Molecular PET/CT Profiling of ACE2 Expression In Vivo: Implications for Infection and Outcome from SARS-CoV-2

Impact of biocontainment on small animal PET performance adapted for BSL-2/3 infectious disease imaging research

BACKGROUND

Biocontainment protocols are critical for conducting infectious disease (ID) research, particularly when using small animal models in biosafety level (BSL) 2/3 environments. This study evaluates the impact of poly-methyl methacrylate (PMMA) containment vessels on the performance of preclinical positron emission tomography (PET) systems. We tested containment vessels designed with varying wall thicknesses (3, 6, and 9 mm) to simulate ID imaging facility equipment and protocols. Through the use of multicomponent phantoms and in vivo mouse models of Staphylococcus aureus infection, we assessed key performance metrics including count rate, image quality, activity recovery, and spatial resolution.

RESULTS

The results indicate that the use of PMMA containment causes only minor reductions in imaging performance. The thickest PMMA (9 mm) led to a maximum 6.8% decrease in count rate, which remains well within the acceptable range of variation. Effects on spatial resolution were most noticeable for smaller structures within the phantom study, with a 19.65% difference in full width at half maximum (FWHM) for the thickest walled vessel. In vivo, using infected mice, the containment devices had modest effects on the task of activity concentration to be detected at the infection site, even with the thickest PMMA tube.

CONCLUSION

These findings suggest that PMMA biocontainment vessels have small but measurable impact on preclinical PET system performance, making them a viable and cost-effective solution for conducting infectious disease imaging under BSL-2/3 conditions. Specifically, the thinnest containment (3 mm) had only minor effects on all tested parameters, suggesting it is well-suited for use in ID enclosures while maintaining accurate qualitative and quantitative assessments. This approach may reduce the burden for fully separate and specialized modifications for BSL-3 imaging facilities, and can be broadly applied to preclinical research involving pathogenic organisms.

Development of radiofluorinated MLN-4760 derivatives for PET imaging of the SARS-CoV-2 entry receptor ACE2

PURPOSE

The angiotensin converting enzyme 2 (ACE2) plays a regulatory role in the cardiovascular system and serves SARS-CoV-2 as an entry receptor. The aim of this study was to synthesize and evaluate radiofluorinated derivatives of the ACE2 inhibitor MLN-4760. [18F]F-MLN-4760 and [18F]F-Aza-MLN-4760 were demonstrated to be suitable for non-invasive imaging of ACE2, potentially enabling a better understanding of its expression dynamics.

METHODS

Computational molecular modeling, based on the structures of human ACE2 (hACE2) and mouse ACE2 (mACE2), revealed that the ACE2-binding modes of F-MLN-4760 and F-Aza-MLN-4760 were similar to that of MLN-4760. Co-crystallization of the hACE2/F-MLN-4760 protein complex was performed for confirmation. Displacement experiments using [3H]MLN-4760 enabled the determination of the binding affinities of the synthesized F-MLN-4760 and F-Aza-MLN-4760 to hACE2 expressed in HEK-ACE2 cells. Aryl trimethylstannane-based and pyridine-based radiofluorination precursors were synthesized and used for the preparation of the respective radiotracers. [18F]F-MLN-4760 and [18F]F-Aza-MLN-4760 were evaluated with regard to the uptake in HEK-ACE2 and HEK-ACE cells and in vitro binding to tissue sections of HEK-ACE2 xenografts and normal organs of mice. Biodistribution and PET/CT imaging studies of [18F]F-MLN-4760 and [18F]F-Aza-MLN-4760 were performed using HEK-ACE2 and HEK-ACE xenografted nude mice.

