Naphthalimmide (NI) and 1,4,5,8-naphthalentetracarboxylic diimide (NDI) derivatives were synthesized and evaluated for their antiproliferative activity. NDI derivatives 1-9 were more cytotoxic than the corresponding NI derivatives 10-18. The molecular mechanisms of 1 and 2 were investigated in comparison to mitonafide. They interacted with DNA, were not topoisomerase IIalpha poisons, triggered caspase activation, caused p53 protein accumulation, and down-regulated AKT survival. Furthermore, 1 and 2 caused a decrease of ERK1/2 and, unlike mitonafide, inhibited ERKs phosphorylation.