The role of DNA repair in nitrogen mustard drug resistance

Genomic Adaption and Mutational Patterns in a HaCaT Subline Resistant to Alkylating Agents and Ionizing Radiation

Sulfur mustard (SM) is a chemical warfare agent that can damage DNA via alkylation and oxidative stress. Because of its genotoxicity, SM is cancerogenic and the progenitor of many chemotherapeutics. Previously, we developed an SM-resistant cell line via chronic exposure of the popular keratinocyte cell line HaCaT to increasing doses of SM over a period of 40 months. In this study, we compared the genomic landscape of the SM-resistant cell line HaCaT/SM to its sensitive parental line HaCaT in order to gain insights into genetic changes associated with continuous alkylation and oxidative stress. We established chromosome numbers by cytogenetics, analyzed DNA copy number changes by means of array Comparative Genomic Hybridization (array CGH), employed the genome-wide chromosome conformation capture technique Hi-C to detect chromosomal translocations, and derived mutational signatures by whole-genome sequencing. We observed that chronic SM exposure eliminated the initially prevailing hypotetraploid cell population in favor of a hyperdiploid one, which contrasts with previous observations that link polyploidization to increased tolerance and adaptability toward genotoxic stress. Furthermore, we observed an accumulation of chromosomal translocations, frequently flanked by DNA copy number changes, which indicates a high rate of DNA double-strand breaks and their misrepair. HaCaT/SM-specific single-nucleotide variants showed enrichment of C > A and T > A transversions and a lower rate of deaminated cytosines in the CpG dinucleotide context. Given the frequent use of HaCaT in toxicology, this study provides a valuable data source with respect to the original genotype of HaCaT and the mutational signatures associated with chronic alkylation and oxidative stress.

Synthesis of flavonoids nitrogen mustard derivatives and study on their antitumor activity in vitro

Several novel flavonoids nitrogen mustard derivatives were synthesized and evaluated for antiproliferative activity against seven human cancer cell lines (HeLa, A549, HepG2, MCF7, SH-SY5Y, PC-3, DU145) by the MTT assay in vitro. The resulting IC₅₀ showed that most compounds exhibited better inhibitory activity against seven cell lines. IC₅₀ values of some compounds were lower than well-known melphalan. In particular, compound 8b was the most promising compound which inhibited HeLa cells with IC₅₀ value of 1.43 μM. It showed excellent antitumor activity against these seven cell lines. Besides, it could arrest cell cycle of HeLa in G2/M phase and induce cell apoptosis. The loss of mitochondrial membrane potential may be an apoptotic mediating factor.

Behind the Mirror: Chirality Tunes the Reactivity and Cytotoxicity of Chloropiperidines as Potential Anticancer Agents

The pressing demand for sustainable antitumor drugs prompted us to investigate 3-chloropiperidines as potential mustard-based anticancer agents. In this study, an explorative set of variously decorated monofunctional 3-chloropiperidines (M-CePs) was efficiently synthesized through a fast and affordable route providing high yields of pure racemates and enantiomers. Consistently with their reactivity, M-CePs were demonstrated to alkylate DNA in vitro. On a panel of carcinoma cell lines, M-CePs exhibited low nanomolar cytotoxicity indexes, which showed their remarkable activity against pancreatic cancer cells and in all cases performed strikingly better than the chlorambucil control. Very interestingly, stereochemistry modulated the activity of M-CePs in unexpected ways, pointing to additional molecular mechanisms of action beyond the direct damage of genomic DNA. This encouraging combination of efficacy and sustainability suggests they are valid candidates for anticancer agent development.

Synthesis, 99m Tc-labeling, and preliminary biological evaluation of DTPA-melphalan conjugates

Melphalan (MFL) is a typical nitrogen mustard for the treatment of many types of cancer. For the purpose to develop novel ^99m Tc-labeled tumor imaging agents with SPECT, MFL was directly labeled by ^99m Tc using diethylene triamine pentacetate acid (DTPA) as bifunctional chelating agent. The novel ligands were successfully synthesized by conjugation of DTPA to MFL to get monosubstituted DTPA-MFL and bis-substituted DTPA-2MFL. Radiolabeling was performed in high yield to get ^99m Tc-DTPA-MFL and ^99m Tc-DTPA-2MFL, respectively, which were hydrophilic and stable at room temperature. The high initial tumor uptake with retention, good tumor/muscle ratios, and satisfactory scintigraphic images suggested the potential of ^99m Tc-DTPA-MFL and ^99m Tc-DTPA-2MFL for tumor imaging. However, the slow normal tissue clearance would be a great obstacle. Further modification on the linker and/or ^99m Tc-chelate to improve the tumor targeting efficacy and in vivo kinetic profiles is currently in progress.

