Redox cycling of potential antitumor aziridinyl quinones

Dissecting the role of multiple reductases in bioactivation and cytotoxicity of the antitumor agent 2,5-diaziridinyl-3-(hydroxymethyl)-6-methyl-1,4-benzoquinone (RH1)

2,5-Diaziridinyl-3-(hydroxymethyl)-6-methyl-1,4-benzoquinone (RH1) is a novel antitumor diaziridinyl benzoquinone derivative designed to be bioactivated by the two-electron reductase NAD(P)H:quinone oxidoreductase (NQO1) and is currently in clinical trials. NQO1 is expressed at high levels in many solid tumors. RH1 cytotoxicity has been shown previously to be NQO1-dependent. The purpose of this study was to investigate whether other reducing enzymes such as cytochrome b(5) reductase (b5R), cytochrome P450 reductase (P450R), dihydronicotinamide riboside:quinone oxidoreductase 2 (NQO2), and xanthine oxidase/xanthine dehydrogenase (XO/XDH) also contribute to the bioactivation and cytotoxicity of RH1 in human tumor cells. For these studies, we established a series of stable MDA468 breast cancer cell lines overexpressing various levels of NQO1, b5R, P450R, and NQO2 and compared RH1-induced growth inhibition [3-(4,5-dimethylthiazol-2,5-diphenyl)tetrazolium and sulforhodamine B analysis] and interstrand DNA cross-linking (comet analysis) in both parental MDA468 cells and transfected clones. RH1 toxicity correlated with NQO1 and NQO2 but not with either b5R or P450R activity levels in the respective series of transfected MDA468 cell clones. Enzymatic assays showed that RH1 was an in vitro substrate for xanthine oxidase. However, XO/XDH protein and activity could not be detected in a variety of human tumor cell lines. These studies suggest that NQO1 and NQO2 are the principal enzymatic determinants of RH1 bioactivation in MDA468 tumor cells and that b5R, P450R, and XDH/XO are unlikely to play major roles. Our studies also suggest that NQO2 may be particularly relevant as a bioactivation system for RH1 in NQO1-deficient tumors such as leukemias and lymphomas.

Reductive activation of terpenylnaphthoquinones

Four terpenylnaphthoquinones were found to enhance the rate of superoxide production in the presence of ascorbate as detected from the superoxide dismutase (SOD)-inhibitable initial oxygen consumption rates. Initial rates of oxygen consumption in the presence of ascorbate plus quinone increase with an increase in the half-wave reduction potentials of the quinones. These quinones also enhance the rate of Cyt(III)c reduction by xanthine/xanthine oxidase (X/XO) in both air- and nitrogen-saturated aqueous solutions at pH 7.4. Maximum rates of Cyt(III)c reduction in nitrogen and oxygen-saturated solutions (V(max)), in the presence of X/XO, increase with an increase in the half-wave reduction potentials of the quinones. SOD inhibits Cyt(III)c reduction rates in the presence of these quinones and X/XO in a manner which is also dependent on the quinone half-wave redox potential. The relative antineoplastic activity of two of these quinones follows the order in rates of oxygen consumption or Cyt(III)c reduction. This is consistent with an antineoplastic action of these quinones through the mechanism of redox cycling or possible interference or inhibition of mitochondrial respiration.

In vivo activity and hydrophobicity of cytostatic aziridinyl quinones

For a series of 3,6-disubstituted bisaziridinylbenzoquinones the in vivo and in vitro activities against murine tumors, as well as the in vivo toxicity, are analyzed. Properties describing biochemical and physicochemical reactions are also incorporated in the analyses. The important 1-octanol/water partition coefficients were determined, using a fast variation of the shake flask method. New pi'-values were calculated for the substituents in this series. These quinone pi'-values deviate strongly from the standard pi-values, especially for hydrogen-bonding substituents. To discriminate between the toxic and therapeutic activity of the compounds, principal components and partial least squares analyses were applied. Evidence is presented for selective antitumor action of the investigated compounds. The L1210 clonogenic assay only seems to relate to the general cytotoxicity and has no predictive value for in vivo activity for these compounds. The activity is correlated to the hydrophobicity of the quinones. The toxicity correlates with the ease of reduction, contrary to the hypothesis of bioreductive activation as a mechanism for selectivity.

