# Exploring the Coronavirus Exoribonuclease as an Antiviral Target

> **NIH NIH R21** · UNIVERSITY OF CONNECTICUT SCH OF MED/DNT · 2021 · $463,700

## Abstract

Over the last 20 years, three major zoonotic coronavirus (CoV) infections have emerged all causing acute
respiratory illness, leading to significant morbidity and mortality. SARS-CoV-1 emerged in Asia in late 2002
while Middle East Respiratory Syndrome (MERS-CoV) was first reported in Saudi Arabia in 2012. In late 2019,
SARS-CoV-2 was reported in China and has now spread globally causing over 673,000 deaths in less than eight
months. The currently-raging COVID-19 pandemic presents an urgent need to explore new targets and
approaches. Many RNA viruses such as Hepatitis C virus (HCV) and Respiratory Syncytial virus (RSV) can be
treated with ribavirin and other broad-spectrum antiviral nucleoside analogues. Ribavirin and other nucleoside
analogues are misincorporated into progeny genomes by the virally encoded RNA-dependent RNA-polymerase
(RdRp), resulting in lethal mutagenesis. Interestingly, ribavirin has minimal effect against SARS or MERS due to
unique aspects of CoV replication. The 30kb CoV genomes are the largest of all RNA viral genomes, more than
three times the typical size. All CoVs encode 16 non-structural proteins (nsps) required for the production of
progeny RNA. Nsp14 contains an exoribonuclease domain (ExoN), which has been shown to ensure replication
fidelity of the large genome and provide resistance to drugs like ribavirin. ExoN, a conserved 3’ to 5’ proofreading
exoribonuclease, removes misincorporated ribavirin, rendering it ineffective. Active site mutants of human CoV
ExoN result in severe defects in viral RNA synthesis. Thus, ExoN inhibitors are expected to be effective anti-
coronavirus agents either as monotherapy or in synergistic combination with nucleoside analogues. This
proposal aims to discover first-in-class ExoN inhibitors for further drug development.
Our current program is focused on developing broad-spectrum small molecule inhibitors of essential viral
exonucleases as antiviral drugs. These exonucleases, like CoV ExoN, possess an acidic active site containing
dual magnesium ions that coordinate substrate binding and catalyze bond cleavage. This ExoN key structural
motif presents an excellent opportunity to expand our antiviral program to CoVs. We have synthesized hundreds
of herpesvirus exonuclease inhibitors tailored to the bi-metallic binding site, which will be evaluated for activity
against bacterially expressed ExoN. We propose that inhibitors of ExoN proof-reading activity would exert strong
antiviral effects as single agents and would powerfully synergize with ribonucleoside analogs such as ribavirin
and remdesivir. Key outcomes of the work would be (1) establishing the druggability of the active site of ExoN
for small molecule inhibitors, (2) determining the antiviral activity produced by direct ExoN inhibition, (3)
evaluating the potential synergism between ExoN inhibitors and nucleoside analogs and (4) identifying lead
candidates for follow on in vivo drug development efforts.

## Key facts

- **NIH application ID:** 10238324
- **Project number:** 1R21AI161230-01
- **Recipient organization:** UNIVERSITY OF CONNECTICUT SCH OF MED/DNT
- **Principal Investigator:** SANDRA K WELLER
- **Activity code:** R21 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2021
- **Award amount:** $463,700
- **Award type:** 1
- **Project period:** 2021-07-19 → 2024-06-30

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10238324

## Citation

> US National Institutes of Health, RePORTER application 10238324, Exploring the Coronavirus Exoribonuclease as an Antiviral Target (1R21AI161230-01). Retrieved via AI Analytics 2026-05-25 from https://api.ai-analytics.org/grant/nih/10238324. Licensed CC0.

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