tirapazamine and quindoxin

The ˙OH radical is released from 3-trifluoromethyl-quinoxaline 1,4-dioxides upon one-electron reduction by cytochrome P450 oxidoreductase. This process effectively competes with back oxidation of the intermediate radical anion by oxygen and underlies the increased aerobic cytotoxicity of such compounds compared to that seen for the related clinical bioreductive benzotriazine drug, tirapazamine.

Topics: Antineoplastic Agents; Humans; Hydroxyl Radical; NADPH-Ferrihemoprotein Reductase; Oxidation-Reduction; Oxides; Pyrroles; Quinoxalines; Tirapazamine; Triazines

A new series of quinoxaline 1,4-di-N-oxides was synthesized and evaluated for antitumor and hypoxic-selective cytotoxic activities. Antitumor activity against liver carcinoma (Hepg2) and brain tumor (U251) human cell lines were evaluated, among the tested compounds, 5b and 9b exhibited potential cytotoxic effect against Hepg2 with IC50 values of 0.77 and 0.50 microg/mL respectively, whereas, all the tested compounds lack antitumor activity against U251 human cell line. Moreover, compound 4 was the most potent hypoxia selective-cytotoxin on EAC cell line; IC50 2.5 microg/mL, potency 22 microg/mL, and was approximately 5.4-times more selective cytotoxin (HCR>40) than 3-amino-2-quinoxalinecarbonitrile1,4-dioxide (standard, HCR>7.4). Compounds 8b and 9b were more selective than the standard.

Topics: Antineoplastic Agents; Cell Hypoxia; Cell Line, Tumor; Electrons; Humans; Inhibitory Concentration 50; Quinoxalines

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.

Topics: Antineoplastic Agents; Azides; Computer Simulation; Cyclic N-Oxides; DNA Damage; Electron Spin Resonance Spectroscopy; Free Radicals; Glutathione; NAD; Oxidation-Reduction; Photolysis; Plasmids; Quinoxalines; Radiation-Sensitizing Agents; Singlet Oxygen; Spectrophotometry; Spin Labels; Tirapazamine; Triazines

Hypoxic cells, which are a common feature of solid tumors, but not normal tissues, are resistant to both anticancer drugs and radiation therapy. Thus the identification of drugs with selective toxicity toward hypoxic cells is an important objective in anticancer chemotherapy. The benzotriazine di-N-oxide (SR 4233, Tirapazamine) has been shown to be an efficient and selective cytotoxin for hypoxic cells. Since the bioreductive activation of Tirapazamine is thought to be due to the presence of the 1,4-di-N-oxide moiety, a series of 3-aminoquinoxaline-2-carbonitrile 1,4-di-N-oxides with a range of electron-donating and -withdrawing substitutents in the 6- and/or 7- positions has been synthesized and evaluated for toxicity to hypoxic cells. Electrochemical studies of the quinoxaline di-N-oxides and Tirapazamine showed that as the electron-withdrawing nature of the 6(7)-substituent increases, the reduction potential becomes more positive and the compound is more readily reduced. Apart from the unsubstituted 6a and the 6,7-dimethyl derivative 6c, the quinoxaline di-N-oxides have reduction potentials significantly more positive than Tirapazamine (Epc -0.90 V). The most potent cytotoxins to cells in culture were the 6,7-dichloro and 6,7-difluoro derivatives 6i and 6l, which were 30-fold more potent than Tirapazamine. The 6(7)-fluoro and 6(7)-chloro compounds, 6e and 6h, showed the greatest hypoxia selectivity. Four of the compounds, 63, 6f, 6h and 6i, killed the inner cells of multicellular tumor spheroids in vitro. In vivo Balb/c mice tolerated a dose of these four compounds twice the size of that of Tirapazamine. This study demonstrates that quinoxaline 1,4-di-N-oxides could provide useful hypoxia-selective therapeutic agents.

Topics: Animals; Antineoplastic Agents; Cell Hypoxia; Cell Line; Cell Survival; Cricetinae; Cricetulus; Female; Mice; Mice, Inbred BALB C; Quinoxalines; Tirapazamine; Triazines; Tumor Cells, Cultured