Accepted Manuscript Review DNA Binders in Clinical Trials and Chemotherapy Asfa Ali, Santanu Bhattacharya PII: DOI: Reference:

S0968-0896(14)00379-4 http://dx.doi.org/10.1016/j.bmc.2014.05.030 BMC 11591

To appear in:

Bioorganic & Medicinal Chemistry

Received Date: Revised Date: Accepted Date:

23 February 2014 9 May 2014 14 May 2014

Please cite this article as: Ali, A., Bhattacharya, S., DNA Binders in Clinical Trials and Chemotherapy, Bioorganic & Medicinal Chemistry (2014), doi: http://dx.doi.org/10.1016/j.bmc.2014.05.030

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

DNA Binders in Clinical Trials and Chemotherapy Asfa Ali,1 Santanu Bhattacharya 1,2,* 1

Department of Organic Chemistry, Indian Institute of Science, Bangalore 560 012, India. 2

Also at Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560 064, India. *To whom the correspondence should be addressed. E-mail: [email protected]

Abstract Cancer has always been a dreadful disease and continues to attract extensive research investigations. Various targets have been identified to restrain cancer. Among these DNA happens to be the most explored one. A wide variety of small molecules, often referred as “ligands”, has been synthesized to target numerous structural features of DNA. The sole purpose of such molecular design has been to interfere with the transcriptional machinery in order to drive the cancer cell toward apoptosis. The mode of action of the DNA targeting ligands focuses either on the sequence-specificity by groove binding and strand cleavage, or by identifying the morphologically distinct higher order structures like that of the Gquadruplex DNA. However, in spite of the extensive research, only a tiny fraction of the molecules have been able to reach the clinical trials and only a handful emerges as potential candidates in chemotherapy. This review attempts to record the journey of the DNA binding small molecules from its inception to cancer therapy via various modifications at the molecular level. Nevertheless, factors like limited bioavailability, severe toxicities, unfavorable pharmacokinetics etc. still prove to be the major impediments in the field which warrant considerable scope for further research investigations.

1

Keywords Duplex DNA; G-quadruplex DNA; DNA binders; Topoisomerase; Chemotherapy; Clinical trials; Anticancer.

Contents 1. Introduction............................................................................................................................3 2. DNA as an anticancer target...................................................................................................3 3. Targeting cancer by small molecules.....................................................................................5 4. Groove binders of the duplex DNA.......................................................................................6 5. Metal complex based DNA binders.......................................................................................9 6. DNA cleaving agents...........................................................................................................13 7. Topoisomerase inhibitors.....................................................................................................19 8. Novel DNA-targets for therapeutic intervention..................................................................24 9. Conclusions..........................................................................................................................31 References and notes................................................................................................................33

2

1. Introduction Cancer research focuses on targeting the cell cycle in cancerous cells by various methods, primarily by therapeutic intervention through antibodies or restricting the signaling pathways by numerous DNA damaging drugs.1 DNA serves as the prime target for cancer therapy where small molecules are synthesized aiming at various parts of DNA either by binding to the grooves, or via intercalation, or cross-linking, or even DNA strand-scission. An alternative strategy for anticancer therapy focuses on targeting different DNA morphologies which includes stabilization of higher order DNA structures like the Gquadruplex DNA. However, the effectiveness and specificity of the majority of the drugs toward cancer cells over normal cells, pharmacokinetics, and bioavailability are some of the important issues that need to be addressed carefully. 2. DNA as an anticancer target Extensive research over the years led to the emergence of a few putative targets for cancer. Irregularities in the MET (receptor tyrosine kinase) signaling pathways promote tumor formation.2 Hence the proto-oncogene MET may be considered a potential anticancer target.3 Microtubule is considered to be another interesting target to focus on and several drugs like paclitaxel (Fig. 1A) and vinca alkaloids (Fig. 1B) lead to successful inhibition of cell proliferation.4-6 Recently, Dawson et al. have established that the kinases Mps1 and Ipl1/Aurora B assist in proper segregation of chromosomes during meiosis.7 Hence these may serve as future targets in anticancer therapeutics. However, amidst all the on-going research, DNA is still regarded as one of the most explored and investigated probe to combat cancer.

3

Figure 1. Molecular structures of natural products (A) paclitaxel and (B) vinca alkaloid (vincristine).

Gene mutation is one of the principal underlying factors leading to the genesis and development of cancer. Generally, two to eight mutations in the driver genes (~140 genes) may lead to the formation of a tumor and several consecutive mutations may result in the conversion of a benign tumor into a malignant one.8 In fact, mutation refers to a perturbation in the DNA sequence which may arise from exogenous mutagens and/or endogenous factors and causes improper gene expression with increasing risk of cancer.9 Various cancers derived namely from Epstein-Barr virus (EBV), hepatitis B virus (HBV), hepatitis C virus (HCV), human papilloma virus (HPV), Kaposi’s associated sarcoma virus (KSHV) etc. originate due to a complex mechanism where an improper expression of viral oncoproteins leads to irregularity in the signaling pathways.10 Thus DNA plays a significant role in gene mutation, defects in repair mechanism, uncontrolled proliferation of cells, and hence serve as an important target in cancer therapeutics. DNA double-helix comprises complementary anti-parallel strands with sugar phosphate backbone linked by hydrogen bonds between the nucleotide bases.11 The winding of the strands leads to the formation of major and minor grooves. While the major grooves provide a support for the recognition of proteins,12 the minor grooves provide ample platform for the binding of small molecules.13 Besides duplex DNA, there exists a variety of higher order structures which include triplex,14 i-motif,15 and G-quadruplex DNA.16-18 G-quadruplex

4

DNA structures are present everywhere in the genome and cluster in some repetitive regions like the telomeric 3′-ends.19 The G-quadruplexes are also well distributed in the promoter regions like c-myc,20,21 c-kit,22,23 bcl2 (B cell lymphoma 2),24 KRAS (Kirsten rat sarcoma viral oncogene homolog)25 etc. By keeping in view the structural criterion of each of the DNA conformations, various ligands are designed and synthesized. A perturbation of the DNA structure and function often indirectly influences the replication and transcription, thereby punctuating the abnormal cell growth in cancer cells.14,26 3. Targeting cancer by small molecules Throughout the years, cancer has remained elusive, and hence increasing number of small molecules is being synthesized to target cancer. Toward this end numerous ligands are designed to target DNA of different structural and conformational features.27 Small molecules acting on the duplex DNA, focus on the groove binding, DNA strand cleavage, cross-linking, or restricting the action of DNA topoisomerases (Fig. 2). Several molecules are also developed to achieve stabilization of different morphologies of DNA, especially the higher order structures namely the G-quadruplex DNA.

DNA strand cleavage

DNA Minor Groove Binder Cross-linking of DNA

Topoisomerase I

Topoisomerase II

Figure 2. Various modes of targeting duplex DNA to arrest cancer.

5

4. Groove binders of the duplex DNA The widely available B-DNA comprises a broader major groove (11.6 Å) and a narrower minor groove (3-6 Å)28 depending on the exact sequence. The differences in dimension drive the proteins toward the major grooves12 and small molecules toward the minor grooves.13 DNA groove binders play a pivotal role in cancer drug design and therapy. Besides proteins, triplex-forming oligonucleotide (TFO)14,29 and peptide nucleic acids (PNA)30,31 are known to bind to the wider and deeper major grooves. Major advances have been achieved with the design and synthesis of ligands that bind to the minor groove and few promising compounds have indeed made all the way to the clinical trials. The natural oligopeptides, such as, netropsin (Fig. 3A) and distamycin A (Fig. 3B), isolated from streptomyces netropsis32 and streptomyces distallicus,33 respectively, are among the early minor groove binding ligands. Netropsin exhibits a 1:1 mode of binding to the BDNA (Dickerson’s dodecamer).34 This results in the release of water molecules from the DNA spine of hydration resulting in an entropic contribution to the drug-DNA binding.34 Netropsin is reported to attach to the central 5′-ATAT-3′ region by single hydrogen bonds instead of bifurcated hydrogen bonds.35 Distamycin type oligoamides have been extensively investigated by Dervan and co-workers. These authors examined the role of incorporation of other functionalities and also different combinations of the core moieties on DNA binding.36-39 The choice of the DNA sequence directly dictates the width of the groove. While the AT-rich sequences form a narrower groove (3-4 Å), the sequences enriched in GC bases contribute to a wider groove formation (5-6 Å).40 While netropsin binds to the AT-rich groove in a 1:1 stoichiometry, its functionally modified derivatives indulge in a 2:1 binding mode due to their shallow penetration which lowers the overall free energy of the complexes.37,38 Distamycin derivatives devoid of the leading amide group (shown in Fig. 3B) also associate with significant binding and sequence-specificity toward the duplex DNA.41-44 6

In order to target a contiguous stretch of nucleobases,45 design of an array of spacer linked distamycin derivatives with bent, crescent-shaped, or hairpin geometry has been carried out.46,47 Optimization of linkers played an important role in further progress in their design and covalently linked peptide dimers indeed portrayed better binding capabilities compared to their monomers.48 Imidazole (Im) and pyrrole polyamides were attached in a head-to-tail fashion by various amino acid linkers (glycine, β-alanine, 4-aminobutyric acid) to obtain a distinct category of hairpin minor groove binders.49 NMR analyses have further shown why the β-alanine based linkers are superior to others.50 It appears that their adequate flexibility and minimal steric conflict help the DNA binding.50 Specially designed hairpin molecules identified 10 base pair stretches with multi-fold selectivity.51 A high resolution 2D-NMR study elucidated a clear picture of the hydrogen bonding between Im-N3 and GNH2 along with the role of β-alanine linker in 1:1 binding.52 Bhattacharya and co-workers successfully developed appropriate linkers and side-chain functionalities for the design of dimeric distamycin analogs, some of which are photoactivatable.53-55 These authors found that the tail-to-tail linked dimeric lexitropsins target the duplex DNA in a bidentate manner with substantial binding ability.53 Among a large number of groove binding ligands, tallimustine (Fig. 3C) and brostallicin (Fig. 3E) have reached phase II of the clinical trials. Tallimustine, a distamycin derivative where the terminal formamide group is substituted by benzoyl nitrogen mustard,56 attributes to sequence-specific alkylation (N3-adenine) after attaching to the minor groove unlike the classical alkylating agents which form a lesion at N7-guanine of the major groove.57 It is also reported to impede the transcription of a promoter containing the TATA box (having the core DNA sequence 5′-TATAAA-3′)58 in vitro and proved to be more cytotoxic than either of the individual components, distamycin and nitrogen mustard.59 Preclinical reports claimed tallimustine to be moderately active against human leukemia with 7

notable activity in the case of intraperitoneal (i.p) grafting.56 However, only a minimal activity was observed with the intravenous (i.v) grafts.56 Although phase I trial of the drug showed some anti-tumor activity accompanied by mild toxic effects of neutropenia,60 it failed in phase II trials with patients previously treated for small cell lung cancer (SCLC).61

Figure 3. Molecular structures of some minor groove binders of DNA.

