Reaction of 1,3-bis(2-chloroethyl)-1-nitrosourea with synthetic polynucleotides

Autocorrelation analysis of a phenotypic screen reveals hidden drug activity

Phenotype based screening is a powerful tool to evaluate cellular drug response. Through high content fluorescence imaging of simple fluorescent labels and complex image analysis phenotypic measurements can identify subtle compound-induced cellular changes unique to compound mechanisms of action (MoA). Recently, a screen of 1008 compounds in three cell lines was reported where analysis detected changes in cellular phenotypes and accurately identified compound MoA for roughly half the compounds. However, we were surprised that DNA alkylating agents and other compounds known to induce or impact the DNA damage response produced no measured activity in cells with fluorescently labeled 53BP1-a canonical DNA damage marker. We hypothesized that phenotype analysis is not sensitive enough to detect small changes in 53BP1 distribution and analyzed the screen images with autocorrelation image analysis. We found that autocorrelation analysis, which quantifies fluorescently-labeled protein clustering, identified higher compound activity for compounds and MoAs known to impact the DNA damage response, suggesting altered 53BP1 recruitment to damaged DNA sites. We then performed experiments under more ideal imaging settings and found autocorrelation analysis to be a robust measure of changes to 53BP1 clustering in the DNA damage response. These results demonstrate the capacity of autocorrelation to detect otherwise undetectable compound activity and suggest that autocorrelation analysis of specific proteins could serve as a powerful screening tool.

Alkylating Agents, the Road Less Traversed, Changing Anticancer Therapy

Cancer is considered one of the gruelling challenges and poses a grave health hazard across the globe. According to the International Agency for Research on Cancer (IARC), new cancer diagnoses increased to 18.1 million in 2018, with 9.6 million deaths, bringing the global cancer rate to 23.6 million by 2030. In 1942, the discovery of nitrogen mustard as an alkylating agent was a tremendous breakthrough in cancer chemotherapy. It acts by binding to the DNA, and creating cross linkages between the two strands, leading to arrest of DNA replication and eventual cell death. Nitrogen lone pairs of 'nitrogen mustard' produce an intermediate 'aziridinium ion' at molecular level, which is very reactive towards DNA of tumour cells, resulting in multiple side effects with therapeutic consequences. Owing to its high reactivity and peripheral cytotoxicity, several improvements have been made with structural modifications for the past 75 years to enhance its efficacy and improve the direct transport of drugs to the tumour cells. Alkylating agents were among the first non-hormonal substances proven to be active against malignant cells and also, the most valuable cytotoxic therapies available for the treatment of leukaemia and lymphoma patients. This review focus on the versatile use of alkylating agents and the structure activity relationship (SAR) of each class of these compounds. This could provide an understanding for design and synthesis of new alkylating agents having enhanced target specificity and adequate bioavailability.

Antitumor sulfonylhydrazines: design, structure-activity relationships, resistance mechanisms, and strategies for improving therapeutic utility

1,2-Bis(sulfonyl)-1-alkylhydrazines (BSHs) were conceived as more specific DNA guanine O-6 methylating and chloroethylating agents lacking many of the undesirable toxicophores contained in antitumor nitrosoureas. O(6)-Alkylguanine-DNA alkyltransferase (MGMT) is the sole repair protein for O(6)-alkylguanine lesions in DNA and has been reported to be absent in 5-20% of most tumor types. Many BSHs exhibit highly selective cytotoxicity toward cells deficient in MGMT activity. The development of clinically useful MGMT assays should permit the identification of tumors with this vulnerability and allow for the preselection of patient subpopulations with a high probability of responding. The BSH system is highly versatile, permitting the synthesis of many prodrug types with the ability to incorporate an additional level of tumor-targeting due to preferential activation by tumor cells. Furthermore, it may be possible to expand the spectrum of activity of these agents to include tumors with MGMT activity by combining them with tumor-targeted MGMT inhibitors.

Evaluation of the efficiency of chemotherapy in in vivo orthotopic models of human glioma cells with and without 1p19q deletions and in C6 rat orthotopic allografts serving for the evaluation of surgery combined with chemotherapy

BACKGROUND

Malignant gliomas of the central nervous system remain associated with dismal prognoses because of their diffuse invasion of the brain parenchyma. Very few experimental models that mimic clinical reality are available today to test potentially new therapies. The authors set up experimental in vivo glioma models of anaplastic astrocytomas of human and rat origins and anaplastic oligodendroglioma of human origin. Standard hospital chemotherapies were employed to test the validity of these models.

