Synergistic enhancement of topotecan-induced cell death by ascorbic acid in human breast MCF-7 tumor cells.

Topotecan, a derivative of camptothecin, is an important anticancer drug for the treatment of various human cancers in the clinic. While the principal mechanism of tumor cell killing by topotecan is due to its interactions with topoisomerase I, other mechanisms, e.g., oxidative stress induced by reactive free radicals, have also been proposed. However, very little is known about how topotecan induces free radical-dependent oxidative stress in tumor cells. In this report we describe the formation of a topotecan radical, catalyzed by a peroxidase-hydrogen peroxide system. While this topotecan radical did not undergo oxidation-reduction with molecular O2, it rapidly reacted with reduced glutathione and cysteine, regenerating topotecan and forming the corresponding glutathiyl and cysteinyl radicals. Ascorbic acid, which produces hydrogen peroxide in tumor cells, significantly increased topotecan cytotoxicity in MCF-7 tumor cells. The presence of ascorbic acid also increased both topoisomerase I-dependent topotecan-induced DNA cleavage complex formation and topotecan-induced DNA double-strand breaks, suggesting that ascorbic acid participated in enhancing DNA damage induced by topotecan and that the enhanced DNA damage is responsible for the synergistic interactions of topotecan and ascorbic acid. Cell death by topotecan and the combination of topotecan and ascorbic acid was predominantly due to necrosis of MCF-7 breast tumor cells.

Role of nitric oxide in the chemistry and anticancer activity of etoposide (VP-16,213).

Originally identified as an innate cytotoxin, nitric oxide ((·)NO) formation in tumors can influence chemotherapy and exacerbate cancer progression. Here, we examined the hypothesis that (·)NO generation contributes to cancer cell drug resistance toward the widely used anticancer drug Etoposide (VP-16). The UV-vis spectrum of VP-16 was not changed by exposure of VP-16 to (·)NO in aqueous buffer. In contrast, reddish-orange compound(s) characteristic of o-quinone- and nitroso-VP-16 were readily generated in a hydrophobic medium (chloroform) in an oxygen-dependent manner. Similar products were also formed when the VP-16 radical, generated from VP-16 and horseradish peroxidase/H2O2, was exposed directly to (·)NO in chloroform in the presence of oxygen. Separation and spectral analysis of VP-16 reaction extracts by electron spin resonance and UV-vis indicated the generation of the phenoxy radical and the o-quinone of VP-16, as well as putative nitroxide, iminoxyl, and other nitrogen oxide intermediates. Nitric oxide products of VP-16 displayed significantly diminished topoisomerase II-dependent cleavage of DNA and cytotoxicity to human HL-60 leukemia cells. LPS-mediated induction of nitric oxide synthase in murine macrophages resulted in VP-16 resistance compared to Raw cells. Furthermore, (·)NO products derived from iNOS rapidly reacted with VP-16 leading to decreased DNA damage and cytotoxicity. Together, these observations suggest that the formation of (·)NO in tumors (associated macrophages) can contribute to VP-16 resistance via the detoxification of VP-16.

Real-time visualization of photochemically induced fluorescence of 8-halogenated quinolones: lomefloxacin, clinafloxacin and Bay3118 in live human HaCaT keratinocytes.

Halogenoquinolones are potent and widely used antimicrobials blocking microbial DNA synthesis. However, they induce adverse photoresponses through the absorption of UV light, including phototoxicity and photocarcinogenicity. The phototoxic responses may be the result of photosensitization of singlet oxygen, production of free radicals and/or other reactive species resulting from photodehalogenation. Here, we report the use of laser scanning confocal microscopy to detect and to follow the fluorescence changes of one monohalogenated and three di-halogenated quinolones in live human epidermal keratinocyte cells during in situ irradiation by confocal laser in real time. Fluorescence image analysis and co-staining with the LysoTracker probe showed that lysosomes are a preferential site of drug localization and phototransformations. As the lysosomal environment is relatively acidic, we also determined how low pH may affect the dehalogenation and concomitant fluorescence. With continued UV irradiation, fluorescence increased in the photoproducts from BAY y3118 and clinafloxacin, whereas it decreased for lomefloxacin and moxifloxacin. Our images not only help to localize these phototoxic agents in the cell, but also provide means for dynamic monitoring of their phototransformations in the cellular environment.

