Oxidation of 2-deoxyribose by benzotriazinyl radicals of antitumor 3-amino-1,2,4-benzotriazine 1,4-dioxides

Tirapazamine (3-amino-1,2,4-benzotriazine 1,4-dioxide) is the lead bioreductive drug in clinical trials as an anticancer agent to kill refractory hypoxic cells of solid tumors. It has long been known that, upon metabolic one-electron reduction, tirapazamine induces lethal DNA double strand breaks in hypoxic cells. These strand breaks arise from radical damage to the ribose moiety of DNA, and in this pulse radiolysis and product analysis study we examine mechanistic aspects of the dual function of tirapazamine and analogues in producing radicals of sufficient power to oxidize 2-deoxyribose to form radicals, as well as the ability of the compounds to oxidize the resulting deoxyribose radicals to generate the strand breaks. Both the rate of oxidation of 2-deoxyribose and the radical yield increase with the one-electron reduction potentials of the putative benzotriazinyl radicals formed from the benzotriazine 1,4-dioxides. Subsequent oxidation of the 2-deoxyribose radicals by the benzotriazine 1,4-dioxides and 1-oxides proceeds through adduct formation followed by breakdown to form the radical anions of both species. The yield of the radical anions increases with increasing one-electron reduction potentials of the compounds. We have previously presented evidence that oxidizing benzotriazinyl radicals are formed following one-electron reduction of the benzotriazine 1,4-dioxides. The reactions reported in this work represent the kinetic basis of a short chain reaction leading to increased oxidation of 2-deoxyribose, a process which is dependent on the one-electron reduction potential of the benzotriazinyl radicals that are above a threshold value of ca. 1.24 V.     

3-amino-1,2,4-benzotriazine 4-oxide: characterization of a new metabolite arising from bioreductive processing of the antitumor agent 3-amino-1,2,4-benzotriazine 1,4-dioxide (tirapazamine).

Tirapazamine (1) is a promising antitumor agent that selectively causes DNA damage in hypoxic tumor cells, following one-electron bioreductive activation. Surprisingly, after more than 10 years of study, the products arising from bioreductive metabolism of tirapazamine have not been completely characterized. The two previously characterized metabolites are 3-amino-1,2,4-benzotriazine 1-oxide (3) and 3-amino-1,2,4-benzotriazine (5). In this work, 3-amino-1,2,4-benzotriazine 4-oxide (4) is identified for the first time as a product resulting from one-electron activation of the antitumor agent tirapazamine by the enzymes xanthine/xanthine oxidase and NADPH:cytochrome P450 oxidoreductase. As part of this work, the novel N-oxide (4) was unambiguously synthesized and characterized using NMR spectroscopy, UV-vis spectroscopy, LC/MS, and X-ray crystallography. Under conditions where the parent drug tirapazamine is enzymatically activated, the metabolite 4 is produced but readily undergoes further reduction to the benzotriazine (5). Thus, under circumstances where extensive reductive metabolism occurs, the yield of the 4-oxide (4) decreases. In contrast, the isomeric two-electron reduction product 3-amino-1,2,4-benzotriazine 1-oxide (3) does not readily undergo enzymatic reduction and, therefore, is found as a major bioreductive metabolite under all conditions. Finally, the ability of the 4-oxide metabolite (4) to participate in tirapazamine-mediated DNA damage is considered.     

Activation of 3-amino-1,2,4-benzotriazine 1,4-dioxide antitumor agents to oxidizing species following their one-electron reduction

