Development of a Radioligand for ACE2 PET Imaging - A Tool to Investigate the Expression and Regulation of the SARS-CoV-2 Entry Receptor

Background: The severe acute respiratory syndrome cornavirus-2 (SARS-CoV-2, Covid-19) pandemic shows large variability in disease presentation, progression, and (fatal) outcome. Currently, the development of diagnostic assays for early detection of Covid-19 infection receives much attention, but the risk factors that determine disease severity and outcome are insufficiently addressed. Adequate early risk stratification will be essential to avoid overburdening hospitals and intensive care units in view of eventual second wave of the outbreak or future generations of SARS-CoV. Molecular imaging using Positron Emission Tomography (PET) may serve for risk stratification by quantifying the expression and regulation of key players of the disease, thereby providing personalized therapy options. PET agents have hardly been implemented in clinical research regarding SARS-CoV-2.Hypothesis: The angiotensin converting enzyme 2 (ACE2), the virus' entry receptor to the host cell, has been identified as a pivotal determinant of SARS-CoV-2 transmissibility. Recent findings suggest that the expression and regulation of ACE2 can be used as a biomarker for an individual's susceptibility for viral infection as well as disease progression and severity. Goal: The aim of this project is to develop a radioligand for non-invasive imaging of ACE2 using PET. This novel radioligand should serve clinicians as a tool to investigate the dynamics of ACE2 expression in response to age, gender, preexisting morbidity, medication and environmental factors and, thus, enable (early) risk stratification.Objectives and Approach: Peptides- and small molecule-based ligands targeting ACE2 will be developed for radiolabeling to enable non-invasive in vivo imaging of ACE2. The ligands will be synthesized using solid phase and conventional chemistry. Peptides will be modified with a chelator for radio-metalation, while specific precursors of small molecule-based ligands will be designed for radio-fluorination. The radioligands will be tested for chemical and biological stability, as well as ACE2-binding affinity and selectivity using ACE2-transfected cells. The most promising candidates will be evaluated in vivo using normal mice to determine the pharmacokinetics, xenografted mice for assessing ACE2-specific uptake and genetically-modified mice expressing the human ACE2 to determine the radioligand's potential for quantifying ACE2 expression using PET imaging.Scientific Value and Impact: Molecular imaging agents are essential tools to improve the understanding of a disease, its severity and progression. Successful completion of this research project will deliver a PET agent (peptide or small molecule) for non-invasive imaging of ACE2. Such radioligands will aid researchers to better understand the protein's expression, function and regulation in the context of Covid-19 infection and disease phenotype. The development of an ACE2-targeted PET agent is innovative, novel and may - when combined with existing and on-going projects of ACE2 determination - give insights in the dynamics of ACE2 expression which will be essential for the risk management of individual patients. An ACE2-targeting PET agent may also be utilized towards a better understanding of the ACE2's protective role in cardiovascular disease, which in turn presents the main risk factor for severe disease progression upon Covid-19 injection. Future Implementation: The rapid and cost-effective clinical translation of a novel ACE2-targeting PET agent is feasible due to the unique situation at the Center for Radiopharmaceutical Sciences, which allows production of radiopharmaceuticals under Good Manufacturing Practice (GMP). The investment of resources into the proposed project would serve a rapid development of an ACE2-targeting radioligand to provide physicians with this essential tool in the near future.