A boronic acid moiety was found to be a critical pharmacophore for enhanced in vitro potency against wild-type hepatitis C replicons and known clinical polymorphic and resistant HCV mutant replicons. The synthesis, optimization, and structure-activity relationships associated with inhibition of HCV replication in a subgenomic replication system for a series of non-nucleoside boron-containing HCV RNA-dependent RNA polymerase (NS5B) inhibitors are described. A summary of the discovery of 3 (GSK5852), a molecule which entered clinical trials in subjects infected with HCV in 2011, is included.

GSK2336805 is an orally bioavailable hepatitis C virus (HCV) inhibitor working through an NS5A-mediated mechanism. This first-time-in-human study was conducted to assess the safety, tolerability, pharmacokinetics, metabolism, and efficacy of GSK2336805 in healthy subjects and subjects infected with HCV genotype 1. We performed a three-part, randomized, doubleblind, placebo-controlled study in 46 healthy subjects and 23 HCV-infected subjects. After an overnight fast, healthy subjects received GSK2336805 as 10 mg, 30 mg, 30 mg plus food, and 60 mg in a single dose and 10 mg (7 days), 30 mg (7 days), and 75 mg (14 days) in a once-daily multiple dose. Subjects with HCV received GSK2336805 as a 1-to 120-mg single dose. In subjects with HCV, reductions in HCV RNA were observed within 4 h and a single dose of GSK2336805 of >10 mg resulted in a statistically significant >2-log reduction in HCV RNA compared with placebo at 24 h postdose. GSK2336805 was readily absorbed in all subjects, and the half-life (t 1/2 ) was suitable for once-daily dosing. Administration of GSK2336805 with food had no effect on plasma GSK2336805 exposure; however, absorption was delayed, with a median t max (time to maximum concentration of drug in serum) of 4.5 versus 2.0 h. Twenty subjects who received GSK2336805 experienced mild to moderate adverse events; none were serious. GSK2336805 was well tolerated and exhibited rapid, significant antiviral activity after a single dose in HCV-infected subjects. These results support the conduct of further studies evaluating GSK2336805 administered once daily for longer durations in combination with peginterferon, ribavirin, and other direct-acting antivirals. (This study has been registered at ClinicalTrials.gov under registration no. NCT01277692.) H epatitis C virus (HCV) is the leading cause of cirrhosis, liver failure, and primary hepatocellular carcinoma and the primary indication for liver transplantation (1). Of the six major HCV genotypes, genotype 1 (GT-1) is the most prevalent and is associated with the highest rate of treatment failure (2-4).Administration of two recently approved HCV nonstructural protein 3 (NS3) serine protease inhibitors, telaprevir and boceprevir, with the standard treatment of peginterferon and ribavirin led to sustained viral response rates of 75% and 68%, respectively (5-8). However, increased side effects, especially skin rash and anemia, are still problematic (9, 10). Therefore, antiviral agents with novel modes of action are necessary for an all-oral combination therapy to further improve sustained viral response rates and reduce side effects (11).NS5A is essential for HCV replication, and NS5A inhibitors have shown potent anti-HCV activity relative to interferon-ribavirin regimens in clinical trials (12)(13)(14)(15). GSK2336805 is an orally bioavailable NS5A inhibitor with selective activity against GT-1a and GT-1b subtypes in HCV replicon systems (50% effective concentration [EC 50 ] for GT-1a, 58.5 pM; for GT-1b, 7.4 pM) in vitro (J. Walker, submitted for publicat...

Rapid clinical progress of hepatitis C virus (HCV) replication inhibitors, including these selecting for resistance in the NS5A region (NS5A inhibitors), promises to revolutionize HCV treatment. Herein, we describe our explorations of diverse spiropyrrolidine motifs in novel NS5A inhibitors and a proposed interaction model. We discovered that the 1,4-dioxa-7-azaspiro[4.4]nonane motif in inhibitor 41H (GSK2236805) supported high potency against genotypes 1a and 1b as well as in genotype 1b L31V and Y93H mutants. Consistent with this, 41H potently suppressed HCV RNA in the 20-day RNA reduction assay. Pharmacokinetic and safety data supported further progression of 41H to the clinic.