RESULTS

Crystallography data revealed an equal hACE2-binding mode for F-MLN-4760 as previously found for MLN-4760. Moreover, computer-based modeling indicated that similar binding to hACE2 and mACE2 holds true for both, F-MLN-4760 and F-Aza-MLN-4760, as is the case for MLN-4760. The IC50 values were three-fold and seven-fold higher for F-MLN-4760 and F-Aza-MLN-4760, respectively, than for MLN-4760. [18F]F-MLN-4760 and [18F]F-Aza-MLN-4760 were obtained in 1.4 ± 0.3 GBq and 0.5 ± 0.1 GBq activity with > 99% radiochemical purity in a 5.3% and 1.2% radiochemical yield, respectively. Uptake in HEK-ACE2 cells was higher for [18F]F-MLN-4760 (67 ± 9%) than for [18F]F-Aza-MLN-4760 (37 ± 8%) after 3-h incubation while negligible uptake was seen in HEK-ACE cells (< 0.3%). [18F]F-MLN-4760 and [18F]F-Aza-MLN-4760 accumulated specifically in HEK-ACE2 xenografts of mice (13 ± 2% IA/g and 15 ± 2% IA/g at 1 h p.i.) with almost no uptake observed in HEK-ACE xenografts (< 0.3% IA/g). This was confirmed by PET/CT imaging, which also visualized unspecific accumulation in the gall bladder and intestinal tract.

CONCLUSION

Both radiotracers showed specific and selective binding to ACE2 in vitro and in vivo. [18F]F-MLN-4760 was, however, obtained in higher yields and the ACE2-binding affinity was superior over that of [18F]F-Aza-MLN-4760. [18F]F-MLN-4760 would, thus, be the candidate of choice for further development in view of its use for PET imaging of ACE2.

Evaluation of Angiotensin-Converting Enzyme 2 Expression In Vivo with Novel 68Ga-Labeled Peptides Originated from the Coronavirus Receptor-Binding Domain

Angiotensin-converting enzyme 2 (ACE2) is not only a key to the renin-angiotensin-aldosterone system and related diseases, but also the main entry point on cell surfaces for certain coronaviruses, including severe acute respiratory syndrome coronavirus (SARS-CoV) and SARS-CoV-2. By analyzing the different key binding sites from the receptor-binding domain (RBD) of SARS-CoV and SARS-CoV-2, nine new ACE2-targeting peptides (A1 to A9) were designed, synthesized and connected with a chelator, 1,4,7-triazacyclononane-N,N',N''-triacetic acid (NOTA). NOTA-A1, NOTA-A2, NOTA-A4, NOTA-A5, and NOTA-A8 were successfully labeled with [68Ga]Ga3+ and were used for biological evaluation. [68Ga]Ga-NOTA-A2, [68Ga]Ga-NOTA-A5, and [68Ga]Ga-NOTA-A8 showed specific binding to ACE2 via cell assays, and their binding sites and binding capacity were calculated by molecular docking and molecular dynamics simulations. In tumor-bearing mice, A549 tumors were visualized 60 min postinjection of [68Ga]Ga-NOTA-A2, [68Ga]Ga-NOTA-A5, or [68Ga]Ga-NOTA-A8. These peptides also accumulated in the organs with high-level ACE2 expression, confirmed by immunohistochemical stain. Among them, [68Ga]Ga-NOTA-A5 exhibited the highest tumor uptake and tumor/background ratio, and it successfully tracked the increased ACE2 levels in mice tissues after excessive Losartan treatment. In a first-in-human study, the distribution of [68Ga]Ga-NOTA-A5 was evaluated with positron emission tomography/computed tomography (PET/CT) in three participants without adverse events. 68Ga-labeled peptides originated from the coronavirus RBD, with [68Ga]Ga-NOTA-A5 as a typical representative, seem to be safe and effective for the evaluation of ACE2 expression in vivo with PET/CT, facilitating further mechanism investigation and clinical evaluation of ACE2-related diseases.

Radiotracers for Molecular Imaging of Angiotensin-Converting Enzyme 2

Angiotensin-converting enzymes (ACE) are well-known for their roles in both blood pressure regulation via the renin-angiotensin system as well as functions in fertility, immunity, hematopoiesis, and many others. The two main isoforms of ACE include ACE and ACE-2 (ACE2). Both isoforms have similar structures and mediate numerous effects on the cardiovascular system. Most remarkably, ACE2 serves as an entry receptor for SARS-CoV-2. Understanding the interaction between the virus and ACE2 is vital to combating the disease and preventing a similar pandemic in the future. Noninvasive imaging techniques such as positron emission tomography and single photon emission computed tomography could noninvasively and quantitatively assess in vivo ACE2 expression levels. ACE2-targeted imaging can be used as a valuable tool to better understand the mechanism of the infection process and the potential roles of ACE2 in homeostasis and related diseases. Together, this information can aid in the identification of potential therapeutic drugs for infectious diseases, cancer, and many ACE2-related diseases. The present review summarized the state-of-the-art radiotracers for ACE2 imaging, including their chemical design, pharmacological properties, radiochemistry, as well as preclinical and human molecular imaging findings. We also discussed the advantages and limitations of the currently developed ACE2-specific radiotracers.

