image-guided surgical resection of U87MG tumors. More importantly, our rationale can be used in the construction of other disease-targeting bispecific antibody fragments for early detection and diagnosis of small malignant lesions.

New radiotracers for imaging of vascular targets in angiogenesis-related diseases

Tremendous advances over the last several decades in positron emission tomography (PET) and single photon emission computed tomography (SPECT) allow for targeted imaging of molecular and cellular events in the living systems. Angiogenesis, a multistep process regulated by the network of different angiogenic factors, has attracted world-wide interests, due to its pivotal role in the formation and progression of different diseases including cancer, cardiovascular diseases (CVD), and inflammation. In this review article, we will summarize the recent progress in PET or SPECT imaging of a wide variety of vascular targets in three major angiogenesis-related diseases: cancer, cardiovascular diseases, and inflammation. Faster drug development and patient stratification for a specific therapy will become possible with the facilitation of PET or SPECT imaging and it will be critical for the maximum benefit of patients.

In vivo tumor vasculature targeted PET/NIRF imaging with TRC105(Fab)-conjugated, dual-labeled mesoporous silica nanoparticles

Multifunctional mesoporous silica nanoparticles (MSN) with well-integrated multimodality imaging properties have generated increasing research interest in the past decade. However, limited progress has been made in developing MSN-based multimodality imaging agents to image tumors. We describe the successful conjugation of, copper-64 (⁶⁴Cu, t_1/2 = 12.7 h), 800CW (a near-infrared fluorescence [NIRF] dye), and TRC105 (a human/murine chimeric IgG1 monoclonal antibody) to the surface of MSN via well-developed surface engineering procedures, resulting in a dual-labeled MSN for in vivo targeted positron emission tomography (PET) imaging/NIRF imaging of the tumor vasculature. Pharmacokinetics and tumor targeting efficacy/specificity in 4T1 murine breast tumor-bearing mice were thoroughly investigated through various in vitro, in vivo, and ex vivo experiments. Dual-labeled MSN is an attractive candidate for future cancer theranostics.

Engineering of hollow mesoporous silica nanoparticles for remarkably enhanced tumor active targeting efficacy

Hollow mesoporous silica nanoparticle (HMSN) has recently gained increasing interests due to their tremendous potential as an attractive nano-platform for cancer imaging and therapy. However, possibly due to the lack of efficient in vivo targeting strategy and well-developed surface engineering techniques, engineering of HMSN for in vivo active tumor targeting, quantitative tumor uptake assessment, multimodality imaging, biodistribution and enhanced drug delivery have not been achieved to date. Here, we report the in vivo tumor targeted positron emission tomography (PET)/near-infrared fluorescence (NIRF) dual-modality imaging and enhanced drug delivery of HMSN using a generally applicable surface engineering technique. Systematic in vitro and in vivo studies have been performed to investigate the stability, tumor targeting efficacy and specificity, biodistribution and drug delivery capability of well-functionalized HMSN nano-conjugates. The highest uptake of TRC105 (which binds to CD105 on tumor neovasculature) conjugated HMSN in the 4T1 murine breast cancer model was ~10%ID/g, 3 times higher than that of the non-targeted group, making surface engineered HMSN a highly attractive drug delivery nano-platform for future cancer theranostics.

Multimodality imaging of RNA interference

The discovery of small interfering RNAs (siRNAs) and their potential to knock down virtually any gene of interest has ushered in a new era of RNA interference (RNAi). Clinical use of RNAi faces severe limitations due to inefficiency delivery of siRNA or short hairpin RNA (shRNA). Many molecular imaging techniques have been adopted in RNAi-related research for evaluation of siRNA/shRNA delivery, biodistribution, pharmacokinetics, and the therapeutic effect. In this review article, we summarize the current status of in vivo imaging of RNAi. The molecular imaging techniques that have been employed include bioluminescence/fluorescence imaging, magnetic resonance imaging/ spectroscopy, positron emission tomography, single-photon emission computed tomography, and various combinations of these techniques. Further development of non-invasive imaging strategies for RNAi, not only focusing on the delivery of siRNA/shRNA but also the therapeutic efficacy, is critical for future clinical translation. Rigorous validation will be needed to confirm that biodistribution of the carrier is correlated with that of siRNA/shRNA, since imaging only detects the label (e.g. radioisotopes) but not the gene or carrier themselves. It is also essential to develop multimodality imaging approaches for realizing the full potential of therapeutic RNAi, as no single imaging modality may be sufficient to simultaneously monitor both the gene delivery and silencing effect of RNAi.

