Olaparib is a small molecule inhibitor of ADP ribose polymerase (PARP) with potential chemical sensitization, radiosensitization, and anti-tumor activity. Olaparib selectively binds and inhibits PARP, inhibiting PARP mediated repair of single stranded DNA breaks; The inhibition of PARP may enhance the cytotoxicity of DNA disruptors and potentially reverse the chemotherapy and radiation resistance of tumor cells. PARP catalyzes post-translational ADP nucleosylation of nuclear proteins, which can be activated by single stranded DNA cleavage.