Mechanism of AGT-Mediated Repair of dG-dC Cross-Links in the Drug Resistance to Chloroethylnitrosoureas: Molecular Docking, MD Simulation, and ONIOM (QM/MM) Investigation.

Chloroethylnitrosoureas (CENUs) are important chemotherapies applied in the treatment of cancer. They exert anticancer activity by inducing DNA interstrand cross-links (ICLs) via the formation of two O6-alkylguanine intermediates, O6-chloroethylguanine (O6-ClEtG) and N1,O6-ethanoguanine (N1,O6-EtG). However, O6-alkylguanine-DNA alkyltransferase (AGT), a DNA-repair enzyme, can restore the O6-alkylguanine damages and thereby obstruct the formation of ICLs (dG-dC cross-link). In this study, the inhibitory mechanism of ICL formation was investigated to elucidate the drug resistance of CENUs mediated by AGT in detail. Based on the structures of the substrate-enzyme complexes obtained from docking and MD simulations, two ONIOM (QM/MM) models with different sizes of the QM region were constructed. The model with a larger QM region, which included the substrate (O6-ClEtG or N1,O6-EtG), a water molecule, and five residues (Tyr114, Cys145, His146, Lys165, and Glu172) in the active pocket of AGT, accurately described the repairing reaction and generated the results coinciding with the experimental outcomes. The repair process consists of two sequential steps: hydrogen transfer to form a thiolate anion on Cys145 and alkyl transfer from the O6 site of guanine (the rate-limiting step). The repair of N1,O6-EtG was more favorable than that of O6-ClEtG from both kinetics and thermodynamics aspects. Moreover, the comparison of the repairing process with the formation of dG-dC cross-link and the inhibition of AGT by O6-benzylguanine (O6-BG) showed that the presence of AGT could effectively interrupt the formation of ICLs leading to drug resistance, and the inhibition of AGT by O6-BG that was energetically more favorable than the repair of O6-ClEtG could not prevent the repair of N1,O6-EtG. Therefore, it is necessary to completely eliminate AGT activity before CENUs medication to enhance the chemotherapeutic effectiveness. This work provides reasonable explanations for the supposed mechanism of AGT-mediated drug resistance of CENUs and will assist in the development of novel CENU chemotherapies and their medication strategies.

N-Nitrosation Mechanism Catalyzed by Non-heme Iron-Containing Enzyme SznF Involving Intramolecular Oxidative Rearrangement.

The non-heme iron-dependent enzyme SznF catalyzes a critical N-nitrosation step during the N-nitrosourea pharmacophore biosynthesis in streptozotocin. The intramolecular oxidative rearrangement process is known to proceed at the FeII-containing active site in the cupin domain of SznF, but its mechanism has not been elucidated to date. In this study, based on the density functional theory calculations, a unique mechanism was proposed for the N-nitrosation reaction catalyzed by SznF in which a four-electron oxidation process is accomplished through a series of complicated electron transferring between the iron center and substrate to bypass the high-valent FeIV═O species. In the catalytic reaction pathway, the O2 binds to the iron center and attacks on the substrate to form the peroxo bridge intermediate by obtaining two electrons from the substrate exclusively. Then, instead of cleaving the peroxo bridge, the Cε-Nω bond of the substrate is homolytically cleaved first to form a carbocation intermediate, which polarizes the peroxo bridge and promotes its heterolysis. After O-O bond cleavage, the following reaction steps proceed effortlessly so that the N-nitrosation is accomplished without NO exchange among reaction species.

Thioredoxin-dependent system. Application of inhibitors.

One of the systems responsible for maintaining cellular redox homeostasis is the thioredoxin-dependent system. An equally important function of this system is the regulation of the expression of many proteins by the transcription factor NF-κB or the apoptosis regulating kinase (ASK-1). Since it has been shown that the Trx-dependent system can contribute to both the enhancement of tumour angiogenesis and growth as well as apoptosis of neoplastic cells, the search for compounds that inhibit the level/activity of Trx and/or TrxR and thus modulate the course of the neoplastic process is ongoing. It has been shown that many naturally occurring polyphenolic compounds inactivate elements of the thioredoxin system. In addition, the effectiveness of Trx is inhibited by imidazole derivatives, while the activity of TrxR is reduced by transition metal ions complexes, dinitrohalobenzene derivatives, Michael acceptors, nitrosourea and ebselen. In addition, research is ongoing to identify new selective Trx/TrxR inhibitors.