Direct and Topoisomerase II Mediated DNA Damage by Bis-3-chloropiperidines: The Importance of Being an Earnest G

Bis-3-chloropiperidines are a new class of DNA-active compounds capable of alkylating nucleobases and inducing strand cleavage. In this study, we investigated the reactivity of these mustard-based agents with both single- and double-stranded DNA constructs. Polyacrylamide gel electrophoresis (PAGE) and electrospray ionization mass spectrometry (ESI-MS) were used to obtain valuable insight into their mechanism at the molecular level and to investigate their time- and concentration-dependent activity. The results revealed the preferential formation of mono- and bifunctional adducts at nucleophilic guanine sites. In a stepwise fashion, alkylation was followed by depurination and subsequent strand scission at the ensuing apurinic site. We demonstrated that the covalent modifications introduced by this new class of compounds can inhibit the activity of essential DNA-processing proteins, such as topoisomerase IIα, thereby suggesting that bis-3-chloropiperidines may have excellent anticancer potential.

A role for the base excision repair enzyme NEIL3 in replication-dependent repair of interstrand DNA cross-links derived from psoralen and abasic sites

Interstrand DNA-DNA cross-links are highly toxic lesions that are important in medicinal chemistry, toxicology, and endogenous biology. In current models of replication-dependent repair, stalling of a replication fork activates the Fanconi anemia pathway and cross-links are "unhooked" by the action of structure-specific endonucleases such as XPF-ERCC1 that make incisions flanking the cross-link. This process generates a double-strand break, which must be subsequently repaired by homologous recombination. Recent work provided evidence for a new, incision-independent unhooking mechanism involving intrusion of a base excision repair (BER) enzyme, NEIL3, into the world of cross-link repair. The evidence suggests that the glycosylase action of NEIL3 unhooks interstrand cross-links derived from an abasic site or the psoralen derivative trioxsalen. If the incision-independent NEIL3 pathway is blocked, repair reverts to the incision-dependent route. In light of the new model invoking participation of NEIL3 in cross-link repair, we consider the possibility that various BER glycosylases or other DNA-processing enzymes might participate in the unhooking of chemically diverse interstrand DNA cross-links.

Repairing of N-mustard derivative BO-1055 induced DNA damage requires NER, HR, and MGMT-dependent DNA repair mechanisms

Alkylating agents are frequently used as first-line chemotherapeutics for various newly diagnosed cancers. Disruption of genome integrity by such agents can lead to cell lethality if DNA lesions are not removed. Several DNA repair mechanisms participate in the recovery of mono- or bi-functional DNA alkylation. Thus, DNA repair capacity is correlated with the therapeutic response. Here, we assessed the function of novel water-soluble N-mustard BO-1055 (ureidomustin) in DNA damage response and repair mechanisms. As expected, BO-1055 induces ATM and ATR-mediated DNA damage response cascades, including downstream Chk1/Chk2 phosphorylation, S/G₂ cell-cycle arrest, and cell death. Further investigation revealed that cell survival sensitivity to BO-1055 is comparable to that of mitomycin C. Both compounds require nucleotide excision repair and homologous recombination, but not non-homologous end-joining, to repair conventional cross-linking DNA damage. Interestingly and unlike mitomycin C and melphalan, MGMT activity was also observed in BO-1055 damage repair systems, which reflects the occurrence of O-alkyl DNA lesions. Combined treatment with ATM/ATR kinase inhibitors significantly increases BO-1055 sensitivity. Our study pinpoints that BO-1055 can be used for treating tumors that with deficient NER, HR, and MGMT DNA repair genes, or for synergistic therapy in tumors that DNA damage response have been suppressed.

Epigenetic changes mediated by polycomb repressive complex 2 and E2a are associated with drug resistance in a mouse model of lymphoma

Background

The genetic origins of chemotherapy resistance are well established; however, the role of epigenetics in drug resistance is less well understood. To investigate mechanisms of drug resistance, we performed systematic genetic, epigenetic, and transcriptomic analyses of an alkylating agent-sensitive murine lymphoma cell line and a series of resistant lines derived by drug dose escalation.