METHODS

Three glioma cells lines obtained from the American Type Culture Collection (i.e., human Hs683 and U373 cells and rat C6 cells) were implanted into nude mouse brains (Hs683 and U373 cells) and rat brains (C6 cells). The astrocytic nature, as opposed to the oligodendrocytic nature, of the Hs683 and U373 models was investigated by using quantitative (computer-assisted microscopy) immunohistochemical characterizations of nestin, vimentin, glutathione-S-transferase alpha (GSTalpha), GSTmu, GSTpi, and p53 expression. Comparative genomic hybridization (CGH) was employed to investigate 1p19q losses. Chronic administrations of carmustine (BCNU), fotemustin, or temozolomide were assayed in the xenografted U373 and Hs683 models. Both BCNU-related chemotherapy and surgery were assayed in the C6 model.

RESULTS

The quantitative phenotypic analyses pointed to the oligodendroglial nature of the Hs683 cell line and the astrocytic nature of the U373 cell line. The Hs683 cells exhibited 1p19q losses, whereas the U373 cells did not. BCNU, fotemustin, and temozolomide dramatically increased the time of survival of the Hs683 oligodendroglioma-bearing mice, whereas temozolomide only induced a weak but nevertheless statistically significant increase in the U373 glioma-bearing mice. In the C6 rat glioma model, surgery and BCNU chemotherapy were more efficient than either treatment alone.

CONCLUSIONS

The in vivo models of gliomas of the central nervous system developed in the current work best mimicked clinical reality. They can be used either to identify new therapies against human gliomas or to optimize existing therapies.

The chloroethylnitrosoureas: sensitivity and resistance to cancer chemotherapy at the molecular level

The chloroethylnitrosoureas were developed in a synthetic program that began with the observation that N-methyl-N'-nitro-N-nitrosoguanidine was an effective agent against L1210 cells. The antitumor activity of the chloroethylnitrosoureas is based on their reactions with DNA, especially the formation of a cytosine-guanine crosslink in DNA. Resistance occurs when the enzyme, O6-alkylguanine-DNA alkyltransferase, repairs an intermediate in crosslink formation. Inhibition of O6-alkylguanine-DNA alkyltransferase often restores sensitivity to the chloroethlylnitrosoureas although evidence is accumulating that other repair mechanisms may also contribute to the resistance phenomenon. Continuing investigations in this field center on finding agents whose reactions with DNA are more specific, on elucidating other resistance mechanisms, and on overcoming resistance by developing new inhibitors of repair enzymes.

Metabolism of the chloroethylnitrosoureas

1. The chemical degradation and metabolism of the 2-chloroethylnitrosoureas (CENUSs) have been critically reviewed with the objective of gaining a better understanding of factors that could aid in the design of new, more effective, anticancer drugs. 2. The CENUs are chemically unstable under normal physiological conditions and can rapidly degrade to give a variety of reactive intermediates capable of carbamoylating proteins and/or alkylating both proteins and DNA. 3. Carbamoylation is thought to make a minimal contribution to the cytotoxic effect of the CENUs, although it may be involved in some of the unwanted side-effects. It would seem desirable, therefore, to design new CENUs with low carbamoylating activity. 4. The main action of the CENUs is by alkylation of DNA via a chloroethyldiazo-hydroxide intermediate. Chloroethylation is important, as opposed to hydroxyethylation, since the former leads to inter-strand DNA cross-linking via an intramolecular rearrangement with the removal of chloride. It is this inter-strand cross-linking which prevents subsequent DNA repair and loss of cytotoxicity. 5. Metabolism usually, although not exclusively, leads to deactivation of the CENUs either by dechlorination or denitrosation of the molecule, generally with the former being the dominant route. These reactions occur very rapidly, and before chemical degradation can take place, and can be an important determinant in the final cytotoxicity. Moreover, both these pathways involve the cytochrome P-450 system and can be induced with phenobarbital.(ABSTRACT TRUNCATED AT 250 WORDS)

N-(2-chloroethyl)-N-nitrosoureas covalently bound to nonionic and monocationic lexitropsin dipeptides. Synthesis, DNA affinity binding characteristics, and reactions with 32P-end-labeled DNA

