New quinoxalinecarbonitrile 1,4-di-N-oxide derivatives as hypoxic-cytotoxic agents

Quinoxaline 1,4-Dioxides: Advances in Chemistry and Chemotherapeutic Drug Development

N-Oxides of heterocyclic compounds are the focus of medical chemistry due to their diverse biological properties. The high reactivity and tendency to undergo various rearrangements have piqued the interest of synthetic chemists in heterocycles with N-oxide fragments. Quinoxaline 1,4-dioxides are an example of an important class of heterocyclic N-oxides, whose wide range of biological activity determines the prospects of their practical use in the development of drugs of various pharmaceutical groups. Derivatives from this series have found application in the clinic as antibacterial drugs and are used in agriculture. Quinoxaline 1,4-dioxides present a promising class for the development of new drugs targeting bacterial infections, oncological diseases, malaria, trypanosomiasis, leishmaniasis, and amoebiasis. The review considers the most important methods for the synthesis and key directions in the chemical modification of quinoxaline 1,4-dioxide derivatives, analyzes their biological properties, and evaluates the prospects for the practical application of the most interesting compounds.

Quinoxaline 1,4-di-N-Oxide Derivatives: Are They Unselective or Selective Inhibitors?

BACKGROUND

For decades, the quinoxaline 1,4-di-N-oxide ring has been considered a privileged structure to develop new antibacterial, antitumoural, and antiprotozoal agents, among others, however its mechanism of action is not clear.

OBJECTIVE

The main aim of this mini-review was to analyze the mechanism of action of quinoxaline 1,4-di-N-oxide derivatives reported as antibacterial, antitumoural and antiprotozoal agents.

RESULTS

Initially, the mechanism of action of quinoxaline 1,4-di-N-oxide derivatives against bacteria, tumoural cell lines, and parasites has been described as nonspecific, but recently, the results against different organisms have shown that these compounds have an inhibitory action on specific targets such as trypanothione reductase, triosephosphate isomerase, and other essential enzymes.

CONCLUSION

In summary, quinoxaline 1,4-di-N-oxide is a scaffold to develop new anti-Mycobacterium tuberculosis, antitumoural and antiprotozoal agents, however, understanding the mechanism of action of quinoxaline 1,4-di-N-oxide derivatives in each microorganism could contribute to the development of new, and more potent selective drugs.

Revision of the Regioselectivity of the Beirut Reaction of Monosubstituted Benzofuroxans with Benzoylacetonitrile. 6-Substituted quinoxaline-2-carbonitrile 1,4- dioxides: Structural Characterization and Estimation of Anticancer Activity and Hypoxia Selectivity

BACKGROUND

Quinoxaline 1,4-dioxides have a broad range of biological activity that causes a growing interest in their derivatives for drug discovery. Recent studies demonstrated that quinoxaline 1,4- dioxides have a promising anticancer activity and good hypoxia-selectivity.

OBJECTIVE

The preparation, isolation, structure characterization, and screening for anticancer activity of the first representatives of 6-substituted quinoxaline-2-carbonitrile 1,4-dioxides have been described.

MATERIALS AND METHODS

A series of 7- and 6-halogeno-3-phenylquinoxaline-2-carbonitrile 1,4-dioxides was synthesized by the Beirut reaction. The cytotoxicity was assessed by MTT test (72 h incubation) in normoxia (21% O2) and hypoxia (1% O2) conditions.

RESULTS

We found that during the Beirut reaction between a benzofuroxan bearing an electron withdrawing group and benzoylacetonitrile in the presence of triethylamine, in addition to well-known 7-substituted quinoxaline-2-carbonitrile 1,4-dioxides 7-11a, the 6-isomers 7-11b are formed. Moreover, the yield of the 6- isomers increased with the increase in the electron-withdrawing character of the substituent. For benzofuroxans with CO2Me and CF3 groups, 6-substituted quinoxaline-2-carbonitrile 1,4-dioxides 10-11b were the major products. Despite similarities in physicochemical and spectroscopic properties, the obtained isomers exhibit considerable differences in their anticancer activity and hypoxia selectivity.

CONCLUSION

Substituents and their electronic effects play a key role in the formation of 7- and 6-substituted quinoxaline-2-carbonitrile 1,4-dioxides in the Beirut reaction and in the cytotoxicity properties of the obtained isomers.