Methods

Dose escalation of the alkylating agent mafosfamide was used to create a series of increasingly drug-resistant mouse Burkitt’s lymphoma cell lines. Whole genome sequencing, DNA microarrays, reduced representation bisulfite sequencing, and chromatin immunoprecipitation sequencing were used to identify alterations in DNA sequence, mRNA expression, CpG methylation, and H3K27me3 occupancy, respectively, that were associated with increased resistance.

Results

Our data suggest that acquired resistance cannot be explained by genetic alterations. Based on integration of transcriptional profiles with transcription factor binding data, we hypothesize that resistance is driven by epigenetic plasticity. We observed that the resistant cells had H3K27me3 and DNA methylation profiles distinct from those of the parental lines. Moreover, we observed DNA methylation changes in the promoters of genes regulated by E2a and members of the polycomb repressor complex 2 (PRC2) and differentially expressed genes were enriched for targets of E2a. The integrative analysis considering H3K27me3 further supported a role for PRC2 in mediating resistance. By integrating our results with data from the Immunological Genome Project (Immgen.org), we showed that these transcriptional changes track the B-cell maturation axis.

Conclusions

Our data suggest a novel mechanism of drug resistance in which E2a and PRC2 drive changes in the B-cell epigenome; these alterations attenuate alkylating agent treatment-induced apoptosis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-016-0305-0) contains supplementary material, which is available to authorized users.

Synthesis and evaluation of a bis-3-chloropiperidine derivative incorporating an anthraquinone pharmacophore

With the aim to attain an alkylating agent with enhanced DNA-affinity, we have successfully synthesised lysine-linked bis-3-chloropiperidine 1 bearing an anthraquinone moiety known to bind double-stranded DNA. Consistent with our expectations, compound 1 appears to intercalate into the DNA double helix, which can be observed by conformational changes of plasmid DNA suggesting alkylation and intercalation-induced DNA unwinding. The results of this work can provide a meaningful starting point for investigating the molecular mechanism of action of this novel DNA alkylating conjugate 1 with improved affinity for DNA.

Bis-3-chloropiperidines containing bridging lysine linkers: Influence of side chain structure on DNA alkylating activity

A series of bis-3-chloropiperidines containing lysine linkers was synthesised as DNA alkylating model compounds by using a bidirectional synthetic strategy. These novel piperidine mustard based agents have been evaluated for their alkylating properties towards nucleic acids and were shown to alkylate and cleave DNA with strong preference for guanine residues. Our studies reveal that the introduction of aromatic groups in the side chain of the lysine linker has an impact on DNA alkylating activity. Analysis by ESI mass spectrometry enabled the verification of the reactive aziridinium ion formation. Overall, the results confirm our recently proposed reaction mechanism of bis-3-chloropiperidines.

Evidence for base excision repair processing of DNA interstrand crosslinks

Many bifunctional alkylating agents and anticancer drugs exert their cytotoxicity by producing cross links between the two complementary strands of DNA, termed interstrand crosslinks (ICLs). This blocks the strand separating processes during DNA replication and transcription, which can lead to cell cycle arrest and apoptosis. Cells use multiple DNA repair systems to eliminate the ICLs. Concerted action of repair proteins involved in Nucleotide Excision Repair and Homologous Recombination pathways are suggested to play a key role in the ICL repair. However, recent studies indicate a possible role for Base Excision Repair (BER) in mediating the cytotoxicity of ICL inducing agents in mammalian cells. Elucidating the mechanism of BER mediated modulation of ICL repair would help in understanding the recognition and removal of ICLs and aid in the development of potential therapeutic agents. In this review, the influence of BER proteins on ICL DNA repair and the possible mechanisms of action are discussed.

The mechanism of guanine alkylation by nitrogen mustards: a computational study

The thermodynamics and kinetics for the monofunctional binding of nitrogen mustard class of anticancer drugs to purine bases of DNA were studied computationally using guanine and adenine as model substrates. Mechlorethamine and melphalan are used as model systems in order to better understand the difference in antitumor activity of aliphatic and aromatic mustards, respectively. In good agreement with experiments that suggested the accumulation of a reactive intermediate in the case of mechlorethamine, our model predicts a significant preference for the formation of corresponding aziridinium ion for mechlorethamine, while the formation of the aziridinium ion is not computed to be preferred when melphalan is used. Two effects are found that contribute to this difference. First, the ground state of the drug shows a highly delocalized lone pair on the amine nitrogen of the melphalan, which makes the subsequent cyclization more difficult. Second, because of the aromatic substituent connected to the amine nitrogen of melphalan, a large energy penalty has to be paid for solvation. A detailed study of energy profiles for the two-step mechanism for alkylation of guanine and adenine was performed. Alkylation of guanine is ∼6 kcal mol(-1) preferred over adenine, and the factors contributing to this preference were explained in our previous study of cisplatin binding to purine bases. A detailed analysis of energy profiles of mechlorethamine and melphalan binding to guanine and adenine are presented to provide an insight into rate limiting step and the difference in reactivity and stability of the intermediate in both nitrogen mustards, respectively.