The synthesis and characterization of a series of compounds that contain an N-alkyl-N-nitrosourea functionality linked to DNA minor groove binding bi- and tripeptides (lexitropsins or information-reading peptides) based on methylpyrrole-2-carboxamide subunits are described. The lexitropsins (lex) synthesized have either a 3-(dimethylamino)propyl or propyl substituent on the carboxyl terminus. The preferred DNA affinity binding sequences of these compounds were footprinted in 32P-end-labeled restriction fragments with methidiumpropyl-EDTA.Fe(II), and in common with other structural analogues, e.g., distamycin and netropsin, these nitrosoureas recognize A-T-rich runs. The affinity binding of the compound with the dimethylamino terminus, which is ionized at near-neutral pH, appeared stronger than that observed for the neutral dipeptide. The sequence specificity for DNA alkylation by (2-chloroethyl)nitrosourea-lex dipeptides (Cl-ENU-lex), with neutral and charged carboxyl termini, using 32P-end-labeled restriction fragments, was determined by the conversion of the adducted sites into single-strand breaks by sequential heating at neutral pH and exposure to base. The DNA cleavage sites were visualized by polyacrylamide gel electrophoresis and autoradiography. The alkylation of DNA by Cl-ENU-lex was compared to that by N-(2-chloroethyl)-N'-cyclohexyl-N-nitrosourea (CCNU), which has no DNA affinity binding properties. While all the Cl-ENU compounds generate DNA breaks as a consequence of the formation of N7-alkyl-guanine, the Cl-ENU-lex compounds induced, in a time- and dose-dependent fashion, intense DNA cleavage bands at adenine, cytosine, and thymine residues associated with affinity binding sites. These non-G cleavages induced by Cl-ENU-lex were inhibited by the coaddition of distamycin at concentrations that did not affect G alkylation break sites. CCNU, even at much higher concentrations, does not generate any similar detectable lesions at non-G sites. Therefore, linking the Cl-ENU moiety to minor groove binders is a viable strategy to qualitatively and quantitatively control the delivery and release of the ultimate DNA alkylating agent in a sequence-dependent fashion.

Reaction of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) with guanosine: evidence for a new mechanism of DNA modification

When guanosine reacts with 1,3-bis(2-chloro-2,2-dideuteroethyl)-1-nitrosourea in a mixture of pH 7.1 buffer and DMSO, the 7-chloroethylguanosine which is isolated contains two deuterium atoms located next to the guanine ring and beta to the chlorine atom as shown by electron impact mass spectrometry. It is proposed that initial attack by DNA bases occurs on the number 2 carbon of the haloethylnitrosourea with displacement of the chloride ion. In accordance with this proposed mechanism, 7-bromoethylguanosine is isolated as a major product when BCNU is reacted with guanosine in the presence of high concentrations of KBr. These results suggest that the antitumor activity of various haloethylating antitumor agents may be determined by structural changes which affect their mechanisms of reaction with DNA.

Biochemical and cytotoxic properties of the isomeric forms of N,N'-bis[N-2-chloroethyl)-N-nitrosocarbamoyl] cystamine

Three isomeric forms of a cystamine-containing chloroethylnitrosourea, N,N'-bis[N-(2-chloroethyl)-N-nitrosocarbamoyl]cystamine (CNCC), have been identified and separated by high pressure liquid chromatography. Isomer S, 3,3'-bis[N-(2-chloroethyl)-N-nitrosocarbamoyl] ethyl disulfide, was significantly less cytotoxic than isomer C, 1,1'-bis [N-(2-chloroethyl)-N-nitrosocarbamoyl] ethyl disulfide, or isomer M, 1,3'-bis[N-(2-chloroethyl)-N-nitrosocarbamoyl] ethyl disulfide, in either a human Namalva lymphoblastoid or a rat Walker 256 carcinoma cell line. Both isomers S and C inhibited DNA synthesis at a 50 microM concentration. A structural analysis of the isomeric forms suggested that bioreduction of the disulfide bond would permit both isomers to produce isocyanate moieties which would carbamoylate intracellular proteins and depress nucleic acid synthesis. The reduced cytotoxic potential of isomer S is consistent with a prolongation in the half-life of production of alkylating carbonium species that lack the capacity to cross-link macromolecules. Overall, the relative position of the NH group within each of the nitrosourea isomers appears critical to the biological properties of the drug.