Hypoxia-targeted drug delivery

Hypoxia is a state of low oxygen tension found in numerous solid tumours. It is typically associated with abnormal vasculature, which results in a reduced supply of oxygen and nutrients, as well as impaired delivery of drugs. The hypoxic nature of tumours often leads to the development of localized heterogeneous environments characterized by variable oxygen concentrations, relatively low pH, and increased levels of reactive oxygen species (ROS). The hypoxic heterogeneity promotes tumour invasiveness, metastasis, angiogenesis, and an increase in multidrug-resistant proteins. These factors decrease the therapeutic efficacy of anticancer drugs and can provide a barrier to advancing drug leads beyond the early stages of preclinical development. This review highlights various hypoxia-targeted and activated design strategies for the formulation of drugs or prodrugs and their mechanism of action for tumour diagnosis and treatment.

Design and synthesis of novel quinoxaline derivatives as potential candidates for treatment of multidrug-resistant and latent tuberculosis

Twenty-four quinoxaline derivatives were evaluated for their antimycobacterial activity using BacTiter-Glo microbial cell viability assay. Five compounds showed MIC values <3.1 μM and IC50 values<1.5 μM in primary screening and therefore, they were moved on for further evaluation. Compounds 21 and 18 stand out, showing MIC values of 1.6 μM and IC50 values of 0.5 and 1.0 μM, respectively. Both compounds were the most potent against three evaluated drug-resistant strains. Moreover, they exhibited intracellular activity in infected macrophages, considering log-reduction and cellular viability. In addition, compounds 16 and 21 were potent against non-replicating Mycobacterium tuberculosis and compound 21 was bactericidal. Therefore, quinoxaline derivatives could be considered for making further advances in the future development of antimycobacterial agents.

Quinoxaline 1,4-di-N-Oxides: Biological Activities and Mechanisms of Actions

Quinoxaline 1,4-di-N-oxides (QdNOs) have manifold biological properties, including antimicrobial, antitumoral, antitrypanosomal and antiinflammatory/antioxidant activities. These diverse activities endow them broad applications and prospects in human and veterinary medicines. As QdNOs arouse widespread interest, the evaluation of their medicinal chemistry is still in progress. In the meantime, adverse effects have been reported in some of the QdNO derivatives. For example, genotoxicity and bacterial resistance have been found in QdNO antibacterial growth promoters, conferring urgent need for discovery of new QdNO drugs. However, the modes of actions of QdNOs are not fully understood, hindering the development and innovation of these promising compounds. Here, QdNOs are categorized based on the activities and usages, among which the antimicrobial activities are consist of antibacterial, antimycobacterial and anticandida activities, and the antiprotozoal activities include antitrypanosomal, antimalarial, antitrichomonas, and antiamoebic activities. The structure-activity relationship and the mode of actions of each type of activity of QdNOs are summarized, and the toxicity and the underlying mechanisms are also discussed, providing insight for the future research and development of these fascinating compounds.

Design and synthesis of novel quinoxaline derivatives as potential candidates for treatment of multidrug-resistant and latent tuberculosis

Twenty-four quinoxaline derivatives were evaluated for their antimycobacterial activity using BacTiter-Glo microbial cell viability assay. Five compounds showed MIC values <3.1 μM and IC50 values<1.5 μM in primary screening and therefore, they were moved on for further evaluation. Compounds 21 and 18 stand out, showing MIC values of 1.6 μM and IC50 values of 0.5 and 1.0 μM, respectively. Both compounds were the most potent against three evaluated drug-resistant strains. Moreover, they exhibited intracellular activity in infected macrophages, considering log-reduction and cellular viability. In addition, compounds 16 and 21 were potent against non-replicating Mycobacterium tuberculosis and compound 21 was bactericidal. Therefore, quinoxaline derivatives could be considered for making further advances in the future development of antimycobacterial agents.

Synthesis, 3D-QSAR analysis and biological evaluation of quinoxaline 1,4-di-N-oxide derivatives as antituberculosis agents

A series of quinoxaline 1,4-di-N-oxide derivatives variously substituted at C-2 position were synthesized and evaluated for in vitro antimycobacterial activity. Seventeen compounds exhibited potential activity (MIC ⩽6.25μg/mL) against Mycobacterium tuberculosis (H37Rv), in particular the compounds 3d and 3j having an MIC value of 0.39μg/mL. None of the compounds exhibited cytotoxicity when using an MTT assay in VERO cells. To further investigate the structure-activity relationship, CoMFA (q(2)=0.507, r(2)=0.923) and CoMSIA (q(2)=0.665, r(2)=0.977) models were performed on the basis of antimycobacterial activity data. The 3D-QSAR study of these compounds can provide useful information for further rational design of novel quinoxaline 1,4-di-N-oxides for treatment of tuberculosis.

