Discovery of GS-5245 (Obeldesivir), an Oral Prodrug of Nucleoside GS-441524 That Exhibits Antiviral Efficacy in SARS-CoV-2-Infected African Green Monkeys

Abstract

Remdesivir 1 is an phosphoramidate prodrug that releases the monophosphate of nucleoside GS-441524 (2) into lung cells, thereby forming the bioactive triphosphate 2-NTP. 2-NTP, an analog of ATP, inhibits the SARS-CoV-2 RNA-dependent RNA polymerase replication and transcription of viral RNA. Strong clinical results for 1 have prompted interest in oral approaches to generate 2-NTP. Here, we describe the discovery of a 5'-isobutyryl ester prodrug of 2 (GS-5245, Obeldesivir, 3) that has low cellular cytotoxicity and 3-7-fold improved oral delivery of 2 in monkeys. Prodrug 3 is cleaved presystemically to provide high systemic exposures of 2 that overcome its less efficient metabolism to 2-NTP, leading to strong SARS-CoV-2 antiviral efficacy in an African green monkey infection model. Exposure-based SARS-CoV-2 efficacy relationships resulted in an estimated clinical dose of 350-400 mg twice daily. Importantly, all SARS-CoV-2 variants remain susceptible to 2, which supports development of 3 as a promising COVID-19 treatment.

Figure 1.

Metabolic activation pathways of prodrug 3 and phosphoramidate 1 to the common active 2-NTP metabolite in lungs. Ester prodrug 3 administered orally is metabolized presystemically in the intestine and liver to parent nucleoside 2 which then distributes into cells, including lung cells, where it is metabolized by a nucleoside kinase to the monophosphate. The monophosphate is then metabolized to the active metabolite, 2-NTP, by the action of nucleotide kinases. These steps are reversible and allow the phosphorylated metabolites generated inside all cells to also be broken down to parent nucleoside 2 and released back into systemic circulation. Phosphoramidate 1, following iv administration, rapidly distributes into many cell types, including lung cells, where it is metabolized irreversibly by the action of hydrolases (CES1, Cathepsin A) to the alanine metabolite and then to the same monophosphate by the action of phosphoramidases, e.g., HINT-1. The metabolism of 1 to the monophosphate inside cells is efficient and is the major pathway for generation of 2-NTP following administration of 1.

Figure 2.

Cellular metabolism and in vivo pharmacokinetics of 3. (A) Intracellular metabolism of 3 (blue), 2 (black), and 1 (red) to the active metabolite 2-NTP in A549-hACE2 cells. Average 2-NTP concentrations are indicated after dose normalization to 1 μM. (B) Intracellular metabolism in NHBE cells. Average 2-NTP concentrations are indicated after dose normalization to 1 μM. (C) Plasma concentration–time profile of 2 following iv and oral administration and oral administration of 3 in AGMs. (D) Lung 2-NTP concentrations at 24 h normalized to exposure of 2 in plasma following iv administration of 2 in both cynomolgus monkeys and AGMs and oral dosing of 3 in AGMs.

Figure 3.

Antiviral effect of oral 3 on respiratory tract lavage, swab, and tissue samples in the African green monkey SARS-CoV-2 infection model. (A) Infectious virus in bronchoalveolar lavage fluid (BALF). (B) Genomic RNA in BALF. (C) Infectious viral titer in throat swab. (D) Genomic RNA in throat swab. (E) Genomic RNA in respiratory tissue samples harvested day 6 across the respiratory tract. LLOQ, lower limit of quantification. * p < 0.05; ** p < 0.01; *** p < 0.001; **** p < 0.0001.

Scheme 1.

Synthesis of 5′-Ester Prodrugsa