A tricyclic indole poly(ADP-Ribose) polymerase (PARP1) inhibitor with potential chemosensitizing, radiosensitizing, and antineoplastic activities. Rucaparib selectively binds to PARP1 and inhibits PARP1-mediated DNA repair, thereby enhancing the accumulation of DNA strand breaks and promoting genomic instability and apoptosis. This may enhance the cytotoxicity of DNA-damaging agents and reverse tumor cell resistance to chemotherapy and radiation therapy. PARP1 catalyzes post-translational ADP-ribosylation of nuclear proteins and is activated by single-strand DNA (ssDNA) breaks.