Sustained release of a human PD-L1 single-domain antibody using peptide-based hydrogels

Monoclonal antibodies (mAbs) targeting the immune checkpoint axis, which contains the programmed cell death protein-1 (PD-1) and its ligand PD-L1, revolutionized the field of oncology. Unfortunately, the large size of mAbs and the presence of an Fc fraction limit their tumor penetrative capacities and support off-target effects, potentially resulting in unresponsive patients and immune-related adverse events (irAEs) respectively. Single-domain antibodies (sdAbs) are ten times smaller than conventional mAbs and represent an emerging antibody subclass that has been proposed as next generation immune checkpoint inhibitor (ICI) therapeutics. They demonstrate favorable characteristics, such as an excellent stability, high antigen-binding affinity and an enhanced tumor penetration. Because sdAbs have a short half-life, methods to prolong their presence in the circulation and at the target site might be necessary in some cases to unfold their full therapeutic potential. In this study, we investigated a peptide-based hydrogel as an injectable biomaterial depot formulation for the sustained release of the human PD-L1 sdAb K2. We showed that a hydrogel composed of the amphipathic hexapeptide hydrogelator H-FQFQFK-NH2 prolonged the in vivo release of K2 after subcutaneous (s.c.) injection, up to at least 72 h, as monitored by SPECT/CT and fluorescence imaging. Additionally, after encapsulation in the hydrogel and s.c. administration, a significantly extended systemic presence and tumor uptake of K2 was observed in mice bearing a melanoma tumor expressing human PD-L1. Altogether, this study describes how peptide hydrogels can be exploited to provide the sustained release of sdAbs, thereby potentially enhancing its clinical and therapeutic effects.

The GEM-handle as convenient labeling strategy for bimodal single-domain antibody-based tracers carrying 99mTc and a near-infrared fluorescent dye for intra-operative decision-making

Intra-operative fluorescence imaging has demonstrated its ability to improve tumor lesion identification. However, the limited tissue penetration of the fluorescent signals hinders the detection of deep-lying or occult lesions. Integrating fluorescence imaging with SPECT and/or intra-operative gamma-probing synergistically combines the deep tissue penetration of gamma rays for tumor localization with the precision of fluorescence imaging for precise tumor resection. In this study, we detail the use of a genetically encoded multifunctional handle, henceforth referred to as a GEM-handle, for the development of fluorescent/radioactive bimodal single-domain antibody (sdAb)-based tracers. A sdAb that targets the urokinase plasminogen activator receptor (uPAR) was engineered to carry a GEM-handle containing a carboxy-terminal hexahistidine-tag and cysteine-tag. A two-step labeling strategy was optimized and applied to site-specifically label IRDye800CW and 99mTc to the sdAb. Bimodal labeling of the sdAbs proved straightforward and successful. 99mTc activity was however restricted to 18.5 MBq per nmol fluorescently-labeled sdAb to prevent radiobleaching of IRDye800CW without impeding SPECT/CT imaging. Subsequently, the in vivo biodistribution and tumor-targeting capacity of the bimodal tracer were evaluated in uPAR-positive tumor-bearing mice using SPECT/CT and fluorescence imaging. The bimodal sdAb showed expected renal background signals due to tracer clearance, along with slightly elevated non-specific liver signals. Four hours post-injection, both SPECT/CT and fluorescent images achieved satisfactory tumor uptake and contrast, with significantly higher values observed for the anti-uPAR bimodal sdAb compared to a control non-targeting sdAb. In conclusion, the GEM-handle is a convenient method for designing and producing bimodal sdAb-based tracers with adequate in vivo characteristics.

Safety assessment of fluorescently labeled anti-EGFR Nanobodies in healthy dogs

Introduction: Surgical resection is one of the main treatment options for several types of cancer, the desired outcome being complete removal of the primary tumor and its local metastases. Any malignant tissue that remains after surgery may lead to relapsing disease, negatively impacting the patient's quality of life and overall survival. Fluorescence imaging in surgical oncology aims to facilitate full resection of solid tumors through the visualization of malignant tissue during surgery, following the administration of a fluorescent contrast agent. An important class of targeting molecules are Nanobodies® (Nbs), small antigen-binding fragments derived from camelid heavy chain only antibodies. When coupled with a fluorophore, Nbs can bind to a specific receptor and demarcate tumor margins through a fluorescence camera, improving the accuracy of surgical intervention. A widely investigated target for fluorescence-guided surgery is the epidermal growth factor receptor (EGFR), which is overexpressed in several types of tumors. Promising results with the fluorescently labeled anti-EGFR Nb 7D12-s775z in murine models motivated a project employing the compound in a pioneering study in dogs with spontaneous cancer. Methods: To determine the safety profile of the study drug, three healthy purpose-bred dogs received an intravenous injection of the tracer at 5.83, 11.66, and 19.47 mg/m2, separated by a 14-day wash-out period. Physical examination and fluorescence imaging were performed at established time points, and the animals were closely monitored between doses. Blood and urine values were analyzed pre- and 24 h post administration. Results: No adverse effects were observed, and blood and urine values stayed within the reference range. Images of the oral mucosa, acquired with a fluorescence imaging device (Fluobeam®), suggest rapid clearance, which was in accordance with previous in vivo studies. Discussion: These are the first results to indicate that 7D12-s775z is well tolerated in dogs and paves the way to conduct clinical trials in canine patients with EGFR-overexpressing spontaneous tumors.

Severity Classification of Laboratory Animal Procedures in Two Belgian Academic Institutions

According to the EU Directive 2010/63, all animal procedures must be classified as non-recovery, mild, moderate or severe. Several examples are included in the Directive to help in severity classification. Since the implementation of the Directive, different publications and guidelines have been disseminated on the topic. However, due to the large variety of disease models and animal procedures carried out in many different animal species, guidance on the severity classification of specific procedures or models is often lacking or not specific enough. The latter is especially the case in disease models where the level of pain, suffering, distress and lasting harm depends on the duration of the study (for progressive disease models) or the dosage given (for infectious or chemically induced disease models). This, in turn, may lead to inconsistencies in severity classification between countries, within countries and even within institutions. To overcome this, two Belgian academic institutions with a focus on biomedical research collaborated to develop a severity classification for all the procedures performed. This work started with listing all in-house procedures and assigning them to 16 (sub)categories. First, we determined which parameters, such as clinical signs, dosage or duration, were crucial for severity classification within a specific (sub)category. Next, a severity classification was assigned to the different procedures, which was based on professional judgment by the designated veterinarians, members of the animal welfare body (AWB) and institutional animal ethics committee (AEC), integrating the available literature and guidelines. During the classification process, the use of vague terminology, such as 'minor impact', was avoided as much as possible. Instead, well-defined cut-offs between severity levels were used. Furthermore, we sought to define common denominators to group procedures and to be able to classify new procedures more easily. Although the primary aim is to address prospective severity, this can also be used to assess actual severity. In summary, we developed a severity classification for all procedures performed in two academic, biomedical institutions. These include many procedures and disease models in a variety of animal species for which a severity classification was not reported so far, or the terms that assign them to a different severity were too vague.