Preclinical Evaluation and Pilot Clinical Study of 18F-Labeled Inhibitor Peptide for Noninvasive Positron Emission Tomography Mapping of Angiotensin Converting Enzyme 2

Angiotensin-converting enzyme 2 (ACE2) is the main molecular target for coronavirus SARS-CoV-2 to enter cells. Molecularly specific tracers that bind to ACE2 with high affinity can be used to determine the tissue distribution of this important receptor, noninvasively. A novel targeting PET imaging probe, [18F]AlF-DX600-BCH, was developed to detect the in vivo expression of ACE2 and monitor response to therapy. Preclinical experiments, including biodistribution, PET imaging, and tissue section analysis, were conducted after tests of in vitro and in vivo stability and pharmacokinetics. The agent was advanced to clinical evaluation in 10 volunteers who received [18F]AlF-DX600-BCH PET/CT at 1 and 2 h after injection (NCT04542863). Preclinical results of both biodistribution and PET demonstrated [18F]AlF-DX600-BCH accumulation in rat kidney (standardized uptake value; SUVkidney/normal > 50), along with specific uptake in testes (SUVtestis/normal > 10) tissues. Kidney, gastrointestinal, and bronchial cell labeling were correlated to ACE2 positive by immunohistochemistry (IHC) staining. In clinical imaging, significant tracer accumulation was predominantly observed in the urinary and reproductive system (SUVrenal cortex = 32.00, SUVtestis = 4.56), and the conjunctiva and nasal mucosa saw elevated uptake in several cases. This work is the first report of a radioisotope probe, [18F]AlF-DX600-BCH, targeting ACE2 with promising preliminary preclinical and translational outlook, thereby demonstrating the potential of noninvasive mapping of ACE2.

[99mTc]Tc-labeled cyc-DX600-HYNIC as a SPECT probe for ACE2-specific pancreatic cancer imaging

As a regulator in renin-angiotensin-aldosterone system, angiotensin-converting enzyme 2 (ACE2) closely correlated with tumor progression of pancreatic cancer, meantime, was easily affected by a variety of factors. [99mTc]Tc-cyc-DX600 SPECT was established as an ACE2-specific imaging protocol to figure out the ACE2 status in pancreatic tumor. BALB/C-NU mice were used to prepare the subcutaneous cell derived xenograft (CDX) models with HEK-293T or HEK-293T/hACE2 cells to validate ACE2 specificity of [99mTc]Tc-cyc-DX600 SPECT and establish SPECT imaging protocol. On the basis of [99mTc]Tc-cyc-DX600 SPECT and [18F]F-FDG PET/CT, ACE2-dependence on tumor size and tumor metabolism were further verified on orthotopic pancreatic cancer model with KPC cells. Immunohistochemical analysis was used to demonstrate the findings on ACE2 SPECT. [99mTc]Tc-cyc-DX600 was of superior tumor uptake in HEK-293T/hACE2 CDX than wild type (6.74 ± 0.31 %ID/mL vs 1.83 ± 0.26 %ID/mL at 1.5 h post injection (p.i.); 3.14 ± 0.31 %ID/mL vs 1.16 ± 0.15 %ID/mL at 4.5 h p.i.). For the CDX models with PANC-1 cells, a significant negative correlation between the slope of tumor volume and tumor uptake was observed (r = -0.382 for the 1-4th day; r = -0.146 for the 1-5th day; r = -0.114 for the 1-6th day; r = -0.152 for the 1-7th day; but P > 0.05 for all). For orthotopic pancreatic cancer model, the linear correlation between FDG PET and ACE2 SPECT of the pancreatic lesions was negative (r = -0.878), the quantitative values of ACE2 SPCET was positively correlated with the volume of primary lesions (r = 0.752) and also positively correlated with the quantitative values of ACE2 immunohistochemical analysis (r = 0.991). Conclusively, [99mTc]Tc-cyc-DX600 SPECT is an ACE2-specific imaging protocol with clinical translational potential, adding multidimensional information on the disease progression of pancreatic cancer.

