In vitro characterization and safety assessment of68Ga-HZ20
The procedure of 68Ga-HZ20 radiosynthesis is shown in Fig. 1A. Radio-HPLC showed nondecayed radiolabeling rates at 65 °C, 85 °C, and 95 °C of 74.03%, 96.42%, and 78.60% for the68Ga-HZ20 probe, respectively (Fig. 1B). After purification on a C18 column, the radiochemical purity was higher than 99% (Fig. S1), and the nondecayed radiochemical yields were 61.51±4.47%, 78.99±3.92%, and 66.34±5.51 (n=3), respectively (Fig. 1C). The specific activity was (3.37±0.35) ×104GBq/mmol (n=3). The in vitro stability of the probe was tested using radio-HPLC. The radiochemical purity of68Ga-HZ20 in either normal saline or 5% HSA was higher than 96% within 180 min at room temperature incubation (n=3) (Fig. 1D). To evaluate the safety of the probe,68Ga-HZ20 of 37 MBq was injected into KM mice (n=5) one week in advance, and then the main organs were sampled for HE staining; no organic damage to the organs was observed (Fig. 1E). Immunohistochemical staining was used to assess the expression level of ACE2 in the main organs of mice. Almost no expression of ACE2 was observed in the stomach, heart, or spleen (-) of mice, with microexpression visible in the brain (+). There was no expression of ACE2 in hepatocytes, and notably, it was highly expressed in the hepatic manifold (++); similarly, it was not expressed in alveolar cells but was highly expressed in bronchial epithelial cells (++). In addition, high expression was observed in both the kidneys and small intestine (++), supporting symptoms such as respiratory and fecal-oral transmission of SARS-CoV-2 as well as acute kidney injury and diarrhea following infection (Fig. 1F).