A Peroxodiiron(III/III) Intermediate Mediating Both N-Hydroxylation Steps in Biosynthesis of the N-Nitrosourea Pharmacophore of Streptozotocin by the Multi-domain Metalloenzyme SznF.

The alkylating warhead of the pancreatic cancer drug streptozotocin (SZN) contains an N-nitrosourea moiety constructed from Nω-methyl-l-arginine (l-NMA) by the multi-domain metalloenzyme SznF. The enzyme's central heme-oxygenase-like (HO-like) domain sequentially hydroxylates Nδ and Nω' of l-NMA. Its C-terminal cupin domain then rearranges the triply modified arginine to Nδ-hydroxy-Nω-methyl-Nω-nitroso-l-citrulline, the proposed donor of the functional pharmacophore. Here we show that the HO-like domain of SznF can bind Fe(II) and use it to capture O2, forming a peroxo-Fe2(III/III) intermediate. This intermediate has absorption- and Mössbauer-spectroscopic features similar to those of complexes previously trapped in ferritin-like diiron oxidases and oxygenases (FDOs) and, more recently, the HO-like fatty acid oxidase UndA. The SznF peroxo-Fe2(III/III) complex is an intermediate in both hydroxylation steps, as shown by the concentration-dependent acceleration of its decay upon exposure to either l-NMA or Nδ-hydroxy-Nω-methyl-l-Arg (l-HMA). The Fe2(III/III) cluster produced upon decay of the intermediate has a small Mössbauer quadrupole splitting parameter, implying that, unlike the corresponding product states of many FDOs, it lacks an oxo-bridge. The subsequent decomposition of the product cluster to one or more paramagnetic Fe(III) species over several hours explains why SznF was previously purified and crystallographically characterized without its cofactor. Programmed instability of the oxidized form of the cofactor appears to be a unifying characteristic of the emerging superfamily of HO-like diiron oxidases and oxygenases (HDOs).

Structural-conformational aspects of tRNA complexation with chloroethyl nitrosourea derivatives: A molecular modeling and spectroscopic investigation.

Chloroethyl nitrosourea derivatives (CENUs) represent an important family of anticancer chemotherapeutic agents, which are used in the treatment of different types of cancer such as brain tumors, resistant or relapsed Hodgkin's disease, small cell lung cancer and malignant melanoma. This work focuses towards understanding the interaction of chloroethyl nitrosourea derivatives; lomustine, nimustine and semustine with tRNA using spectroscopic approach in order to elucidate their auxiliary anticancer action mechanism inside the cell. Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), Fourier transform infrared difference spectroscopy, circular dichroism spectroscopy and UV-visible spectroscopy were employed to investigate the binding parameters of tRNA-CENUs complexation. Results of present study demonstrate that all CENUs, studied here, interact with tRNA through guanine nitrogenous base residues and possibly further crosslink cytosine residues in paired region of tRNA. Moreover, spectral data collected for nimustine-tRNA and semustine-tRNA complex formation indicates towards the groove-directed-alkylation as their anti-malignant action, which involves the participation of uracil moiety located in major groove of tRNA. Besides this, tRNA-CENUs adduct formation did not alter the native conformation of biopolymer and tRNA remains in A-form after its interaction with all three nitrosourea derivatives studied. The binding constants (Ka) estimated for tRNA complexation with lomustine, nimustine and semustine are 2.55×102M-1, 4.923×102M-1 and 4.223×102M-1 respectively, which specify weak type of CENU's binding with tRNA. Moreover, molecular modeling simulations were also performed to predict preferential binding orientation of CENUs with tRNA that corroborates well with spectral outcomes. The findings, presented here, recognize tRNA binding properties of CENUs that can further help in rational designing of more specific and efficient RNA targeted chemotherapeutic agents.

Synthesis, SAR and biological evaluation of a novel series of 1-(2-chloroethyl)-1-nitroso-3-(2-(3-oxobenzoelenazol-2(3H)-yl)ethyl) urea: Organoselenium compounds for cancer therapy.