Structure-activity relationship study of anticancer thymidine-quinoxaline conjugates under the low radiance of long wavelength ultraviolet light for photodynamic therapy

Thymidine quinoxaline conjugate (dT-QX) is a thymidine analog with selective cytotoxicity against different cancer cells. In this study, the structure activity relationship study of dT-QX analogs was carried out under the low radiance of black fluorescent (UVA-1) light. Significantly enhanced cytotoxicity was observed under UVA-1 activation among analogs containing both thymidine and quinoxaline moieties with different length of the linker, stereochemical configuration and halogenated substituents. Among these analogs, the thymidine dichloroquinoxaline conjugate exhibited potent activity under UVA-1 activation as the best candidate with EC50 at 0.67 μM and 1.3 μM against liver and pancreatic cancer cells, respectively. In contrast, the replacement of thymidine moiety with a galactosyl residue or the replacement of quinoxaline moiety with a fluorescent pyrenyl residue or a simplified diketone structure resulted in the full loss of activity. Furthermore, it was revealed that the low radiance of UVA-1 at 3 mW/cm(2) for 20 min was sufficient enough to induce the full cytotoxicity of thymidine dichloroquinoxaline conjugate and that the cytotoxic mechanism was achieved through a rapid and steady production of reactive oxygen species.

Synthesis, biological evaluation and structure-activity relationships of new quinoxaline derivatives as anti-Plasmodium falciparum agents

We report the synthesis and antimalarial activities of eighteen quinoxaline and quinoxaline 1,4-di-N-oxide derivatives, eight of which are completely novel. Compounds 1a and 2a were the most active against Plasmodium falciparum strains. Structure-activity relationships demonstrated the importance of an enone moiety linked to the quinoxaline ring.

THEORETICAL CALCULATION OF BOND DISSOCIATION ENERGIES AND HEATS OF FORMATION FOR ALKYL NITRATE AND NITRITE COMPOUNDS WITH DENSITY FUNCTIONAL THEORY AND COMPLETE BASIS SET METHOD

The N – O bond dissociation energies (BDEs) and the heats of formation (HOFs) of alkyl nitrate and nitrite compounds in gas phase at 298.15 K were theoretically calculated. Density functional theory (B3LYP and PBE1PBE) with 6-311+g** and 6-311g** basis sets was employed. It is found that PBE1PBE functional has an average increased BDE of 4.03 kcal/mol from B3LYP functional. What is more, we find the reverse trend in ab initio approach, which is slightly smaller than PBE1PBE. The B3LYP functional is found to be sufficiently reliable to compute the BDEs of alkyl nitrate compounds without the presence of diffusion functions. The BDEs of alkyl nitrite compounds appear to be a constant. The functionals (B3LYP and PBE1PBE) with 6-311g** and 6-311+g** basis sets and CBS-4M ab initio method can all yield good results with respect to the experimental HOFs with the deviation less than 2.0 kcal/mol. As the number of methylene group increases, the HOFs of alkyl nitrate and nitrite compounds increase. In addition, the conclusion of Ventural et al. (J Phys Chem A105:9912, 2001 and Phys Lett245:488, 1995) is confirmed again by our computational results.

Design of anticancer prodrugs for reductive activation

Anticancer prodrugs designed to target specifically tumor cells should increase therapeutic effectiveness and decrease systemic side effects in the treatment of cancer. Over the last 20 years, significant advances have been made in the development of anticancer prodrugs through the incorporation of triggers for reductive activation. Reductively activated prodrugs have been designed to target hypoxic tumor tissues, which are known to overexpress several endogenous reductive enzymes. In addition, exogenous reductive enzymes can be delivered to tumor cells through fusion with tumor-specific antibodies or overexpressed in tumor cells through gene delivery approaches. Many anticancer prodrugs have been designed to use both the endogenous and exogenous reductive enzymes for target-specific activation and these prodrugs often contain functional groups such as quinones, nitroaromatics, N-oxides, and metal complexes. Although no new agents have been approved for clinical use, several reductively activated prodrugs are in various stages of clinical trial. This review mainly focuses on the medicinal chemistry aspects of various classes of reductively activated prodrugs including design principles, structure-activity relationships, and mechanisms of activation and release of active drug molecules.

Selective activity against Mycobacteriumtuberculosis of new quinoxaline 1,4-di-N-oxides

New series of 3-phenylquinoxaline 1,4-di-N-oxide with selective activity against Mycobacterium tuberculosis have been prepared and evaluated. Thirty-four of the seventy tested compounds showed an MIC value less than 0.2 microg/mL, a value on the order of the MIC of rifampicin. Furthermore, 45% of the evaluated derivatives showed a good in vitro activity/toxicity ratio. The most active and selective compounds carry a fluorine atom in the quinoxaline 7-position or in the phenyl substituent para-position. In conclusion, the potency, low cytotoxicity and selectivity of these compounds make them valid lead compounds for synthesizing new analogues, particularly compound 7-methyl-3-(4'-fluoro)phenylquinoxaline-2-carbonitrile 1,4-di-N-oxide (MIC <0.2 microg/mL and SI > 500).