A combined DNA-affinic molecule and N-mustard alkylating agent has an anti-cancer effect and induces autophagy in oral cancer cells

Although surgery or the combination of chemotherapy and radiation are reported to improve the quality of life and reduce symptoms in patients with oral cancer, the prognosis of oral cancer remains generally poor. DNA alkylating agents, such as N-mustard, play an important role in cancer drug development. BO-1051 is a new 9-anilinoacridine N-mustard-derivative anti-cancer drug that can effectively target a variety of cancer cell lines and inhibit tumorigenesis in vivo. However, the underlying mechanism of BO-1051-mediated tumor suppression remains undetermined. In the present study, BO-1051 suppressed cell viability with a low IC(50) in oral cancer cells, but not in normal gingival fibroblasts. Cell cycle analysis revealed that the tumor suppression by BO-1051 was accompanied by cell cycle arrest and downregulation of stemness genes. The enhanced conversion of LC3-I to LC3-II and the formation of acidic vesicular organelles indicated that BO-1501 induced autophagy. The expression of checkpoint kinases was upregulated as demonstrated with Western blot analysis, showing that BO-1051 could induce DNA damage and participate in DNA repair mechanisms. Furthermore, BO-1051 treatment alone exhibited a moderate tumor suppressive effect against xenograft tumor growth in immunocompromised mice. Importantly, the combination of BO-1051 and radiation led to a potent inhibition on xenograft tumorigenesis. Collectively, our findings demonstrated that BO-1051 exhibited a cytotoxic effect via cell cycle arrest and the induction of autophagy. Thus, the combination of BO-1051 and radiotherapy may be a feasible therapeutic strategy against oral cancer in the future.

Probing DNA interstrand cross-link formation by an oxidized abasic site using nonnative nucleotides

The C4'-oxidized abasic site (C4-AP) forms two types of interstrand cross-links with the adjacent nucleotides in DNA. Previous experiments revealed that dG does not react with the lesion and that formation of one type of cross-link is catalyzed by the opposing dA. iso-Guanosine·dC and 2-aminopurine·dT base pairs were used to determine why dG does not cross-link with C4-AP despite its well known reactivity with other bis-electrophiles. 7-Deaza-2'-deoxyadenosine was used to probe the role of the nucleotide opposite C4-AP in the catalysis of interstrand cross-link formation.

Enhancement of radiosensitivity in human glioblastoma cells by the DNA N-mustard alkylating agent BO-1051 through augmented and sustained DNA damage response

Background

1-{4-[Bis(2-chloroethyl)amino]phenyl}-3-[2-methyl-5-(4-methylacridin-9-ylamino)phenyl]urea (BO-1051) is an N-mustard DNA alkylating agent reported to exhibit antitumor activity. Here we further investigate the effects of this compound on radiation responses of human gliomas, which are notorious for the high resistance to radiotherapy.

Methods

The clonogenic assay was used to determine the IC₅₀ and radiosensitivity of human glioma cell lines (U87MG, U251MG and GBM-3) following BO-1051. DNA histogram and propidium iodide-Annexin V staining were used to determine the cell cycle distribution and the apoptosis, respectively. DNA damage and repair state were determined by γ-H2AX foci, and mitotic catastrophe was measure using nuclear fragmentation. Xenograft tumors were measured with a caliper, and the survival rate was determined using Kaplan-Meier method.

Results

BO-1051 inhibited growth of human gliomas in a dose- and time-dependent manner. Using the dosage at IC₅₀, BO-1051 significantly enhanced radiosensitivity to different extents [The sensitizer enhancement ratio was between 1.24 and 1.50 at 10% of survival fraction]. The radiosensitive G₂/M population was raised by BO-1051, whereas apoptosis and mitotic catastrophe were not affected. γ-H2AX foci was greatly increased and sustained by combined BO-1051 and γ-rays, suggested that DNA damage or repair capacity was impaired during treatment. In vivo studies further demonstrated that BO-1051 enhanced the radiotherapeutic effects on GBM-3-beared xenograft tumors, by which the sensitizer enhancement ratio was 1.97. The survival rate of treated mice was also increased accordingly.