Brostallicin (PNU-166196, Fig. 3E) is another distamycin A derivative with four adjoining pyrrole carbamoyl skeleton flanked by α-bromoacrylamide and guanidine moieties at the ends. While severe symptoms of myelotoxicity served as a major hurdle for tallimustine,60 brostallicin exhibited substantial reduction in myelotoxicity. In fact, the latter accounted for ~80-fold enhanced therapeutic efficacy over the former.62 An earlier compound of the distamycin family, PNU-151807 (Fig. 3D), a α-bromoacrylamido distamycin homolog with four pyrrole rings, showed high levels of cytotoxicity and in vivo efficiency. However, it was incapable of alkylating AT-rich sequences in the minor groove.63 The α-halo-acrylic moiety is indispensable for cytotoxicity as native acrylamido derivative lost its efficiency completely. The α-bromoacrylamido derivative also proved to be a potent candidate while αfluoroacrylamido counterpart was found to be inactive.64 In the quest of better activity, substitution of the amidine group with multiple functionalities and replacement of pyrrole 8

with imidazole and pyrazole rings led to the establishment of a collection of oligopeptidic αbromoacrylic derivatives. Among these brostallicin (PNU-166196) emerged as the most successful drug.65 The drug is pro-apoptotic and acts efficiently in tumor cells lacking DNA mismatch repair mechanism, unlike other minor groove binders and alkylating agents.66,67 While many alkylating agents failed in the presence of high levels of glutathione (GSH) in cancer cells, brostallicin exhibited higher cancer cell cytotoxicity and enhanced anti-tumor activity in such conditions.68 This makes brostallicin overcome glutathione and glutathione related enzyme mediated drug resistance. Phase I clinical studies revealed its maximum tolerated dose at 10 mg/m2 in a 21 day cycle.67 The phase II trials showed acceptable levels of toxicity even in the case of pre-treated patients with metastatic soft-tissue carcinoma.69 Clinical combination studies presented the use of of brostallicin and cis-platin (cis-diamine dichloro platinum (II) or CDDP, Fig. 4A) together for a phase II trial in patients afflicted with recurrent squamous cell carcinoma of the head and neck (SCCHN) and claimed to be tolerable in maximum cases with hematological toxicity.70 Presently the research is primarily focused on the usage of brostallicin in combination with other chemotherapeutic drugs to target the triple-negative breast cancer. 5. Metal complex based DNA binders Metal based complexes occupy an important position in cancer research. Many drugs have successfully passed the clinical trials and are used widely as chemotherapeutic agents. The discovery of the anti-neoplastic activity of cis-platin (Fig. 4A) is held as a landmark in cancer therapeutics71 which subsequently marked its entry in clinical trials in 1971.72 This accelerated a major investigation in the field leading to the development of nearly 3000 platinum derivatives, out of which a maximum of only 30 compounds were screened at the clinical trials.73 However, only four drugs (cis-platin, carboplatin, oxaliplatin, nedaplatin)

9

(Fig. 4A-D) are presently employed in actual cancer chemotherapy.73 All the four drugs are basically prodrugs which turn on their activity upon hydrolysis. Two amine groups are present in the cis position with chloride, cyclobutane, or glycolate moiety as leaving groups in the cases of cis-platin, carboplatin, and nedaplatin which yield cis-diamino-diaquoplatinum as the active species on hydration (Fig. 4E). On the other hand, oxaliplatin is activated by the replacement of the oxalate moiety by H2O and chloride ion, which resulted in monochloro-, dichloro-, and diaquo-diaminocyclohexane-platinum (Fig. 4F) complexes. The low intracellular chloride concentration (~4-20 mM) facilitates the replacement of the chloro ligand in cis-platin with water to form the active species.74 The aquated platinum complex selectively targets the adenine and guanine-N7 position, thereby generating intra-strand and/or inter-strand cross-links. Predominantly, cis-platin forms intra-strand cross-links by association of two adjacent guanine bases to platinum to form cis-[Pt(NH3)2{d(GpG)-N7(1), N7(2)}].75 The orientation of the diaquo complex is important. Thus only the cis intermediate is active over the trans isomer.73 Apart from interacting with DNA, cis-platin blocks various signal transduction pathways which impede DNA synthesis and repair mechanisms that prompt the cell cycle arrest followed by apoptosis.76-79

Figure 4. Molecular structures of some of the Pt (II) based DNA binders along with their active species as anticancer drugs.

10

Cis-platin is abundantly used in the treatment of a wide variety of cancers including head-and-neck, lung, bladder, ovarian, and testicular cancer.80,81 However, the benefits of cisplatin are warded off by several major side-effects which include hearing impairment in children,82 severe nephrotoxicity,83 cardiotoxicity,84 and drug resistance85 etc. In order to subdue the demerits of cis-platin, numerous modifications have been carried out. Carboplatin retained the same reactivity as that of cis-platin but the former exhibited much lower toxicity toward kidney and nervous systems.86,87 In combination with paclitaxel, carboplatin is used in the treatment of ovarian cancer and popularly marketed in the US as a better option than the combination of cis-platin with paclitaxel.85,88 Oxaliplatin showed better aqueous solubility along with improved anti-tumor activity.89 However, the reactivity of the drug alone was not superior to combination chemotherapy of cis-platin and carboplatin. Oxaliplatin-5fluorouracil combination has been implemented in the chemotherapeutic treatment of colon cancer90,91 and most importantly, oxaliplatin is credited for its “salvage therapy” to cis-platin resistant cancers.92 Nedaplatin is another second generation analog of cisplatin which revealed reduced toxic effect toward various patients afflicted with uterine cervical, esophageal, head and neck, and non-small cell lung cancer. However, due to the lack of common usage, phase III clinical trials on this compound are yet to be initiated.93 Although since decades, platinum complexes have reigned as the most widely employed anticancer drugs and chemotherapeutic agents, constant efforts are being made to reduce the toxicities with the use of other metal complexes. Ruthenium is often considered to be an eligible alternative with some properties quite similar to that of the Pt (II) complexes.94 Variable oxidation states, high kinetic stability, and the characteristic of ruthenium to mimic iron in interacting with biomacromolecules comprise some of the salient features which render it a potential therapeutic agent.95 Ruthenium effectively targets tumor cells over normal ones by utilizing its flexible redox property and affinity toward transferrin. 11

“Activation-by reduction” is the phenomenon which drives the relatively inactive Ru (III) to the neoplasmic site where hypoxia and acidic environment induce its reduction to Ru (II).96 Thus the Ru (III) complex acts as a prodrug which acquires its activity on reaching the tumor cells and the reduced Ru (II) due to its high affinity toward the unprotonated imines binds specifically to the purine N7 of the DNA.97 Cancer cells possess high demand for iron which is required for their rapid proliferation. This is quenched by the over-expression of the transferrin receptors on the cell surface.98 Ru (III) complexes bind to transferrin, identify the transferrin receptor, and finally penetrate into the malignant cells via endocytosis (Fig. 5).99

Figure 5. A schematic representation of the proposed mechanism of interaction of ruthenium complexes to transferrin, showing its cellular uptake, and delivery of the active species inside the cell. Adapted with the permission from reference 101, copyright 2013, American Chemical Society.

In spite of all the in vitro data, mainly two compounds, NAMI-A (New Anti-tumor Metastasis Inhibitor-A, Fig. 6A) and KP1019 (indazolium trans-[tetrachlorobis(1Hindazole)ruthenate (III)], Fig. 6B), have reached phase I clinical trials.100 While KP1019 showed activity against primary tumors with lower toxicity, NAMI-A exhibited activity toward metastatic or secondary cancer cells promting phase II trials.100

12

Figure 6. Molecular structures of some DNA cross-linking agents.

6. DNA cleaving agents Scission of the DNA strands (responsible for cellular oncogenesis) is another significant mode to achieve modification of DNA and inhibit cancer. Although the hydrolysis of the phosphodiester bonds, which form the skeletal framework of the DNA helix, is thermodynamically favourable (∆G°′ = -5.3 kcal mol-1),102 the reaction is extremely slow at physiological conditions.103 So at physiological pH, the hydrolysis is non-spontaneous but the presence of phosphodiesterases and metal ions make the reaction feasible in cells.104-107 While the hydrolytic cleavage of the DNA deals with the cleavage of the phosphodiester bonds (Fig. 7A), oxidative DNA cleavage occurs principally by the oxidation of the deoxyribose sugar or the nucleotide bases. The latter activity is induced by various active species which include singlet oxygen, light, ionizing radiations, hydroxyl radicals, or other unstable radicals.108-110 In the case of deoxyribonucleotide oxidation, the most easily oxidizable nucleobase is guanine due to its lowest oxidation potential.111 Oxidation of nucleobases in DNA could also be achieved by the metal-mediated oxo-transfer by various oxygen donors or reactive oxygen species (ROS) (Fig. 7C).112 On the other hand, oxidation of the sugar unit in DNA does not follow the unfavorable electron abstraction from the deoxyribose sugar. Rather it is accomplished by the more facile hydrogen abstraction resulting in the damage of the moiety (Fig. 7B).112

13

Figure 7. A schematic representation of the various modes of DNA strand cleavage. The red oval in (A) marks the cleavage of the phosphodiester bond and the red circles in (B) denote the abstraction of C4′ and C1′ hydrogens. The nucleotide bases are represented as B1, B2, B3, and B4 in (A) and (B).