Substitutions of fluorine atoms and phenoxy groups in the synthesis of quinoxaline 1,4-di-N-oxide derivatives

The unexpected substitution of fluorine atoms and phenoxy groups attached to quinoxaline or benzofuroxan rings is described. The synthesis of 2-benzyl- and 2-phenoxy-3-methylquinoxaline 1,4-di-N-oxide derivatives was based on the classical Beirut reaction. The tendency of fluorine atoms linked to quinoxaline or benzofuroxan rings to be replaced by a methoxy group when dissolved in an ammonia saturated solution of methanol was clearly demonstrated. In addition, 2-phenoxyquinoxaline 1,4-di-N-oxide derivatives became 2-aminoquinoxaline 1,4-di-N-oxide derivatives in the presence of gaseous ammonia.

Unexpected reduction of ethyl 3-phenylquinoxaline-2- carboxylate 1,4-di-N-oxide derivatives by amines

The unexpected tendency of amines and functionalized hydrazines to reduce ethyl 3-phenylquinoxaline-2-carboxylate 1,4-di-N-oxide (1) to afford a quinoxaline 1c and mono-oxide quinoxalines 1a and 1b is described. The experimental conditions were standardized to the use of two equivalents of amine in ethanol under reflux for two hours, with the aim of studying the distinct reductive profiles of the amines and the chemoselectivity of the process. With the exception of hydrazine hydrate, which reduced compound 1 to a 3-phenyl-2-quinoxalinecarbohydrazide derivative, the amines only acted as reducing agents.

Synthesis and biological evaluation of new 2-arylcarbonyl-3-trifluoromethylquinoxaline 1,4-di-N-oxide derivatives and their reduced analogues

As a continuation of our research in the quinoxaline 1,4-di-N-oxide new series of 2-arylcarbonyl-3-trifluoromethylquinoxaline, 1,4-di-N-oxide derivatives have been synthesized and evaluated in a full panel of 60 human tumor cell lines. Selective reductions were carried out on two compounds which allowed us to determine the compound structures by comparison of the 1H NMR spectra. In general, all the di-N-oxidized compounds showed good cytotoxic parameters. The best activity was observed in derivatives with electron-withdrawing groups in position 6 or 7 on the quinoxaline ring and in the unsubstituted analogues, whereas loss of one or two oxygens reduced the cytotoxicity. The best five compounds were selected for evaluation for the in vivo hollow fiber assays. In vitro studies reveal that compound 5h efficiently generates reactive oxygen species via redox cycling in the presence of the NADPH/cytochrome P450 enzyme system, providing a plausible molecular mechanism for the observed aerobic cytotoxicity of these quinoxaline N-oxides.

A quinoxaline 1,4-di-N-oxide derivative induces DNA oxidative damage not attenuated by vitamin C and E treatment

Some anticancer compounds are pro-drugs which give rise to toxic species through enzymatic reduction. The quinoxaline-di-N-oxide derivative Q-85 HCl (7-chloro-3-[[(N,N-dimethylamino)propyl]amino]-2-quinoxalinecarbonitrile 1,4-di-N-oxide hydrochloride) is a bioreductive compound selectively toxic in hypoxia. Due to the possibility of secondary tumors the study of the genotoxic capability of antitumoral drugs is very important. The aim of this study was to assess the ability of Q-85 HCl to produce reactive oxygen species (ROS) and oxidative DNA damage in Caco-2 cells, both in hypoxia and in well-oxygenated conditions. Secondly, we attempted to evaluate the effect of vitamins C and E under hypoxic and normoxic conditions, in order to determine if these antioxidant substances modify Q-85 HCl effect in hypoxic cells or possibly exert a protective action in normal cells. Caco-2 cells were treated with Q-85 HCl for 2h, at high concentrations in normoxia (0.1-5 microM) and at low concentrations in hypoxia (0.002-0.1 microM). In normoxia, a dose-related significant increase in intracellular ROS level was evident; in hypoxia all the concentrations produced very high level of ROS. Just after the treatment and 24h later, oxidative DNA damage was evaluated by the modified comet assay after post-digestion of the cells with formamidopyrimidine-DNA glycosylase (FPG) and endonuclease III (Endo III). Q-85 HCl treatment evoked a significant dose-dependent increase in the total comet score of the cells both in hypoxia and normoxia, indicating that this compound or some metabolite is able to oxidize purine and pyrimidine bases. After 24h DNA damage caused by the compound was completely repaired with only one exception: cells treated with the highest concentration of Q-85 HCl in hypoxia and post-digested with FPG. Vitamin C (5-100 microM) and vitamin E (500-400 microM) did not have a pro-oxidant effect in Caco-2 cells. Treatment of cells with vitamin C (10 microM) or vitamin E (100 microM) did not significantly reduce oxidative DNA damage in hypoxia and normoxia. In conclusion, the use of these vitamins would not hinder toxicity against hypoxic cells, but a protective effect in normoxic cells was not evident.