Conclusions

These results indicate that BO-1051 can effectively enhance glioma cell radiosensitivity in vitro and in vivo. It suggests that BO-1051 is a potent radiosensitizer for treating human glioma cells.

Using synthetic DNA interstrand crosslinks to elucidate repair pathways and identify new therapeutic targets for cancer chemotherapy

Many cancer chemotherapeutic agents form DNA interstrand crosslinks (ICLs), extremely cytotoxic lesions that form covalent bonds between two opposing DNA strands, blocking DNA replication and transcription. However, cellular responses triggered by ICLs can cause resistance in tumor cells, limiting the efficacy of such treatment. Here we discuss recent advances in our understanding of the mechanisms of ICL repair that cause this resistance. The recent development of strategies for the synthesis of site-specific ICLs greatly contributed to these insights. Key features of repair are similar for all ICLs, but there is increasing evidence that the specifics of lesion recognition and synthesis past ICLs by DNA polymerases are dependent upon the structure of ICLs. These new insights provide a basis for the improvement of antitumor therapy by targeting DNA repair pathways that lead to resistance to treatment with crosslinking agents.

Cross-link structure affects replication-independent DNA interstrand cross-link repair in mammalian cells

DNA interstrand cross-links (ICLs) are cytotoxic products of common anticancer drugs and cellular metabolic processes, whose mechanism(s) of repair remains poorly understood. In this study, we show that cross-link structure affects ICL repair in nonreplicating reporter plasmids that contain a mispaired N(4)C-ethyl-N(4)C (C-C), N3T-ethyl-N3T (T-T), or N1I-ethyl-N3T (I-T) ICL. The T-T and I-T cross-links obstruct the hydrogen bond face of the base and mimic the N1G-ethyl-N3C ICL created by bis-chloroethylnitrosourea, whereas the C-C cross-link does not interfere with base pair formation. Host-cell reactivation (HCR) assays in human and hamster cells showed that repair of these ICLs primarily involves the transcription-coupled nucleotide excision repair (TC-NER) pathway. Repair of the C-C ICL was 5-fold more efficient than repair of the T-T or I-T ICLs, suggesting the latter cross-links hinder lesion bypass following initial ICL unhooking. The level of luciferase expression from plasmids containing a C-C cross-link remnant on either the transcribed or nontranscribed strand increased in NER-deficient cells, indicating NER involvement occurs at a step prior to remnant removal, whereas expression from similar T-T remnant plasmids was inhibited in NER-deficient cells, demonstrating NER is required for remnant removal. Sequence analysis of repaired plasmids showed a high proportion of C residues inserted at the site of the T-T and I-T cross-links, and HCR assays showed that Rev1 was likely responsible for these insertions. In contrast, both C and G residues were inserted at the C-C cross-link site, and Rev1 was not required for repair, suggesting replicative or other translesion polymerases can bypass the C-C remnant.

Dasatinib sensitizes primary chronic lymphocytic leukaemia lymphocytes to chlorambucil and fludarabine in vitro

The dual c-abl/Src kinase inhibitor, dasatinib, utilized to treat chronic myeloid leukaemia (CML) when used at clinically attainable sublethal concentrations, synergistically sensitized primary chronic lymphocytic leukaemia (CLL) lymphocytes to chlorambucil and fludarabine. In contrast, dasatinib alone demonstrated toxicity to CLL lymphocytes at concentrations that are generally not clinically attainable. Dasatinib resistance and poorer dasatinib-mediated sensitization to chlorambucil and fludarabine was associated with higher expression of c-abl protein levels. In contrast, chlorambucil and fludarabine resistance correlated with basal p53 protein levels. Moreover, Western blot analysis after in vitro treatment of primary CLL lymphocytes with dasatinib, chlorambucil and/or fludarabine, showed that dasatinib: (i) inhibited c-abl function (e.g. downregulation of c-abl protein levels and decreased the phosphorylation of a c-abl downstream target, Dok2), (ii) decreased chlorambucil/fludarabine induced accumulation of p53 protein levels, (iii) altered the response to chlorambucil/fludarabine induced DNA-damage as evidenced by an increase in chlorambucil/fludarabine-induced H2AX phosphorylation, and (iv) accentuated the c-abl downregulation induced by chlorambucil/fludarabine. Our results suggest that dasatinib in combination with chlorambucil or fludarabine may improve the therapy of CLL.