Molecular PET/CT mapping of rhACE2 distribution and quantification in organs to aid in SARS-CoV-2 targeted therapy

Coronavirus disease 2019 (COVID-19), which is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, poses a significant threat to public health. Angiotensin-converting enzyme 2 (ACE2) is a key receptor for SARS-CoV-2 infection. Recombinant human ACE2 (RhACE2), as a soluble supplement for human ACE2, can competitively block SARS-CoV-2 infection. In this study, a mouse organ in situ rhACE2 high aggregation model was constructed for the first time, and in vivo real-time positron emission tomography (PET) imaging of rhACE2 in the mouse model was performed using an ACE2-specific agent 68 Ga-HZ20. This radiotracer exhibits reliable radiochemical properties in vitro and maintains a high affinity for rhACE2 in vivo. In terms of probe uptake, 68 Ga-HZ20 showed a good target-to-nontarget ratio and was rapidly cleared from the circulatory system and excreted by the kidneys and urinary system. PET imaging with this radiotracer can noninvasively and accurately monitor the content and distribution of rhACE2 in the body, which clarifies that rhACE2 can aggregate in multiple organs, suggesting the preventive and therapeutic potential of rhACE2 for SARS-CoV-2 and COVID-19.

Advances and Challenges in Molecular Imaging of Viral Infections

Molecular imaging of viral infection, using a variety of advanced imaging techniques such as optical and nuclear imaging, can and has been used for direct visualization of the virus as well as assessment of virus-host interactions. Unlike imaging of other pathogens such as bacteria and fungi, challenging aspects of imaging viral infections include the small size of viruses, the complexity of viral infection animal models (eg, species dependence), and the high-level containment needs for many high-consequence pathogens, among others. In this review, using representative viral infections, we discuss how molecular imaging can reveal real-time infection dynamics, improve our understanding of disease pathogenesis, and guide optimization of treatment and prevention strategies. Key findings from human and animal studies are highlighted.

Molecular Imaging of ACE2 Expression in Infectious Disease and Cancer

Angiotensin-converting enzyme 2 (ACE2) is a cell-surface receptor that plays a critical role in the pathogenesis of SARS-CoV-2 infection. Through the use of ligands engineered for the receptor, ACE2 imaging has emerged as a valuable tool for preclinical and clinical research. These can be used to visualize the expression and distribution of ACE2 in tissues and cells. A variety of techniques including optical, magnetic resonance, and nuclear medicine contrast agents have been developed and employed in the preclinical setting. Positron-emitting radiotracers for highly sensitive and quantitative tomography have also been translated in the context of SARS-CoV-2-infected and control patients. Together this information can be used to better understand the mechanisms of SARS-CoV-2 infection, the potential roles of ACE2 in homeostasis and disease, and to identify potential therapeutic modulators in infectious disease and cancer. This review summarizes the tools and techniques to detect and delineate ACE2 in this rapidly expanding field.

Radiochemistry for positron emission tomography

Positron emission tomography (PET) constitutes a functional imaging technique that is harnessed to probe biological processes in vivo. PET imaging has been used to diagnose and monitor the progression of diseases, as well as to facilitate drug development efforts at both preclinical and clinical stages. The wide applications and rapid development of PET have ultimately led to an increasing demand for new methods in radiochemistry, with the aim to expand the scope of synthons amenable for radiolabeling. In this work, we provide an overview of commonly used chemical transformations for the syntheses of PET tracers in all aspects of radiochemistry, thereby highlighting recent breakthrough discoveries and contemporary challenges in the field. We discuss the use of biologicals for PET imaging and highlight general examples of successful probe discoveries for molecular imaging with PET - with a particular focus on translational and scalable radiochemistry concepts that have been entered to clinical use.