A number of synthetic transition metal complexes, possessing variable redox potentials and DNA affinities are capable of inducing DNA cleavage.111 Various Ru (II) polypyridyl DNA photocleavers,113 singlet oxygen sensitization by ruthenium complexes like [Ru(bpy)3]2+, [Ru(dppz)] (bpy = 2,2′-bipyridine, dppz = dipyrido[3,2-α : 2′,3′-c]phenazine) etc. are instrumental in achieving the oxidation of guanine causing degradation of the duplex DNA.114 Several ruthenium compounds have also been utilized via photodynamic therapy (PDT) for the degradation of cancer cells.115 Rhodium is another transition metal on which an extensive work has been performed. Site-specific cleavage chemistry occurs by C3′ hydrogen abstraction from the sugar unit as in the case of phenanthrene quinine diimine (phi) of rhodium.116 While dirhodium carboxylate compounds exhibit positive activity toward many cancer cell lines, compounds such as [Rh2(O2CCH3)2(N-N)2(H2O)2]2+ (N-N = bpy or phen)

14

show anticancer activities against oral carcinoma KB cells.117,118 Dirhodium complexes containing intercalating dppz moiety also behave as promising photosensitizers toward the photodynamic therapy, where they cause DNA scission on activation by visible light even in the absence of oxygen. Such compounds thus provide ample scope in the degradation of hypoxic cancer tissues.119-121 Gallium nitrate is a predominantly used Ga (III) drug. It is extensively checked for its anti-tumor properties against various cancers as revealed from phase II clinical trials and is often employed against bladder cancer and non-Hodgkin’s lymphoma.122,123 US FDA authorized the drug to be used for the treatment of cancer associated hypercalcemia.124 Although it showed efficacy against patients resistant to classical chemotherapeutic drugs, its use is restricted to intravenous treatment only, as its oral bioavailability is considerably poor.125 Similar to gallium nitrate, gallium chloride also bears anti-neoplastic properties. However, clinical trials reported its partial success against ovarian cancer but not against lung cancer.126,127 Oral administration of the drug demonstrated poor bioavailability limiting its efficiency in anti-tumor activity.127 Consequently gallium maltolate [tris(3-hydroxy-2methyl-4H-pyran-4-onato)gallium or GaM, Fig. 8A], an orally active drug, was identified to possess therapeutic activities against various cancers due to its characteristic property of mimicking ferric ion.125,128 Since Fe (III) is indispensable for the synthesis of DNA, GaM impedes DNA synthesis and cell division.128 Interestingly, GaM possesses an incredible capacity to treat metastasis as it associates with the transferrin of the serum to be carried away to all possible cells.128 Among other gallium compounds, KP46 [tris(8quinolonato)gallium (III), Fig. 8B] and G4544 (an oral tablet formulation of gallium nitrate) are in the clinical trials and have considerable scope for future research.125 Compounds containing titanium (IV) offer promise as potential anticancer drugs and serve as a replacement for the classical platinum chemotherapy. Titanocene dichloride 15

[cyclopentadienyl-dichloro titanium (IV), Fig. 8C] and budotitane [cis-diethoxybis(1phenylbutane-1, 3-dionato)titanium (IV), Fig. 8D] served as the two most potent candidates which were extensively studied and entered phase I clinical trials. Titanocene dichloride works by causing single-strand DNA scission with lower DNA cross-linking and produces cytostatic effects by restricting the cell cycle.129 It showed remarkable inhibition against cisplatin- and doxorubicin-resistant human ovarian carcinoma cell lines.130 Although titanocene dichloride has cleared phase I trials with a maximum tolerated dose of 315 mg/m2 accompanied by nephrotoxicity, it failed drastically at phase II clinical trials when treated with patients suffering from the metastatic breast cancer.131,132 Budotitane, on the other hand, showed some anti-neoplastic activity in vitro but could not produce encouraging results after phase I clinical trials.133 The main reason behind the failure of the above two compounds in the clinical trials is possibly due to their rapid hydrolysis at physiological conditions.134,135 Further

studies

claimed

considerable

activity

of

bis-[(p-

methoxybenzyl)cyclopentadienyl]titanium dichloride or titanocene Y (Fig. 8E) against renal and human breast cancer cells.136,137 Recently, a new class of titanium salan complexes (Fig. 8F) were identified which endured aqueous conditions and showed considerable toxicity toward colon, ovarian, and cervical cancer, thereby paving a new path for the development of metallotherapeutics in the near future.138,139

16

Figure 8. Molecular structures of some DNA cleaving agents.

Extensive research has been carried out with several other metal ions which serve as potential cleavage agents. Some of these may be useful toward the development of better cancer therapeutics. Various iron complexes are regarded as well-known cleavers of DNA. Among these the most important is the natural chemotherapeutic drug, iron-bleomycin, which induces DNA oxidative cleavage by targeting the sugar unit.140 Isolated from streptomyces verticillus, bleomycin (Fig. 9A) is a natural antibiotic which exhibits anti-tumor activity against all types of lymphoma, squamous cell, and testicular carcinoma.141 DNA strandscission was observed by N2 release from tris[3-hydroxy-1, 2, 3-benzotriazine-4(3H)one]iron (III) (Fig. 9B) complex upon ligand-to-metal charge-transfer (LMCT) excitation in the visible wavelength range.142 Various Fe (II) chelators have also showed both in vitro and in vivo anti-neoplastic properties.143-148 Recently, tris(diimine)iron (II) complexes rendered DNA cleavage which exhibited promising anticancer activities through apoptosis and necrosis mechanism.149 Like Co (III)-bleomycin, which induces DNA strand-scission in presence of light, various Co-complexes like tris(phenanthroline)Co (III) (Fig. 9C) and 17

tris(diphenylphenanthroline)Co (III)-complexes (Fig. 9D) show photoactivated doublestranded DNA cleavage.150,151 Water-soluble cobalt-bis-picolylamine complexes (Fig. 9E) also cause DNA strand-scission on slight exposure to visible light.152,153 Metal salens comprise another class of compounds which are highly effective as DNA cleaving agents.154156

Mn (III) salen and salphen complexes facilitate programmed cell death via mitochondrial

pathway and exhibited anti-neoplastic behavior toward breast cancer and colon cancer cells.157,158

Figure 9. Molecular structures of various DNA cleaving agents.

Besides directly targeting DNA, efforts have been also made to probe the enzymes which maintain the vital biological processes that efficiently alter the topology of chromosomal DNA. These enzymes namely the DNA topoisomerases are responsible for any

18

topological irregularity originating during transcription, replication, and recombination etc.159 Consequently, these have been widely investigated as prime anti-neoplastic targets.159 7. Topoisomerase inhibitors Cells are equipped with highly sophisticated enzymes that coordinate the supercoiling of DNA by the winding and unwinding of the DNA double-helical strands.160 The topology of a closed circular DNA is identified by the number of times one duplex strand passes over the other which is termed as the linking number.27 Supercoiling is defined by the linking number (L), which is the sum of twist (Tw, local pitch of the helix) and writhe (Wr, geometric in space).161 When Wr is zero, Tw is the number of base-pairs (N) divided by the pitch (N/10.5) and the molecule is in a completely relaxed form (L = L°). Various factors like ionic strength, temperature and certain DNA intercalating drugs can modify the extent of twist and writhe.27,162 The topoisomerase enzymes are important in the DNA replication, transcription, and chromatin remodeling.160 During a DNA strand-scission, a DNA phosphate ester is attacked by a tyrosyl oxygen of the enzyme which disrupts a phosphodiester bond and forms a new phosphotyrosine linkage (Fig. 10).163 Subsequently, a second trans-esterification reaction occurs in which the oxygen of the previously formed DNA hydroxyl group attacks the phosphorus center of the phosphotyrosine bond, thereby reconciling the DNA strand.163 Topoisomerases can be broadly classified into at least two kinds, type I and II which, in turn, can be further subdivided into type IA, IB, IIA, and IIB etc.160 Largely type I topoisomerases perform scission of one strand of the DNA while type II accomplish the double-strand lesions.160 Topoisomerase I (Topo I) is crucial for the development of the cell and also plays a pivotal role in the cell division.164,165 Type IA topoisomerase cuts the DNA by associating the ends of the DNA to the enzyme via the formation of a 5′-phosphodiester bond to the tyrosine.160 It also triggers the relaxation of negative supercoils along with the winding, 19

unwinding, and interlinking of the single-stranded ends of the DNA.160 However, type IB enzymes relax both the positive and negative supercoils, and the attachment of the tyrosine site occurs to the 3′-phosphate end. On the other hand, type II topoisomerases (Topo II) cleave the two strands and the dimeric enzyme subunit gets connected to the 5′-end of the DNA via the phosphotyrosine linkage. A gated or G-segment is created by the two terminals of the cleaved DNA. Subsequent passage of a distinct duplex region from the same or different molecule called transported or T-segment occurs through this open gate. In addition, Topo II necessitates the presence and hydrolysis of ATP for its proper functioning.

Figure 10. Formation of transient phosphotyrosine linkage by topoisomerase I during DNA strand cleavage. “B” denotes a solvent water molecule with potential basic character.