Thioredoxin reductase 1 (TrxR1) is an important potential anticancer drug target and closely related to both carcinogenesis and cancer progression. Ethaselen (BBSKE), a novel organoselenium compound inhibiting TrxR1 with selective antitumor effect, while its symmetrical structure results in poor solubility. Carmustine (BCNU), a DNA cross-link agent and also a deactivator of TrxR, is with high toxicity and low selectivity which limit its clinical application to some extents. Herein, a novel compound, 1-(2-chloroethyl)-1-nitroso-3-(2-(3-oxobenzoelenazol-2(3H)-yl)ethyl)urea(4a-1), which was designed through the combination of Ethaselen and Carmustine, showed good solubility, good tagetability, low toxicity and excellent antitumor activity by synergism. Using the structure of 4a-1 as a key active scaffold, a series of novel 1-(2-chloroethyl)-1-nitroso-3-(2-(3-oxobenzoelenazol-2(3H)-yl)ethyl)urea was designed, synthesized and evaluated to explore the structure-activity relationships (SARs) of these inhibitors and to improve their antitumor activities. Notably, 1-(2-chloroethyl)-3-(2-(6-fluoro-3-oxobenzoselenazol-2(3H)-yl)ethyl)-1-nitrosourea(4b-1) was found to exhibit more potent antitumor activities comparable to 4a-1 against all the four cancer cell lines, including Mia PaCa-2, PANC-1, RKO, LoVo. These results have highlighted compound 4b-1 as a new potential lead candidate for future development of novel potent broad-spectrum antitumor agents. In addition, a SAR model was established to conduct further structural modification.

DFT investigations of phosphotriesters hydrolysis in aqueous solution: a model for DNA single strand scission induced by N-nitrosoureas.

DNA phosphotriester adducts are common alkylation products of DNA phosphodiester moiety induced by N-nitrosoureas. The 2-hydroxyethyl phosphotriester was reported to hydrolyze more rapidly than other alkyl phosphotriesters both in neutral and in alkaline conditions, which can cause DNA single strand scission. In this work, DFT calculations have been employed to map out the four lowest activation free-energy profiles for neutral and alkaline hydrolysis of triethyl phosphate (TEP) and diethyl 2-hydroxyethyl phosphate (DEHEP). All the hydrolysis pathways were illuminated to be stepwise involving an acyclic or cyclic phosphorane intermediate for TEP or DEHEP, respectively. The rate-limiting step for all the hydrolysis reactions was found to be the formation of phosphorane intermediate, with the exception of DEHEP hydrolysis in alkaline conditions that the decomposition process turned out to be the rate-limiting step, owing to the extraordinary low formation barrier of cyclic phosphorane intermediate catalyzed by hydroxide. The rate-limiting barriers obtained for the four reactions are all consistent with the available experimental information concerning the corresponding hydrolysis reactions of phosphotriesters. Our calculations performed on the phosphate triesters hydrolysis predict that the lower formation barriers of cyclic phosphorane intermediates compared to its acyclic counter-part should be the dominant factor governing the hydrolysis rate enhancement of DEHEP relative to TEP both in neutral and in alkaline conditions.

Highly enantioselective reduction of ß-amino nitroolefins with a simple N-sulfinyl urea as bifunctional catalyst.

Simple but effective: A structurally simple N-sulfinyl urea was found to be a highly efficient bifunctional catalyst, which allows for the development of a novel pathway for the construction of chiral ß-amino nitroalkanes through enantioselective reduction of ß-amino nitroolefins by trichlorosilane. High yields and excellent enantioselectivities were obtained for a broad range of ß-arylamino nitroolefin substrates (see scheme).

[Cytotoxicity of lysomustine and its isomers, and their potential use for selection of cells].

N epsilon-Nitroso-N epsilon- [N'-(2-chloroethyl)carbamoyl]-L-lysine (I) and N epsilon- [N'-(2-chloroethyl)-N'-nitrosocarbamoyl]-L-lysine (II), the isomers being the constituents of antitumor agent Lysomustine, were obtained by RFHPLC. The study of cytotoxicity of the above compounds against K562 cells showed that the lesions induced by isomer (II) produce a significant cytotoxic effect but can be efficiently repaired by the action of MGMT (O6-methylaguanine DNA methyltransferase). Under similar conditions, the lesions induced by isomer (I) produce substantially smaller effect but are weakly if at all repairable by MGMT. The effects of a clinically approved agent Lysomustine, which is the mixture of isomers (I) and (II), are similar to those of isomer (II). The results obtained point to a different chemical nature of DNA lesions induced by two Lysomustine isomers. Our data indicate that Lysomustine and its isomer (II) can be used for in vitro selection of cells expressing MGMT.