Method To Diversify Cyanine Chromophore Functionality Enables Improved Biomolecule Tracking and Intracellular Imaging

Heptamethine indocyanines are invaluable probes for near-infrared (NIR) imaging. Despite broad use, there are only a few synthetic methods to assemble these molecules, and each has significant limitations. Here, we report the use of pyridinium benzoxazole (PyBox) salts as heptamethine indocyanine precursors. This method is high yielding, simple to implement, and provides access to previously unknown chromophore functionality. We applied this method to create molecules to address two outstanding objectives in NIR fluorescence imaging. First, we used an iterative approach to develop molecules for protein-targeted tumor imaging. When compared to common NIR fluorophores, the optimized probe increases the tumor specificity of monoclonal antibody (mAb) and nanobody conjugates. Second, we developed cyclizing heptamethine indocyanines with the goal of improving cellular uptake and fluorogenic properties. By modifying both the electrophilic and nucleophilic components, we demonstrate that the solvent sensitivity of the ring-open/ring-closed equilibrium can be modified over a wide range. We then show that a chloroalkane derivative of a compound with tuned cyclization properties undergoes particularly efficient no-wash live cell imaging using organelle-targeted HaloTag self-labeling proteins. Overall, the chemistry reported here broadens the scope of accessible chromophore functionality, and, in turn, enables the discovery of NIR probes with promising properties for advanced imaging applications.

Near-Infrared Fluorescence Imaging of Pancreatic Cancer Using a Fluorescently Labelled Anti-CEA Nanobody Probe: A Preclinical Study

Molecular fluorescence-guided surgery using near-infrared light has the potential to improve the rate of complete resection of cancer. Typically, monoclonal antibodies are being used as targeting moieties, however smaller fragments, such as single-domain antibodies (i.e., Nanobodies®) improve tumor specificity and enable tracer injection on the same day as surgery. In this study, the feasibility of a carcinoembryonic antigen-targeting Nanobody (NbCEA5) conjugated to two zwitterionic dyes (ZW800-1 Forte [ZW800F] and ZW800-1) for visualization of pancreatic ductal adenocarcinoma (PDAC) was investigated. After site-specific conjugation of NbCEA5 to the zwitterionic dyes, binding specificity was evaluated on human PDAC cell lines with flow cytometry. A dose escalation study was performed for both NbCEA5-ZW800F and NbCEA5-ZW800-1 in mice with subcutaneously implanted pancreatic tumors. Fluorescence imaging was performed up to 24 h after intravenous injection. Furthermore, the optimal dose for NbCEA5-ZW800-1 was injected in mice with orthotopically implanted pancreatic tumors. A dose-escalation study showed superior mean fluorescence intensities for NbCEA5-ZW800-1 compared to NbCEA5-ZW800F. In the orthotopic tumor models, NbCEA5-ZW800-1 accumulated specifically in pancreatic tumors with a mean in vivo tumor-to-background ratio of 2.4 (SD = 0.23). This study demonstrated the feasibility and potential advantages of using a CEA-targeted Nanobody conjugated to ZW800-1 for intraoperative PDAC imaging.

Size-advantage of monovalent nanobodies against the macrophage mannose receptor for deep tumor penetration and tumor-associated macrophage targeting

Rationale: Nanobodies (Nbs) have emerged as an elegant alternative to the use of conventional monoclonal antibodies in cancer therapy, but a detailed microscopic insight into the in vivo pharmacokinetics of different Nb formats in tumor-bearers is lacking. This is especially relevant for the recognition and targeting of pro-tumoral tumor-associated macrophages (TAMs), which may be located in less penetrable tumor regions. Methods: We employed anti-Macrophage Mannose Receptor (MMR) Nbs, in a monovalent (m) or bivalent (biv) format, to assess in vivo TAM targeting. Intravital and confocal microscopy were used to analyse the blood clearance rate and targeting kinetics of anti-MMR Nbs in tumor tissue, healthy muscle tissue and liver. Fluorescence Molecular Tomography was applied to confirm anti-MMR Nb accumulation in the primary tumor and in metastatic lesions. Results: Intravital microscopy demonstrated significant differences in the blood clearance rate and macrophage targeting kinetics of (m) and (biv)anti-MMR Nbs, both in tumoral and extra-tumoral tissue. Importantly, (m)anti-MMR Nbs are superior in reaching tissue macrophages, an advantage that is especially prominent in tumor tissue. The administration of a molar excess of unlabelled (biv)anti-MMR Nbs increased the (m)anti-MMR Nb bioavailability and impacted on its macrophage targeting kinetics, preventing their accumulation in extra-tumoral tissue (especially in the liver) but only partially influencing their interaction with TAMs. Finally, anti-MMR Nb administration not only allowed the visualization of TAMs in primary tumors, but also at a distant metastatic site. Conclusions: These data describe, for the first time, a microscopic analysis of (m) and (biv)anti-MMR Nb pharmacokinetics in tumor and healthy tissues. The concepts proposed in this study provide important knowledge for the future use of Nbs as diagnostic and therapeutic agents, especially for the targeting of tumor-infiltrating immune cells.

Impact of doubling peptide length on in vivo hydrogel stability and sustained drug release

Peptide-based hydrogels represent promising systems for the sustained release of different types of drugs, ranging from small molecules to biologicals. Aiming at subcutaneous injection, which is a desirable parenteral administration route, especially for biologicals, we herein focus on physically crosslinked systems possessing thixotropic behaviour. The purpose of this study was to evaluate the in vitro and in vivo properties of hydrogels based on the amphipathic hexapeptide H-FQFQFK-NH₂, which served as the lead sequence. Upon doubling the length of this peptide, the dodecapeptide H-FQFQFKFQFQFK-NH₂ gave a significant improvement in terms of in vivo stability of the hydrogel post-injection, as monitored by nuclear SPECT/CT imaging. This increased hydrogel stability also led to a more prolonged in vivo release of encapsulated peptide cargoes. Even though no direct link with the mechanical properties of the hydrogels before injection could be made, an important effect of the subcutaneous medium was noticed on the rheological properties of the hydrogels in post in vivo injection measurements. The results were validated in vivo for a therapeutically relevant analgesic peptide using the hot-plate test as an acute pain model. It was confirmed that elongation of the hydrogelator sequence induced more extended antinociceptive effects. Altogether, this simple structural modification of the hydrogelating peptide could provide a basis for reaching longer durations of action upon use of these soft biomaterials.

Emerging applications of nanobodies in cancer therapy

Cancer is a heterogeneous disease, requiring treatment tailored to the unique phenotype of the patient's tumor. Monoclonal antibodies (mAbs) and variants thereof have enabled targeted therapies to selectively target cancer cells. Cancer cell-specific mAbs have been used for image-guided surgery and targeted delivery of radionuclides or toxic agents, improving classical treatment strategies. Cancer cell-specific mAbs can further inhibit tumor cell growth or can stimulate immune-mediated destruction of cancer cells, a feature that has also been achieved through mAb-mediated manipulation of immune cells and pathways. Drawbacks of mAbs and their variants, together with the discovery of camelid heavy chain-only antibodies and the many advantageous features of their variable domains, referred to as V_HHs, single domain antibodies or nanobodies (Nbs), resulted in the exploration of Nbs as an alternative targeting moiety. We therefore review the state-of-the-art as well as novel exploitation strategies of Nbs for targeted cancer therapy.

Fluorescent Anti-CEA Nanobody for Rapid Tumor-Targeting and Imaging in Mouse Models of Pancreatic Cancer

Tumor-specific targeting with fluorescent probes can enhance contrast for identification of cancer during surgical resection and visualize otherwise invisible tumor margins. Nanobodies are the smallest naturally-occurring antigen-binding molecules with rapid pharmacokinetics. The present work demonstrates the efficacy of a fluorescent anti-CEA nanobody conjugated to an IR800 dye to target and label patient derived pancreatic cancer xenografts. After intravenous administration, the probe rapidly localized to the pancreatic cancer tumors within an hour and had a tumor-to-background ratio of 2.0 by 3 h. The fluorescence signal was durable over a prolonged period of time. With the rapid kinetics afforded by fluorescent nanobodies, both targeting and imaging can be performed on the same day as surgery.