Thermochemical Studies on 3-Methyl-quinoxaline-2-carboxamide-1,4-dioxide Derivatives: Enthalpies of Formation and of N−O Bond Dissociation

The standard molar enthalpies of formation of the 3-methyl-N-R-2-quinoxalinecarboxamide-1,4-dioxides (R = H, phenyl, 2-tolyl) in the gas phase were derived using the values for the enthalpies of combustion of the crystalline compounds, measured by static bomb combustion calorimetry, and for the enthalpies of sublimation, measured by Knudsen effusion, at T = 298.15 K. These values have also been used to calibrate a computational procedure that has been employed to estimate the gas-phase enthalpies of formation of the corresponding 3-methyl-N-R-2-quinoxalinecarboxamides and also to compute the first, second, and mean N-O bond dissociation enthalpies in the gas phase. It is found that the size of the substituent almost does not influence the computed N-O bond dissociation enthalpies; the maximum enthalpic difference is approximately 5 kJ.mol-1.

Radiosensitization by 2-benzoyl-3-phenyl-6,7-dichloroquinoxaline 1,4-dioxide under oxia and hypoxia in human colon cancer cells

Background

The sensitizing effects of 2-benzoyl-3-phenyl-6,7-dichloroquinoxaline 1,4-dioxide (DCQ) and ionizing radiation (IR) were determined in four colon cancer cells and in FHs74Int normal intestinal cells.

Methods

Cell cycle modulation, TUNEL assay, clonogenic survival and DNA damage were examined under oxia or hypoxia. Effects on apoptotic molecules and on p-Akt and Cox-2 protein expression were investigated.

Results

The four cell lines responded differently to DCQ+IR; HT-29 cells were most resistant. Combination treatment caused significant increases in preG₁ (apoptosis) in HCT-116, while G₂/M arrest occurred in DLD-1. DCQ potentiated IR effects more so under hypoxia than oxia. Pre-exposure of DLD-1 to hypoxia induced 30% apoptosis, and G₂/M arrest in oxia. The survival rate was 50% lower in DCQ+IR than DCQ alone and this rate further decreased under hypoxia. FHs74Int normal intestinal cells were more resistant to DCQ+IR than cancer cells.Greater ssDNA damage occurred in DLD-1 exposed to DCQ+IR under hypoxia than oxia. In oxia, p-Akt protein expression increased upon IR exposure and drug pre-treatment inhibited this increase. In contrast, in hypoxia, exposure to IR reduced p-Akt protein and DCQ restored its expression to the untreated control. Apoptosis induced in hypoxic DLD-1 cells was independent of p53-p21 modulation but was associated with an increase in Bax/Bcl-2 ratio and the inhibition of the Cox-2 protein.

Conclusion

DCQ is a hypoxic cell radiosensitizer in DLD-1 human colon cancer cells.

New quinoxaline 1,4-di-N-oxides. Part 1: Hypoxia-selective cytotoxins and anticancer agents derived from quinoxaline 1,4-di-N-oxides

Hypoxic cells which are common feature of solid tumors are resistant to both anticancer drugs and radiation therapy. Thus, the identification of drugs with the selective toxicity toward hypoxic cells is an important target in anticancer chemotherapy. Tirapazamine has been shown to be an efficient and selective cytotoxin after bioreductive activation in hypoxic cells which is thought to be due to the presence of the 1,4-di-N-oxide. A new series of quinoxaline 1,4-di-N-oxides and fused quinoxaline di-N-oxides were synthesized and evaluated for hypoxic-cytotoxic activity on EAC cell line. Compound 10a was the most potent cytotoxin IC(50) 0.9 microg/mL, potency 75 microg/mL, and was approximately 15 times more selective cytotoxin (HCR>111) than 3-aminoquinoxaline-2-carbonitrile which has been used as a standard (HCR>7.5). Compounds 4 and 3a,b were more selective than the standard. In addition, antitumor activity against Hepg2 (liver) and U251 (brain) human cell lines was evaluated, compounds 9c and 8a were the most active against Hepg2 with IC(50) values 1.9 and 2.9 microg/mL, respectively, however, all the tested compounds were nontoxic against U251 cell line.

