Nirmatrelvir Resistance in SARS-CoV-2 Omicron_BA.1 and WA1 Replicons and Escape Strategies (preprint)

The antiviral component of Paxlovid, nirmatrelvir (NIR), forms a covalent bond with Cys145 of SARS-CoV-2 nsp5. To explore NIR resistance we designed mutations to impair binding of NIR over substrate. Using 12 Omicron (BA.1) and WA.1 SARS-CoV-2 replicons, cell-based complementation and enzymatic assays, we showed that in both strains, E166V imparted high NIR resistance (~55-fold), with major decrease in WA1 replicon fitness (~20-fold), but not BA.1 (~2-fold). WA1 replicon fitness was restored by L50F. These differences may contribute to a potentially lower barrier to resistance in Omicron than WA1. E166V is rare in untreated patients, albeit more prevalent in paxlovid-treated EPIC-HR clinical trial patients. Importantly, NIR-resistant replicons with E166V or E166V/L50F remained susceptible to a) the flexible GC376, and b) PF-00835231, which forms additional interactions. Molecular dynamics simulations show steric clashes between the rigid and bulky NIR t-butyl and {beta}-branched V166 distancing the NIR warhead from its Cys145 target. In contrast, GC376, through-wiggling and jiggling- accommodates V166 and still covalently binds Cys145. PF-00835231 uses its strategically positioned methoxy-indole to form a beta-sheet and overcome E166V. Drug design based on strategic flexibility and main chain-targeting may help develop second-generation nsp5-targeting antivirals efficient against NIR-resistant viruses.

Biochemical and structural insights into SARS-CoV-2 polyprotein processing by Mpro (preprint)

SARS-CoV-2, a human coronavirus, is the causative agent of the COVID-19 pandemic. Its ~30 kb RNA genome is translated into two large polyproteins subsequently cleaved by viral papain-like protease and main protease (Mpro/nsp5). Polyprotein processing is essential yet incompletely understood. We studied Mpro-mediated processing of the nsp7-10/11 polyprotein, whose mature products are cofactors of the viral replicase, identifying the order of cleavages as: 1) nsp9-10, 2) nsp8-9/nsp10-11, and 3) nsp7-8. Integrative modeling based on mass spectrometry (including hydrogen-deuterium exchange and cross-linking) and X-ray scattering yielded three-dimensional models of the nsp7-10/11 polyprotein. Our data suggest that the nsp7-10/11 structure in complex with Mpro strongly resembles the unbound polyprotein, and that both polyprotein conformation and junction accessibility determine the preference and order of cleavages. Finally, we used limited proteolysis assays to characterize the effect of a series of inhibitors/binders on Mpro processing of nsp7-11 and Mpro inhibition using a polyprotein substrate.

Subgenomic SARS-CoV-2 replicon and reporter replicon cell lines enable ultrahigh throughput antiviral screening and mechanistic studies with antivirals, viral mutations or host factors that affect COVID-19 replication (preprint)

Replicon-based technologies were used to develop reagents and assays for advanced drug discovery efforts against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and for examining all facets of the SARS-CoV-2 replication cycle at reduced biocontainment level. Specifically: a) 21 replicons were cloned in bacterial artificial chromosomes (BACs) and delivered as transfectable plasmid DNA or transcribed RNA in various cell types. Replicons carrying mutations that affect the activity or antiviral susceptibility of SARS-CoV-2 enzymes were used to establish utility for mechanistic studies while reducing the community risks associated with gain-of-function studies in fully infectious virus. b) A BHK-21 stable cell line harboring SARS-CoV-2 replicon was generated and characterized in robust high/ultra-high throughput assays of antiviral efficacy with orthogonal SARS-CoV-2 replication reporter genes (Nano luciferase and enhanced green fluorescent protein-eGFP); the estimated antiviral potencies in the fully infectious SARS-CoV-2 system and in the transient or stable replicon systems were similar. HEK293 and Calu1 stable cell lines expressing SARS-CoV-2 replicon have also been prepared. Finally, c) we generated trans-encapsidated replicons by co-expression with SARS-CoV-2 structural proteins, thus producing single-round infectious SARS-CoV-2 virus-like particles able to transduce susceptible cell types, thus expanding utility to enable study of virion assembly and entry into target cells. Hence, these SARS-CoV-2 replicon-based reagents include a novel approach to replicon-harboring cell line generation and are valuable tools that can be used at lower biosafety level (BSL2) for drug discovery efforts, characterization of SARS-CoV-2 and variant evolution in the COVID-19 pandemic, mechanisms of inhibition and resistance, and studies on the role of SARS-CoV-2 genes and host dependency factors.