Thread by @mega_pindang71

Road to #Paxlovid #Pfizer #COVID19 new drug. pubmed.ncbi.nlm.nih.gov/34029993/
3CLpro inhibitors commonly target host proteases, which presents a potential liability for unexpected side effects.

Challenges : enzyme inhibition was only achieved in the absence of physiologically relevant reducing agents, and the reported antiviral effect in cells infected with SARS-CoV-2 could not be reproduced and was therefore suspected to be an indirect consequence of cell death.

Additional challenges in assessing 3CLpro inhibition potency in infected cells arose from the role of host proteases in virus entry. SARSCoV-2 uses human ACE2 as its entry receptor and human proteases as entry activators including cell surface transmembrane protease/serine

(TMPRSS) proteases, furin, cathepsins, plasmin, elastase, and trypsin.
In vitro, the antiviral activity of protease inhibitors largely depends on the mechanism of SARS-CoV-2 cellular entry and is driven by the levels of host protease expression in the cells. For example,

the rhinovirus inhibitor rupintrivir and the cathepsin inhibitor K11777 block SARS-CoV-2 replication in A549 lung epithelial cells, but their antiviral effect was greatly diminished when TMRPSS2 was overexpressed.
First-generation SARS-CoV-2 3CLpro inhibitors were PF-07304814,

GC-376 (for feline coronavirus, low oral bioavailability in rats), and CDI-45205. The antiviral effect of GC-376 was improved when combined with GS441524, the parent nucleoside of the remdesivir prodrug. #cat #coronavirus
In particular, 3CLpro inhibitors that lack selectivity

toward cathepsin L may potentially have counter-effective side effects by dampening the immune response against S2. The broad spectrum of virus protease inhibition previously reported for GC-376 combined with the cathepsin L inhibition effect suggest its activity may extend to

other human proteases.