In vitro activity: MK-5172 (Grazoprevir) is effective in biochemical assays against major genotypes and variants engineered with common resistant mutations, with Ki of 0.01±<0.01 nM (gt1b), 0.01±0.01 nM (gt1a), 0.08±0.02 nM (gt2a), 0.15±0.06 nM (gt2b), 0.90±0.2 nM (gt3a), 0.07±0.01 nM (gt1bR155K), 0.14±0.03 nM (gt1bD168V), 0.30±0.04 nM (gt1bD168Y), 5.3±0.9 nM (gt1bA156T), and 12±2 nM (gt1bA156V), respectively. In the replicon assay, MK-5172 demonstrates subnanomolar to low-nanomolar EC50s against genotypes 1a, 1b, and 2a, with EC50s of 0.5±0.1 nM, 2±1 nM, and 2±1 nM for gt1bcon1, gt1a, and gt2a, respectively. MK-5172 is potent against a panel of HCV replication mutants NS5A (Y93H) (EC50=0.7±0.3 nM), NS5B nucleosides (S282T) (EC50=0.3±0.1 nM), and NS5B (C316Y) (EC50=0.4±0.2). MK-5172 maintains the excellent potency against the gt 3a enzyme as well as a broad panel of mutant enzymes, has excellent potency in the replicon system [gt1b IC50(50% NHS)=7.4 nM; gt1a IC50(40% NHS)=7 nM], and shows excellent rat liver exposure.

Kinase Assay: recombinant HCV NS3/4A enzymes are expressed and purified from E. Coli. Enzyme sequences are derived from genotype 1a (gt1a) H77, gt1b con1, gt2a JFH1, gt2b HCJ8, and gt3a NZL1. Inhibition of HCV NS3/4A protease activity in reaction mixtures containing MK-5172 (Grazoprevir), Vaniprevir, or the reference compounds Danoprevir and TMC435 is determined in a time-resolved fluorescence assay. Cell-based HCV replicon assays are conducted in genotype 1b (con1) stable cell line HB1 or a gt2a cell line (JFH) in the presence of either 10% fetal bovine serum (FBS) or 40% normal human serum (NHS). Determinations of 50% effective concentrations (EC50s) against the panel of genotype or mutant replicon cell lines are conducted using a TaqMan-based assay. The 50% cytotoxic concentration (CC50) is determined in the HCV replicon cell line with the use of an MTS assay. Potency determinations against clinical genotype 1 NS3/4A sequences are made using a transient cell-based phenotype assay. The NS3/4A patient isolates are cloned from human plasma infected with HCV. Broad counterscreening, in which MK-5172 is evaluated for its inhibitory potency at a concentration of 10 μM, is conducted at MDS Pharma Services.

Cell Assay: HB1 cells (30,000 per well) are seeded of a 6-well tissue culture plate per drug concentration. The next day (day 0), the medium is replenished with fresh medium and MK-5172 at the appropriate drug concentration. Cells from a single well per drug concentration are harvested on days 0, 1, and 2, washed, and stored frozen until evaluation. The fourth well is similarly harvested on day 3.5 except that 30,000 cells are reseeded with fresh medium and MK-5172 at the appropriate drug concentration. For additional time points, cells are passaged and harvested every one-half week for 2 weeks. For the third week, cells are similarly treated except that cells received replenishing medium which contained 0.5 mg/ml G418 without protease inhibitor.