Various topoisomerase inhibiting drugs actually interfere with the cleavage and ligation mechanism of the topoisomerases by stabilizing the DNA-topoisomerase “cleavage complex”.159 This results in the conversion of the enzyme into a cellular poison.159 A lot of work has been carried out by targeting both Topo I and II with small molecules. The Topo I inhibitors like camptothecin (Fig. 11A) and its derivatives serve as the major anti-tumor agents. Camptothecin, a natural pentacycylic alkaloid isolated from camptotheca acuminate, possess excellent anti-neoplastic properties and is used in the treatment of various types of cancers.166 The first total synthesis of dl-camptothecin was performed by Stork and Schultz in 1971 and the first asymmetric synthesis of 20(S)-camptothecin was successfully 20

accomplished by Tagawa et al. in 1989.167,168 After this, numerous synthetic strategies have been utilized toward its total synthesis.169 However, the compound is water-insoluble and has been modified to a water-soluble sodium salt by lactone hydrolysis which has entered phase II clinical trials. Unfortunately, this strategy also proved to be futile for patients suffering from melanoma and gastrointestinal malignancies.170 Structure-activity relationship has deduced the following essential criteria for the design of camptothecin analogs which include, i) the stability in blood; ii) a reduced cytotoxicity; iii) an accessibility toward enzymatic cleavage, and most importantly, iv) an enhanced water solubility.171 Irinotecan (CPT-11 or 7-ethyl-10-(4-[-piperidino)methyl-10-hydroxycamptothecin, Fig. 11B) is the first water-soluble camptothecin analog and is a prodrug which transforms into a highly active SN-38 (Fig. 11C) by hydrolysis reaction in plasma.170 Irinotecan exhibited dramatic efficacy in a wide array of cancers like colorectal, non-small-cell lung, and cervical cancers as revealed from phase II clinical trials.170 Topotecan [9-(dimethylamino)methyl-10hydroxycamptothecin, Fig. 11D], on the other hand, is soluble in water due to the presence of the charged amino group at the 9-substituent. Both topotecan and irinotecan are authorized by the FDA for the treatment of ovarian, small cell and lung cancer,172 and colorectal cancer, respectively.173

Figure 11. Molecular structures of some topoisomerase I inhibitors: 20(S)-Camptothecin and its modified derivatives (irinotecan and topotecan). SN-38, an active metabolite of irinotecan, is also shown.

21

Camptothecin renders no reaction with Topo II but shows marked inhibition of Topo I.166 The mechanism of action of camptothecin operates via the stabilization of the DNAtopoisomerase enzyme complex which, in turn, inhibits the re-ligation step of the topoisomerase.166 In fact, camptothecin restricts the catalytic behavior of the Topo I and impedes the reunion step of the enzyme, thereby resulting in the accumulation of the cleavage complex. Moreover, the drug suppresses the DNA synthesis in cells and causes scission of the DNA strands. Camptothecin promotes severe S-phase-specific cytotoxicity which may be due to an interaction between the replication fork and the drug stabilized cleavage complex.174 This leads to the detention and breakage of the replication fork which finally kills the cancer cells.174 Several minor groove binders (MGBs) of DNA are also reported which interact with the Topo I enzymes and produce specific single-strand cleavage of the duplex DNA.175 The enzyme remains associated with the 3′-single-strand fragment of the DNA by a covalent linkage.175 Unlike camptothecin, the MGBs induce DNA scission specifically at AT-rich sequences leading to the formation of a ternary complex comprising Topo I, drug, and the DNA. Among the MGBs, Hoechst 33342 and 33258 show better activity toward Topo I compared to distamycin A, berenil, or netropsin.175 Toxic chemotherapy, drug resistance, and co-infection of leishmaniasis along with HIV resulted in research on parasite (L. donovani) topoisomerase. Certain small molecules have been successfully developed which proved to be quite active and specific toward the Topo I of the parasite.176,177

22

Figure 12. Molecular structures of some topoisomerase II inhibitors.

Various Topo II inhibiting molecules have also been developed. Etoposide, teniposide, doxorubicin, idarubicin, epirubicin, and mitoxantrone (Fig. 12) are some of the drugs which have been approved by the US FDA.178 Topo II enzymes bear much importance in the condensation and segregation of chromosomes and DNA replication.179 Epipodophyllotoxins comprise a class of naturally occurring compounds which encompass the two major drugs, etoposide and teniposide (Fig. 12A, B). Etoposide restricts the rejoining step of the Topo II enzyme; however, the drug induced DNA degradation is also dependent on caspase and mitochondrial apoptotic route.180 While etoposide is implemented for the treatment of malignancies including lung, germ-cell, soft-tissue carcinoma etc., teniposide is used effectively for the childhood lymphoma and CNS malignancies.181 However, toxicities like bone marrow suppression, nausea, and vomiting etc. are accompanied with the incorporation of these drugs. Another class of compounds that has proved its potency against multiple malignancies are the anthracyclines (Fig. 12C-E). The mode of action of the anthracyclines involves restriction of the Topo II enzyme via the generation of single- and double-strand DNA scissions.182 The anthracycline family of Topo II inhibitors comprise 23

doxorubicin, daunorubicin, idarubicin, and epirubicin among others. Although myelosuppression is a dose limiting toxicity, the most severe side-effect of the anthracyclines is cardiotoxicity. Accordingly various chemical modifications have been performed to reduce the latter.183 The most widely used drug in the family is doxorubicin and is used against solid tumors.184 While epirubicin has similar anticancer properties as doxorubicin, idarubicin has considerable oral bioavailability and is mainly used in acute myeloid leukemia (AML) therapy.185 The cardiotoxicity of the anthracycline family of drugs could be overcome by mitoxantrone (Fig. 12F) due to the absence of quinone type moiety which is capable of forming free radicals.186 Chemotherapy used for the treatment of breast cancer often afflicts the patients with a secondary cancer called acute promyelocytic leukemia (APL) and mitoxantrone plays a significant role in APL related therapy as well.187 8. Novel DNA-targets for therapeutic intervention Recently, emergence of different targets for cancer like receptor tyrosine kinases (RTK) and other cell surface receptors have occurred.188 Stabilization of G-quadruplex DNA by small molecules is considered a novel approach with significant potential in the field of anticancer drug design.189 However, this is still not clinically applicable. Although the discovery of the G-quadruplex DNA dates back to 1962, it was established only in the late 1980s that the single-stranded guanine rich DNA sequences associate to yield parallel or anti-parallel four-stranded structures called the G-quadruplex DNA.190-192 In these structures, the guanine bases are held together by Hoogsteen type hydrogen bonds.191 Monovalent metal ions (K+, Na+, Cs+, NH4+ but not Li+) and divalent ions like Sr2+ can template the formation of the G-tetrad via co-ordination with the central oxygens.193-195 Polymorphism is an essential characteristic of the G-quadruplex DNA by which it can adopt various conformations. The strands intertwine to form parallel,196 antiparallel,18 or mixed hybrid 197 conformations (Fig. 13A) which result in different grooves and 24

loops. Besides the formation of monomeric intermolecular and intramolecular conformations, dimeric quadruplexes are also reported.198-202 Telomeric G-quadruplex DNA can either form by the dimerization of hairpin loops or by directly folding into intramolecular structures.192,203 Although telomeres play a vital role in preserving the termini of the eukaryotic chromosomes, its degradation over repeated cell divisions lead to cellular senescence and death.204-208 Cancer cells induce a pronounced overexpression of the telomerase enzyme which inhibits the cell aging process and leads to the immortalization of cells.209,210 So inhibiting the telomerase enzyme or stabilizing the formation of G-quadruplex DNA may be strategically used to restrain the growth of cancer.211

A

Parallel (PDB: 1KF1)

Antiparallel (PDB: 143D)

Hybrid (PDB: 2HY9)

B

c-myc (PDB: 1XAV)

c-kit (PDB: 2O3M)

bcl2 (PDB: 2F8U)

Figure 13. Some of the G-quadruplex DNA structures observed at the (A) telomere and (B) promoter regions. The backbone is viewed as tubes (red) and the bases as ribbons (blue) as shown in VMD software.

The G-quadruplex DNA organizations have high abundance in several protooncogenes and are less available in tumor suppressor genes.212,213 Various promoter genes are reported to participate in the G-quadruplex DNA formation which include c-myc,20,21,214 ckit,22,23,215 KRAS,25,216,217 bcl2,24,218 VEGF (vascular endothelial growth factor),219 25

HIF1 α (hypoxia-inducible factor 1α),220 PDGFA (platelet-derived growth factor αpolypeptide),221 PDGFRβ (platelet-derived growth factor receptor β polypeptide),222 hTERT (human telomerase reverse transcriptase),223 RB1 (retinoblastoma protein 1)224,225 etc (Fig. 13B). Although it is reasonable for a single-stranded telomeric overhang to fold into a Gquadruplex, it is substantially difficult to comprehend the formation of these secondary structures in the genome which is predominantly duplex in nature.189 Cellular mechanisms like transcription, replication, and recombination involve unwinding of the duplex DNA and provide ample scope for the G-quadruplex structure formation (Fig. 14).226 The driving forces for the duplex to the G-quadruplex DNA transition are the transcription induced negative superhelicity, molecular crowding conditions, and stabilization of the resultant G-quadruplex by proteins and ligands.189,227

Figure 14. Unwinding of the duplex DNA in the genomic region with subsequent formation of G-quadruplex DNA structures. The backbone is viewed as tubes (blue) and the bases as ribbons (green) as generated in VMD software.

Several small molecules are synthesized to stabilize the G-quadruplex DNA structures. Porphyrin has always been an appealing molecule in the cancer treatment for its use in the photodynamic therapy (PDT).

228

G-quadruplex DNA stabilizing ligands bind

26

either via π-π stacking with planar tetrads (end-stacking), or by interacting with the grooves, or non-helical loops.26,229 Porphyrins possess a large planar surface due to its aromatic core and four easily substitutable meso positions that make them putative candidates for targeting the G-quadruplex DNA.230 Among the earlier compounds of porphyrin, H2-TMPyP4 [mesotetrakis-(4-N-methylpyridiniumyl)porphyrin] exerts stacking interaction with the G-tetrads and also triggers telomerase inhibition.231 However, such compounds fail drastically in identifying the G-quadruplex DNA over the duplex DNA.232 Chemical modification by insertion of a metal into non-metalated H2-TMPyP4233-235 resulted in minimal improvement in the specificity of the ligands toward the G-quadruplex DNA structures. However, introduction of cationic side-chains at the meso positions resulted in better selectivity toward G-quadruplex DNA.236-238 Interestingly, a manganese (III) derivative of porphyrin (Fig. 15A) showed specific binding to the G-quadruplex DNA over the duplex DNA with a preference of nearly 10000 times.237 Porphyrazines and phthalocyanines, close relatives of porphyrin structurally, have been shown to stabilize the G-quadruplex DNA quite efficiently.239-242 A new class of guanidino phthalocyanines (GPcs, Fig. 15B) showed cellular uptake along with the G-quadruplex-mediated promoter inhibition.241 Telomestatin (Fig. 15C) is a neutral polycyclic compound with some structural similarity to the G-tetrad.243 Various modifications were performed on this molecular structure to improve the solubility and/or activity by changing the side-chain or by dimerization of the macrocyclic scaffolds.244-248 Acridines render a broad class of G-quadruplex DNA stabilizing compounds with an inherent planar surface.249-253 BRACO-19 (Fig. 15D), a member of 3, 6, 9-trisubstituted acridine family, proved to be highly efficient in restricting telomerase activity both in vitro and in vivo.251 Many ligands based on anthraquinones,254-257 phenanthrolines258-262 quinacridines,263

carbazoles,264-266

bis-indole

carboxamides,267,268

triazoles,269

and

benzimidazoles270-277 also show considerable G-quadruplex DNA affinity, stabilizing the 27

corresponding secondary structures. Several tetra-substituted naphthalene diimides,278,279 perlyene tetracarboxylic diimide derivative (PIPER),280 and 2, 7-disubstituted fluorenones281 also play a pivotal role in the reduction of telomerase activity as evidenced from their interactions with the G-quadruplex DNA. Various modifications were performed on berberine (Fig. 15E) and the 9-substituted derivatives exhibited better performance in restricting telomerase activity compared to berberine itself.282-285 Quindoline derivative, SYUIQ-5 (Fig. 15F), a G-quadruplex DNA stabilizer and telomerase inhibitor, showed considerable arrest in the cell proliferation in human leukemia and colon cancer cell lines.286,287

Figure 15. Molecular structures of some of the G-quadruplex stabilizers and/or telomerase inhibitors.