Selection of genetically modified hematopoietic cells in vitro and in vivo using alkylating agent lysomustine.

Efficient gene transfer into hematopoietic stem cells is vital for the success of gene therapy of hematopoietic and immune system disorders. An in vivo selection system based on a mutant form of the O(6)-methylguanine-DNA-methyltransferase gene (MGMTm) is considered one of the more promising strategies for expansion of hematopoietic cells transduced with viral vectors. Here we demonstrate that MGMTm-expressing cells can be efficiently selected using lysomustine, a nitrosourea derivative of lysine. K562 and murine bone marrow cells expressing MGMTm are protected from the cytotoxic action of lysomustine in vitro. We also show in a murine model that MGMTm-transduced hematopoietic cells can be expanded in vivo on transplantation into sublethally irradiated recipients followed by lysomustine treatment. These results indicate that lysomustine can be used as a potent novel chemoselection drug applicable for gene therapy of hematopoietic and immune system disorders.

Proline analogue of nitrosourea as a new cytotoxic prodrug.

Carmustine is frequently used as anticancer drug. High toxicity and low selectivity reduces the application of this drug. Though, there is a necessity to find new compounds characterized by similar therapeutic effects but a higher selectivity and safety. As a result, the proline analogue of nitrosourea, N-[N'-(2-bromophenyl)-N'-nitrosocarbamoyl]proline (AC), has been synthesized. The aim of this study was to compare the influence of carmustine and the proline analogue of nitrosourea on the antioxidant abilities of fibroblasts and leukemia cells, MOLT4. It was shown that carmustine as well as AC cause an increase in hydrogen peroxide concentration in normal and neoplastic cells. Incubation with both compounds led to a diminution of the activity of antioxidant enzymes such as superoxide dismutase, glutathione peroxidase, and reductase. Changes in activity / level of antioxidant parameters were accompanied by augmentation of lipid and oxidative protein modifications. In conclusion, carmustine and AC cause changes in the antioxidative system of normal and MOLT4 cells and are a reason of oxidative stress formation.

Estudio morfológico de la neurocarcinogénesis experimental inducida por nitroso-ureas / Morphological study of the experimental neurocarcinogenesis induced by nitrosoureas

En el presente trabajo se hace una revisión de la neurocarcinogénesis química,con especial referencia a los tumores del Sistema Nervioso inducidos por etil-nitrosoureaen la rata Wistar. El estudio morfológico de los tumores del Sistema NerviosoCentral y del Sistema Nervioso Periférico inducidos por exposición prenatala este carcinógeno (50 mg/kg día 17 de gestación) muestra la aparición de tumorescompatibles con un diagnóstico histológico de schwannoma maligno o de oligodendroglioma.Sin embargo, los estudios inmunohistoquímicos y ultraestructuralespermiten clasificar a estos tumores, independientemente de su morfología, comotumores malignos, de tipo neuroectodérmico indiferenciado, con posibilidad dediferenciación hacia líneas neuronales y gliales. Sus características biológicas y suposible diagnóstico in vivo, por medio de Resonancia Magnética, hacen que estemodelo de neurocarcinogénesis sea extraordinariamente útil para valorar el efectode nuevos tratamientos farmacológicos, aplicables a tumores cerebrales malignosen el hombre

This work shows a revision of the chemical neurocarcinogenesis, with specialreference to the Nervous System tumors induced by ethyl-nitrosourea in the Wistarrat. Prenatal administration of this carcinogen (50 mg/kg, day 17 of pregnancy)induces Nervous System tumors with histological features of malignantschwannomas or oligodendrogliomas. However, and independently of theirmorphology, the immunohistochemical and ultrastructural studies allow to classifyto these tumors as undifferentiated malignant tumors, with tumor cells showingpossible differentiation toward neuronal and glial phenotypes. Their biologicproperties and their possible diagnosis in vivo, by means of Magnetic Resonance,make this neurocarcinogenesis model to be extraordinarily useful to value theeffect of new drug treatments, applicable to human malignant brain tumors

Synthesis and evaluation of ethylnitrosoureas of substituted naphthalimides as anticancer compounds.

