Relationship between inhibition of mitochondrial respiration by naphthoquinones, their antitumor activity, and their redox potential

The physicochemical properties of a series of 1,4-naphthoquinones were correlated with their activities against Sarcoma-180 by Hodnett et al. [J. med. Chem. 26, 570 (1983)]. Redox potential was the most important molecular parameter determining antitumor activity in this series of compounds, suggesting that interference with electron transport contributes to their cytotoxicity. We evaluated this same series of quinones for their abilities to inhibit the beef heart mitochondrial succinoxidase and NADH-oxidase enzyme systems. They exhibited a broad range of inhibitory potencies. There was a strong relationship between succinoxidase inhibition, antitumor activity (T/C ratio), and redox potential. The redox potentials of the quinones which inhibited succinoxidase lay within the narrow range of endogenous components of the respiratory chain. In contrast, inhibition of NADH-oxidase was related to redox potential but did not significantly predict antitumor activity. These results suggest that inhibition of mitochondrial succinoxidase may be a useful preliminary screen for antitumor activity.