In vivo Visualization of M2 Macrophages in the Myocardium After Myocardial Infarction (MI) Using 68Ga-NOTA-Anti-MMR Nb: Targeting Mannose Receptor (MR, CD206) on M2 Macrophages

Introduction and Objectives

Wound healing after myocardial infarction (MI) is a dynamic and complex multiple phase process, and a coordinated cellular response is required for proper scar formation. The current paradigm suggests that pro-inflammatory monocytes infiltrate the MI zone during the initial pro-inflammatory phase and differentiate into inflammatory macrophages, and then switch their phenotypes to anti-inflammatory during the reparative phase. Visualization of the reparative phase post-MI is of great interest because it may reveal delayed resolution of inflammation, which in turn predicts adverse cardiac remodeling. Imaging of anti-inflammatory macrophages may also be used to assess therapy approaches aiming to modulate the inflammatory response in order to limit MI size. Reparative macrophages can be distinguished from inflammatory macrophages by the surface marker mannose receptor (MR, CD206). In this study we evaluated the feasibility of ⁶⁸Ga-NOTA-anti-MMR Nb for imaging of MR on alternatively activated macrophages in murine MI models.

Methods

Wildtype and MR-knockout mice and Wistar rats were subjected to MI via permanent ligation of the left coronary artery. Non-operated or sham-operated animals were used as controls. MR expression kinetics on cardiac macrophages was measured in mice using flow cytometry. PET/CT scans were performed 1 h after intravenous injection of ⁶⁸Ga-NOTA-anti-MMR Nb. Mice and rats were euthanized and hearts harvested for ex vivo PET/MRI, autoradiography, and staining. As a non-targeting negative control, ⁶⁸Ga-NOTA-BCII10 was used.

Results

In vivo-PET/CT scans showed focal radioactivity signals in the infarcted myocardium for ⁶⁸Ga-NOTA-anti-MMR Nb which were confirmed by ex vivo-PET/MRI scans. In autoradiography images, augmented uptake of the tracer was observed in infarcts, as verified by the histochemistry analysis. Immunofluorescence staining demonstrated the presence and co-localization of CD206- and CD68-positive cells, in accordance to infarct zone. No in vivo or ex vivo signal was observed in the animals injected with control Nb or in the sham-operated animals. ⁶⁸Ga-NOTA-anti-MMR Nb uptake in the infarcts of MR-knockout mice was negligibly low, confirming the specificity of ⁶⁸Ga-NOTA-anti-MMR Nb to MR.

Conclusion

This exploratory study highlights the potential of ⁶⁸Ga-NOTA-anti-MMR Nb to image MR-positive macrophages that are known to play a pivotal role in wound healing that follows acute MI.

Design and Validation of Site-Specifically Labeled Single-Domain Antibody-Based Tracers for in Vivo Fluorescence Imaging and Image-Guided Surgery

Near-infrared fluorescence molecular imaging has become an established preclinical technique to investigate molecular processes in vivo and to study novel therapies. Furthermore, fluorescence molecular imaging is gaining significant interest from clinicians as an intra-operative guidance tool. This technique makes use of targeted fluorescent tracers as contrast agents that recognize specific biomarkers expressed at the site of disease. Single-domain antibodies have shown to possess excellent properties for in vivo imaging in comparison to conventional antibodies. In this chapter, we describe a method for site-specific conjugation of a near-infrared fluorophore to single-domain antibodies by exploiting cysteine-maleimide chemistry. As opposed to random conjugation, site-specific conjugation results in a homogenously labeled fluorescent tracer and avoids inference with antigen binding.

Rapid tumor-labeling kinetics with a site-specific near-infrared anti-CEA nanobody in a patient-derived orthotopic xenograft mouse model of colon cancer

BACKGROUND/OBJECTIVES

Nanobodies are the smallest biologic antigen-binding fragments derived from camelid-derived antibodies. Nanobodies effect a peak tumor signal within minutes of injection and present a novel opportunity for fluorescence-guided surgery (FGS). The present study demonstrates the efficacy of an anti-CEA nanobody conjugated to near-infrared fluorophore LICOR-IRDye800CW for rapid intraoperative tumor labeling of colon cancer.

METHODS

LS174T human colon cancer cells or fragments of patient-derived colon cancer were implanted subcutaneously or orthotopically in nude mice. Anti-CEA nanobodies were conjugated with IRDye800CW and 1-3 nmol were injected intravenously. Mice were serially imaged over time. Peak fluorescence signal and tumor-to-background ratio (TBR) were recorded.

RESULTS

Colon cancer tumors were detectable using fluorescent anti-CEA nanobody within 5 min of injection at all three doses. Maximal fluorescence intensity was observed within 15 min-3 h for all three doses with TBR values ranging from 1.3 to 2.3. In the patient-derived model of colon cancer, fluorescence was detectable with a TBR of 4.6 at 3 h.

CONCLUSIONS

Fluorescent anti-CEA nanobodies rapidly and specifically labeled colon cancer in cell-line-based and patient-derived orthotopic xenograft (PDOX) models. The kinetics of nanobodies allow for same day administration and imaging. Anti-CEA-nb-800 is a promising and practical molecule for FGS of colon cancer.

Reducing the renal retention of low- to moderate-molecular-weight radiopharmaceuticals

The field of nuclear imaging and therapy is rapidly progressing with the development of targeted radiopharmaceuticals that show rapid targeting and rapid clearance with minimal background. Unfortunately, they are often reabsorbed in the kidneys, leading to possible nephrotoxicity, limiting the therapeutic dose, and/or reducing imaging quality. The blocking of endocytic receptors has been extensively used as a strategy to reduce kidney radiation. Alternatively, the physicochemical properties of radiotracers can be modulated to either prevent their reuptake or promote the excretion of radiometabolites. Other interesting strategies focus on the insertion of a cleavable linker between the radiolabel and the targeting moiety or pretargeting approaches in which the targeting moiety and radiolabel are administered separately. In the context of this review, we will discuss the latest advances and insights on strategies used to reduce renal retention of low- to moderate-molecular-weight radiopharmaceuticals.

Targeted Repolarization of Tumor-Associated Macrophages via Imidazoquinoline-Linked Nanobodies

Tumor-associated macrophages (TAMs) promote the immune suppressive microenvironment inside tumors and are, therefore, considered as a promising target for the next generation of cancer immunotherapies. To repolarize their phenotype into a tumoricidal state, the Toll-like receptor 7/8 agonist imidazoquinoline IMDQ is site-specifically and quantitatively coupled to single chain antibody fragments, so-called nanobodies, targeting the macrophage mannose receptor (MMR) on TAMs. Intravenous injection of these conjugates result in a tumor- and cell-specific delivery of IMDQ into MMRhigh TAMs, causing a significant decline in tumor growth. This is accompanied by a repolarization of TAMs towards a pro-inflammatory phenotype and an increase in anti-tumor T cell responses. Therefore, the therapeutic benefit of such nanobody-drug conjugates may pave the road towards effective macrophage re-educating cancer immunotherapies.