28

Quarfloxin (CX-3543, Fig. 15G), a fluoroquinolone compound, has successfully entered phase II clinical trials against carcinoid/neuroendocrine neoplasms arising from neural crest cells.288 CX-3543 is a commercially available molecule and a G-quadruplex DNA binder that interrupts the association of the nucleolin protein to the human ribosomal DNA (rDNA) G-quadruplex.288 The putative drug has been well tolerated in humans with solid tumors in phase I clinical trials and such promising results led to further evaluation in phase II trials. Despite such advances, it could not progress further due to its limited bioavailability.189 3′-Azido-2′, 3′-dideoxythymidine [zidovudine or azidothymidine (AZT), Fig. 15H] has proved to be a potent pharmacophore against a number of virus-associated cancers like AIDS-related Kaposi sarcoma, Epstein-Barr-associated lymphoma, and adult Tcell leukemia etc.289 AZT bears a special affinity toward the telomeres and arrests the activity of telomerase by inducing tumor cells to undergo senescence and apoptosis as seen by the significant telomere shortening in Tetrahymena and immortalized human cell lines.290,291 The promising drug has been evaluated in phase I and II cancer clinical trials, either alone or in combination with other chemotherapeutic drugs (methotrexate, cis-platin, 5-fluorouracil, leucovorin etc.), against a wide range of cancers like pancreatic, metastatic colorectal malignancies.292 G-quadruplex DNA is an emerging DNA target for cancer and till date, no drug has entered the cancer clinical trials solely based on the G-quadruplex DNA stabilization mechanism. However, several G-quadruplex DNA stabilizers have been successfully used in the clinical trials in combination with other drugs. Biroccio and co-workers have shown that the G-quadruplex stabilizing drug, RHPS4 (3,11-difluoro-6,8,13-trimethyl-8H-quino[4,3,2kl]acridinium methosulfate, Fig. 16A), exhibits a high synergistic action when used in combination with camptothecin.293 While combination with Topo I inhibitor (SN-38, Fig. 11C) it showed impressive results, no such synergistic effect was observed in the presence of 29

a Topo II inhibitor (doxorubicin, Fig. 12C).294 Moreover, the sequence of administering the drug combination also bears importance and camptothecin is preferably given before RHPS4.294 Mechanistically, the DNA damage caused by the G-quadruplex DNA stabilizers specifically require the presence of Topo I for its repair. Camptothecin impairs this repair process and hence the drug combination acts synergistically. In fact, the treatment with these ligands increased Topo I at the telomeric ends.294 While camptothecin analog, ST1481 (Fig. 16B), has completed phase II clinical trial with advanced ovarian cancer, a combination of ST1481/RHPS4 showed better tumor regression and increased survival in mice.294 It is also reported that RHPS4 induced the formation of “uncapped telomeres” and specifically activated poly-adenosine diphosphate (ADP) ribose polymerase I (PARP1) at the telomeres.295 A multi-component strategy of using GPI/Irinotecan/RHPS4 (GPI or GPI 15427 is a PARP inhibitor, Fig. 16C) resulted in a total inhibition of tumor growth with mice treated at a very late stage of tumor.295 The high efficacy of this triple combination suggests that PARP inhibition can be used as a strategy to modify G-quadruplex based therapy toward tumors.295

Figure 16. Molecular structures of compounds used in synergistic interaction and/or synthetic lethality.

30

Synthetic lethality is another therapeutic approach in which individual mutations in two genes fail to cause cell death whereas successive mutations of both the genes lead to apoptosis.296 It may be accomplished synthetically by combining drugs which individually act at different targets. Yet they function cooperatively leading to the inhibition of cancer.296 Telomestatin (Fig. 15C), a G-quadruplex ligand, competes with the binding of shelterin proteins at the telomeres leading to uncapping of the telomeric ends.297 On the other hand, werner syndrome helicase (WRN) can unwind the G-quadruplex DNA structures formed at the telomere.297 So a combination of WRN inhibitor, 1-(propoxymethyl)-maleimide (NSC 19630, Fig. 16D), and telomestatin was co-treated in U2OS cells which caused inhibition of ~70% cancer cell growth, thus revealing the synergistic interaction.297 Recently, it has been shown that pyridostatin (PDS, Fig. 16E), another G-quadruplex binder, acts synergistically with a DNA-dependent protein kinase (DNA-PKcs) inhibitor, 2-N-morpholino-8dibenzothiophenyl-chromen-4-one (NU7441, Fig. 16F), in causing synthetic lethality to cancer cells.296 G-quadruplex stabilizers induce double-strand breaks which are primarily repaired by homologous recombination (HR) or by non-homologous end joining (NHEJ). The PDS analog, PDSI (Fig. 16E), exhibited synergistic action with cells either impaired with HR repair mechanism or chemical inhibition of NHEJ as in the case of NU7441.296 The combination of the G-quadruplex binding ligands with other anti-proliferative agents has improved their efficiency which warrants further investigation toward their use in cancer therapeutics. 9. Conclusions Even if a number of synthesized ligands show high cytotoxicity toward cancer cells in vitro, only a handful of drugs have reached the clinical trial stages. The basis of approval of a new drug in the market is that the drug must have a satisfactory safety profile in substantial clinical trials, which is often at the expense of hundreds of millions of dollars, thousands of 31

patients, and a time span of over a decade.298 Not surprisingly, very few drugs emerge as the promising candidates for cancer chemotherapy. Precisely, the primary reasons behind the failure of cancer clinical trials may be due to biological and pharmacological inactivity of the concerned molecule. There may be strategic problems involving improper design of the study, and/or inadequate clinical safety profile.299 The ligands generally induce unpredictable side-effects and severe toxicities toward normal cells of the treated patients. Such unpredictability poses a big challenge for the drugs in clinical trials and chemotherapy.300 Several other factors like oral bioavailability, pharmacokinetics which involve absorption, distribution, metabolism, and excretion (ADME), and proper extrapolation of its activity from animals to humans must also be taken into consideration.301 Moreover, appropriate design of the clinical trial plays a significant role toward the success of the experiment which requires the choice of right endpoint (survival, response rate, quality of life assessment etc.) and the right pool of patients.298 Thus development of a drug to arrest cancer must start with a careful journey from its molecular design and synthesis. However, understanding of pharmacology and pharmacokinetics in human organs are extremely important. Acknowledgments AA thanks CSIR for a Senior Research Fellowship. SB is thankful to the Department of Science and Technology, Govt. of India, for the J. C. Bose Fellowship.

32

References and notes 1.

Helleday, T.; Petermann, E.; Lundin, C.; Hodgson, B.; Sharma, R. A. Nat. Rev. Cancer 2008, 8, 193.

2.

Bottaro, D. P.; Rubin, J. S.; Faletto, D. L.; Chan, A. M.; Kmiecik, T. E.; Vande Woude, G. F.; Aaronson, S. A. Science 1991, 251, 802.

3.

Peters, S.; Adjei, A. A. Nat. Rev. Clin. Oncol. 2012, 9, 314.

4.

Jordan, M. A.; Toso, R. J.; Thrower, D.; Wilson, L. Proc. Natl. Acad. Sci. U.S.A 1993, 90, 9552.

5.

Yvon, A.-M. C.; Wadsworth, P.; Jordan, M. A. Mol. Biol. Cell 1999, 10, 947.

6.

Jordan, M. A.; Thrower, D.; Wilson, L. Cancer Res. 1991, 51, 2212.

7.

Meyer, R. E.; Kim, S.; Obeso, D.; Straight, P. D.; Winey, M.; Dawson, D. S. Science 2013, 339, 1071.

8.

Vogelstein, B.; Papadopoulos, N.; Velculescu, V. E.; Zhou, S.; Diaz, L. A. J.; Kinzler, K. W. Science 2013, 339, 1546.

9.

De Bont, R.; van Larebeke, N. Mutagenesis 2004, 19, 169.

10.

Saha, A.; Kaul, R.; Murakami, M.; Robertson, E. S. Cancer Biol. Ther. 2010, 10, 961.

11.

Dickerson, R. E.; Drew, H. R.; Conner, B. N.; Wing, R. M.; Fratini, A. V.; Kopka, M. L. Science 1982, 216, 475.

12.

Schleif, R. Science 1988, 241, 1182.

13.

Wemmer, D. E.; Dervan, P. B. Curr. Opin. Struct. Biol. 1997, 7, 355.

14.

Jain, A. K.; Bhattacharya, S. Bioconjug. Chem. 2010, 21, 1389.

15.

Gehring, K.; Leroy, J. L.; Guéron, M. Nature 1993, 363, 561.

16.

Kang, C.; Zhang, X.; Ratliff, R.; Moyzis, R.; Rich, A. Nature 1992, 356, 126.

17.

Smith, F. W.; Feigon, J. Nature 1992, 356, 164.

18.