Application of the Nicoya OpenSPR to Studies of Biomolecular Binding: A Review of the Literature from 2016 to 2022

The Nicoya OpenSPR is a benchtop surface plasmon resonance (SPR) instrument. As with other optical biosensor instruments, it is suitable for the label-free interaction analysis of a diverse set of biomolecules, including proteins, peptides, antibodies, nucleic acids, lipids, viruses, and hormones/cytokines. Supported assays include affinity/kinetics characterization, concentration analysis, yes/no assessment of binding, competition studies, and epitope mapping. OpenSPR exploits localized SPR detection in a benchtop platform and can be connected with an autosampler (XT) to perform automated analysis over an extended time period. In this review article, we provide a comprehensive survey of the 200 peer-reviewed papers published between 2016 and 2022 that use the OpenSPR platform. We highlight the range of biomolecular analytes and interactions that have been investigated using the platform, provide an overview on the most common applications for the instrument, and point out some representative research that highlights the flexibility and utility of the instrument.

A tool for nuclear imaging of the SARS-CoV-2 entry receptor: molecular model and preclinical development of ACE2-selective radiopeptides

PURPOSE

The angiotensin converting enzyme-2 (ACE2)-entry receptor of SARS-CoV-2-and its homologue, the angiotensin-converting enzyme (ACE), play a pivotal role in maintaining cardiovascular homeostasis. Potential changes in ACE2 expression levels and dynamics after SARS-CoV-2 infection have been barely investigated. The aim of this study was to develop an ACE2-targeting imaging agent as a noninvasive imaging tool to determine ACE2 regulation.

METHODS

DOTA-DX600, NODAGA-DX600 and HBED-CC-DX600 were obtained through custom synthesis and labeled with gallium-67 (T_1/2 = 3.26 d) as a surrogate radioisotope for gallium-68 (T_1/2 = 68 min). ACE2- and ACE-transfected HEK cells were used for the in vitro evaluation of these radiopeptides. The in vivo tissue distribution profiles of the radiopeptides were assessed in HEK-ACE2 and HEK-ACE xenografted mice and imaging studies were performed using SPECT/CT.

RESULTS

The highest molar activity was obtained for [⁶⁷Ga]Ga-HBED-CC-DX600 (60 MBq/nmol), whereas the labeling efficiency of the other peptides was considerably lower (20 MBq/nmol). The radiopeptides were stable over 24 h in saline (> 99% intact peptide). All radiopeptides showed uptake in HEK-ACE2 cells (36-43%) with moderate ACE2-binding affinity (K_D value: 83-113 nM), but no uptake in HEK-ACE cells (< 0.1%) was observed. Accumulation of the radiopeptides was observed in HEK-ACE2 xenografts (11-16% IA/g) at 3 h after injection, but only background signals were seen in HEK-ACE xenografts (< 0.5% IA/g). Renal retention was still high 3 h after injection of [⁶⁷Ga]Ga-DOTA-DX600 and [⁶⁷Ga]Ga-NODAGA-DX600 (~ 24% IA/g), but much lower for [⁶⁷Ga]Ga-HBED-CC-DX600 (7.2 ± 2.2% IA/g). SPECT/CT imaging studies confirmed the most favorable target-to-nontarget ratio for [⁶⁷Ga]Ga-HBED-CC-DX600.

CONCLUSIONS

This study demonstrated ACE2 selectivity for all radiopeptides. [⁶⁷Ga]Ga-HBED-CC-DX600 was revealed as the most promising candidate due to its favorable tissue distribution profile. Importantly, the HBED-CC chelator enabled ⁶⁷Ga-labeling at high molar activity, which would be essential to obtain images with high signal-to-background contrast to detect (patho)physiological ACE2 expression levels in patients.

