PSI-661

PSI-661 is a purine nucleotide analog (specifically a guanine nucleotide analog) developed by Pharmasset as an investigational antiviral agent for chronic hepatitis C virus (HCV) infection. It acts as an alternative substrate that inhibits HCV RNA synthesis by targeting the viral NS5B RNA-dependent RNA polymerase, demonstrating preclinical potency against wild-type HCV and the S282T resistance mutation associated with other nucleotide analogs like PSI-7977 and RG7128. The compound reached advanced preclinical development around 2010-2011 but did not progress to clinical trials, with its highest reported R&D status as pending.[1][2][3][4][5][7][9][10][15][16]