Wang, Y.; Patel, D. J. Structure 1993, 1, 263. 33

19.

Tang, J.; Kan, Z. Y.; Yao, Y.; Wang, Q.; Hao, Y. H.; Tan, Z. Nucleic Acids Res. 2008, 36, 1200.

20.

Simonsson, T.; Pecinka, P.; Kubista, M. Nucleic Acids Res. 1998, 26, 1167.

21.

Siddiqui-Jain, A.; Grand, C. L.; Bearss, D. J.; Hurley, L. H. Proc. Natl. Acad. Sci. U.S.A 2002, 99, 11593.

22.

Rankin, S.; Reszka, A. P.; Huppert, J.; Zloh, M.; Parkinson, G. N.; Todd, A. K.; Ladame, S.; Balasubramanian, S.; Neidle, S. J. Am. Chem. Soc. 2005, 127, 10584.

23.

Phan, A. T.; Kuryavyi, V.; Burge, S.; Neidle, S.; Patel, D. J. J. Am. Chem. Soc. 2007, 129, 4386.

24.

Dai, J.; Chen, D.; Jones, R. A.; Hurley, L. H.; Yang, D. Nucleic Acids Res. 2006, 34, 5133.

25.

Cogoi, S.; Xodo, L. E. Nucleic Acids Res. 2006, 34, 2536.

26.

Jain, A. K.; Bhattacharya, S. Bioconjug. Chem. 2011, 22, 2355.

27.

Paul, A.; Bhattacharya, S. Curr. Sci. 2012, 102, 212.

28.

Neidle, S. Nat. Prod. Rep. 2001, 18, 291.

29.

Thuong, N. T.; Hélène, C. Angew. Chem. Int. Ed. Engl. 1993, 32, 666.

30.

Nielsen, P. E. Curr. Opin. Struct. Biol. 1999, 9, 353.

31.

Kumar, V. A.; Ganesh, K. N. Acc. Chem. Res. 2005, 38, 404.

32.

Thrum, H. Naturwissenschaften 1959, 46, 87.

33.

DiMarco, A.; Gaetani, M.; Orezzi, P.; Scotti, T.; Arcamone, F. Cancer Chemother. Rep. 1962, 18, 15.

34.

Kopka, M. L.; Yoon, C.; Goodsell, D.; Pjura, P.; Dickerson, R. E. Proc. Natl. Acad. Sci. U.S.A 1985, 82, 1376.

35.

Coll, M.; Aymami, J.; van der Marel, G. A.; van Boom, J. H.; Rich, A.; Wang, A. H. Biochemistry 1989, 28, 310.

34

36.

White, S.; Szewczyk, J. W.; Turner, J. M.; Baird, E. E.; Dervan, P. B. Nature 1998, 391, 468.

37.

Wade, W. S.; Mrksich, M.; Dervan, P. B. J. Am. Chem. Soc. 1992, 114, 8783.

38.

Mrksich, M.; Wade, W. S.; Dwyer, T. J.; Geierstanger, B. H.; Wemmer, D. E.; Dervan, P. B. Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 7586.

39.

Dervan, P. B. Bioorg. Med. Chem. 2001, 9, 2215.

40.

Yoon, C.; Privé, G. G.; Goodsell, D. S.; Dickerson, R. E. Proc. Natl. Acad. Sci. U.S.A. 1988, 85, 6332.

41.

Bhattacharya, S.; Thomas, M. Biochem. Biophys. Res. Commun. 2000, 267, 139.

42.

Bhattacharya, S.; Thomas, M. Tetrahedron Lett. 2001, 42, 3499.

43.

Bhattacharya, S.; Thomas, M. Tetrahedron Lett. 2000, 41, 5571.

44.

Bhattacharya, S.; Thomas, M. Tetrahedron Lett. 2001, 42, 5525.

45.

Dervan, P. B. Science 1986, 232, 464.

46.

Trauger, J. W.; Baird, E. E.; Dervan, P. B. Nature 1996, 382, 559.

47.

Gottesfeld, J. M.; Neely, L.; Trauger, J. W.; Baird, E. E.; Dervan, P. B. Nature 1997, 387, 202.

48.

Mrksich, M.; Dervan, P. B. J. Am. Chem. Soc. 1993, 115, 9892.

49.

Mrksich, M.; Parks, M. E.; Dervan, P. B. J. Am. Chem. Soc. 1994, 116, 7983.

50.

de Clairac, R. P. L.; Seel, C. J.; Geierstanger, B. H.; Mrksich, M.; Baird, E. E.; Dervan, P. B.; Wemmer, D. E. J. Am. Chem. Soc. 1999, 121, 2956.

51.

Weyermann, P.; Dervan, P. B. J. Am. Chem. Soc. 2002, 124, 6872.

52.

Urbach, A. R.; Love, J. J.; Ross, S. A.; Dervan, P. B. J. Mol. Biol. 2002, 320, 55.

53.

Bhattacharya, S.; Thomas, M. Chem. Commun. 2001, 1464.

54.

Ghosh, S.; Usharani, D.; Paul, A.; De, S.; Jemmis, E. D.; Bhattacharya, S. Bioconjug. Chem. 2008, 19, 2332.

35

55.

Ghosh, S.; Usharani, D.; De, S.; Jemmis, E. D.; Bhattacharya, S. Chem. Asian J. 2008, 3, 1949.

56.

Pezzoni, G.; Grandi, M.; Biasoli, G.; Capolongo, L.; Ballinari, D.; Giuliani, F. C.; Barbieri, B.; Pastori, A.; Pesenti, E.; Mongelli, N. et al. Br. J. Cancer 1991, 64, 1047.

57.

Broggini, M.; Coley, H. M.; Mongelli, N.; Pesenti, E.; Wyatt, M. D.; Hartley, J. A.; D'Incalci, M. Nucleic Acids Res. 1995, 23, 81.

58.

Bellorini, M.; Moncollin, V.; D'Incalci, M.; Mongelli, N.; Mantovani, R. Nucleic Acids Res. 1995, 23, 1657.

59.

Arcamone, F. M.; Animati, F.; Barbieri, B.; Configliacchi, E.; D'Alessio, R.; Geroni, C.; Giuliani, F. C.; Lazzari, E.; Menozzi, M.; Mongelli, N. J. Med. Chem. 1989, 32, 774.

60.

Sessa, C.; Pagani, O.; Zurlo, M. G.; de Jong, J.; Hofmann, C.; Lassus, M.; Marrari, P.; Strolin Benedetti, M.; Cavalli, F. Ann. Oncol. 1994, 5, 901.

61.

Viallet, J.; Stewart, D.; Shepherd, F.; Ayoub, J.; Cormier, Y.; DiPietro, N.; Steward, W. Lung Cancer. 1996, 15, 367.

62.

Geroni, C.; Pennella, G.; Capolongo, L.; Moneta, D.; Rossi, R.; Farao, M.; Marchini, S.; Cozzi, P. Proc. Am. Assoc. Cancer Res. 2000, 41, 265.

63.

Marchini, S.; Cirò, M.; Gallinari, F.; Geroni, C.; Cozzi, P.; D'Incalci, M.; Broggini, M. Br. J. Cancer 1999, 80, 991..

64.

Cozzi, P.; Beria, I.; Caldarelli, M.; Capolongo, L.; Geroni, C.; Mongelli, N. Bioorg. Med. Chem. Lett. 2000, 10, 1269.

65.

Cozzi, P. Farmaco 2003, 58, 213.

66.

Colella, G.; Marchini, S.; D'Incalci, M.; Brown, R.; Broggini, M. Br. J. Cancer 1999, 80, 338.

36

67.

Ten Tije, A. J.; Verweij, J.; Sparreboom, A.; Van Der Gaast, A.; Fowst, C.; Fiorentini, F.; Tursi, J.; Antonellini, A.; Mantel, M.; Hartman, C. M.; Stoter, G.; Planting, A. S. T.; De Jonge, M. J. A. Clin. Cancer Res. 2003, 9, 2957.

68.

Geroni, C.; Marchini, S.; Cozzi, P.; Galliera, E.; Ragg, E.; Colombo, T.; Battaglia, R.; Howard, M.; D'Incalci, M.; Broggini, M. Cancer Res. 2002, 62, 2332.

69.

Leahy, M.; Ray-Coquard, I.; Verweij, J.; Le Cesne, A.; Duffaud, F.; Hogendoorn, P. C.; Fowst, C.; de Balincourt, C.; di Paola, E. D.; van Glabbeke, M.; Judson, I.; Blay, J. Y. Eur. J. Cancer 2007, 43, 308.

70.

Caponigro, F.; Lorusso, D.; Fornari, G.; Barone, C.; Merlano, M.; Airoldi, M.; Schena, M.; MacArthur, R.; Weitman, S.; Jannuzzo, M. G.; Crippa, S.; Fiorentini, F.; Petroccione, A.; Comis, S. Cancer Chemother. Pharmacol. 2010, 66, 389.

71.

Rosenberg, B.; Van Camp, L.; Krigas, T. Nature 1965, 205, 698.

72.

Rosenberg, B. Plat. Met. Rev. 1971, 15, 42.

73.

Desoize, B.; Madoulet, C. Crit. Rev. Oncol. Hematol. 2002, 42, 317.

74.

Alderden, R. A.; Hall, M. D.; Hambley, T. W. J. Chem. Educ. 2006, 83, 728.

75.

Poklar, N.; Pilch, D. S.; Lippard, S. J.; Redding, E. A.; Dunham, S. U.; Breslauer, K. J. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 7606.

76.

Ormerod, M. G.; O'Neill, C.; Robertson, D.; Kelland, L. R.; Harrap, K. R. Cancer Chemother. Pharmacol. 1996, 37, 463.

77.

Li, G. M. Oncol. Res. 1999, 11, 393.

78.

Wu, J.; Gu, L.; Wang, H.; Geacintov, N. E.; Li, G. M. Mol. Cell. Biol. 1999, 19, 8292.

79.

Florea, A.-M.; Büsselberg, D. Cancers 2011, 3, 1351.

80.

Oliver, T.; Mead, G. Curr. Opin. Oncol. 1993, 5, 559.

81.

Stathopoulos, G. P.; Rigatos, S.; Malamos, N. A. Oncol. Rep. 1999, 6, 797.

