Disclosed are tetraaza phenalen-3-one compounds of formula (I):(see formula I),or a pharmaceutically acceptable salt thereof, wherein R is NR1R2, anoptionally substitutedaryloxy or optionally substituted heterocyclyl, and wherein R1 and R2 are asdescribed herein.These compounds may be used to inhibit poly (ADP-ribose) polymerase (PARP).