Antioxidant and cytotoxic tocopheryl quinones in normal and cancer cells

We found previously that [d]-alpha-tocopherol (alpha-T) and [d]-gamma-tocopherol (gamma-T) are lipid antioxidants (thiobarbituric acid test) in model systems containing arachidonic acid (AA), cumene hydroperoxide, and Fe3+ and in smooth muscle cell (SMC) cultures challenged with AA. We now show that [d]-alpha-tocopherylquinone (alpha-TQ), [d]-delta-tocopherylquinone (delta-TQ), and [d]-gamma-tocopherylquinone (gamma-TQ) are antioxidants at low concentrations and prooxidants at high concentrations in the model system. Prooxidant activity is greater with gamma-TQ than either alpha-TQ or delta-TQ. Low concentrations of alpha-TQ, delta-TQ, and gamma-TQ are also antioxidants in SMC cultures challenged with AA. Unlike alpha-TQ, partially substituted gamma-TQ and glutathione (GSH) form a Michael adduct which has been purified and characterized. We found previously that alpha-T, gamma-T, and alpha-TQ are mitogenic in SMC. We now report that both delta-TQ and gamma-TQ but not alpha-TQ show concentration-dependent cytotoxicity (changes in morphology, propidium iodide stain) in SMC cultures. Cytotoxicity is greater with gamma-TQ than delta-TQ. An acute lymphoblastic leukemia (ALL) cell line shows greater chemosensitivity (MTT and Neutral Red assays) to gamma-TQ than to either doxorubicin (DOX) or vinblastine (VLB). An ALL cell line resistant to both DOX and VLB retains the same chemosensitivity to gamma-TQ as the drug-sensitive ALL cell line. ALL cell lines are unaffected by either alpha-TQ or the GSH Michael adduct of gamma-TQ. These data show that partially substituted tocopheryl quinones capable of forming Michael adducts are potential chemotherapeutic agents for multidrug-resistant cancer cells.

DT-diaphorase in activation and detoxification of quinones. Bioreductive activation of mitomycin C

A role of DTD in the bioreductive activation of mitomycin C was supported by indirect evidence utilizing enzyme inhibitors in cellular systems. Using a cell-free system, we have confirmed that DTD can bioactivate mitomycin C using both purified rat and human DTD. Metabolism and bioactivation of mitomycin C by DTD is pH-dependent. At pH 7.8 alkylation of DTD leading to enzyme inhibition and DTD crosslinking occurs whereas at pH values of 7.4 and below metabolite formation, preservation of catalytic activity of DTD and sequence-selective DNA crosslinking occurs. Bioactivation of mitomycin C by DTD and the cytotoxicity of this drug in DTD-rich cell lines is oxygen-independent. Mitomycin C induces greater DNA crosslinking, even after chemical reduction, at lower pH values. This suggests that if mitomycin C is used in tumors with elevated DTD activity, greater therapeutic activity may be obtained by lowering intratumoral pH. Human NSCLC has elevated DTD activity relative to SCLC and normal lung and may be a target for the development of drugs which can be efficiently bioactivated by DTD. Because of the pH-dependent inactivation of DTD by mitomycin C, however, other drugs which are efficiently metabolized and bioactivated by DTD may be better candidates for the therapy of tumors high in DTD such as NSCLC.

Ultrasonic absorption frequency dependence of two widely used anti-cancer drugs: doxorubicin and daunorubicin

Low intensity ultrasound (approximately 10(-6) W cm-2) in the frequency range 0.5-6.0 MHz was employed to investigate the ultrasound absorption properties of doxorubicin (DOX) at several temperatures. At physiological temperatures, we found enhanced ultrasound absorption from DOX, and its closely related analogue daunorubicin (DNR), in the upper kilohertz frequency range. The findings do not conform to classical theory of ultrasound absorption, thus suggesting an ultrasound coupling with the drug molecules via structural and/or chemical relaxation processes. The absorption spectra are analysed from the point of view of the non-classical theory of sound absorption due to physical and/or chemical relaxations. Only one spectral difference between the two anti-cancer agents is observed, around 2 MHz, and may be attributed to the sole difference in the chemical make-up of the side chain of the two antibiotics.