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Reply to Yan et al.: Quercetin possesses a fluorescence quenching effect but is a weak inhibitor against SARS-CoV-2 main protease

Xu et al., Proceedings of the National Academy of Sciences, doi:10.1073/pnas.2309870120
Sep 2023  
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Quercetin for COVID-19
22nd treatment shown to reduce risk in July 2021
 
*, now with p = 0.0031 from 11 studies.
No treatment is 100% effective. Protocols combine treatments. * >10% efficacy, ≥3 studies.
4,300+ studies for 75 treatments. c19early.org
In Vitro study1 and associated response from the original authors2, collectively showing that quercetin and echinatin had weak SARS-CoV-2 protease inhibition in SDS-PAGE assays2, despite false positive FRET results from MCA-AVLQ quenching1,2. Authors note that the compounds may act via other targets to achieve reported anti-COVID-19 effects2, and underscore the importance of meticulous validation with multiple assays when identifying SARS-CoV-2 protease inhibitors1,2.
Bioavailability. Quercetin has low bioavailability and studies typically use advanced formulations to improve bioavailability which may be required to reach therapeutic concentrations.
55 preclinical studies support the efficacy of quercetin for COVID-19:
In Silico studies predict inhibition of SARS-CoV-2, or minimization of side effects, with quercetin or metabolites via binding to the spikeA,30,50,13,33,21,15,29,16,51,32, MproB,11,25,28,33,21,15,29,14,34,52,9,51,27,36,8,6,4,3, RNA-dependent RNA polymeraseC,23, PLproD,28,36, ACE2E,13,28,14,51,32,19, TMPRSS2F,13, helicaseG,25,20, endoribonucleaseH,30, cathepsin LI,17, Wnt-3J,13, FZDK,13, LRP6L,13, ezrinM,31, ADRPN,29, NRP1O,32, EP300P,7, PTGS2Q,14, HSP90AA1R,14,7, matrix metalloproteinase 9S,22, IL-6T,12,26, IL-10U,12, VEGFAV,26, and RELAW,26 proteins. In Vitro studies demonstrate efficacy in Calu-3X,39, A549Y,12, HEK293-ACE2+Z,46, Huh-7AA,16, Caco-2AB,38, Vero E6AC,33,10,38, mTECAD,41, and RAW264.7AE,41 cells. Animal studies demonstrate efficacy in K18-hACE2 miceAF,43, db/db miceAG,41,49, BALB/c miceAH,48, and rats53. Quercetin reduced proinflammatory cytokines and protected lung and kidney tissue against LPS-induced damage in mice48.
Xu et al., 5 Sep 2023, peer-reviewed, 7 authors. Contact: huangluqi01@126.com.
In Vitro studies are an important part of preclinical research, however results may be very different in vivo.
This PaperQuercetinAll
Reply to Yan et al.: Quercetin possesses a fluorescence quenching effect but is a weak inhibitor against SARS-CoV-2 main protease
Haiyu Xu, Yute Zhong, Jinyue Yang, Lifeng Fu, Yi Shi, Luqi Huang, George F Gao
Proceedings of the National Academy of Sciences, doi:10.1073/pnas.2309870120
Recently, we published an article (1) showing that several bioactive compounds were identified from Huashi Baidu decoction (Q-14) that showed both antiviral activity and antiinflammatory effects against COVID-19. The main protease (M pro ) of SARS-CoV-2 is an attractive target for the development of antiviral drugs. Our results showed that quercetin and echinatin from Q-14 displayed moderate inhibition activities against SARS-CoV-2 M pro at submicromolar levels by the fluorescence resonance energy transfer (FRET)-based protease activity assay. However, Yan et al. suggested that quercetin is a promiscuous M pro inhibitor because of its fluorescence quenching effect (2). We took this suspicion very seriously. Therefore, we repeated the FRET-based protease activity assay and tested the fluorescence quenching effects with the MCA-AVLQ-Lys (Dnp)-Lys-NH2 (MCA-AVLQ) fragment. The FRET assay results showed that both quercetin and echinatin have M pro inhibition activity (Fig. 1 A and E ), but both quercetin and echinatin displayed a fluorescence quenching effect on MCA-AVLQ fragment (Fig. 1 B and F ). The inhibition ratio and quenching ratio were further calculated (Fig. 1 C and G). We found that the inhibition ratio is significantly greater than the quenching rate, especially at low concentrations, which indicates that quercetin and echinatin should possess certain M pro inhibition activity. To further validate the M pro inhibition activity of
References
Xu, Bioactive compounds from Huashi Baidu decoction possess both antiviral and anti-inflammatory effects against COVID-19, Proc. Natl. Acad. Sci. U.S.A
Yan, Production of a versatile SARS-CoV-2 main protease biosensor based on a dimerization-dependent red fluorescent protein, J. Med. Virol
Yan, Zhang, Liu, Wang, Chen, Reframing quercetin as a promiscuous inhibitor against SARS-CoV-2 main protease, Proc. Natl. Acad. Sci. U.S.A
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