Decorating sdAbs with Chelators: Effect of Conjugation on Biodistribution and Functionality

Single domain antibodies (sdAbs) have proven to be valuable probes for molecular imaging. In order to produce such probes, one strategy is the functionalization of the reactive amine side chain of lysines with a chelator, resulting in a mixture of compounds with a different degree of conjugation. In this study, we implemented anion exchange chromatography (AEX) to separate the different compounds or fractions that were further characterized and evaluated to study the impact of the conjugation degree on pharmacokinetic properties and functionality. Anti-HER2 and anti-MMR sdAbs were functionalized with NOTA or DTPA chelator. Anion exchange chromatography was performed using 0.02 mol/L Tris pH 7.5 as the first solvent and 0.25 M or 0.4 M NaCl (in case of NOTA chelator or DTPA chelator, respectively) as the second solvent applied as a gradient. The fractions were characterized via mass spectrometry (MS), surface plasmon resonance (SPR), and isoelectric focusing gel electrophoresis (IEF), while in vivo studies were performed after radiolabeling with either ⁶⁸Ga (NOTA) or ¹¹¹In (DTPA) to assess the impact of the conjugation degree on pharmacokinetics. AEX could successfully be applied to separate fractions of (chelator)_n-anti-HER2 and (chelator)_n-anti-MMR sdAb constructs. MS confirmed the identity of different peaks obtained in the separation process. SPR measurement suggests a small loss of affinity for (chelator)₃-anti-sdAb, while IEF revealed a correlated decrease in isoelectric point (pI) with the number of conjugated chelators. Interestingly, both the reduction in affinity and in pI was stronger with the DTPA chelator than with NOTA for both sdAbs. In vivo data showed no significant differences in organ uptake for any construct, except for (DTPA)_n-anti-MMR, which showed a significantly higher liver uptake for (DTPA)₁-anti-MMR compared to (DTPA)₂-anti-MMR and (DTPA)₃-anti-MMR. For all constructs in general, high kidney uptake was observed, due to the typical renal clearance of sdAb-based tracers. The kidney uptake showed significant differences between fractions of a same construct and indicates that a higher conjugation degree improves kidney clearance. AEX allows the separation of sdAbs with a different degree of conjugation and provides the opportunity to further characterize individual fractions. The conjugation of a chelator to sdAbs can alter some properties of the tracers, such as pI; however, the impact on the general biodistribution profile and tumor targeting was minimal.

The Design and Preclinical Evaluation of a Single-Label Bimodal Nanobody Tracer for Image-Guided Surgery

Intraoperative guidance using targeted fluorescent tracers can potentially provide surgeons with real-time feedback on the presence of tumor tissue in resection margins. To overcome the limited depth penetration of fluorescent light, combining fluorescence with SPECT/CT imaging and/or gamma-ray tracing has been proposed. Here, we describe the design and preclinical validation of a novel bimodal nanobody-tracer, labeled using a “multifunctional single attachment point” (MSAP) label, integrating a Cy5 fluorophore and a diethylenetriaminepentaacetic acid (DTPA) chelator into a single structure. After conjugation of the bimodal MSAP to primary amines of the anti-HER2 nanobody 2Rs15d and ¹¹¹In-labeling of DTPA, the tracer’s characteristics were evaluated in vitro. Subsequently, its biodistribution and tumor targeting were assessed by SPECT/CT and fluorescence imaging over 24 h. Finally, the tracer’s ability to identify small, disseminated tumor lesions was investigated in mice bearing HER2-overexpressing SKOV3.IP1 peritoneal lesions. [¹¹¹In]In-MSAP.2Rs15d retained its affinity following conjugation and remained stable for 24 h. In vivo SPECT/CT and fluorescence images showed specific uptake in HER2-overexpressing tumors with low background. High tumor-to-muscle ratios were obtained at 1h p.i. and remained 19-fold on SPECT/CT and 3-fold on fluorescence images over 24 h. In the intraperitoneally disseminated model, the tracer allowed detection of larger lesions via nuclear imaging, while fluorescence enabled accurate removal of submillimeter lesions. Bimodal nuclear/fluorescent nanobody-tracers can thus be conveniently designed by conjugation of a single-molecule MSAP-reagent carrying a fluorophore and chelator for radioactive labeling. Such tracers hold promise for clinical applications.

Tumor-specific near-infrared nanobody probe rapidly labels tumors in an orthotopic mouse model of pancreatic cancer

BACKGROUND

Nanobodies, derived from camelid antibodies made of only heavy chains, are the smallest, biologic, antigen-binding fragments (~15kDa) with faster pharmacokinetics and better tumor penetration efficiency than standard antibodies. The present study evaluates the efficacy of a fluorescent, anti-carcinoembryonic antigen (CEA) nanobody for rapid tumor labeling in an orthotopic mouse model of pancreatic cancer.

METHODS

Anti-CEA or control nanobodies were conjugated with the near-infrared fluorophore IRDye 800CW. Fragments of BxPC-3 (high-CEA expressing) or MiaPACA-2 (low-CEA expressing) human pancreatic cancer cell lines were orthotopically implanted into the pancreatic tail of nude mice. After tumors reached 7 to 10 mm in size, 2 nmol anti-CEA or control nanobody-IRDye800CW were injected intravenously. Mice were imaged at various time points hours post-injection.

RESULTS

Anti-CEA nanobodies clearly labeled BxPC3 orthotopic pancreatic tumors 3 hours after injection. The signal was present as early as 15 minutes after injection and was robust at 1 to 3 hours after injection with a tumor-to-background ratio of 2.66. In contrast, there was very low accumulation in the low CEA-expressing, MiaPACA2 pancreatic orthotopic tumors. The fluorophore-conjugated nanobody was specific for CEA-expressing tumors, while the control nanobody did not show any tumor-specific signal. Both nanobodies had strong kidney uptake as expected for small-molecule probes. The fluorescence signal was detectable using 2 clinical, Food and Drug Administration-approved, 800 nm imaging devices as well as small animal imaging systems.

CONCLUSION

This anti-CEA, nanobody-based, fluorescent probe labeled pancreatic orthotopic tumors within 15 minutes of intravenous injection. Fluorescent anti-CEA nanobodies have labeling kinetics that approach the speed of nonspecific dyes such as indocyanine green but with the specificity of antibodies. The use of fluorescently-labeled, intact antibodies leads to a labeling delay of 48 to 96 hours between probe administration and the necessarily delayed time of operation, which can be avoided with nanobodies. The kinetics of a nanobody-based probe makes it a practical agent for same-day, patient administration and fluorescence-guided surgery.

Improved Detection of Molecular Markers of Atherosclerotic Plaques Using Sub-Millimeter PET Imaging

Since atherosclerotic plaques are small and sparse, their non-invasive detection via PET imaging requires both highly specific radiotracers as well as imaging systems with high sensitivity and resolution. This study aimed to assess the targeting and biodistribution of a novel fluorine-18 anti-VCAM-1 Nanobody (Nb), and to investigate whether sub-millimetre resolution PET imaging could improve detectability of plaques in mice. The anti-VCAM-1 Nb functionalised with the novel restrained complexing agent (RESCA) chelator was labelled with [¹⁸F]AlF with a high radiochemical yield (>75%) and radiochemical purity (>99%). Subsequently, [¹⁸F]AlF(RESCA)-cAbVCAM1-5 was injected in ApoE^-/- mice, or co-injected with excess of unlabelled Nb (control group). Mice were imaged sequentially using a cross-over design on two different commercially available PET/CT systems and finally sacrificed for ex vivo analysis. Both the PET/CT images and ex vivo data showed specific uptake of [¹⁸F]AlF(RESCA)-cAbVCAM1-5 in atherosclerotic lesions. Non-specific bone uptake was also noticeable, most probably due to in vivo defluorination. Image analysis yielded higher target-to-heart and target-to-brain ratios with the β-CUBE (MOLECUBES) PET scanner, demonstrating that preclinical detection of atherosclerotic lesions could be improved using the latest PET technology.

Identifying Biomarkers in Lymph Node Metastases of Esophageal Adenocarcinoma for Tumor-Targeted Imaging

INTRODUCTION

Tumor-targeted imaging is a promising technique for the detection of lymph node metastases (LNM) and primary tumors. It remains unclear which biomarker is the most suitable target to distinguish malignant from healthy tissue in esophageal adenocarcinoma (EAC).