The functional views on response of host rabbit post coronavirus vaccination via ACE2 PET

Molecular imaging can dynamically and quantitatively record the biochemical changes in a systemic view. In this research, SARS-CoV-2 pseudovirus was intramuscularly injected to simulate the vaccination with inactivated virus. New Zealand white rabbits were evaluated with ¹⁸F-FDG PET for inflammation and ⁶⁸Ga-cyc-DX600 PET for ACE2 fluctuation, which were performed before and at 3, 7 and 14 days post injection (d P.I.); furthermore, one rabbit was vaccinated with two cycles with interval of 14 days for a longer period evaluation. Different with the vaccination-induced inflammatory response that was random and individual, ACE2 regulation was systemic and organ-specific: the liver and spleen were of a moderate decrease post injection but rebound at 14 d P.I., while there were a downward trend in heart, testis and bone marrow; besides, similar pattern of ACE2 regulation were recorded after the second injection with a relatively greater volatility. In conclusion, ACE2 PET gave a more comprehensive view on host response post vaccination, hold substantial promise in continuous monitoring of coronavirus vaccine administration and effectiveness.

A Positron Emission Tomography Tracer Targeting the S2 Subunit of SARS-CoV-2 in Extrapulmonary Infections

Tracking the pathogen of coronavirus disease 2019 (COVID-19) in live subjects may help estimate the spatiotemporal distribution of SARS-CoV-2 infection in vivo. This study developed a positron emission tomography (PET) tracer of the S2 subunit of spike (S) protein for imaging SARS-CoV-2. A pan-coronavirus inhibitor, EK1 peptide, was synthesized and radiolabeled with copper-64 after being conjugated with 1,4,7-triazacyclononane-1,4,7-triyl-triacetic acid (NOTA). The in vitro stability tests indicated that [64Cu]Cu-NOTA-EK1 was stable up to 24 h both in saline and in human serum. The binding assay showed that [64Cu]Cu-NOTA-EK1 has a nanomolar affinity (Ki = 3.94 ± 0.51 nM) with the S-protein of SARS-CoV-2. The cell uptake evaluation used HEK293T/S+ and HEK293T/S- cell lines that showed that the tracer has a high affinity with the S-protein on the cellular level. For the in vivo study, we tested [64Cu]Cu-NOTA-EK1 in HEK293T/S+ cell xenograft-bearing mice (n = 3) and pseudovirus of SARS-CoV-2-infected HEK293T/ACE2 cell bearing mice (n = 3). The best radioactive xenograft-to-muscle ratio (X/Nxenograft 8.04 ± 0.99, X/Npseudovirus 6.47 ± 0.71) was most evident 4 h postinjection. Finally, PET imaging in the surrogate mouse model of beta-coronavirus, mouse hepatic virus-A59 infection in C57BL/6 J mice showed significantly enhanced accumulation in the liver than in the uninfected mice (1.626 ± 0.136 vs 0.871 ± 0.086 %ID/g, n = 3, P < 0.05) at 4 h postinjection. In conclusion, our experimental results demonstrate that [64Cu]Cu-NOTA-EK1 is a potential molecular imaging probe for tracking SARS-CoV-2 in extrapulmonary infections in living subjects.

Oncology and cardiology positron emission tomography/computed tomography faced with COVID-19: A review of available literature data

The COVID-19 pandemic has forced people to significantly change their lifestyles and attitudes, and has greatly burdened healthcare delivery systems worldwide. The redistribution of the medical delivery system to maintain normal medical care while responding generously to COVID-19 is a continuing challenge that weighs heavily on medical institutions. Among imaging modalities, chest X-rays and computed tomography (CT) examinations have clearly made a large contribution to treatment of COVID-19. In contrast, it is difficult to express the standpoint of nuclear medicine examinations in a straightforward manner, as the greatest emphasis in this modality has been on how necessary medical care can continue to be provided. Many clinical reports of nuclear medicine examinations related to COVID-19 have been published, and knowledge continues to accumulate. This review provides a summary of the current state of oncology and cardiology positron emission tomography (PET) examinations related to COVID-19, and includes preparation of the nuclear medicine department, trends in PET examinations, specific imaging findings on 18F-fluorodeoxyglucose (FDG) PET/CT, imaging of complications of COVID-19, PET tracers other than FDG, and the effects of vaccines on PET imaging findings.

