ABSTRACT
Objectives: With over 90 million cases reported in the globe, the COVID-19 pandemic caused by SARS-CoV-2 has been a serious public health crisis. Development of novel and specific antiviral drugs against the SARS-CoV-2 has been an urgent demand. One such drug is Favipiravir, initially developed as an antiviral drug against influenza. Now Favipiravir has received approvals for emergency use against SARS-CoV-2 in many countries. A better understanding of Favipiravir’s biodistribution and pharmacokinetics in vivo will facilitate the clinical development of antiviral drugs against the SARS-CoV-2. Herein, we reported the evaluation of [18 F]Favipiravir with PET in cross-species studies to demonstrate the drug’s biodistribu-tion and pharmacokinetics and investigate the potentially increased risk of neurodegenerative diseases and/or neuroinflammation to COVID-19. Methods: The radiosynthesis of [18 F]Favipiravir was via labeling a commercially available precursor, methyl-5-chloroisoxazolo[4,5-b] pyrazine-3-carboxylate with K[18F]F/K222 and K2CO3 in DMSO at 130°C for 10 min, followed by hydrolysis with NaOH (aq.) at 110°C for 15 min.1 The whole body distribution on CD-1 mice was performed at four time points (5, 15, 30, 60 min). PET studies were carried out in CD-1 mice and AD mice (5XFAD) and naïve rhesus monkeys. We also performed the radiometabolite analysis of [18 F]Favipiravir in plasma and brain of CD-1 mice at 30 min post-injection. Results: [18 F]Favipiravir was obtained in 29% isolated radiochemi-cal yield (decay corrected). The radiochemical purity of the tracer was greater than 99%. No sign of radiolysis was observed for [18F] Favipiravir up to 120 min after formulation with 10% EtOH/saline. High radioactivity accumulation was observed in blood, lung, liver, kidney, and bone (around or more than 5% ID/g, injected dose per gram of wet tissue). The radioactivity level reached a plateau in small intestine, kidney and liver at 30,15 and 5 min, respectively, followed by slow washout, indicating that [18F]Favipiravir was possibly eliminated via the hepatobiliary and urinary pathway. For the radio-metabolic analysis of [18F]Favipiravir, average 41% and 89% of the radioactivity was parent fraction in the mice brain and plasma at 30 min post-injection (n=2), respectively. In PET imaging of CD-1 mice, the standard uptake value (SUV) of [18F]Favipiravir in brain reached its max value of 0.5 at 10 min and slowly reduced to 0.4 at 60 min. The results of PET imaging of AD mice with [18 F]Favipiravir were similar with that of CD-1 mice. In PET imaging of Rhesus monkeys, the brain uptake of [18 F]Favipiravir reached the max value of 0.5 SUV at 5 min and subsequently decreased to 50-60% of the maximum at 60 min. Conclusion: The evaluation of [18F]Favipiravir has demonstrated with bio-distribution and PET in mice and NHPs. Further evaluation of pharmacokinetics of [18F]Favipiravir in whole body monkey scans and LPS-induced neuroinflammation models is underway.
ABSTRACT
Purpose: Research has shown that ACE2 is one of the important targets of SARS - CoV - 2 virus infection. And receptor binding domain (receptorbindingdomain, RBD), a recombinant spike protein extracted from SARS - CoV - 2 virus, can bind to the angiotensin converting enzyme 2 (ACE2) in human body. This study aims to observe the biodistribution of [124I]I-RBD in mammals, evaluate the ability of [124I]I-RBD to detect ACE2-positive lesions, and explore the potential of RBD against COVID-19. Method: The binding ability of RBD to human ACE2 receptor was studied by surface plasmon resonance (SPR). RBD was labeled with 124I by N-Bromosuccinimide (NBS) mediated method, and high quality radiopharmaceutics [124I]I-RBD were obtained after purification. The binding potency of [124I]I-RBD to human ACE2 was detected by a modified enzyme linked immunosorbent assay (ELISA) method. The biodistribution of [124I]I-RBD in normal BALC/c mice was observed at 0.5 h, 2 h, 24 h and 60 h after tail vein injection. The human radiation dosimetry was estimated based on the animal studies. Results: The labeled [124I] I-RBD was purified by size-exclusion chromatography (PD-10), yielding a radio-chemical purity over 99%, as tested by radio-TLC. Specific activity of the product [124I]I-RBD was 28.9 GBq/nmol, and the radio chemistry purity (RCP) was over in saline for seven days. RBD binds to ACE2 with K<INFD= 14.08 nM, while the binding potency of [124I]I-RBD to ACE2 was calculated as 75.7 nM. Biodistribution data in normal balc/c mice showed that, RBD had a moderate metabolic rate, and its 24-hours-p.i. distribution was basically consistent with the high expression distribution of ACE2 in human body. Human radiation dosimetry estimates an indicated effective dose of 6.42 x 10-2 mSv/MBq. Conclusion: These experimental results confirmed the penitential of 124I-RBD as a novel molecular targeting probe for COVID-19. (Figure Presented) The probing of ACE2 expression via radiolabeled RBD may not only be used for non-invasive ACE2 mapping in mammals, but also have the prospect of radiotherapy to suppress the pandemic of COVID-19 by simply changing the iodine isotopes.