Four new ethylnitrosourea derivatives of substituted naphthalimides 3a-d have been synthesized from the respective N-(2-ethylamino) naphthalimides. Their chemical alkylating activity compared with the clinical drug CCNU and an experimental compound Mitonafide indicated that they possess lower alkylating activity than CCNU and comparable activity with the latter. Their anti-tumor efficacies were assessed in vivo in two murine ascites tumors namely Sarcoma-180 (S-180) and Ehrlich ascites carcinoma (EAC) by measuring the increase in median survival times (MST) of drug treated (T) over untreated control (C) mice. CCNU and Mitonafide were used as positive controls for comparison. The representative compound 3a has displayed marginal anti-tumoral activity in these tumors. Three compounds were further screened in vitro in 4 different human tumor cell lines but no significant activity was observed in those lines. These compounds moderately inhibit the synthesis of DNA and RNA in S-180 tumor cells.

The combi-targeting concept: synthesis of stable nitrosoureas designed to inhibit the epidermal growth factor receptor (EGFR).

According to the "combi-targeting" concept, the EGFR tyrosine kinase (TK) inhibitory potency of compounds termed "combi-molecules" is critical for selective growth inhibition of tumor cells with disordered expression of EGFR or its closest family member erbB2. Here we report on the optimization of the EGFR TK inhibitory potency of the combi-molecules of the nitrosourea class by comparison with their aminoquinazoline and ureidoquinazoline precursors. This led to the discovery of a new structural parameter that influences their EGFR TK inhibitory potency, i.e., the torsion angle between the plane of the quinazoline ring and the ureido or the nitrosoureido moiety of the synthesized drugs. Compounds (3'-Cl and Br series) with small angles (0.5-3 degrees ) were generally stronger EGFR TK inhibitors than those with large angles (18-21 degrees ). This was further corroborated by ligand-receptor van der Waals interaction calculations that showed significant binding hindrance imposed by large torsion angles in the narrow ATP cleft of EGFR. Selective antiproliferative studies in a pair of mouse fibroblast NIH3T3 cells, one of which NIH3T3/neu being transfected with the erbB2 oncogene, showed that IC(50) values for inhibition of EGFR TK could be good predictors of their selective potency against the serum-stimulated growth of the erbB2-tranfected cell line (Pearson r = 0.8). On the basis of stability (t(1/2)), EGFR TK inhibitory potency (IC(50)), and selective erbB2 targeting, compound 23, a stable nitrosourea, was considered to have the structural requirements for further development.

Theoretical study of some nitrososulfamide compounds with antitumor activity.

The lowest-energy conformations of four 2-chloroethylnitrososulfamides were determined using the MM+ molecular mechanics method as implemented in Hyperchem 6.0. Some of the calculated structural parameters, angles and bonds lengths were compared with the crystal structure data of N-nitroso-N-(2-chloroethyl)-N'-sulfamoyl- proline. Using MM+, AM1 and PM3 the anti conformation was predicted to be more stable than the syn conformation in each of these compounds. With these methods we found that the relative energy of the transition state (TS) was considerably higher, but with the ab initio method using RHF with minimal basic function STO-3G we found that the syn conformation is predicted to be slightly more stable. The determination of some atomic charges of a selection of atoms on the syn, anti and TS structures of the various compounds provided some details about the nature of the transition state.

Cytotoxicity, DNA damage, and apoptosis induced by new fotemustine analogs on human melanoma cells in relation to O6-methylguanine DNA-methyltransferase expression.

