Discovery of the First <i>C</i>‑Nucleoside HCV Polymerase Inhibitor (GS-6620) with Demonstrated Antiviral Response in HCV Infected Patients

Hepatitis C virus (HCV) infection presents an unmet medical need requiring more effective treatment options. Nucleoside inhibitors (NI) of HCV polymerase (NS5B) have demonstrated pan-genotypic activity and durable antiviral response in the clinic, and they are likely to become a key component of future treatment regimens. NI candidates that have entered clinical development thus far have all been <i>N</i>-nucleoside derivatives. Herein, we report the discovery of a <i>C</i>-nucleoside class of NS5B inhibitors. Exploration of adenosine analogs in this class identified 1′-cyano-2′-<i>C</i>-methyl 4-aza-7,9-dideaza adenosine as a potent and selective inhibitor of NS5B. A monophosphate prodrug approach afforded a series of compounds showing submicromolar activity in HCV replicon assays. Further pharmacokinetic optimization for sufficient oral absorption and liver triphosphate loading led to identification of a clinical development candidate GS-6620. In a phase I clinical study, the potential for potent activity was demonstrated but with high intra- and interpatient pharmacokinetic and pharmacodynamic variability

Discovery of GS-9669, a Thumb Site II Non-Nucleoside Inhibitor of NS5B for the Treatment of Genotype 1 Chronic Hepatitis C Infection

Investigation of thiophene-2-carboxylic acid HCV NS5B site II inhibitors, guided by measurement of cell culture medium binding, revealed the structure–activity relationships for intrinsic cellular potency. The pharmacokinetic profile was enhanced through incorporation of heterocyclic ethers on the <i>N</i>-alkyl substituent. Hydroxyl groups were incorporated to modulate protein binding. Intrinsic potency was further improved through enantiospecific introduction of an olefin in the <i>N</i>-acyl motif, resulting in the discovery of the phase 2 clinical candidate GS-9669. The unexpected activity of this compound against the clinically relevant NS5B M423T mutant, relative to the wild type, was shown to arise from both the <i>N</i>-alkyl substituent and the <i>N</i>-acyl group

Discovery and Synthesis of a Phosphoramidate Prodrug of a Pyrrolo[2,1‑<i>f</i>][triazin-4-amino] Adenine <i>C</i>‑Nucleoside (GS-5734) for the Treatment of Ebola and Emerging Viruses

The recent Ebola virus (EBOV) outbreak in West Africa was the largest recorded in history with over 28,000 cases, resulting in >11,000 deaths including >500 healthcare workers. A focused screening and lead optimization effort identified <b>4b</b> (GS-5734) with anti-EBOV EC₅₀ = 86 nM in macrophages as the clinical candidate. Structure activity relationships established that the 1′-CN group and <i>C</i>-linked nucleobase were critical for optimal anti-EBOV potency and selectivity against host polymerases. A robust diastereoselective synthesis provided sufficient quantities of <b>4b</b> to enable preclinical efficacy in a non-human-primate EBOV challenge model. Once-daily 10 mg/kg iv treatment on days 3–14 postinfection had a significant effect on viremia and mortality, resulting in 100% survival of infected treated animals [Nature 2016, 531, 381−385]. A phase 2 study (PREVAIL IV) is currently enrolling and will evaluate the effect of <b>4b</b> on viral shedding from sanctuary sites in EBOV survivors