Design and evaluation of “3 + 1” mixed ligand oxorhenium and oxotechnetium complexes bearing a nitroaromatic group with potential application in nuclear medicine oncology

The synthesis and evaluation of a series of oxotechnetium and oxorhenium complexes containing a nitroaromatic moiety as potential radiopharmaceuticals for targeting tumour hypoxia is presented. 99mTc labelling was performed in high yield (>85%) and radiochemical purity (>90%). Their structure was corroborated by means of the rhenium complexes. Reduction potentials were in the range for bioreducible compounds. 99mTc complexes III-VI were selected for "in vivo" experiments in view of the results of cytotoxicity studies. Biodistribution in normal animals was characterized by high initial blood, lung and liver uptake, fast blood and soft tissue depuration and preferential excretion via the hepatobiliary system. Initial tumour uptake was moderate but tumour/muscle ratios for complexes III and IV, were favourable at all time points. Although the results are encouraging further development is still necessary in order to achieve higher tumour uptake and lower gastrointestinal activity.

Improving anti-trypanosomal activity of 3-aminoquinoxaline-2-carbonitrile N1,N4-dioxide derivatives by complexation with vanadium

New vanadium complexes of the type [V(IV)O(L)(2)], where L are 3-aminoquinoxaline-2-carbonitrile N(1),N(4)-dioxide derivatives, were prepared as an effort to obtain new anti-trypanosomal agents improving the bioactivity of the free ligands. Complexation to vanadium of the quinoxaline ligands leads to excellent antiprotozoal activity, similar to that of the reference drugs nifurtimox and benznidazole and in all cases higher than that of the corresponding free ligands. In addition, it is for the first time that the V((IV))O-quinoxaline complexes are reported as a family of anti-Trypanosoma cruzi agents. Finally, the anti-trypanosomal activity of these vanadium complexes could be explained on the basis of their lipophilicity and the electronic characteristics of the quinoxaline substituents.

Selective hypoxia-cytotoxins based on vanadyl complexes with 3-aminoquinoxaline-2-carbonitrile-N1,N4-dioxide derivatives

A new vanadyl complex with the formula VO(L1)2, where L1=3-amino-6(7)-chloroquinoxaline-2-carbonitrile N(1), N(4)-dioxide, has been synthesized and characterized by elemental analyses, conductometry, fast atom bombardment mass spectroscopy (FAB-MS) and electronic, Fourier transform infrared (FTIR), Raman, nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopies. Results were compared with those previously reported for analogous vanadium complexes with other 3-aminoquinoxaline-2-carbonitrile N1,N4-dioxide derivatives as ligands. As an effort to develop novel metal-based selective hypoxia-cytotoxins and to improve bioavailability and pharmacological and toxicological properties of aminoquinoxaline carbonitrile N-dioxides bioreductive prodrugs, the new complex and VO(L)2 complexes, with L=3-amino-6(7)-bromoquinoxaline-2-carbonitrile N1,N4-dioxide (L2) and 3-amino-6(7)-methylquinoxaline-2-carbonitrile N1,N4-dioxide (L3), were subjected to cytotoxic evaluation in V79 cells in hypoxic and aerobic conditions. The complexes resulted in vitro more potent cytotoxins than the free ligands (i.e. potencies P(VO(L1)2)=3.0, P(L1)=9.0 microM) and Tirapazamine (P=30.0 microM) and showed excellent selective cytotoxicity in hypoxia, being no cytotoxic in oxia. In addition, the solubility in hydrophilic solvents resulted significantly higher for the vanadyl complexes than for the free ligands. These results could be indicative that complexation of the quinoxaline-2-carbonitrile N1,N4-dioxide derivatives with vanadium could improve their bioavailability. In addition, a new aspect of the series has been investigated. A detailed comparison of the electrochemical behavior of the free ligands and the complexes has been performed searching for a correlation between reduction potentials of the complexes and their activities and hypoxia selectivities.

Synthesis of new quinoxaline-2-carboxylate 1,4-dioxide derivatives as anti-Mycobacterium tuberculosis agents

Twenty-nine new 6(7)-substituted quinoxaline-2-carboxylate 1,4-dioxide derivatives were synthesized and evaluated for in vitro antituberculosis activity. In general, the in vitro activity is significantly affected by substituents on the quinoxaline nucleus. It has been observed that the presence of a chloro, methyl, or methoxy group in position 7 of the benzene moiety reduces the MIC and IC(50) values. However, antituberculosis activity principally depends on the substituents in the carboxylate group, improving in the following order: benzyl > ethyl > 2-methoxyethyl > allyl > tert-butyl. Fourteen compounds have been selected for macrophage assay, and the results show that ethyl and benzyl 3-methylquinoxaline-2-carboxylate 1,4-dioxide derivatives with the chlorine group in position 7 of the benzene moiety (compounds 10 and 26) and the unsubstituted derivatives (compounds 11 and 27) have good antitubercular activity, including activity in macrophages. In addition, compounds 7 and 28 (the only ones tested up to now) are active against drug-resistant strains of M. tuberculosis H(37)Rv. In conclusion, the potency, selectivity, and low cytotoxicity of these compounds make them valid leads for synthesizing new compounds that possess better activity.