Photosensitized oxidation of tetrabromobisphenol a by humic acid in aqueous solution.

The brominated flame retardant 3,3',5,5'-tetrabromobisphenol A (TBBPA) may accumulate in the environment, including surface waters, and degrade there to potentially toxic products. We have previously shown that singlet oxygen (1O2), produced by irradiation of rose bengal with visible light, oxidizes Triton X-100-solubilized TBBPA to yield the 2,6-dibromo-p-benzosemiquinone anion radical while consuming oxygen (Environ. Sci. Technol.42, 166, 2008). Here, we report that a similar 1O2-induced oxidation can be initiated in aqueous solutions by the irradiation of TBBPA dissolved in a humic acid (HA) solution. HA is a known weak 1O2 photosensitizer and we indeed detected the infrared 1O2 phosphorescence from HA preparations in D2O. When an aqueous preparation of HA was irradiated (lambda > 400 nm) in the presence of TBBPA, oxygen was consumed, and the 2,6-dibromo-p-benzosemiquinone anion radical was generated and detected using electron paramagnetic resonance. Radical formation and oxygen consumption were inhibited by sodium azide, a singlet oxygen quencher. Our results suggest that solar radiation, in the presence of HA, may play an important role in the photodegradation of TBBPA in the aquatic environment.

An in vivo study of free radicals generated in murine skin by protoporphyrin IX and visible light.

Lipids extracted from the skin of C57BL/6J mice injected subcutaneously with alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN) and exposed to topical protoporphyrin IX (PPIX) and visible light had significantly higher levels of POBN spin adducts compared with dark PPIX exposed or vehicle-treated controls. Computer analysis of the POBN adduct electron paramagnetic (spin) resonance (EPR) spectra indicated that two radical species were present in each extract, one of which was a lipid-derived carbon-centered adduct (1, a(N) = 14.8 G and a(H) = 2.6 G), whereas the other (2, a(N) = 13.8 G and a(H) = 1.8 G) was probably oxygen centered. Adduct 2 was present in greater proportion in lipids extracted from PPIX/light-exposed mice compared with dark or vehicle-treated controls. These findings suggest that PPIX/light generates free radicals in mouse skin, thus providing a radical mechanism for PPIX-induced photosensitivity. Our approach may be useful for the detection of free radicals generated by other skin photosensitizers and may also provide a means for testing putative skin-protecting agents.

Photorearrangement of alpha-azoxy ketones and triplet sensitization of azoxy compounds.

[reaction: see text] Although some aspects of azoxy group radical chemistry have been investigated, unhindered alpha-azoxy radicals remain poorly understood. Here we report the generation of alpha-azoxy radicals under mild conditions by irradiation of alpha-azoxy ketones 4a,b. These compounds undergo alpha-cleavage to yield radicals 5a,b, whose oxygen atom then recombines with benzoyl radicals to produce presumed intermediate 15. Formal Claisen rearrangement gives alpha-benzoyloxyazo compounds 8a,b, which are themselves photolabile, leading to both radical and ionic decomposition. The ESR spectrum of 5a was simulated to extract the isotropic hyperfine splitting constants, which showed its resonance stabilization energy to be exceptionally large. Azoxy compounds have been found for the first time to be good quenchers of triplet excited acetophenone, the main sensitized photoreaction of 7Z in benzene being deoxygenation. While this reaction has been reported previously, it was always in hydrogen atom donating solvents, where chemical sensitization occurred. The principal direct irradiation product of 4bZ and model azoxyalkane 7Z is the E isomer, whose thermal reversion to Z is much faster than that of previously studied analogues.