OBJECTIVE

We performed an immunohistochemistry study to identify viable tumor markers for tumor-targeted imaging of EAC.

METHODS

We used samples from 72 patients with EAC to determine the immunohistochemical expression of ten potential tumor biomarkers for EAC (carbonic anhydrase IX [CA-IX], carcinoembryonic antigen [CEA], hepatic growth factor receptor, epidermal growth factor receptor, epithelial membrane antigen [EMA], epithelial cell adhesion molecule [EpCAM], human epidermal growth factor receptor 2 [HER-2], urokinase plasminogen activator receptor, vascular endothelial growth factor-A [VEGF-A], and VEGF receptor 2). Immunohistochemistry was performed on tissue microarrays of LNM (n = 48), primary EACs (n = 62), fibrotic tissues (n = 11), nonmalignant lymph nodes (n = 24), and normal esophageal and gastric tissues (n = 40). Tumor marker staining was scored on intensity and percentage of positive cells.

RESULTS

EMA and EpCAM showed strong expression in LNM (> 95%) and primary EACs (> 95%). Significant expression was also observed for LNM and EAC using VEGF-A (85 and 92%), CEA (68 and 54%), and CA-IX (4 and 34%). The other tumor biomarkers showed expression of 0-15% for LNM and primary EAC. Except for VEGF-A, nonmalignant lymph node staining was scored as slight or absent.

CONCLUSIONS

High expression rates and correlation between LNM in EAC combined with low expression rates in healthy lymph nodes and esophagus tissues were observed for EpCAM and CEA, meaning these are promising targets for tumor-targeted imaging approaches for lymph nodes in patients with EAC.

Size and affinity kinetics of nanobodies influence targeting and penetration of solid tumours

A compound's intratumoural distribution is an important determinant for the effectiveness of molecular therapy or imaging. Antibodies (Abs), though often used in the design of targeted compounds, struggle to achieve a homogenous distribution due to their large size and bivalent binding mechanism. In contrast, smaller compounds like nanobodies (Nbs) are expected to distribute more homogenously, though this has yet to be demonstrated in vivo at the microscopic level. We propose an intravital approach to evaluate the intratumoural distribution of different fluorescently labeled monomeric and dimeric Nb tracers and compare this with a monoclonal antibody (mAb). Monomeric and dimeric formats of the anti-HER2 (2Rb17c and 2Rb17c-2Rb17c) and control (R3B23 and R3B23-R3B23) Nb, as well as the dimeric monovalent Nb 2Rb17c-R3B23 were generated and fluorescently labeled with a Cy5 fluorophore. The mAb trastuzumab-Cy5 was also prepared. Whole-body biodistribution of all constructs was investigated in mice bearing subcutaneous xenografts (HER2+ SKOV3) using in vivo epi-fluorescence imaging. Next, for intravital experiments, GFP-expressing SKOV3 cells were grown under dorsal window chambers on athymic nude mice (n = 3/group), and imaged under a fluorescence stereo microscope immediately after intravenous injection of the tracers. Consecutive fluorescence images within the tumour were acquired over the initial 20 min after injection and later, single images were taken at 1, 3 and 24 h post-injection. Additionally, two-photon microscopy was used to investigate the colocalization of GFP (tumour cells) and Cy5 fluorescence (tracers) at higher resolution. Whole-body images showed rapid renal clearance of all Nbs, and fast tumour targeting for the specific Nbs. Specific tumour uptake of the mAb could only be clearly distinguished from background after several hours. Intravital imaging revealed that monomeric Nb tracers accumulated rapidly and distributed homogenously in the tumour mere minutes after intravenous injection. The dimeric compounds initially achieved lower fluorescence intensities than the monomeric. Furthermore, whereas the HER2-specific dimeric bivalent compound remained closely associated to the blood vessels over 24 h, the HER2-specific dimeric monovalent tracer achieved a more homogenous tumour distribution from 1 h post-injection onwards. Non-specific tracers were not retained in the tumour. Trastuzumab had the most heterogenous intratumoural distribution of all evaluated compounds, while -due to the long blood retention- achieving the highest overall tumour uptake at 24 h post-injection. In conclusion, monomeric Nbs very quickly and homogenously distribute through tumour tissue, at a rate significantly greater than dimeric Nbs and mAbs. This underlines the potential of monomeric Nb tracers and therapeutics in molecular imaging and targeted therapies.

Radiometal-labeled anti-VCAM-1 nanobodies as molecular tracers for atherosclerosis - impact of radiochemistry on pharmacokinetics

Radiolabeling of nanobodies with radiometals by chelation has the advantage of being simple, fast and easy to implement in clinical routine. In this study, we validated 68Ga/111In-labeled anti-VCAM-1 nanobodies as potential radiometal-based tracers for molecular imaging of atherosclerosis. Both showed specific targeting of atherosclerotic lesions in ApoE-/- mice. Nevertheless, uptake in lesions and constitutively VCAM-1 expressing organs was lower than previously reported for the 99mTc-labeled analog. We further investigated the impact of different radiolabeling strategies on the in vivo biodistribution of nanobody-based tracers. Comparison of the pharmacokinetics between 68Ga-, 18F-, 111In- and 99mTc-labeled anti-VCAM-1 nanobodies showed highest specific uptake for 99mTc-nanobody at all time-points, followed by the 68Ga-, 111In- and 18F-labeled tracer. No correlation was found with the estimated number of radioisotopes per nanobody, and mimicking specific activity of other radiolabeling methods did not result in an analogous biodistribution. We also demonstrated specificity of the tracer using mice with a VCAM-1 knocked-down phenotype, while showing for the first time the in vivo visualization of a protein knock-down using intrabodies. Conclusively, the chosen radiochemistry does have an important impact on the biodistribution of nanobodies, in particular on the specific targeting, but differences are not purely due to the tracer's specific activity.

In Vivo Assessment of VCAM-1 Expression by SPECT/CT Imaging in Mice Models of Human Triple Negative Breast Cancer

Recent progress in breast cancer research has led to the identification of Vascular Cell Adhesion Molecule-1 (VCAM-1) as a key actor of metastatic colonization. VCAM-1 promotes lung-metastases and is associated with clinical early recurrence and poor outcome in triple negative breast cancer (TNBC). Our objective was to perform the in vivo imaging of VCAM-1 in mice models of TNBC. The Cancer Genomic Atlas (TCGA) database was analyzed to evaluate the prognostic role of VCAM-1 in TNBC. MDA-MB-231 (VCAM-1+) and control HCC70 (VCAM-1-) TNBC cells were subcutaneously xenografted in mice and VCAM-1 expression was assessed in vivo by single-photon emission computed tomography (SPECT) imaging using ^99mTc-cAbVCAM1-5. Then, MDA-MB-231 cells were intravenously injected in mice and VCAM-1 expression in lung metastasis was assessed by SPECT imaging after 8 weeks. TCGA analysis showed that VCAM-1 is associated with a poor prognosis in TNBC patients. In subcutaneous tumor models, ^99mTc-cAbVCAM1-5 uptake was 2-fold higher in MDA-MB-231 than in HCC70 (p < 0.01), and 4-fold higher than that of the irrelevant control (p < 0.01). Moreover, ^99mTc-cAbVCAM1-5 uptake in MDA-MB-231 lung metastases was also higher than that of ^99mTc-Ctl (p < 0.05). ^99mTc-cAbVCAM1-5 is therefore a suitable tool to evaluate the role of VCAM-1 as a marker of tumor aggressiveness of TNBC.

