Discussion
According to the Johns Hopkins Center for Health Security, as of December 20, 2022, there were 650 million cumulative COVID-19 cases and 6.65 million deaths worldwide. As a functional receptor of SARS-CoV-2, ACE2 plays a great role in viral infection and prognosis in vivoand has thereby gained widespread attention.
The main ACE2 detection assay relies on IHC staining, which is invasive and relies heavily on experienced physicians; unfortunately, its inability to comprehensively probe lesions throughout the body limits its wider application. Nuclear medicine molecular probes can be used in noninvasive imaging of critical molecules in the human body and thus may provide a valuable tool in ACE2 examination.
Here, we described a novel ACE2-specific nuclide molecular probe,68Ga-HZ20, which can be used to detect the distribution and content of rhACE2 in vivo . In a previous study, we performed the first quantitative analysis of ACE2 expression in human organs by PET/CT imaging using the 68Ga-HZ20 probe, which contributed to the understanding of the pathogenesis of SARS-CoV-2 infection, assessing the susceptibility of different populations to SARS-CoV-2, the severity and duration of symptoms after infection, etc.32. In this study, HZ20, a specific inhibitory peptide with high affinity for ACE2, was selected as the PET imaging agent. Micro-PET/CT images of the 68Ga-HZ20 probe in normal KM mice showed that it had ideal pharmacokinetic characteristics and low uptake in background organs, and the probe was mainly metabolized by the kidney through the bladder. This is consistent with the abundant expression of ACE2 in the kidney, as shown by IHC. After evaluating the safety of the probe, we built subcutaneous and organ intraperitonealin situ injection models of rhACE2 for research. To fully evaluate the sensitivity of the probe, we injected different doses of rhACE2 into each model. The results showed that the uptake of the probe was highly positively correlated with the content of rhACE2. To evaluate the specificity of the probe, we also conducted a blocking study in which excessive non-68Ga-labeled HZ20 could competitively bind to rhACE2 in vivo , demonstrating the specificity of the probe against rhACE2.
The latest study published in Nature shows that ursodeoxycholic acid (UDCA), a drug for treating liver disease, can block the ACE2 receptor and thus viral entry into host cells, and because the drug targets host cells rather than the virus, it may prevent infection with future new variants of the virus as well as other coronaviruses that may emerge34. As rhACE2 therapy may become a preventive and therapeutic modality for critically ill COVID-19 patients23, monitoring exogenous rhACE2 is indispensable. The use of nuclear medicine molecular probes and PET imaging technology to monitor the content and distribution of rhACE2in vivo , noninvasively and repeatedly, helps to clarify the mechanism by which rhACE2 blocks viral infection and analyze the preventive ability of rhACE2 against SARS-CoV-2 and the effectiveness of therapies for COVID-19. This technique is expected to provide an effective analytical method for quantifying the dynamic distribution of rhACE2 in vivo and a scientific basis for the effectiveness of rhACE2 therapy.
The most prominent symptom of SARS-CoV-2 infection is respiratory tract infection, but much evidence shows that the virus can cause multisystem damage. Acute kidney injury (AKI) is one of the most common and serious organ complications of novel coronavirus8. IHC staining showed high expression of ACE2 in the kidney. PET images also showed the super uptake of probe in the kidney. Although the high uptake of the kidneys is partly because the kidneys are the metabolic organs of the probe, after subcutaneous injection of 0.4 nmol rhACE2, HZ20 blocking and nonblocking studies showed that the kidneys do have a high specific uptake of the probe, and the SUVmax of the kidneys 60 min after probe injection was 3.81±0.32 to 4.92±0.05 (P=0.004). This demonstrates the high expression of ACE2 in the kidneys, which partly supports AKI caused by SARS-CoV-2 infection.
Approximately one-fifth of patients suffer from liver injury during SARS-CoV-2 infection, and the incidence of liver decompensation and acute and chronic liver failure is higher in patients with liver cirrhosis10,35–37. Our 68Ga-HZ20 probe can be used to accurately locate and quantify rhACE2 in the liver to provide support for the prevention of serious liver complications by exogenous ACE2.
The virus may accumulate in the olfactory bulb and vagus nerve through the blood brain barrier (BBB) to infiltrate the central nervous system (CNS)38,39. Some studies have confirmed the existence of SARS-CoV-2 in the brain and cerebrospinal fluid40. A recent meta-analysis report showed that more than 40% of patients had psychiatric and neuropsychiatric symptoms associated with severe coronavirus infection at an early stage39,41–46. We injected rhACE2 into the anterior fontanel of the mouse brain at a fixed point. PET imaging demonstrated that 68Ga-HZ20 can reach brain tissue, target lesions at fixed points, and detect changes in rhACE2, which supports its use in monitoring rhACE2 changes in the brain.
ACE2 is highly expressed in many tumors and is a potential protective factor against cancer progression3,11,47. However, as the host cell receptor of SARS-CoV-2, the high expression of ACE2 may increase the risk of SARS-CoV-2 infection35,48. Studies have found that the SARS-CoV-2 spike protein not only binds to ACE2 to mediate infection of host cells but also activates intracellular signals to degrade ACE2 mRNA after entering host cells. After downregulation of ACE2 expression in infected cells, the level of angiotensin II increases. High-dose angiotensin II induces the death of arterial endothelial cells and exacerbates cardiovascular disease49. Therefore, it may not be a wise choice to use ACE2 inhibitors to prevent and treat COVID-19. For tumor patients, rhACE2 may be a reliable therapy. The 68Ga-HZ20 probe can also be used to monitor the tumor site of these patients, and exogenous infusion of rhACE2 can help achieve more precise treatment.
Based on the positive correlation between the uptake value of the probe SUVmax and the content of rhACE2 as well as the precise positioning ability of the probe at the target site, PET imaging can be used for noninvasive real-time quantification of the content and spatial distribution of the receptor, which provides scientific guidance for the formulation of prevention and treatment measures for SARS-CoV-2.