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.