Vaniprevir (MK-7009) is primarily used in the treatment of chronic hepatitis C virus (HCV) infection. It functions as a protease inhibitor, specifically targeting the NS3/4A protease enzyme, which is crucial for viral replication. By inhibiting this enzyme, Vaniprevir effectively reduces the viral load in patients, aiding in the management and potential eradication of the virus. It is often used in combination with other antiviral agents to enhance efficacy and reduce the likelihood of resistance development. This makes it a valuable component in therapeutic regimens aimed at achieving sustained virologic response (SVR) in HCV-infected individuals.