Photochemical and photobiological studies of tirapazamine (SR 4233) and related quinoxaline 1,4-Di-N-oxide analogues.

Tirapazamine, 3-amino-1,2,4-benzotriazine 1,4-di-N-oxide (TPZ; SR 4233), is currently undergoing phase II and III clinical trials as an antitumor agent. We have studied the photochemical properties of TPZ, and the related analogues 3-amino-2-quinoxalinecarbonitrile 1,4-di-N-oxide (TPZCN) and quinoxaline-1,4-di-N-oxide (quindoxin) with respect to their potential to photodamage DNA both oxidatively and reductively. We have found that TPZ, TPZCN, and quindoxin photosensitized the generation of singlet oxygen with quantum yields of 0.007, 0.19, and 0.02, respectively, in acetonitrile. Irradiation (lambda > 300 nm) of TPZ at pH 9.4 in the presence of a reducing agent, NADH, generated the corresponding nitroxide radical. At pH 7.4, photoirradiation of either TPZ or TPZCN in the presence of NADH in air saturated buffer gave the superoxide radical, which was trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO). In the absence of a reducing agent, singlet oxygen generated from TPZCN oxidized DMPO to 5,5-dimethyl-2-oxopyrrolin-1-oxyl (DMPOX). No spin adducts were detected during photoirradiation of TPZ, NADH, and DMPO in nitrogen-saturated buffer. However, when DMSO was also present, the DMPO/(*)CH(3) adduct was observed, indicating the generation of the free hydroxyl radical. Both TPZ and TPZCN photooxidized reduced glutathione and azide to the glutathiyl and azidyl radicals, respectively. Under anaerobic conditions, NADH increased photoinduced strand breaks in pBR322 plasmid DNA caused by TPZ or TPZCN. For TPZ, the reactive species is probably the aforementioned nitroxide radical or the hydroxyl radical generated from its decomposition. In contrast, DNA damage by quindoxin was not affected by NADH, suggesting a different mechanism, possibly involving a photogenerated oxaziridine intermediate. These studies show that the photochemistry of TPZ, TPZCN, and quindoxin is complex and depends on the redox environment and whether oxygen is present.

The role of A2E in prevention or enhancement of light damage in human retinal pigment epithelial cells.

The process of sight (photostasis) produces, as a by-product, a chromophore called 2-[2,6-dimethyl-8-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E,3E, 5E,7E-octatetraenyl]-1-(2-hydroxyethyl)-4-[4-methyl-6-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E, 3E, 5E-hexatrienyl]-pyridinium (A2E), whose function in the eye has not been defined as yet. In youth and adulthood, A2E is removed from human retinal pigment epithelial (h-RPE) cells as it is made, and so it is present in very low concentrations, but with advanced age, it accumulates to concentrations reaching 20 microM. In the present study we have used photophysical techniques and in vitro cellular measurements to explore the role of A2E in h-RPE cells. We have found that A2E has both pro- and antioxidant properties. It generated singlet oxygen (phiso = 0.004) much less efficiently than its precursor trans-retinal (phiso = 0.24). It also quenched singlet oxygen at a rate (10(8) M(-1) s(-1)) equivalent to two other endogenous quenchers of reactive oxygen species in the eye: alpha-tocopherol (vitamin E) and ascorbic acid (vitamin C). The endogenous singlet oxygen quencher lutein, whose quenching rate is two orders of magnitude greater than that of A2E, completely prevented light damage in vitro, suggesting that singlet oxygen does indeed play a role in light-induced damage to aged human retinas. We have used multiphoton confocal microscopy and the comet assay to measure the toxic, phototoxic and protective capacity of A2E in h-RPE cells. At 1-5 microM, A2E protected these cells from UV-induced breaks in DNA; at 20 microM, A2E no longer exerted this protective effect. These results imply that the role of A2E is not simple and may change over the course of a lifetime. A2E itself may play a protective role in the young eye but a toxic role in older eyes.