The present invention relates to tetraaza phenalen-3-one compounds which inhibit poly (ADP-ribose) polymerase (PARP) and are useful in the chemosensitization of cancer therapeutics. The induction of peripheral neuropathy is a common side-effect of many of the conventional and newer chemotherapies. The present invention further provides means to reliably prevent or cure chemotherapy-induced neuropathy. The invention also relates to the use of the disclosed PARP inhibitor compounds in enhancing the efficacy of chemotherapeutic agents such as temozolomide. The invention also relates to the use of the disclosed PARP inhibitor compounds to radiosensitize tumor cells to ionizing radiation. The invention also relates to the use of the disclosed PARP inhibitor compounds for treatment of cancers with DNA repair defects.