Mericitabine (R-7128) is a nucleoside HCV NS5B polymerase inhibitor. It acts as an RNA chain terminator and prevents the elongation of RNA transcripts during replication.

PSI-6130 is converted through phosphorylation by cellular kinases to an active 5′-triphosphate metabolite, which inhibits the NS5B RNA polymerase of HCV. Mericitabine is an oral cytidine nucleoside analog prodrug that exhibits strong antiviral effectiveness against the HCV polymerase across all HCV genotypes. Mericitabine is a first-in-class nucleoside polymerase inhibitor (NPI), which requires intracellular uptake and phosphorylation to two active triphosphates. Mericitabine, a cytidine analogue, is an oral prodrug of PSI-6130. PSI-6130 has an EC90 value of 4.6±2 μM in the HCV replicon assay. Mericitabine displays high specificity for HCV, minimal cytotoxicity, and does not affect mitochondrial DNA. Mericitabine shows a relatively good safety profile and significant potency against HCV-1 [2][3].