The mechanism by which a benzotriazine 1,4-dioxide class of anticancer drugs produce oxidizing radicals following their one-electron reduction has been investigated using tirapazamine (3-amino-1,2,4-benzotriazine 1,4-dioxide, 1) and its 6-methoxy (6), 7-dimethylamino (7), and 8-methyl (8) analogues. By measuring the changes in absorption with pH, we found that the radical anions undergo protonation with radical pK(r) values of 6.19 +/- 0.05, 6.10 +/- 0.03, 6.45 +/- 0.04, and 6.60 +/- 0.04, respectively. The one-electron reduced species underwent a first-order reaction, with increased rate constants from 112 +/- 23 s(-)(1) for 1 to 777 +/- 12 s(-)(1)(6), 1120 +/- 29 s(-)(1) (7), and 825 +/- 89 s(-)(1) (8) at pH 7. No overall change in conductance was observed following the one-electron reduction of 6, and 8 at pH 4.5, consistent with the protonation of the radical anions, but a loss in conductance was seen for one-electron reduced 7 because of further protonation of the initially formed radical. This is assigned to the protonation of the dimethylamino group of the radical species, which has a pK(a) of 8.8 +/- 0.3. All conductance changes take place on a time-scale shorter than those of the above first-order reactions, which are not associated with the formation or loss of charged species. The absorption spectra present at the end of the unimolecular reactions were found to be similar to those formed immediately upon the one-electron oxidation of the respective substituted 3-amino-1,2,4-benzotriazine 1-oxides, and it is suggested that common benzotriazinyl radicals are formed by both routes. All these intermediate radicals underwent dismutation to produce final spectra matched by equal contributions of the parent compound and their respective substituted 3-amino-1,2,4-benzotriazine 1-oxides. By establishing redox equilibria between the intermediate radicals formed on the one-electron oxidation of the respective 3-amino-1,2,4-benzotriazine 1-oxides of the compounds and reference compounds, we found the one-electron reduction potential of the oxidizing radicals to range from 0.94 to 1.31 V. The benzotriazinyl radical of tirapazamine was found to oxidize dGMP and 2-deoxyribose with rate constants of (1.4 +/- 0.2) x 10(8) M(-)(1) s(-)(1) and (3.7 +/- 0.5) x 10(6) M(-)(1) s(-)(1), respectively.     

Stille coupling reactions in the synthesis of hypoxia-selective 3-alkyl-1,2,4-benzotriazine 1,4-dioxide anticancer agents

The introduction of a 3-alkyl substituent is a key step in the synthesis of 1,2,4-benzotriazine 1,4-dioxide hypoxia-selective anticancer agents, such as SN29751. The Stille reaction of 3-chloro-1,2,4-benzotriazine 1-oxides (BTOs) 5 was inhibited by the presence of electron donating substituents on the benzo ring, thus limiting the range of compounds available for SAR studies. The use of 3-iodo-BTOs 8 did not provide a significant improvement in the yields of 3-ethyl-BTOs 6. Microwave-assisted Stille coupling of chlorides 5 gave dramatically improved yields, which were consistently superior to those from the corresponding iodides 8. The application of microwave-assisted synthesis extended the range of substituted BTOs available for SAR studies and provided an efficient, scalable synthesis of the investigational anticancer agent, SN29751 (1).     

New and versatile syntheses of 3-alkyl- and 3-aryl-1,2,4-benzotriazine 1,4-dioxides: preparation of the bioreductive cytotoxins SR 4895 and SR 4941

Palladium-mediated coupling of 3-chloro-1,2,4-benzotriazine 1-oxide with a variety of stannanes in the presence of Pd(PPh₃)₄ gives 3-alkyl derivatives in good yields. Suzuki reaction of the 3-chloro compound with phenylboronic acids gives 3-aryl-1,2,4-benzotriazine 1-oxides. Oxidation of 1-oxides with trifluoroperacetic acid gives the 1,4-dioxides. This method provides a better route to the potential anti-cancer agents SR 4895 and SR 4941.     

Oxides of 3-methyl-1,2,4-benzotriazine

The previously prepared 3-methyl-1,2,4-benzotriazine oxide¹ is shown to be the 4-oxide 5. Synthesis and structures of other isomeric and related oxides are described. A modification of a previously described synthesis of 1,2,4-benzotriazines produces purer products in higher yields.     

Synthesis of 5-alkoxy-3-amino-7-nitro-1,2,4-benzotriazine 1-oxides from 1,3,5-trinitrobenzene

1-Alkoxy-3,5-dinitrobenzenes were nitrated to give 1-alkoxy-2,3,5-trinitrobenzenes. The reaction of the latter with guanidine affordsN-(2-alkoxy-4,6-dinitrophenyl)guanidines, which undergo cyclization under the action of KOH to form 5-alkoxy-3-amino-7-nitro-1,2,4-benzotriazine 1-oxides. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1650–1652, September, 2000.     

Metal-Assisted Oxidative Cyclization of Arylamidrazones I. Synthesis of 3-Acetyl-1,4-dihydro-1-phenyl-1,2,4-benzotriazine

Summary. In the presence of RuCl₃, N-phenylamidrazone underwent oxidative cyclization into 1,4-dihydro-1-phenyl-1,2,4-benzotriazine, the structure of which is established by spectral and X-ray diffraction data.     

