Redox regulation of tumor cell toxicity by flavones from Lethedon tannaensis

The purpose of the present work has been to seek a correlation of potential predictive value, demonstrating redox control of cytotoxicity toward human nasopharynx carcinoma (KB) cells by seven 5-hydroxy-7-methoxyflavones, tog ether with two glycoside derivatives, all extracted from Lethedon tannaensi s, In this approach, redox control is characterized by a physicochemical pa rameter expressing quantitatively the relative electron-donating power of t he flavones, this parameter being the second order rate constant, k(Delta), for quenching of singlet oxygen O-2 ((1)Delta g). This rate constant k(Del ta) is usually related to the ability of a given molecule D to donate an el ectron and, thus, with the reduction potential E of the couple (D.+/D). Our results show that the flavone toxicity is linearly correlated with ease of oxidation: the higher the rate constant of reaction with singlet oxygen, t he easier the oxidation, the less positive or more negative the reduction p otential (D.+/D), the higher the cytotoxicity. The results suggest new scre ening strategies to identify and improve potential antitumor drugs. (C) 199 9 Elsevier Science Inc.