Cross-linking of DNA induced by chloroethylnitrosourea is presented by O6-methylguanine-DNA methyltransferase

The DNA repair enzyme O6-methylguanine-DNA methyltransferase has been used as a reagent to analyse the initial reaction sites of alkylating agents such as chloroethylnitrosourea that cross-link DNA. The transferase can be employed for this purpose because it removes substituted ethyl groups from DNA, as shown by its ability to act on O6-hydroxyethylguanine residues in DNA. The enzyme counteracts the formation of interstrand cross-links induced by bis-chloroethylnitrosourea, but not those induced by nitrogen mustard. Once formed, chloroethylnitrosourea-induced cross-links are not broken by the enzyme. In agreement with deductions from experiments with living cells, it is concluded that chloroethylnitrosourea act by forming reactive monoadducts at the O6 position of guanine and/or the O4 position of thymine, which subsequently generate -CH2CH2- bridges to the complementary DNA strand. A new method for quantitating interstrand cross-links in DNA has been employed.

Studies on the miscoding properties of 1,N6-ethenoadenine and 3,N4-ethenocytosine, DNA reaction products of vinyl chloride metabolites, during in vitro DNA synthesis

1,N6-Ethenoadenine (epsilon A) and 3,N4-ethenocytosine (epsilon C) are formed when electrophilic vinyl chloride (VC) metabolites, chloroethylene oxide (CEO) or chloroacetaldehyde (CAA) react with adenine and cytosine residues in DNA. They were assayed for their miscoding properties in an in vitro system using Escherichia coli DNA polymerase I and synthetic templates prepared by reaction of poly(dA) and poly(dC) with increasing concentrations of CEO or CAA. Following the introduction of etheno groups, an increasing inhibition of DNA synthesis was observed. dGMP was misincorporated on CAA- or CEO-treated poly(dA) templates and dTMP was misincorporated on CAA- or CEO-treated poly(dC) templates, suggesting that epsilon A and epsilon C may miscode. The error rates augmented with the extent of reaction of CEO or CAA with the templates. Base-pairing models are proposed for the epsilon A.G. and epsilon C.T pairs. The potentially miscoding properties of epsilon A and epsilon C may explain why metabolically-activated VC and its reactive metabolites specifically induce base-pair substitution mutations in Salmonella typhimurium. Promutagenic lesions may represent one of the initial steps in VC- or CEO-induced carcinogenesis.

Mechanistic approaches to new nitrosourea development

Because of their broad range of effectiveness against experimental tumors in rodents, nitrosoureas could have a greater potential against human cancer than has so far been achieved with these agents. Greater effectiveness against human cancer might be attained from an understanding of the mechanisms by which these drugs kill cells. Some clues to possible mechanisms have been obtained and can be pursued using existing experimental techniques. The most promising leads center on the chemical effects of nitrosoureas on DNA. Several types of nitrosourea-induced DNA damage have been identified, including interstrand crosslinks, DNA-protein crosslinks and single-strand breaks. These DNA lesions can be sensitively measured in mammalian cells by the alkaline elution technique. The experimental results so far support the idea that cell killing is related to the formation and repair of DNA lesions. Antitumor activity would then depend on differences in sensitivity of neoplastic versus normal cells in regard to certain types of DNA damage and/or differences in the ability to repair this damage.

Crosslinking of DNA by busulfan. Formation of diguanyl derivatives

High-pressure liquid chromatography has been used to separate the derivatives of guanosine which are formed when this nucleoside is reacted with busulfan. Derivatives which have been identified by a combination of ultraviolet and mass spectrometry include: 7-(delta-hydroxybutyl)guanosine, 1,4-di(7-guanosyl)butane, and 1-(7-guanyl)-4-(7-guanosinyl)butane. The latter two derivatives can be converted to 1,4-di(7-guanyl)butane by mild acid hydrolysis. Using 1,4-di(7-guanyl)butane as a marker for high-pressure liquid chromatography, we have identified this compound as a product of the reaction between DNA and busulfan. These findings verify the previously unconfirmed report that busulfan is a crosslinking agent for DNA and strengthen the hypothesis that differences in the biological properties of alkylating agents result from differences in their reactions with DNA.

Synthesis of polynucleotides which contain 3,N4-ethanocytidine, a nucleoside modification resulting from the action of bis(chloroethyl)-nitrosourea

BCNU,N,N1-bis(2-chloroethyl)-N-nitrosourea is known to produce 3-hydroxyethylcytidine and 3,N4-ethanocytidine when it reacts with polycytidylic acid. The nucleoside, 3,N4-ethanocytidine, presumably results from cyclization of 3-chloroethylcytidine formed initially by transfer of chloroethyl carboniumions from BCNU to cytidine. In order to study the significance of this unusual derivative, we have synthesized 3,N4-ethanocytidine and converted it to the corresponding 5'-mono- and diphosphates. 3,N4-Ethanocytidine diphosphate has been successfully converted to a high molecular weight polymer.