82.

Knoll, C.; Smith, R. J. H.; Shores, C.; Blatt, J. Laryngoscope 2006, 116, 72.

37

83.

Momekov, G.; Ferdinandov, D.; Bakalova, A.; Zaharieva, M.; Konstantinov, S.; Karaivanova, M. Arch. Toxicol. 2006, 80, 555.

84.

Keller, G. A.; Ponte, M. L.; Di Girolamo, G. Curr. Drug Saf. 2010, 5, 105.

85.

Boulikas, T.; Vougiouka, M. Oncol. Rep. 2003, 10, 1663.

86.

Anderson, H.; Wagstaff, J.; Crowther, D.; Swindell, R.; Lind, M. J.; McGregor, J.; Timms, M. S.; Brown, D.; Palmer, P. Eur. J. Cancer Clin. Oncol. 1988, 24, 1471.

87.

Adams, M.; Kerby, I. J.; Rocker, I.; Evans, A.; Johansen, K.; Franks, C. R. Acta. Oncol. 1989, 28, 57.

88.

Sandercock, J.; Parmar, M. K.; Torri, V.; Qian, W. Br. J. Cancer 2002, 87, 815.

89.

Rixe, O.; Ortuzar, W.; Alvarez, M.; Parker, R.; Reed, E.; Paull, K.; Fojo, T. Biochem. Pharmacol. 1996, 52, 1855.

90.

Ibrahim, A.; Hirschfeld, S.; Cohen, M. H.; Griebel, D. J.; Williams, G. A.; Pazdur, R. Oncologist 2004, 9, 8.

91.

Culy, C. R.; Clemett, D.; Wiseman, L. R. Drugs 2000, 60, 895.

92.

Stordal, B.; Pavlakis, N.; Davey, R. Cancer Treat. Rev. 2007, 33, 347.

93.

Shimada, M.; Itamochi, H.; Kigawa, J. Cancer Manag. Res. 2013, 5, 67.

94.

Heffeter, P.; Jungwirth, U.; Jakupec, M.; Hartinger, C.; Galanski, M.; Elbling, L.; Micksche, M.; Keppler, B.; Berger, W. Drug Resist. Updat. 2008, 11, 1.

95.

Clarke, M. J.; Zhu, F.; Frasca, D. R. Chem. Rev. 1999, 99, 2511.

96.

Schluga, P.; Hartinger, C. G.; Egger, A.; Reisner, E.; Galanski, M.; Jakupec, M. A.; Keppler, B. K. Dalton Trans. 2006, 14, 1796.

97.

Frasca, D.; Ciampa, J.; Emerson, J.; Umans, R. S.; Clarke, M. J. Met. Based Drugs 1996, 3, 197.

98.

Sun, H.; Li, H.; Sadler, P. J. Chem. Rev. 1999, 99, 2817.

38

99.

Pongratz, M.; Schluga, P.; Jakupec, M. A.; Arion, V. B.; Hartinger, C. G.; Allmaier, G.; Keppler, B. K. J. Anal. At. Spectrom. 2004, 19, 46.

100.

Antonarakis, E.; Emadi, A. Cancer Chemother. Pharmacol. 2010, 66, 1.

101.

Guo, W.; Zheng, W.; Luo, Q.; Li, X.; Zhao, Y.; Xiong, S.; Wang, F. Inorg. Chem. 2013, 52, 5328.

102.

Dickson, K. S.; Burns, C. M.; Richardson, J. P. J. Biol. Chem. 2000, 275, 15828.

103.

Schroeder, G. K.; Lad, C.; Wyman, P.; Williams, N. H.; Wolfenden, R. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 4052.

104.

Williams, N. H.; Takasaki, B.; Wall, M.; Chin, J. Acc. Chem. Res. 1999, 32, 485.

105.

Chen, W.; Kitamura, Y.; Zhou, J. M.; Sumaoka, J.; Komiyama, M. J. Am. Chem. Soc. 2004, 126, 10285.

106.

Galburt, E. A.; Stoddard, B. L. Biochemistry 2002, 41, 13851.

107.

Sreedhara, A.; Freed, J. D.; Cowan, J. A. J. Am. Chem. Soc. 2000, 122, 8814.

108.

Stubbe, J.; Kozarich, J. W. Chem. Rev. 1987, 87, 1107.

109.

Hutchinson, F. Prog. Nucleic Acid Res. Mol. Biol. 1985, 32, 115.

110.

Breen, A. P.; Murphy, J. A. Free Radic. Biol. Med. 1995, 18, 1033.

111.

Burrows, C. J.; Muller, J. G. Chem. Rev. 1998, 98, 1109.

112.

Pratviel, G.; Bernadou, J.; Meunier, B. Angew. Chem. Int. Ed. Engl. 1995, 34, 746.

113.

Hergueta-Bravo, A.; Jiménez-Hernández, M. E.; Montero, F.; Oliveros, E.; Orellana, G. J. Phys. Chem. B. 2002, 106, 4010.

114.

Erkkila, K. E.; Odom, D. T.; Barton, J. K. Chem. Rev. 1999, 99, 2777.

115.

Zhou, Q. X.; Lei, W. H.; Li, C.; Hou, Y. J.; Wang, X. S.; Zhang, B. W. New J. Chem. 2010, 34, 137.

116.

Chow, C. S.; Barton, J. K. Methods Enzymol. 1992, 212, 219.

117.

Pruchnik, F.; Duś, D. J. Inorg. Biochem. 1996, 61, 55.

39

118.

Angeles-Boza, A. M.; Chifotides, H. T.; Aguirre, J. D.; Chouai, A.; Fu, P. K.; Dunbar, K. R.; Turro, C. J. Med. Chem. 2006, 49, 6841.

119.

Bradley, P. M.; Angeles-Boza, A. M.; Dunbar, K. R.; Turro, C. Inorg. Chem. 2004, 43, 2450.

120.

Angeles-Boza, A. M.; Bradley, P. M.; Fu, P. K.; Wicke, S. E.; Bacsa, J.; Dunbar, K. R.; Turro, C. Inorg. Chem. 2004, 43, 8510.

121.

Angeles-Boza, A. M.; Bradley, P. M.; Fu, P. K.; Shatruk, M.; Hilfiger, M. G.; Dunbar, K. R.; Turro, C. Inorg. Chem. 2005, 44, 7262.

122.

Einhorn, L. Semin. Oncol. 2003, 30(2 Suppl 5), 34.

123.

Chitambar, C. R. Expert Opin. Investig. Drugs 2004, 13, 531.

124.

Straus, D. J. Semin. Oncol. 2003, 30(2 Suppl 5), 25.

125.

Chitambar, C. R. Future Med. Chem. 2012, 4, 1257.

126.

Collery, P.; Keppler, B.; Madoulet, C.; Desoize, B. Crit. Rev. Oncol. Hematol. 2002, 42, 283.

127.

Collery, P.; Millart, H.; Lamiable, D.; Vistelle, R.; Rinjard, P.; Tran, G.; Gourdier, B.; Cossart, C.; Bouana, J. C.; Pechery, C. et al. Anticancer Res. 1989, 9, 353.

128.

Bernstein, L. R.; van der Hoeven, J. J.; Boer, R. O. Anticancer Agents Med. Chem. 2011, 11, 585.

129.

Christodoulou, C. V.; Eliopoulos, A. G.; Young, L. S.; Hodgkins, L.; Ferry, D. R.; Kerr, D. J. Br. J. Cancer 1998, 77, 2088.

130.

Harstrick, A.; Schmoll, H. J.; Sass, G.; Poliwoda, H.; Rustum, Y. Eur. J. Cancer 1993, 29A, 1000.

131.

Korfel, A.; Scheulen, M. E.; Schmoll, H. J.; Gründel, O.; Harstrick, A.; Knoche, M.; Fels, L. M.; Skorzec, M.; Bach, F.; Baumgart, J.; Sass, G.; Seeber, S.; Thiel, E.; Berdel, W. E. Clin. Cancer Res. 1998, 4, 2701.

40

132.

Kröger, N.; Kleeberg, U. R.; Mross, K.; Edler, L.; Hossfeld, D. K. Onkologie 2000, 23, 60.

133.

Schilling, T.; Keppler, K. B.; Heim, M. E.; Niebch, G.; Dietzfelbinger, H.; Rastetter, J.; Hanauske, A. R. Invest. New Drugs 1995, 13, 327.

134.

Toney, J. H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 947.

135.

Köpf, H.; Grabowski, S.; Voigtlander, R. Organomet. Chem. 1981, 216, 185.

136.

Beckhove, P.; Oberschmidt, O.; Hanauske, A. R.; Pampillon, C.; Schirrmacher, V.; Sweeney, N. J.; Strohfeldt, K.; Tacke, M. Anticancer Drugs 2007, 18, 311.

137.

Oberschmidt, O.; Hanauske, A. R.; Pampillon, C.; Sweeney, N. J.; Strohfeldt, K.; Tacke, M. Anticancer Drugs. 2007, 18, 317.

138.

Peri, D.; Meker, S.; Manna, C. M.; Tshuva, E. Y. Inorg. Chem. 2011, 50, 1030.

139.

Immel, T. A.; Groth, U.; Huhn, T.; Öhlschläger, P. PLoS One 2011, 6, e17869.

140.

Umezawa, H. Prog. Biochem. Pharmacol. 1976, 11, 18.

141.

Blum, R. H.; Carter, S. K.; Agre, K. Cancer 1973, 31, 903.

142.

Maurer, T. D.; Kraft, B. J.; Lato, S. M.; Ellington, A. D.; Zaleski, J. M. Chem. Commun. 2000, 69.

143.

Bernhardt, P. V.; Caldwell, L. M.; Chaston, T. B.; Chin, P.; Richardson, D. R. J. Biol. Inorg. Chem., 2003, 8.

144.

Richardson, D. R.; Sharpe, P. C.; Lovejoy, D. B.; Senaratne, D.; Kalinowski, D. S.; Islam, M.; Bernhardt, P. V. J. Med. Chem. 2006, 49, 6510.

145.

Kalinowski, D. S.; Sharpe, P. C.; Bernhardt, P. V.; Richardson, D. R. J. Med. Chem. 2007, 50, 6212.

