The invention discloses 1-N-substituted-6-(hetero)aryl-1H-thieno[3,2-d]pyrimidin-4-one derivatives of formula
wherein R
1
, R
2
, R
3
and R
4
are as defined herein that inhibit Hepatitis C virus NS5b polymerase inhibitors. Also disclosed are compositions and methods for treating an HCV infection and inhibiting HCV replication.