Metal-Assisted Oxidative Cyclization of Arylamidrazones I. Synthesis of 3-Acetyl-1,4-dihydro-1-phenyl-1,2,4-benzotriazine

In the presence of RuCl₃, N-phenylamidrazone underwent oxidative cyclization into 1,4-dihydro-1-phenyl-1,2,4-benzotriazine, the structure of which is established by spectral and X-ray diffraction data.     

Synthesis and properties of 3-substituted 2h-1,2,4-benzothiadiazine 1,1-dioxides

The ethyl ester and amides of 2H-1,2,4-benzothiadiazine-3-carboxylic acid 1,1-dioxide were obtained by reaction of 2-aminobenzenesulfonamide with diethyl oxalate or oxamic acid esters in the presence of sodium methoxide. The hydrolysis, animation, and hydrazinolysis of the ester were studied.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 479–483, April, 1976.     

Synthesis of 3-amino-1,2,4-benzothiadi- azine 1,1-dioxides via a tandem aza-Wittig/heterocumulene annulation

[reaction: see text] Reaction of o-azidobenzenesulfonamides with polymer-supported triphenylphosphine affords the corresponding iminophosphoranes. Subsequent reaction with isocyanates gives 3-amino-1,2,4-benzothiadiazine 1,1-dioxides in high yields and purities. The reaction has been successfully applied to the synthesis of derivatives with various substituents at the 2- and 3-positions and in the benzenoid ring.     

Structure and properties of 3-amino-1-vinyl-1,2,4-triazole

The reaction of acetylene with 3-amino-1,2,4-triazole forms two isomeric 3-amino-N-vinyl-1,2,4-triazoles. Their structures have been studied by PMR and IR spectroscopy and dipole moments. The high activity of the new vinyl compounds of the azole series in the addition of electrophilic reagents, in polymerization, and in complex formation has been shown.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1566–1569, November, 1973.     

Aromatization of 2,3-dihydro-3-hydroxypyrazine 1,4-dioxides in HSo3F-SbF5 superacid: A new pathway to pyrazine 1,4-dioxides

Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1697–1698, December, 1993.     

Synthesis of 3-substituted 4,1,2-benzothiadiazine 4,4-dioxides and 2 or 3-substituted 1,4-benzothiazine 1,1-dioxides

The title compounds were prepared starting from the synthones 6 using two different synthetic approaches: 3-substituted 1,4-benzothiazine 1,1-dioxides were synthesised by cyclisation with phosphorus oxychloride as the Vilsmeier reagent, while 4,1,2-benzothiadiazine 4,4-dioxides and 3-substituted 1,4-benzothiazine 1,1-dioxides were obtained by direct diazotisation or by cyclisation with triethyl orthoformate.     

New approach to the synthesis of 3-amino-1,2-benzothiazine 1,1-dioxides

The possibility of obtaining 3-amino-1,2-benzothiazine 1,1-dioxides by nucleophilic substitution of the chlorine atom in 5-nitro-2-chlorobenzenesulfonamides by carbanions generated from substituted acetonitriles was studied.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 264–266, February, 1992.     

Solid-phase synthesis of 3-amino-1,2,4-triazoles

A solid-phase synthesis of 3-amino-1,2,4-triazoles is described. Reaction of resin bound S-methylisothiourea 2 with carboxylic acids yielded resin-bound S-methyl-N-acylisothioureas 3, which reacted with hydrazines under mild conditions to afford the corresponding resin-bound 3-amino-1,2,4-triazoles 4 with regioselectivity. Following cleavage, the desired products 5 were obtained in good yields and purities.     

Acyl derivatives of 3-amino-1,2,4-triazole

Only 3-acylamino-1,2,4-triazoles were isolated in the acylation of 3-amino-1,2,4-triazole with acid chlorides that contain strong electron acceptor substituents. Acylation takes place in the 2 position when aliphatic and aromatic acid chlorides are used as the acylating agents. The action of methoxy- and ethoxyformyl chlorides leads to the formation of 1- and 2-alkoxycarbonyl-3-amino-1,2,4-triazoles. When N-acyl-3-amino-1,2,4-triazoles are heated, they undergo intermolecular trans-aminoacylation to 3-acylamino-1,2,4-triazoles, which exist in the solid phase in the form of amido and imido tautomers. Under the conditions of massspectroscopic analysis the percentage of the imido form increases as the electronacceptor capacity of the substituent increases.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1414–1419, October, 1980.