Fotemustine is a third generation chloroethylnitrosourea that has demonstrated significant antitumoral effects in malignant melanoma. However, its use is somewhat limited by its toxic side effects and chemoresistance caused by direct repair of O6-alkyl groups by the enzyme O6-methylguanine DNA-methyltransferase (MGMT). The aim of this work was to determine to what extent the expression of MGMT influences cytotoxicity, DNA damage, and apoptosis induced by new nitrososulfamide analogs of fotemustine (compounds 4 and 8), which have previously demonstrated interesting antiproliferative properties. We carried out complementary strategies that consisted of MGMT cDNA transfection in CAL77 Mer- melanoma cells and of MGMT inhibition with O6-benzylguanine (BG) in A375 Mer+ melanoma cells. MGMT-transfected cells were 7 to 9 times less sensitive to fotemustine than parent cells, whereas no difference between the transfected and parent cells was observed for nitrososulfamide analogs. The cytotoxicity of these analogs vis à vis a MGMT-proficient A375 melanoma cell line was approximately 3 times greater than that of fotemustine. Coincubation of these cells with O6-benzylguanine significantly increased the cytotoxicity of fotemustine and compound 8, whereas BG had little effect on the cytotoxicity of compound 4. Furthermore, DNA fragmentation determined by a comet assay was greater with nitrososulfamide analogs than with fotemustine. O6-benzylguanine increased DNA fragmentation for fotemustine and compound 8, but not for compound 4, which induced comets with a typical apoptotic appearance. The ability of this compound to induce apoptosis in the absence of BG was confirmed by a specific enzyme-linked immunosorbent assay apoptotic assay using a single-stranded DNA monoclonal antibody.

Nitroxyl radicals and malignant pigment melanoma.

Our studies showed that the spin-labeled (SL) compounds (free stable nitroxyl radical derivatives) accumulate predominantly in pigment melanoma and some of them possess high antimelanoma activity (100% curability of tumor bearing animals) and lower general toxicity. Taking into account our results we describe a proposed mechanism of the spin-labeled accumulation in melanoma, their antimelanoma activity and lower toxicity.

Structure-based design of nitrosoureas containing tyrosine derivatives as potential antimelanoma agents.

Two new nitrosoureas (TNUs), containing tyrosine derivatives as carriers of nitrosourea cytotoxic group have been synthesised. The physicochemical properties such as half-life time (tau(0.5)), alkylating and carbamoylating activities were determined. The nitrosoureas showed a higher inhibiting effect on the DOPA-oxidase activity of mushroom tyrosinase than that of the antitumour drug N'-cyclohexyl-N-(2-chloroethyl)-N-nitrosourea (lomustine, CCNU). In vitro cytotoxic effects of newly synthesised tyrosine containing nitrosoureas have been studied and compared to those of CCNU. A higher cytotoxicity to B16 melanoma cells than to YAC-1 and to lymphocytes was demonstrated for the tyrosine containing nitrosoureas in comparison with CCNU. Based on the results presented, we accept that a new trend for synthesis of more selective and less toxic nitrosourea derivatives as potential antimelanomic drugs might be developed.

Two glycine containing 2-chloroethylnitrosoureas--a comparative study on some physicochemical properties, in vivo antimelanomic effects and immunomodulatory properties.

Physicochemical properties such as alkylating and carbamoylating activity and in vivo antimelanomic effects against B16 melanoma of the spin labeled (nitroxyl free radical containing) glycine nitrosourea (SLCNUgly) and its nonlabeled analogue (ChCNUgly), synthesized in our laboratory are studied and compared to those of antitumour drug 3-cyclohexyl-1-(2-chloroethyl)-1-nitrosourea (CCNU). We have demonstrated that introducing of glycine moiety in the nitrosourea structure in practice does not affect either alkylating or carbamoylating activity. On the other hand replacement of cyclohexyl moiety in ChCNUgly structure with nitroxyl free radical leads to a decrease in carbamoylating activity and an increase in alkylating activity. Compound ChCNUgly showed in vivo a higher antimelanomic activity against B16 melanoma in comparison with CCNU and SLCNUgly. It completely inhibited B16 melanoma growth (TGI=100%) at a dose 64.0 mg/kg. Moreover, we established that joint i.p. application in normal mice of SLCNUgly plus a new immunostimulator (C3bgp) formerly isolated in our laboratory led to a 75% restoration in immune function with respect to antibody production measured by Jerne hemolytic plaque assay. In contrast, no immunostimulation was found after joint application of C3bgp plus ChCNUgly or CCNU at the same experimental conditions. Based on these preliminary results, a possibility for developing of new combination immunochemotherapy schemes for treatment of human cancers is discussed.