Emerging Fluorescent Molecular Tracers to Guide Intra-Operative Surgical Decision-Making

Fluorescence imaging is an emerging technology that can provide real-time information about the operating field during cancer surgery. Non-specific fluorescent agents, used for the assessment of blood flow and sentinel lymph node detection, have so far dominated this field. However, over the last decade, several clinical studies have demonstrated the great potential of targeted fluorescent tracers to visualize tumor lesions in a more specific way. This has led to an exponential growth in the development of novel molecular fluorescent contrast agents. In this review, the design of fluorescent molecular tracers will be discussed, with particular attention for agents and approaches that are of interest for clinical translation.

Improved Debulking of Peritoneal Tumor Implants by Near-Infrared Fluorescent Nanobody Image Guidance in an Experimental Mouse Model

PURPOSE

Debulking followed by combination chemotherapy is currently regarded as the most effective treatment for advanced ovarian cancer. Prognosis depends drastically on the degree of debulking. Accordingly, near-infrared (NIR) fluorescence imaging has been proposed to revolutionize cancer surgery by acting as a sensitive, specific, and real-time tool enabling visualization of cancer lesions. We have previously developed a NIR-labeled nanobody that allows fast, specific, and high-contrast imaging of HER2-positive tumors. In this study, we applied this tracer during fluorescence-guided surgery in a mouse model and investigated the effect on surgical efficiency.

PROCEDURES

0.5 × 10⁶ SKOV3.IP1-Luc+ cells were inoculated intraperitoneally in athymic mice and were allowed to grow for 30 days. Two nanomoles of IRDye800CW-anti-HER2 nanobody was injected intravenously. After 1h30, mice were killed, randomized in two groups, and subjected to surgery. In the first animal group (n = 7), lesions were removed by a conventional surgical protocol, followed by excision of remaining fluorescent tissue using a NIR camera. The second group of mice (n = 6) underwent directly fluorescence-guided surgery. Bioluminescence imaging was performed before and after surgery. Resected tissue was categorized as visualized during conventional surgery or not, fluorescent or not, and bioluminescent positive or negative.

RESULTS

Fluorescence imaging allowed clear visualization of tumor nodules within the abdomen, up to submillimeter-sized lesions. Fluorescence guidance resulted in significantly reduced residual tumor as compared to conventional surgery. Moreover, sensitivity increased from 59.3 to 99.0 %, and the percentage of false positive lesions detected decreased from 19.6 to 7.1 %.

CONCLUSIONS

This study demonstrates the advantage of intraoperative fluorescence imaging using nanobody-based tracers on the efficiency of debulking surgery.

Targeting mannose receptor expression on macrophages in atherosclerotic plaques of apolipoprotein E-knockout mice using 68Ga-NOTA-anti-MMR nanobody: non-invasive imaging of atherosclerotic plaques

Background

Rupture-prone atherosclerotic plaques are characterized by heavy macrophage infiltration, and the presence of certain macrophage subsets might be a sign for plaque vulnerability. The mannose receptor (MR, CD206) is over-expressed in several types of alternatively activated macrophages. In this study, our objective was to evaluate the feasibility of a Gallium-68 (⁶⁸Ga)-labelled anti-MR nanobody (⁶⁸Ga-anti-MMR Nb) for the visualization of MR-positive (MR⁺) macrophages in atherosclerotic plaques of apolipoprotein E-knockout (ApoE-KO) mice.

Results

NOTA-anti-MMR Nb was labelled with ⁶⁸Ga with radiochemical purity > 95%. In vitro cell-binding studies demonstrated selective and specific binding of the tracer to M2a macrophages. For in vivo atherosclerotic plaque imaging studies, ⁶⁸Ga-NOTA-anti-MMR Nb was injected into ApoE-KO and control mice intravenously (i.v.) and scanned 1 h post-injection for 30 min using a dedicated animal PET/CT. Focal signals could be detected in aortic tissue of ApoE-KO mice, whereas no signal was detected in the aortas of control mice. ⁶⁸Ga-NOTA-anti-MMR Nb uptake was detected in atherosclerotic plaques on autoradiographs and correlated well with Sudan-IV-positive areas. The calculated ratio of plaque-to-normal aortic tissue autoradiographic signal intensity was 7.7 ± 2.6 in aortas excised from ApoE-KO mice. Immunofluorescence analysis of aorta cross-sections confirmed predominant MR expression in macrophages located in the fibrous cap layer and shoulder region of the plaques.

Conclusions

⁶⁸Ga-NOTA-anti-MMR Nb allows non-invasive PET/CT imaging of MR expression in atherosclerotic lesions in a murine model and may represent a promising tool for clinical imaging and evaluation of plaque (in)stability.

Sentinel lymph node mapping by near-infrared fluorescence imaging and contrast-enhanced ultrasound in healthy dogs

Sentinel lymph node (SLN) mapping is a valuable and crucial diagnostic procedure in staging malignancies. We compared two non-invasive techniques, near-infrared (NIR) fluorescence imaging and contrast-enhanced ultrasound (CEUS), to identify the SLNs in three superficial anatomical regions in an animal model. Six healthy laboratory dogs were included in a proof-of-concept trial. A NIR fluorescent dye (Indocyanine Green) and microbubbles (Sonovue) were consecutively injected subdermally in the Inguinal, axillary and popliteal region to map the SLNs. Transcutaneous NIR fluorescence imaging identified SLNs in 17 out of a total of 18 occasions. CEUS identified SLNs in all regions (18/18). Whereas NIR fluorescence imaging performed better in the visualization of the afferent lymphatic tract, CEUS demonstrated different filling patterns of the SLNs, a feature potentially critical for the concept of SLN mapping in cancer patients. Both NIR fluorescence imaging and CEUS are safe, non-invasive, practical and accurate methods to perform real-time transcutaneous SLN mapping with potential in a clinical setting.

Nanobody-Facilitated Multiparametric PET/MRI Phenotyping of Atherosclerosis

OBJECTIVES

This study sought to develop an integrative positron emission tomography (PET) with magnetic resonance imaging (MRI) procedure for accurate atherosclerotic plaque phenotyping, facilitated by clinically approved and nanobody radiotracers.

BACKGROUND

Noninvasive characterization of atherosclerosis remains a challenge in clinical practice. The limitations of current diagnostic methods demonstrate that, in addition to atherosclerotic plaque morphology and composition, disease activity needs to be evaluated.

METHODS

We screened 3 nanobody radiotracers targeted to different biomarkers of atherosclerosis progression, namely vascular cell adhesion molecule (VCAM)-1, lectin-like oxidized low-density lipoprotein receptor (LOX)-1, and macrophage mannose receptor (MMR). The nanobodies, initially radiolabeled with copper-64 (⁶⁴Cu), were extensively evaluated in Apoe^–/– mice and atherosclerotic rabbits using a combination of in vivo PET/MRI readouts and ex vivo radioactivity counting, autoradiography, and histological analyses.

RESULTS

The 3 nanobody radiotracers accumulated in atherosclerotic plaques and displayed short circulation times due to fast renal clearance. The MMR nanobody was selected for labeling with gallium-68 (⁶⁸Ga), a short-lived radioisotope with high clinical relevance, and used in an ensuing atherosclerosis progression PET/MRI study. Macrophage burden was longitudinally studied by ⁶⁸Ga-MMR–PET, plaque burden by T2-weighted MRI, and neovascularization by dynamic contrast-enhanced (DCE) MRI. Additionally, inflammation and microcalcifications were evaluated by fluorine-18 (¹⁸F)-labeled fluorodeoxyglucose (¹⁸F-FDG) and ¹⁸F-sodium fluoride (¹⁸F-NaF) PET, respectively. We observed an increase in all the aforementioned measures as disease progressed, and the imaging signatures correlated with histopathological features.

