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

Remdesivir 1 is an amidate 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 three to seven-fold improved oral delivery of 2 in monkeys. Prodrug 3 is cleaved pre-systemically 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. ### Competing Interest Statement Some authors may be stockholders of Gilead Sciences. * AB : apical to basolateral AGM : African green monkey ATP : adenosine triphosphate BALF : bronchioalveolar lavage fluid BEGM : bronchial epithelial growth medium CES : carboxyesterase Cyno : cynomolgus DIC : N , N ′-Diisopropylcarbodiimide DMAP : 4-dimethylaminopyridine DMEM : Dulbecco’s modified eagle medium dpi, day post-infection FBS : fetal bovine serum GI : gasterointestinal gRNA : genomic RNA Hep : Hepatocytes hr, human recombinant IACUC : International animal care and use committee IV : intravenous LLOQ : lower limit of quantification MERS : Middle East Respiratory Syndrome MOI : multiplicity of infection NHBE : normal human bronchial epithelial NHP : non-human primate NTP : nucleotide triphosphate PBMC : peripheral blood mononuclear cells PBS : phosphate-buffered saline PFA : Plaque forming assay PFU : plaque forming unit PHH : primary human hepatocyte PMSF : phenylmethylsulfonyl fluoride RdRp : RNA-dependent RNA polymerase RSV : respiratory syncytial virus SARS : severe acute respiratory syndrome VOC : variant of concern WHO : World Health Organization