DNA interstrand cross-link repair in Saccharomyces cerevisiae

DNA interstrand cross-links (ICL) present a formidable challenge to the cellular DNA repair apparatus. For Escherichia coli, a pathway which combines nucleotide excision repair (NER) and homologous recombination repair (HRR) to eliminate ICL has been characterized in detail, both genetically and biochemically. Mechanisms of ICL repair in eukaryotes have proved more difficult to define, primarily as a result of the fact that several pathways appear compete for ICL repair intermediates, and also because these competing activities are regulated in the cell cycle. The budding yeast Saccharomyces cerevisiae has proven a powerful tool for dissecting ICL repair. Important roles for NER, HRR and postreplication/translesion synthesis pathways have all been identified. Here we review, with reference to similarities and differences in higher eukaryotes, what has been discovered to date concerning ICL repair in this simple eukaryote.

Molecular mechanism of nitrogen mustard induced leukocyte(s) chemotaxis

Nitrogen mustard(s) (NM) are the primary chemotherapeutic agents (Chlorambucil, BSO) that are used in treating the B-cell chronic lymphocyte leukemia (B-CLL). It is known that NM imparts oxidative stress by inducing the reactive oxygen species (ROS) and reactive nitrogen species (RNS) production in B-lymphocytes of B-CLL. It is likely that these nitrogen mustards can cause the oxidative stress in the peripheral blood leukocytes (PBL) of B-CLL patients. It is possible that chronic exposure of PBL to NM would induce drug resistance in B-CLL patients. However, the precise molecular mechanism(s) by which NM affect the PBL are not known at present. Here, an effort is made to present plausible mechanisms by which chronic NM exposure could lead to increased PBL chemotaxis and vascular tissue damage in B-CLL patients The clinical implication of this phenomenon is most likely the unwanted organ dysfunction in B-CLL patients. Identification of viable therapeutic targets to attenuate the PBL activation during the NM therapy would be beneficial for the clinical remission of B-CLL.

Free radical theory of autoimmunity

Background

Despite great advances in clinical oncology, the molecular mechanisms underlying the failure of chemotherapeutic intervention in treating lymphoproliferative and related disorders are not well understood.

Hypothesis

A hypothetical scheme to explain the damage induced by chemotherapy and associated chronic oxidative stress is proposed on the basis of published literature, experimental data and anecdotal observations. Brief accounts of multidrug resistance, lymphoid malignancy, the cellular and molecular basis of autoimmunity and chronic oxidative stress are assembled to form a basis for the hypothesis and to indicate the likelihood that it is valid in vivo.

Conclusion

The argument set forward in this article suggests a possible mechanism for the development of autoimmunity. According to this view, the various sorts of damage induced by chemotherapy have a role in the pattern of drug resistance, which is associated with the initiation of autoimmunity.

Structure???anti-leukemic activity relationship study of B- and D-ring modified and non-modified steroidal esters of chlorambucil

In order to study the role of the steroidal moiety on the expression of anti-leukemic activity, we synthesized six derivatives of chlorambucil (CHL), and tested them on leukemias P388 and L1210 in vivo and in normal human lymphocytes in vitro. Five of the six tested compounds produced submultiple toxicity, while the measured anti-leukemic potency was significantly increased. The lactamization of the B-steroidal ring rendered the molecules more potent, but the corresponding 7-oxidized derivatives proved better in both leukemias tested. The lactamization of the D-steroidal ring afforded potent compounds, regardless of the configuration of the B-ring. The best among all derivatives contains both chemical modifications and is intended as a promising key molecule that must be further studied. We speculate that in leukemic cells a tumor-specific protein is overexpressed, the steroid has the ability to bind and block this protein from carrying out its normal function, and the drug-protein complex prevents the repair of the adducts. The synthesis, physicochemical and spectroscopic data of these compounds and a modified route for the synthesis of CHL are also reported.

