Generation of Reactive Oxygen Radicals through Bioactivation of Mitomycin Antibiotics1

Mitomycin C (MC) is a naturally occurring anticancer agent which has been shownto be more cytotoxicto hypoxictumor cells than to their aerobic counterparts. The mechanism of action of this agent is thought to involvebiologicalreductiveactivation,to a speciesthat alkylutes DNA. A comparisonof the cytotoxicityof MC to EMT6 tumor cells with that of the structural analogues porfiromycin(PM), .•V-(,V',Ai"-diiiielhylaniinomethylene)amineanalogueof mitomycinC (BMY-25282),and ;\'-(.!\", .V'-ilimeth>laniinoinctliylciie(aminoanalogueof porfiromycin(BL-6783) has demonstrated that PM is considerably less cytotoxic to aerobic EMT6 cells than MC, whereas BMY-25282 and BL-6783 are signifi cantly more toxic. The relative abilities of each of these compounds to generate oxygen free radicals followingbiologicalactivation were mea sured. Tumor cell sonicates, reduced nicotinamideadenine dinucleotide phosphate-cytochromec reducÃ-ase, xanthine oxidase, and mitochondria were used as the biologicalreducing systems. All four mitomycinanti bioticsproducedoxygenradicals followingbiologicalreduction,a process that may account for the aerobic cytotoxicityof agents of this class. The generation of relame amounts of Superoxideand hydroxyl radical were also measured in EMT6 cell sonicates. BMY-2S282and BL-6783 pro duced significantly greater quantities of oxygen free radicals with the EMT6 cell sonicate, reduced nicotinamide adenine dinucleotide phos phate-cytochromec reducÃ-ase, and mitochondriathan did MC and PM. In contrast, BMY-25282and BL-6783 did not generate detectable levels of free radicals in the presenceof xanthine oxidase, whereas this enzyme was capable of generating free radicals with MC and PM as substrates. MC consistentlyproducedgreater amounts of free radicals than PM with all of the reducingsystems. BMY-25282,BL-6783,and MC all generated hydroxyl radicals, while PM did not appear to form these radicals. The findings indicate that a correlation exists between the ability of the mitomycinantibiotics to generate oxygen radicals and their cytotoxicity to aerobic EMT6 tumor cells.