Evaluation of 68Ga- and 177Lu-Labeled HZ20 Angiotensin-Converting Enzyme 2-Targeting Peptides for Tumor-Specific Imaging

Angiotensin-converting enzyme 2 (ACE2) is closely related to tumor formation. We developed the radiolabeled peptide pair ⁶⁸Ga/¹⁷⁷Lu-labeled 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-conjugated DX600 (⁶⁸Ga/¹⁷⁷Lu-HZ20), for the targeting and mapping of ACE2-overexpressing tumors. ⁶⁸Ga/¹⁷⁷Lu-HZ20 was prepared with a routine labeling method. HepG2^ACE2+/HepG2^WT cell lines were used to evaluate the specificity of ⁶⁸Ga/¹⁷⁷Lu-HZ20. Pharmacokinetics, biodistribution, and micro-PET/CT and -SPECT/CT imaging were performed, and radiation dosimetry was estimated. Immunohistochemistry (IHC) staining was performed to assess the expression of ACE2 in tumors. The radiolabeling yields of ⁶⁸Ga/¹⁷⁷Lu-HZ20 were 88.49 ± 8.57% (n > 10) and 84.71 ± 9.75% (n > 10), with specific activities of (18.74 ± 3.72) × 10⁶ and (17.85 ± 1.62) × 10⁶ GBq/mol, respectively. ⁶⁸Ga/¹⁷⁷Lu-HZ20 showed significant differences in the cellular uptake of HepG2^ACE2+/HepG2^WT cells and fast clearance in KM mice. Moreover, HepG2^ACE2+ tumors were clearly visualized in ⁶⁸Ga/¹⁷⁷Lu-HZ20 micro-PET/SPECT images. Based on micro-PET/CT, the standard uptake value (SUVmax) of HepG2^ACE2+ tumors was 0.66 ± 0.02 at 30 min postinjection, IHC confirmed the high expression of ACE2 in HepG2^ACE2+ tumors. In PET/CT images, the SUVmean of volunteer 1 was higher than the ¹⁸F-FDG value in the same lesion. ⁶⁸Ga/¹⁷⁷Lu-HZ20 was successfully obtained and showed high and specific uptake in tumors overexpressing ACE2. They may serve as paired probes for ACE2-targeting theranostics.

Evaluation of SARS-CoV-2-Neutralizing Nanobody Using Virus Receptor Binding Domain-Administered Model Mice

Due to the rapid spread of coronavirus disease 2019 (COVID-19), there is an urgent requirement for the development of additional diagnostic tools for further analysis of the disease. The isolated nanobody Nb11-59 binds to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) receptor-binding domain (RBD) with high affinity to neutralize the virus and block the angiotensin-converting enzyme 2- (ACE2-) RBD interaction. Here, we introduce a novel nanobody-based radiotracer named 68Ga-Nb1159. The radiotracer retained high affinity for the RBD and showed reliable radiochemical characteristics both in vitro and in vivo. Preclinical positron emission tomography (PET) studies of 68Ga-Nb1159 in mice revealed its rapid clearance from circulation and robust uptake into the renal and urinary systems. Fortunately, 68Ga-Nb1159 could specifically reveal the distribution of the RBD in mice. This study also helped to evaluate the pharmacodynamic effects of the neutralizing nanobody. Moreover, 68Ga-Nb1159 may be a promising tool to explore the distribution of the RBD and improve the understanding of the virus. In particular, this study identified a novel molecular radioagent and established a reliable evaluation method for specifically investigating the RBD through noninvasive and visual PET technology.

Noninvasive Mapping of Angiotensin Converting Enzyme-2 in Pigeons Using Micro Positron Emission Tomography

The ACE2 receptor, as the potential entrance site of SARS-CoV-2-affected cells, plays a crucial role in spreading infection. The DX600 peptide is a competitive inhibitor of ACE2. We previously constructed the ⁶⁸Ga-labeled DOTA-DX600 (also known as ⁶⁸Ga-HZ20) peptide and confirmed its ACE2 binding ability both in vitro and in vivo. In this research, we aimed to investigate the noninvasive mapping of ACE2 expression in fowl using ⁶⁸Ga-HZ20 micro-PET. We chose pigeons as an animal model and first studied the administration method of ⁶⁸Ga-HZ20 by direct site injection or intravenous injection. Then, the dynamic micro-PET scan of ⁶⁸Ga-HZ20 was conducted at 0–40 min. Additionally, ¹⁸F-FDG was used for comparison. Finally, the pigeons were sacrificed, and the main organs were collected for further immunoPET and IHC staining. Micro PET/CT imaging results showed that ⁶⁸Ga-HZ20 uptake was distributed from the heart at the preliminary injection to the kidneys, liver, stomach, and lungs over time, where the highest uptake was observed in the kidneys (SUV_max = 6.95, 20 min) and lung (SUV_max = 1.11, 20 min). Immunohistochemical experiments were carried out on its main organs. Compared to the SUVmax data, the IHC results showed that ACE2 was highly expressed in both kidneys and intestines, and the optimal imaging time was determined to be 20 min after injection through correlation analysis. These results indicated that ⁶⁸Ga-HZ20 is a potential target molecule for SARS-CoV-2 in fowl, which is worthy of promotion and further study.

