Simmiparib is a highly potent and orally active PARP1 and PARP2 inhibitor with IC50 values of 1.75 nM and 0.22 nM, respectively. Simmiparib has more potent PARP1/2 inhibition than its parent Olaparib (HY-10162). Simmiparib induces DNA double-strand breaks (DSB) accumulation and G2/M arrest in homologous recombination repair (HR)-deficient cells, thereby inducing apoptosis. Simmiparib exhibits remarkable anticancer activities in cells and nude mice bearing xenografts.