Novel Cu(II) quinoxaline N1,N4-dioxide complexes as selective hypoxic cytotoxins

As an effort to develop novel selective hypoxia-cytotoxins and to improve bioavailability and pharmacological and toxicological properties of quinoxaline N1,N4-dioxide derivatives (L1 = 3-amino-6(7)-chloroquinoxaline-2-carbonitrile N1,N4-dioxide, L2 = 3-amino-6(7)-bromoquinoxaline-2-carbonitrile N1,N4-dioxide and L3 = 3-amino-6(7)-methylquinoxaline-2-carbonitrile N1,N4-dioxide) and to get a synergism among metals and these type of bioreductive agents, L2 and three novel Cu(II) complexes of general formulae [Cu(II)(H2O)x(L - H)2], where L = L1 (x = 1), L2 (x = 0) or L3 (x = 2) were developed. L2 and complexes were synthesized and structurally characterized by elemental and thermal analyses, and FTIR, electronic, MS, NMR, and EPR spectroscopies. The new compounds were subjected to cytotoxic evaluation in V79 cells in hypoxic and aerobic conditions. The complexes showed excellent selective cytotoxicity in hypoxia, being their cytotoxicity similar to or higher than that of the ligands L1-L3. Besides, the copper complexes were so poorly cytotoxic in oxia as the free ligands. In addition, for the first time Cu(II)-quinoxaline complexes are reported as a family of hypoxic cytotoxins.

Synthesis and anticancer activity evaluation of new 2-alkylcarbonyl and 2-benzoyl-3-trifluoromethyl-quinoxaline 1,4-di-N-oxide derivatives

As a continuation of our research in quinoxaline 1,4-di-N-oxide and with the aim of obtaining new anticancer agents, which can improve the current chemotherapeutic treatments, new series of 2-alkylcarbonyl and 2-benzoyl-3-trifluoromethylquinoxaline 1,4-di-N-oxide derivatives have been synthesized and evaluated for in vitro antitumor activity against a 3-cell line panel, consisting of MCF7 (breast), NCI-H460 (lung), and SF-268 (CNS). These active compounds were then evaluated in the full panel of 60 human tumor cell lines derived from nine cancer cell types. The results have shown that, in general, anticancer activity depends on the substituents in the carbonyl group, improving in the order: ethyl<isopropyl<tert-butyl<phenyl-ones. Among these, the compounds 4c, 6e, their difluorinated analogs (4g and 6g), and 5c were the most active, with mean GI(50) values of 1.02, 0.42, 0.52, 0.15, and 0.49microM, respectively. All of them were also found to inhibit the growth of the all of the Leukemia cell lines studied (with 75% of the GI(50) values less than 0.15microM) and therefore, were selected for further evaluation for the in vivo hollow fiber assays.

Thermodynamic properties of quinoxaline-1,4-dioxide derivatives: a combined experimental and computational study

The mean (N-O) bond dissociation enthalpies were derived for three 2-methyl-3-(R)-quinoxaline 1,4-dioxide (1) derivatives, with R = methyl (1a), ethoxycarbonyl (1b), and benzyl (1c). The standard molar enthalpies of formation in the gaseous state at T = 298.15 K for the three 1 derivatives were determined from the enthalpies of combustion of the crystalline solids and their enthalpies of sublimation. In parallel, accurate density functional theory-based calculations were carried out in order to estimate the gas-phase enthalpies of formation for the corresponding quinoxaline derivatives. Also, theoretical calculations were used to obtain the first and second N-O dissociation enthalpies. These dissociation enthalpies are in excellent agreement with the experimental results herewith reported.

Quinoxaline N , N ′-dioxide derivatives and related compounds as growth inhibitors of Trypanosoma cruzi . Structure–activity relationships

Quinoxaline derivatives presented good inhibitor activity of growth of Trypanosoma cruzi in in vitro assays. The 50% inhibitory doses were of the same order of that of Nifurtimox. Derivative 13, a quinoxaline N,N'-dioxide derivative, and the reduced derivatives 19 and 20 were the most cytotoxic compounds against the protozoan. Structural requirements for optimal activity were studied by computational methods. From statistical analysis we could establish a multiple correlation between activity and lipophilic properties and LUMO energy.