China’s radiopharmaceuticals on expressway: 2014–2021

This review provides an essential overview on the progress of rapidly-developing China’s radiopharmaceuticals in recent years (2014–2021). Our discussion reflects on efforts to develop potential, preclinical, and in-clinical radiopharmaceuticals including the following areas: (1) brain imaging agents, (2) cardiovascular imaging agents, (3) infection and inflammation imaging agents, (4) tumor radiopharmaceuticals, and (5) boron delivery agents (a class of radiopharmaceutical prodrug) for neutron capture therapy. Especially, the progress in basic research, including new radiolabeling methodology, is highlighted from a standpoint of radiopharmaceutical chemistry. Meanwhile, we briefly reflect on the recent major events related to radiopharmaceuticals along with the distribution of major R&D forces (universities, institutions, facilities, and companies), clinical study status, and national regulatory supports. We conclude with a brief commentary on remaining limitations and emerging opportunities for China’s radiopharmaceuticals.

Medical imaging of pulmonary disease in SARS-CoV-2-exposed non-human primates

Chest X-ray (CXR), computed tomography (CT), and positron emission tomography-computed tomography (PET-CT) are noninvasive imaging techniques widely used in human and veterinary pulmonary research and medicine. These techniques have recently been applied in studies of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-exposed non-human primates (NHPs) to complement virological assessments with meaningful translational readouts of lung disease. Our review of the literature indicates that medical imaging of SARS-CoV-2-exposed NHPs enables high-resolution qualitative and quantitative characterization of disease otherwise clinically invisible and potentially provides user-independent and unbiased evaluation of medical countermeasures (MCMs). However, we also found high variability in image acquisition and analysis protocols among studies. These findings uncover an urgent need to improve standardization and ensure direct comparability across studies.

Potential COVID-19 therapeutic approaches targeting angiotensin-converting enzyme 2; An updated review

COVID-19 has spread swiftly throughout the world posing a global health emergency. The significant numbers of deaths attributed to this pandemic have researchers battling to understand this new, dangerous virus. Researchers are looking to find possible treatment regimens and develop effective therapies. This study aims to provide an overview of published scientific information on potential treatments, emphasizing angiotensin-converting enzyme II (ACE2) inhibitors as one of the most important drug targets. SARS-CoV-2 receptor-binding domain (RBD); as a viral attachment or entry inhibitor against SARS-CoV-2, human recombinant soluble ACE2; as a genetically modified soluble form of ACE2 to compete with membrane-bound ACE2, and microRNAs (miRNAs); as a negative regulator of the expression of ACE2/TMPRSS2 to inhibit SARS-CoV2 entry into cells, are the potential therapeutic approaches discussed thoroughly in this article. This review provides the groundwork for the ongoing development of therapeutic agents and effective treatments against SARS-COV-2.

Current status and future perspective of radiopharmaceuticals in China

Radiopharmaceuticals are essential components of nuclear medicine and serve as one of the cornerstones of molecular imaging and precision medicine. They provide new means and approaches for early diagnosis and treatment of diseases. After decades of development and hard efforts, a relatively matured radiopharmaceutical production and management system has been established in China with high-quality facilities. This review provides an overview of the current status of radiopharmaceuticals on production and distribution, clinical application, and regulatory supervision and also describes some important advances in research and development and clinical translation of radiopharmaceuticals in the past 10 years. Moreover, some prospects of research and development of radiopharmaceuticals in the near future are discussed.