Genetic barrier and variant fitness in hepatitis C as critical parameters for drug resistance development.

The approval of direct-acting antiviral agents (DAAs) has marked a pivotal change in the treatment landscape of chronic hepatitis C. As for DAAs targeting other viral diseases, there are concerns regarding the development of resistant viral variants. Their selection allows the virus to escape from drug pressure with subsequent treatment failure. The emergence of resistant variants depends on multiple factors that range from genetic barriers to mutations to the fitness of viral variants. This article illustrates the basic mechanisms underlying development of resistance to specific antiviral agents with a special emphasis on NS3 protease inhibitors. The role of fitness deficits and compensation for variant selection and persistence is discussed together with technical issues in sequencing as well as clinical implications in the use of DAAs now and in the future.