Ruthenium (II) nitrofurylsemicarbazone complexes: new DNA binding agents

Complexes of the type [Ru(II)Cl(2)(DMSO)(2)L], where L are 5-nitrofurylsemicarbazone derivatives, were prepared in an effort to combine the potential anti-tumor activity of the metal and the free ligands. The new complexes are excellent DNA binding agents for calf thymus DNA. So, their in vitro anti-tumor activity was tested in cellular models and the complexes were found to be non-cytotoxic on the tumor cell lines assayed, neither in aerobic conditions nor in the bio-reductive assay performed. Redox behavior, lipophilicity and stability were studied in order to explain the lack of cellular cytotoxic effects. The complexes resulted 10-100 times more hydrophilic than the parent ligands thus the bio-activity of these compounds would be compromised by their inadequate lipophilic properties.

Thermochemistry of 2-amino-3-quinoxalinecarbonitrile-1,4-dioxide. Evaluation of the mean dissociation enthalpy of the (N–O) bond

The standard enthalpy of formation of the 2-amino-3-quinoxalinecarbonitrile-1,4-dioxide compound in the gas-phase was derived from the enthalpies of combustion of the crystalline solid measured by static bomb combustion calorimetry and its enthalpy of sublimation determined by Knudsen mass-loss effusion at T= 298.15 K. This value is (383.8 +/- 5.4) kJ mol(-1) and was subsequently combined with the experimental gas-phase enthalpy of formation of atomic oxygen and with the computed gas-phase enthalpy of formation of 2-amino-3-quinoxalinecarbonitrile, (382.0 +/- 6.3) kJ mol(-1), in order to estimate the mean (N-O) bond dissociation enthalpy in the gas-phase of 2-amino-3-quinoxalinecarbonitrile-1,4-dioxide. The result obtained is (248.3 +/- 8.3) kJ mol(-1), which is in excellent agreement with the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G(d) computed value.

Synthesis of new 2-acetyl and 2-benzoyl quinoxaline 1,4-di-N-oxide derivatives as anti-Mycobacterium tuberculosis agents

A series of 2-acetyl and 2-benzoyl-6(7)-substituted quinoxaline 1,4-di-N-oxide derivatives were synthesized and evaluated for in vitro antituberculosis activity. The results show that 2-acetyl-3-methylquinoxaline 1,4-di-N-oxide derivatives with chlorine, methyl or methoxy group in position 7 of the benzene moiety (compounds 2, 4 and 6, respectively) and unsubstituted (3) have good antitubercular activity, exhibiting EC(90)/MIC values between 0.80 and 4.29. In conclusion, the potency, selectivity and low cytotoxicity of these compounds make them valid leads for synthesizing new compounds that possess better activity.

Synthesis and antimycobacterial activity of new quinoxaline-2-carboxamide 1,4-di-N-oxide derivatives

As a continuation of our research and with the aim of obtaining new antituberculosis agents which can improve the current chemotherapeutic antituberculosis treatments, new series of quinoxaline-2-carboxamide 1,4-di-N-oxide derivatives were synthesized and evaluated for in vitro antituberculosis activity against Mycobacterium tuberculosis strain H(37)Rv, using the radiometric BACTEC 460-TB methodology. Active compounds were also screened by serial dilution to assess toxicity to a VERO cell line. The results indicate that some compounds exhibited a good antituberculosis activity and the arylcarboxamide analogues 3, 8, and 9 were the most active compounds (EC(90)/MIC1). Also, the cytotoxic effects indicate that these compounds have a good Selectivity Index (SI).

Pyrazolo[3,4-b]quinoxalines. A new class of cyclin-dependent kinases inhibitors

Protein kinases are involved in most physiological processes and in numerous diseases. Therefore, inhibitors of protein kinases have therefore a wide therapeutic potential. While screening for inhibitors of cyclin-dependent kinases (CDK's) and glycogen synthase kinase-3 (GSK-3), we identified pyrazolo[3,4-b]quinoxalines as sub-micromolar inhibitors of CDK1/cyclin B. A preliminary structure-activity relationship study suggests that this family of compounds can be optimized to inhibit CDK's and GSK-3. Compounds were tested for their anti-proliferative activity and the results show that several of them displayed a significant inhibitory effect on CDK1/cyclin B. The most active compound (1) was also tested against the brain kinases CDK5/p25 and GSK-3, and proved to be a good inhibitor of both of them. On the contrary, none of the compounds showed any activity in the CDC25 phosphatase assay. As an additional approach, affinity chromatography on immobilized pyrazolo[3,4-b]quinoxalines will be used to identify the intracellular targets of this family of compounds.