Pharmacogenetics for individualized cancer chemotherapy

The same doses of medication cause considerable heterogeneity in efficacy and toxicity across human populations. Genetic factors are thought to represent important determinants of drug efficacy and toxicity. Pharmacogenetics focuses on the prediction of the response of tumor and normal tissue to standard therapy by genetic profiling and, thereby, to select the most appropriate medication at optimal doses for each individual patient. In the present review, we discuss the relevance of single nucleotide polymorphisms (SNP) in genes, whose gene products act upstream of the actual drug target sites, that is, drug transporters and drug metabolizing phase I and II enzymes, or downstream of them, that is, apoptosis-regulating genes and chemokines. SNPs in relevant genes, which encode for proteins that interact with anticancer drugs, were also considered, that is, enzymes of DNA biosynthesis and metabolism, DNA repair enzymes, and proteins of the mitotic spindle. A significant body of evidence supports the concept of predicting drug efficacy and toxicity by SNP genotyping. As the efficacy of cancer chemotherapy, as well as the drug-related toxicity in normal tissues is multifactorial in nature, sophisticated approaches such as genome-wide linkage analyses and integrate drug pathway profiling may improve the predictive power compared with genotyping of single genes. The implementation of pharmacogenetics into clinical routine diagnostics including genotype-based recommendations for treatment decisions and risk assessment for practitioners represents a challenge for the future.

Genetic Variants of DNA Repair Genes and Prostate Cancer: A Population-Based Study

As part of a population-based case-control study in Shanghai, China, we investigated whether variants in several DNA repair genes, either alone or in conjunction with other risk factors, are associated with prostate cancer risk. Genomic DNA from 162 patients newly diagnosed with prostate cancer and 251 healthy men randomly selected from the population were typed for five nonsynonymous DNA repair markers. We found that the XRCC1-Arg399Gln AA and the MGMT-Leu84Phe CT+TT genotypes were associated with an increased risk of prostate cancer [odds ratio (OR), 2.18; 95% confidence interval (CI), 0.99-4.81 and OR, 1.99; 95% CI, 1.19-3.34, respectively]. In contrast, XRCC3-Thr241Met, XPD-Lys751Gln, and MGMT-Ile143Val markers showed no significant associations with risk, although due to the much lower frequency of their variant alleles in this population we cannot rule out small to modest effects. There was a significant interaction between the MGMT-84 marker and insulin resistance (P(interaction) = 0.046). Relative to men with the MGMT-84 CC genotype and a low insulin resistance (<0.097), those having the CT-TT genotype and a greater insulin resistance had a 5.4-fold risk (OR, 5.39; 95% CI, 2.46-11.82). In addition, for the XRCC3-241 marker, relative to men with the CC genotype and a low intake of preserved foods (<12.7 g/d), those harboring the CT+TT genotype and having a higher intake of preserved foods (>12.7 g/d), which contain nitrosamines and nitrosamine precursors, had a significantly increased risk of prostate cancer risk (OR, 2.62; 95% CI, 1.13-6.06). In contrast, men with the CT+TT genotype and a low intake of preserved foods had a 69% reduction in risk (OR, 0.31; 95% CI, 0.10-0.96; P(interaction) = 0.005). These results suggest that genetic variants in the DNA repair pathways may be involved in prostate cancer etiology and that other risk factors, including preserved foods and insulin resistance, may modulate prostate cancer risk in combination with genetic susceptibility in these repair pathways. Replication in larger studies is necessary to preclude chance findings, particularly those among subgroups, and clarify the mechanisms involved.

Modulation of DNA Repair In vitro after Treatment with Chemotherapeutic Agents by the Epidermal Growth Factor Receptor Inhibitor Gefitinib (ZD1839)

PURPOSE

The epidermal growth factor receptor (EGFR) is commonly expressed in human tumors and provides a target for therapy. Gefitinib (Iressa, ZD1839) is a quinazoline derivative that inhibits EGFR tyrosine kinase activity. Gefitinib demonstrated anticancer efficacy in vivo, and although experiments in vitro have suggested that inhibition of EGFR modulates the activity of chemotherapeutic agents, the mechanism of this interaction is unclear. We investigated mechanisms for this modulation.

EXPERIMENTAL DESIGN

The antiproliferative effect of gefitinib alone or combined with cisplatin, melphalan, and etoposide was determined in a human breast (MCF-7) cancer cell line. Using the alkaline single-cell gel electrophoresis (comet) assay, we investigated kinetics of DNA damage and repair after treatment with the chemotherapeutic drugs combined with gefitinib. To investigate whether the phosphatidylinositol 3'-kinase pathway was contributing to repair-inhibition produced by gefitinib, cells were exposed to chemotherapy in combination with the phosphatidylinositol 3'-kinase inhibitor LY294002.