CONCLUSIONS

We have evaluated nanobody-based radiotracers in rabbits and developed an integrative PET/MRI protocol that allows noninvasive assessment of different processes relevant to atherosclerosis progression. This approach allows the multiparametric study of atherosclerosis and can aid in early stage anti-atherosclerosis drug trials.

Antimicrobial peptides in frog poisons constitute a molecular toxin delivery system against predators

Animals using toxic peptides and proteins for predation or defense typically depend on specialized morphological structures, like fangs, spines, or a stinger, for effective intoxication. Here we show that amphibian poisons instead incorporate their own molecular system for toxin delivery to attacking predators. Skin-secreted peptides, generally considered part of the amphibian immune system, permeabilize oral epithelial tissue and enable fast access of cosecreted toxins to the predator's bloodstream and organs. This absorption-enhancing system exists in at least three distantly related frog lineages and is likely to be a widespread adaptation, determining the outcome of predator-prey encounters in hundreds of species.

Interaction of renal failure and dyslipidaemia in the development of calcific aortic valve disease in rats

OBJECTIVE

Calcific aortic valve disease (CAVD) is currently the most common heart valve disease worldwide and is known to be an active process. Both renal failure and dyslipidaemia are considered to be promoting factors for the development of valvular calcifications. The aim of this study is to prospectively evaluate the respective contribution and interaction of renal failure and dyslipidaemia on CAVD in a rat model, using echocardiography and compared with histology.

METHODS AND RESULTS

Sixty-eight male Wistar rats were prospectively divided in eight groups, each fed a different diet to induce renal failure alone and combined with hyperlipidaemia or hypercholesterolemia. CAVD was detected and quantified by calibrated integrated backscatter of ultrasound (cIB) and compared with the histological calcium score. The study follow-up was 20 weeks. At the end of the study, the cIB value and the calcium score of the aortic valve were significantly increased in the group with isolated renal failure but not with dyslipidaemia. The combination of renal failure with high cholesterol or high-fat diet did not significantly increase calcifications further.

CONCLUSIONS

Renal failure alone does induce aortic valve calcifications in a rat model of CAVD, whereas dyslipidaemia alone does not. The combination of renal failure with dyslipidaemia does not increase calcification further. These findings suggest that a combination of atherosclerotic and calcifying factors is not required to induce aortic valve calcifications in this model.

Effect of Dye and Conjugation Chemistry on the Biodistribution Profile of Near-Infrared-Labeled Nanobodies as Tracers for Image-Guided Surgery

Advances in optical imaging technologies have stimulated the development of near-infrared (NIR) fluorescently labeled targeted probes for use in image-guided surgery. As nanobodies have already proven to be excellent candidates for molecular imaging, we aimed in this project to design NIR-conjugated nanobodies targeting the tumor biomarker HER2 for future applications in this field and to evaluate the effect of dye and dye conjugation chemistry on their pharmacokinetics during development. IRDye800CW or IRdye680RD were conjugated either randomly (via lysines) or site-specifically (via C-terminal cysteine) to the anti-HER2 nanobody 2Rs15d. After verification of purity and functionality, the biodistribution and tumor targeting of the NIR-nanobodies were assessed in HER2-positive and -negative xenografted mice. Site-specifically IRDye800CW- and IRdye680RD-labeled 2Rs15d as well as randomly labeled 2Rs15d-IRDye680RD showed rapid tumor accumulation and low nonspecific uptake, resulting in high tumor-to-muscle ratios at early time points (respectively 6.6 ± 1.0, 3.4 ± 1.6, and 3.5 ± 0.9 for HER2-postive tumors at 3 h p.i., while <1.0 for HER2-negative tumors at 3 h p.i., p < 0.05). Contrarily, using the randomly labeled 2Rs15d-IRDye800CW, HER2-positive and -negative tumors could only be distinguished after 24 h due to high nonspecific signals. Moreover, both randomly labeled 2Rs15d nanobodies were not only cleared via the kidneys but also partially via the hepatobiliary route. In conclusion, near-infrared fluorescent labeling of nanobodies allows rapid, specific, and high contrast in vivo tumor imaging. Nevertheless, the fluorescent dye as well as the chosen conjugation strategy can affect the nanobodies' properties and consequently have a major impact on their pharmacokinetics.

In Vivo Imaging of the Stability and Sustained Cargo Release of an Injectable Amphipathic Peptide-Based Hydrogel

Hydrogels are promising materials for biomedical applications such as tissue engineering and controlled drug release. In the past two decades, the peptide hydrogel subclass has attracted an increasing level of interest from the scientific community because of its numerous advantages, such as biocompatibility, biodegradability, and, most importantly, injectability. Here, we report on a hydrogel consisting of the amphipathic hexapeptide H-FEFQFK-NH₂, which has previously shown promising in vivo properties in terms of releasing morphine. In this study, the release of a small molecule, a peptide, and a protein cargo as representatives of the three major drug classes is directly visualized by in vivo fluorescence and nuclear imaging. In addition, the in vivo stability of the peptide hydrogel system is investigated through the use of a radiolabeled hydrogelator sequence. Although it is shown that the hydrogel remains present for several days, the largest decrease in volume takes place within the first 12 h of subcutaneous injection, which is also the time frame wherein the cargos are released. Compared to the situation in which the cargos are injected in solution, a prolonged release profile is observed up to 12 h, showing the potential of our hydrogel system as a scaffold for controlled drug delivery. Importantly, this study elucidates the release mechanism of the peptide hydrogel system that seems to be based on erosion of the hydrogel providing a generally applicable controlled release platform for small molecule, peptide, and protein drugs.

Sortase A-mediated site-specific labeling of camelid single-domain antibody-fragments: a versatile strategy for multiple molecular imaging modalities

A generic site-specific conjugation method that generates a homogeneous product is of utmost importance in tracer development for molecular imaging and therapy. We explored the protein-ligation capacity of the enzyme Sortase A to label camelid single-domain antibody-fragments, also known as nanobodies. The versatility of the approach was demonstrated by conjugating independently three different imaging probes: the chelating agents CHX-A"-DTPA and NOTA for single-photon emission computed tomography (SPECT) with indium-111 and positron emission tomography (PET) with gallium-68, respectively, and the fluorescent dye Cy5 for fluorescence reflectance imaging (FRI). After a straightforward purification process, homogeneous single-conjugated tracer populations were obtained in high yield (30-50%). The enzymatic conjugation did not affect the affinity of the tracers, nor the radiolabeling efficiency or spectral characteristics. In vivo, the tracers enabled the visualization of human epidermal growth factor receptor 2 (HER2) expressing BT474M1-tumors with high contrast and specificity as soon as 1 h post injection in all three imaging modalities. These data demonstrate Sortase A-mediated conjugation as a valuable strategy for the development of site-specifically labeled camelid single-domain antibody-fragments for use in multiple molecular imaging modalities. Copyright © 2016 John Wiley & Sons, Ltd.

Quantification of Calcium Amount in a New Experimental Model: A Comparison between Ultrasound and Computed Tomography

Purpose

Calcification is an important prognostic factor in aortic valve stenosis. However, there is no ultrasound (US) method available to accurately quantify calcification in this setting to date. We aimed to validate a new US method for measuring the amount of calcium in an in vitro model, and compare it to computed tomography (CT), the current imaging gold standard.

