# Modifying Remdesivir Prodrug Design to Enhance the Active Metabolite Accumulation in the Lung (Resubmission)

> **NIH NIH R21** · UNIVERSITY OF MICHIGAN AT ANN ARBOR · 2022 · $234,000

## Abstract

Abstract
Although remdesivir is among few medications approved by the FDA for treating patients with COVID-19, a
significant portion of patients did not respond adequately to the treatment. Remdesivir is formed as a 2-
ethylbutyl ester prodrug of the nucleoside analog GS-441524 to overcome the poor cell permeability of
nucleosides and bypass the rate-limiting mono-phosphorylation metabolism required for GS-441524 activation.
Remdesivir is extensively hydrolyzed in human plasma, raising the concerns that only a small portion of
remdesivir can reach the lung in its intact prodrug form. Thus, the benefits associated with the prodrug design
may be unattainable for the treatment of COVID-19. Another limitation is that remdesivir is only available in an
intravenous injection dosage form due to its intensive first-pass effect. Because of those limitations, some
investigators advocate for using the parent compound GS-441524 as an alternative to remdesivir to treat
COVID-19 patients. However, it remains unknown how GS-441524's poor cell permeability affects intracellular
drug accumulation and the extent to which the required rate-limiting mono-phosphorylation step could affect
GS-441524 activation. During remdesivir development, several GS-441524 ester prodrugs were synthesized
and evaluated for their plasma stability and in vitro antiviral activity. Remdesivir was chosen as the drug
candidate for further development because of its superior anti-Ebola activity relative to other prodrugs in
several non-lung cell lines. Among those, the isopropyl ester prodrug of GS-441524 showed lower antiviral
potency but much greater plasma stability than remdesivir. Given that all the prodrugs have the same active
metabolite, the antiviral activity differences among the tested prodrugs are likely attributed to their differences
in intracellular accumulation and activation. Existing evidence suggests that carboxylesterase 1 (CES1) and
cathepsin A (CatA) are involved in the activation of ester nucleoside prodrugs, and both CES1 and CatA are
highly expressed in the human lung. Thus, isopropyl-GS-441524 could be efficiently activated in the lung and
consequently exert its anti-SARS-CoV-2 effect. Moreover, the excellent stability of isopropyl-GS-441524 in
plasma, liver, and intestine could improve the intracellular drug accumulation in the lung and allow the
compound to be developed into an oral dosage form. In this project, we will conduct a pharmacokinetics (PK)
study in mice to compare the activation and disposition between isopropyl-GS-441524, remdesivir, and GS-
441524 in various tissues. We will also determine the activation rates of the three compounds in the human
lung and other PK-related organs and identify and characterize the hydrolases responsible for the prodrug
activation. Furthermore, we will evaluate the anti-SARS-CoV-2 activity of the three compounds using an in vitro
model. The project will provide evidence supporting that isopropyl-GS-441524 is advantageo...

## Key facts

- **NIH application ID:** 10450227
- **Project number:** 1R21AI163425-01A1
- **Recipient organization:** UNIVERSITY OF MICHIGAN AT ANN ARBOR
- **Principal Investigator:** Haojie Zhu
- **Activity code:** R21 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2022
- **Award amount:** $234,000
- **Award type:** 1
- **Project period:** 2022-05-13 → 2024-04-30

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10450227

## Citation

> US National Institutes of Health, RePORTER application 10450227, Modifying Remdesivir Prodrug Design to Enhance the Active Metabolite Accumulation in the Lung (Resubmission) (1R21AI163425-01A1). Retrieved via AI Analytics 2026-05-23 from https://api.ai-analytics.org/grant/nih/10450227. Licensed CC0.

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