RESULTS

A superadditive (synergistic) increase in growth inhibition for combined treatment with gefitinib was found for cisplatin and etoposide, but not with melphalan. There was delayed repair of DNA strand breaks after treatment with etoposide combined with gefitinib, and repair of DNA interstrand cross-links produced by cisplatin is delayed in combination with gefitinib. Inhibition of cell proliferation and DNA repair was identical in cells treated with LY294002. Immunoprecipitation of cell extracts demonstrated that after exposure to gefitinib, there was an association between EGFR and DNA-PK(CS).

CONCLUSION

Gefitinib acts through inhibition of repair of cisplatin and etoposide-induced DNA damage; this effect is mimicked by inhibitors of the phosphatidylinositol 3'-kinase suggesting similar mechanisms of action.

DNA Damage-Processing Pathways Involved in the Eukaryotic Cellular Response to Anticancer DNA Cross-Linking Drugs

We used a panel of isogenic Saccharomyces cerevisiae strains compromised in several different DNA damage-processing pathways to assess in vivo processing of DNA adducts induced by four cross-linking anticancer drugs. By examining cytotoxicity profiles, cell cycle arrest patterns, and determining recombination and mutation frequencies, we found that cisplatin-, nitrogen mustard-, mitomycin-, and carmustine-induced DNA adducts in S. cerevisiae are processed by components of the nucleotide excision repair (NER), recombination repair (RR), and translesion synthesis (TLS) pathways, with substantially different contributions of each pathway for the drugs studied here. In contrast to previous studies that used single pathway-compromised strains to identify genes that mediate sensitivity to DNA cross-linking drugs, we used strains that were compromised in multiple pathways. By doing so, we were able to establish several functions that were previously unknown and interconnections between different DNA damage-processing pathways. To our surprise, we found that for cisplatin-induced cytotoxicity, TLS and RR contribute to survival to a significant extent. In the case of nitrogen mustard DNA adduct processing, equal involvement of two major pathways was established: one that requires functional RR and NER components and one that requires functional TLS and NER components. These data reveal the complexity of DNA cross-link processing that, in many cases, requires interactions of components from several different DNA damage-processing systems. We demonstrate the usefulness of yeast strains with multiple simultaneous defects in DNA damage-processing pathways for studying the modes of action of anticancer drugs.

Reduced DNA double strand breaks in chlorambucil resistant cells are related to high DNA-PKcs activity and low oxidative stress

Modulation of DNA repair represents a strategy to overcome acquired drug resistance of cells to genotoxic chemotherapeutic agents, including nitrogen mustards (NM). These agents induce DNA inter-strand cross-links, which in turn produce double strand breaks (dsbs). These breaks are primarily repaired via the nonhomologous end-joining (NHEJ) pathway. A DNA-dependent protein kinase (DNA-PK) complex plays an important role in NHEJ, and its increased level/activity is associated with acquired drug resistance of human tumors. We show in this report that the DNA-PK complex has comparable levels and kinase activity of DNA-PK catalytic subunit (DNA-PKcs) in a nearly isogenic pair of drug-sensitive (A2780) and resistant (A2780/100) cells; however, treatment with chlorambucil (Cbl), a NM-type of drug, induced differential effects in these cells. The kinase activity of DNA-PKcs was increased up to 2h after Cbl treatment in both cell types; however, it subsequently decreased only in sensitive cells, which is consistent with increased levels of DNA dsbs. The decreased kinase activity of DNA-PKcs was not due to a change in its amount or the levels of Ku70 and Ku86, their subcellular distribution, cell cycle progression or caspase-mediated degradation of DNA-PK. In addition to DNA cross-links, Cbl treatment of cells causes a 2.2-fold increase in the level of reactive oxygen species (ROS) in both cell types. However, the ROS in A2780/100 cells were reduced to the basal level after 3-4h, while sensitive cells continued to produce ROS and undergo apoptosis. Pre-treatment of A2780 cells with the glutathione (GSH) precursor, N-acetyl-L-cysteine prevented Cbl-induced increase in ROS, augmented the kinase activity of DNA-PKcs, decreased the levels of DNA dsbs and increased cell survival. Depletion in GSH from A2780/100 cells by L-buthionine sulfoximine (BSO) resulted in sustained production of ROS, lowered DNA-PKcs kinase activity, enhanced levels of DNA dsbs, and increased cell killing by Cbl. We propose that oxidative stress decreases repair of DNA dsbs via lowering kinase activity of DNA-PKcs and that induction of ROS could be the basis for adjuvant therapies for sensitizing tumor cells to nitrogen mustards and other DNA cross-linking drugs.