Materials and Methods

An agar phantom (2% agar) was made, containing 9 different amounts of calcium-hydroxyapatite Ca₅(PO₄)₃OH (2 to 50mg). The phantoms were imaged with micro-CT and US (10 MHz probe). The calcium area (area_calcium) and its maximum pixel value (PV_max) were obtained. These values were summed to calculate CT and US calcium scores (∑(area_calcium × PV_max)) and volumes (∑area_calcium). Both US- and CT-calcium scores were compared with the calcium amounts, and with each other.

Results

Both calcium scores correlated significantly with the calcium amount (R² = 0.9788, p<0.0001 and R² = 0.8154, p<0.0001 for CT and US respectively). Furthermore, there was a significant correlation between US and CT for calcium volumes (R² = 0.7392, p<0.0001) and scores (R² = 0.7391, p<0.0001).

Conclusion

We developed a new US method that accurately quantifies the amount of calcium in an in vitro model. Moreover it is strongly correlated with CT.

Targeting of vascular cell adhesion molecule-1 by 18F-labelled nanobodies for PET/CT imaging of inflamed atherosclerotic plaques

AIMS

Positron emission tomography-computed tomography (PET-CT) is a highly sensitive clinical molecular imaging modality to study atherosclerotic plaque biology. Therefore, we sought to develop a new PET tracer, targeting vascular cell adhesion molecule (VCAM)-1 and validate it in a murine atherosclerotic model as a potential agent to detect atherosclerotic plaque inflammation.

METHODS AND RESULTS

The anti-VCAM-1 nanobody (Nb) (cAbVCAM-1-5) was radiolabelled with Fluorine-18 ((18)F), with a radiochemical purity of >98%. In vitro cell-binding studies showed specific binding of the tracer to VCAM-1 expressing cells. In vivo PET/CT imaging of ApoE(-/-) mice fed a Western diet or control mice was performed at 2h30 post-injection of [(18)F]-FB-cAbVCAM-1-5 or (18)F-control Nb. Additionally, plaque uptake in different aorta segments was evaluated ex vivo based on extent of atherosclerosis. Atherosclerotic lesions in the aortic arch of ApoE(-/-) mice, injected with [(18)F]-FB-anti-VCAM-1 Nb, were successfully identified using PET/CT imaging, while background signal was observed in the control groups. These results were confirmed by ex vivo analyses where uptake of [(18)F]-FB-cAbVCAM-1-5 in atherosclerotic lesions was significantly higher compared with control groups. Moreover, uptake increased with the increasing extent of atherosclerosis (Score 0: 0.68 ± 0.10, Score 1: 1.18 ± 0.36, Score 2: 1.49 ± 0.37, Score 3: 1.48 ± 0.38%ID/g, Spearman's r(2) = 0.675, P < 0.0001). High lesion-to-heart, lesion-to-blood, and lesion-to-control vessel ratios were obtained (12.4 ± 0.4, 3.3 ± 0.4, and 3.1 ± 0.6, respectively).

CONCLUSION

The [(18)F]-FB-anti-VCAM-1 Nb, cross-reactive for both mouse and human VCAM-1, allows non-invasive PET/CT imaging of VCAM-1 expression in atherosclerotic plaques in a murine model and may represent an attractive tool for imaging vulnerable atherosclerotic plaques in patients.

Echocardiographic integrated backscatter for the differentiation between aortic valve calcification and valvular myxoid degeneration in rats

AIMS

Calcification is an independent predictor of mortality in aortic valve (AV) stenosis. Echocardiographic calibrated integrated backscatter (cIB) is a promising parameter for quantifying AV calcification. However, the ability of cIB to differentiate between calcification and valvular thickening has been questioned. Therefore, we aimed to use cIB to study AV calcification compared with non-calcified AV thickening in rats, with histology as reference.

METHODS AND RESULTS

Twenty male Wistar rats were studied. Group 1 (N = 6) received subcutaneous (SC) serotonin injections (60 mg/kg/day) for 12 weeks to induce myxoid non-calcified AV thickening. Group 2 (N = 7) received vitamin D3 (25,000 UI/kg/day) SC to induce AV calcification, and Group 3 (N = 7) received only vehicle SC for 10 weeks. cIB of the AV was calculated at the end of the study, followed by measurement of the percentage of the histological AV calcification. At the end of the study, cIB values and calcification percentages were significantly higher in vitamin D3-injected rats compared with serotonin-injected rats and controls. There was no significant difference in cIB values between serotonin-injected rats and controls (vitamin D3: 21.5 ± 3.0 dB*; serotonin: 11.8 ± 3.1 dB; control: 10.3 ± 3.4 dB; *P < 0.05). The percentage of histological calcification was significantly higher in the vitamin D3 group compared with the other groups. Serotonin-injected rats developed significant AV thickening.

CONCLUSION

Increased cIB values of the AV are related to increased calcification at histology and not to myxoid non-calcified valvular thickening. Therefore, cIB may be considered as a sensitive technique to quantify calcification of AV rather than for detecting non-calcified valvular thickening.

Use of contrast echocardiography in intensive care and at the emergency room

Bedside echocardiography in emergency room (ER) or in intensive care unit (ICU) is an important tool for managing critically ill patients, to obtain a timely accurate diagnosis and to immediately stratify the risk to the patient’s life. It may also render invasive monitoring unnecessary. In these patients, contrast echocardiography may improve quality of imaging and also may provide additional information, especially regarding myocardial perfusion in those with suspected coronary artery disease. This article focuses on the principle of contrast echocardiography and the clinical information that can be obtained according to the most frequent presentations in ER and ICU.

Nanobodies targeting mouse/human VCAM1 for the nuclear imaging of atherosclerotic lesions

RATIONALE

A noninvasive tool allowing the detection of vulnerable atherosclerotic plaques is highly needed. By combining nanomolar affinities and fast blood clearance, nanobodies represent potential radiotracers for cardiovascular molecular imaging. Vascular cell adhesion molecule-1 (VCAM1) constitutes a relevant target for molecular imaging of atherosclerotic lesions.

OBJECTIVE

We aimed to generate, radiolabel, and evaluate anti-VCAM1 nanobodies for noninvasive detection of atherosclerotic lesions.

METHODS AND RESULTS

Ten anti-VCAM1 nanobodies were generated, radiolabeled with technetium-99m, and screened in vitro on mouse and human recombinant VCAM1 proteins and endothelial cells and in vivo in apolipoprotein E-deficient (ApoE(-/-)) mice. A nontargeting control nanobody was used in all experiments to demonstrate specificity. All nanobodies displayed nanomolar affinities for murine VCAM1. Flow cytometry analyses using human human umbilical vein endothelial cells indicated murine and human VCAM1 cross-reactivity for 6 of 10 nanobodies. The lead compound cAbVCAM1-5 was cross-reactive for human VCAM1 and exhibited high lesion-to-control (4.95±0.85), lesion-to-heart (8.30±1.11), and lesion-to-blood ratios (4.32±0.48) (P<0.05 versus control C57Bl/6J mice). Aortic arch atherosclerotic lesions of ApoE(-/-) mice were successfully identified by single-photon emission computed tomography imaging. (99m)Tc-cAbVCAM1-5 binding specificity was demonstrated by in vivo competition experiments. Autoradiography and immunohistochemistry further confirmed cAbVCAM1-5 uptake in VCAM1-positive lesions.

CONCLUSIONS

The (99m)Tc-labeled, anti-VCAM1 nanobody cAbVCAM1-5 allowed noninvasive detection of VCAM1 expression and displayed mouse and human cross-reactivity. Therefore, this study demonstrates the potential of nanobodies as a new class of radiotracers for cardiovascular applications. The nanobody technology might evolve into an important research tool for targeted imaging of atherosclerotic lesions and has the potential for fast clinical translation.