HDAC9 and neoplasm: Compared with the FDA‐approved HDACi Vorinostat and PARPi Olaparib, P1 and P2 exhibited markedly enhanced inhibition of both HDAC and PARP activities (Figure S5B, Supporting Information), and demonstrated relatively high antiproliferative efficacy across a panel of human and murine tumor cell lines (Figure S5C,D, Supporting Information).