Positron emission tomography imaging of CD105 expression in a rat myocardial infarction model with (64)Cu-NOTA-TRC105

Biological changes following myocardial infarction (MI) lead to increased secretion of angiogenic factors that subsequently stimulate the formation of new blood vessels as a compensatory mechanism to reverse ischemia. The goal of this study was to assess the role of CD105 expression during MI-induced angiogenesis by positron emission tomography (PET) imaging using (64)Cu-labeled TRC105, an anti-CD105 monoclonal antibody. MI was induced by ligation of the left anterior descending (LAD) artery in female rats. Echocardiography and (18)F-fluoro-2-deoxy-D-glucose ((18)F-FDG) PET scans were performed on post-operative day 3 to confirm the presence of MI in the infarct group and intact heart in the sham group, respectively. Ischemia-induced angiogenesis was non-invasively monitored with (64)Cu-NOTA-TRC105 (an extensively validated PET tracer in our previous studies) PET on post-operative days 3, 10, and 17. Tracer uptake in the infarct zone was highest on day 3 following MI, which was significantly higher than that in the sham group (1.41 ± 0.45 %ID/g vs 0.57 ± 0.07 %ID/g; n=3, p<0.05). Subsequently, tracer uptake in the infarct zone decreased over time to the background level on day 17, whereas tracer uptake in the heart of sham rats remained low at all time points examined. Histopathology documented increased CD105 expression following MI, which corroborated in vivo findings. This study indicated that PET imaging of CD105 can be a useful tool for MI-related research, which can potentially improve MI patient management in the future upon clinical translation of the optimized PET tracers.

Positron emission tomography imaging of angiogenesis in a murine hindlimb ischemia model with 64Cu-labeled TRC105

The goal of this study was to assess ischemia-induced angiogenesis with (64)Cu-NOTA-TRC105 positron emission tomography (PET) in a murine hindlimb ischemia model of peripheral artery disease (PAD). CD105 binding affinity/specificity of NOTA-conjugated TRC105 (an anti-CD105 antibody) was evaluated by flow cytometry, which exhibited no difference from unconjugated TRC105. BALB/c mice were anesthetized, and the right femoral artery was ligated to induce hindlimb ischemia, with the left hindlimb serving as an internal control. Laser Doppler imaging showed that perfusion in the ischemic hindlimb plummeted to ∼ 20% of the normal level after surgery and gradually recovered to near normal level on day 24. Ischemia-induced angiogenesis was noninvasively monitored and quantified with (64)Cu-NOTA-TRC105 PET on postoperative days 1, 3, 10, 17, and 24. (64)Cu-NOTA-TRC105 uptake in the ischemic hindlimb increased significantly from the control level of 1.6 ± 0.2 %ID/g to 14.1 ± 1.9 %ID/g at day 3 (n = 3) and gradually decreased with time (3.4 ± 1.9 %ID/g at day 24), which correlated well with biodistribution studies performed on days 3 and 24. Blocking studies confirmed the CD105 specificity of tracer uptake in the ischemic hindlimb. Increased CD105 expression on days 3 and 10 following ischemia was confirmed by histology and reverse transcription polymerase chain reaction (RT-PCR). This is the first report of PET imaging of CD105 expression during ischemia-induced angiogenesis. (64)Cu-NOTA-TRC105 PET may play multiple roles in future PAD-related research and improve PAD patient management by identifying the optimal timing of treatment and monitoring the efficacy of therapy.

Imaging tumor angiogenesis in breast cancer experimental lung metastasis with positron emission tomography, near-infrared fluorescence, and bioluminescence

The goal of this study was to develop a molecular imaging agent that can allow for both positron emission tomography (PET) and near-infrared fluorescence (NIRF) imaging of CD105 expression in metastatic breast cancer. TRC105, a chimeric anti-CD105 monoclonal antibody, was labeled with both a NIRF dye (i.e., IRDye 800CW) and (64)Cu to yield (64)Cu-NOTA-TRC105-800CW. Flow cytometry analysis revealed no difference in CD105 binding affinity/specificity between TRC105 and NOTA-TRC105-800CW. Serial bioluminescence imaging (BLI) was carried out to non-invasively monitor the lung tumor burden in BALB/c mice, after intravenous injection of firefly luciferase-transfected 4T1 (i.e., fLuc-4T1) murine breast cancer cells to establish the experimental lung metastasis model. Serial PET imaging revealed that fLuc-4T1 lung tumor uptake of (64)Cu-NOTA-TRC105-800CW was 11.9 ± 1.2, 13.9 ± 3.9, and 13.4 ± 2.1 %ID/g at 4, 24, and 48 h post-injection respectively (n = 3). Biodistribution studies, blocking fLuc-4T1 lung tumor uptake with excess TRC105, control experiments with (64)Cu-NOTA-cetuximab-800CW (which served as an isotype-matched control), ex vivo BLI/PET/NIRF imaging, autoradiography, and histology all confirmed CD105 specificity of (64)Cu-NOTA-TRC105-800CW. Successful PET/NIRF imaging of tumor angiogenesis (i.e., CD105 expression) in the breast cancer experimental lung metastasis model warrants further investigation and clinical translation of dual-labeled TRC105-based agents, which can potentially enable early detection of small metastases and image-guided surgery for tumor removal.