146.

Kalinowski, D. S.; Yu; Sharpe, P. C.; Islam, M.; Liao, Y. T.; Lovejoy, D. B.; Kumar, N.; Bernhardt, P. V.; Richardson, D. R. J. Med. Chem. 2007, 50, 3716.

41

147.

Buss, J. L.; Greene, B. T.; Turner, J.; Torti, F. M.; Torti, S. V. Curr. Top. Med. Chem. 2004, 4, 1623.

148.

Richardson, D. R. Curr. Med. Chem. 2005, 12, 2711.

149.

Ramakrishnan, S.; Suresh, E.; Riyasdeen, A.; Akbarsha, M. A.; Palaniandavar, M. Dalton Trans. 2011, 40, 3524.

150.

Barton, J. K.; Raphael, A. L. J. Am. Chem. Soc. 1984, 106, 2466.

151.

Barton, J. K.; Paranawithana, S. R. Biochemistry 1986, 25, 2205.

152.

Arounaguiri, S.; Maiya, B. G. Inorg. Chem. 1996, 35, 4267.

153.

Bhattacharya, S.; Mandal, S. S. Chem. Commun. 1996, 1515.

154.

Mandal, S. S.; Vinay Kumar, N.; Varshney, U.; Bhattacharya, S. J. Inorg. Biochem. 1996, 63, 265.

155.

Mandal, S. S.; Varshney, U.; Bhattacharya, S. Bioconjug. Chem. 1997, 8, 798.

156.

Bhattacharya, S.; Mandal, S. S. J. Chem. Soc., Chem. Commun. 1995, 2489.

157.

Ansari, K. I.; Grant, J. D.; Kasiri, S.; Woldemariam, G.; Shrestha, B.; Mandal, S. S. J. Inorg. Biochem. 2009, 103, 818.

158.

Ansari, K. I.; Kasiri, S.; Grant, J. D.; Mandal, S. S. Dalton Trans. 2009, 8525.

159.

Bodley, A. L.; Liu, L. F. Nat. Biotech. 1988, 6, 1315.

160.

Champoux, J. J. Annu. Rev. Biochem. 2001, 70, 369.

161.

Maxwell, A.; Gellert, M. Adv. Protein Chem. 1986, 38, 69.

162.

Crick, F. H. C.; Wang, J. C.; Brucer, W. R. J. Mol. Biol., 1979, 129, 449.

163.

Wang, J. C. Nat. Rev. Mol. Cell Biol. 2002, 3, 430.

164.

Lee, M. P.; Brown, S. D.; Chen, A.; Hsieh, T. S. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 6656.

165.

Morham, S. G.; Kluckman, K. D.; Voulomanos, N.; Smithies, O. Mol. Cell. Biol. 1996, 16, 6804.

42

166.

Hsiang, Y. H.; Hertzberg, R.; Hecht, S. M.; Liu, L. F. J. Biol. Chem. 1985, 260, 14873.

167.

Stork, G.; Schultz, A. G. J. Am. Chem. Soc. 1971, 93, 4074.

168.

Ejima, A.; Terasawa, H.; Sugimori, M.; Tagawa, H. Tetrahedron Lett. 1989, 30, 2639.

169.

Li, Q. Y.; Zu, Y. G.; Shi, R. Z.; Yao, L. P. Curr. Med. Chem. 2006, 13, 2021.

170.

Dancey, J.; Eisenhauer, E. A. Br. J. Cancer. 1996, 74, 327.

171.

Leu, Y. L.; Roffler, S. R.; Chern, J. W. J. Med. Chem. 1999, 42, 3623.

172.

Herzog, T. J. Oncologist. 2002, 7 (Suppl. 5), 3.

173.

Garcia-Carbonero, R.; Supko, J. G. Clin. Cancer Res. 2002, 8, 641.

174.

Hsiang, Y. H.; Lihou, M. G.; Liu, L. F. Cancer Res. 1989, 49, 5077.

175.

Chen, A. Y.; Yu, C.; Gatto, B.; Liu, L. F. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 8131.

176.

Chaudhuri, P.; Majumder, H. K.; Bhattacharya, S. J. Med. Chem. 2007, 50, 2536.

177.

Jean-Moreno, V.; Rojas, R.; Goyeneche, D.; Coombs, G. H.; Walker, J. Exp. Parasitol. 2006, 112, 21.

178.

Hande, K. R. Eur. J. Cancer. 1998, 34, 1514.

179.

Cortés, F.; Pastor, N.; Mateos, S.; Domínguez, I. Mutat. Res. 2003, 543, 59.

180.

Robertson, J. D.; Enoksson, M.; Suomela, M.; Zhivotovsky, B.; Orrenius, S. J. Biol. Chem. 2002, 277, 29803.

181.

Sonneveld, P. Semin. Oncol. 1992, 19(2 Suppl 6), 59.

182.

Swift, L. P.; Rephaeli, A.; Nudelman, A.; Phillips, D. R.; Cutts, S. M. Cancer Res. 2006, 66, 4863.

183.

Shan, K.; Lincoff, A. M.; Young, J. B. Ann. Intern. Med. 1996, 125, 47.

184.

Hortobágyi, G. N. Drugs 1997, 54(Suppl. 4), 1.

43

185.

Crivellari, D.; Lombardi, D.; Spazzapam, S.; Veronesi, A.; Toffoli, G. Crit. Rev. Oncol. Hematol. 2004, 49, 153.

186.

Hande, K. R. Update Cancer Ther. 2008, 3, 13.

187.

Beaumont, M.; Sanz, M.; Carli, P. M.; Maloisel, F.; Thomas, X.; Detourmignies, L.; Guerci, A.; Gratecos, N.; Rayon, C.; San Miguel, J.; Odriozola, J.; Cahn, J. Y.; Huguet, F.; Vekhof, A.; Stamatoulas, A.; Dombret, H.; Capote, F.; Esteve, J.; Stoppa, A. M.; Fenaux, P. J. Clin. Oncol. 2003, 21, 2123.

188.

Gschwind, A.; Fischer, O. M.; Ullrich, A. Nat. Rev. Cancer 2004, 4, 361

189.

Balasubramanian, S.; Hurley, L. H.; Neidle, S. Nat. Rev. Drug. Discov. 2011, 10, 261.

190.

Gellert, M.; Lipsett, M. N.; Davies, D. R. Proc. Natl. Acad. Sci. U.S.A. 1962, 48, 2013.

191.

Sen, D.; Gilbert, W. Nature 1988, 334, 364.

192.

Sundquist, W. I.; Klug, A. Nature 1989, 342, 825.

193.

Williamson, J. R.; Raghuraman, M. K.; Cech, T. R. Cell 1989, 59, 871.

194.

Chen, F. M. Biochemistry 1992, 31, 3769.

195.

Nagesh, N.; Chatterji, D. J. Biochem. Biophys. Methods 1995, 30, 1.

196.

Parkinson, G. N.; Lee, M. P.; Neidle, S. Nature 2002, 417, 876.

197.

Dai, J.; Punchihewa, C.; Ambrus, A.; Chen, D.; Jones, R. A.; Yang, D. Nucleic Acids Res. 2007, 35, 2440.

198.

Patel, D. J.; Phan, A. T.; Kuryavyi, V. Nucleic Acids Res. 2007, 35, 7429.

199.

Phan, A. T.; Kuryavyi, V.; Ma, J. B.; Faure, A.; Andréola, M. L.; Patel, D. J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 634.

200.

Kettani, A.; Gorin, A.; Majumdar, A.; Hermann, T.; Skripkin, E.; Zhao, H.; Jones, R.; Patel, D. J. J. Mol. Biol. 2000, 297, 627.

44

201.

Zhang, N.; Gorin, A.; Majumdar, A.; Kettani, A.; Chernichenko, N.; Skripkin, E.; Patel, D. J. J. Mol. Biol. 2001, 311, 1063.

202.

Paul, A.; Jain, A. K.; Misra, S. K.; Maji, B.; Muniyappa, K.; Bhattacharya, S. PLoS One 2012, 7, e39467.

203.

Simonsson, T. Biol. Chem. 2001, 382, 621.

204.

Greider, C. W. Curr. Opin. Genet. Dev. 1994, 4, 203.

205.

Blackburn, E. H. Cell 1994, 77, 621.

206.

Rhodes, D.; Giraldo, R. Curr. Opin. Struct. Biol. 1995, 5, 311.

207.

Verdun, R. E.; Karlseder, J. Nature 2007, 447, 924.

208.

Herbig, U.; Jobling, W. A.; Chen, B. P. C.; Chen, D. J.; Sedivy, J. M. Mol. Cell 2004, 14, 501.

209.

Kim, N. W.; Piatyszek, M. A.; Prowse, K. R.; Harley, C. B.; West, M. D.; Ho, P. L.; Coviello, G. M.; Wright, W. E.; Weinrich, S. L.; Shay, J. W. Science 1994, 266, 2011.

210.

Shay, J. W.; Wright, W. E. Nat. Rev. Drug Discov. 2006, 5, 577.

211.

Maji, B.; Bhattacharya, S. Chem. Commun. 2014, doi: 10.1039/C4CC00611A.

212.

Eddy, J.; Maizels, N. Nucleic Acids Res. 2006, 34, 3887.

213.

Huppert, J. L.; Balasubramanian, S. Nucleic Acids Res. 2007, 35, 406.

214.

Seenisamy, J.; Rezler, E. M.; Powell, T. J.; Tye, D.; Gokhale, V.; Joshi, C. S.; Siddiqui-Jain, A.; Hurley, L. H. J. Am. Chem. Soc. 2004, 126, 8702.

215.

Fernando, H.; Reszka, A. P.; Huppert, J.; Ladame, S.; Rankin, S.; Venkitaraman, A. R.; Neidle, S.; Balasubramanian, S. Biochemistry 2006, 45, 7854.

216.

Cogoi, S.; Paramasivam, M.; Spolaore, B.; Xodo, L. E. Nucleic Acids Res. 2008, 36, 3765.

217.

Paramasivam, M.; Membrino, A.; Cogoi, S.; Fukuda, H.; Nakagama, H.; Xodo, L. E. Nucleic Acids Res. 2009, 37, 2841.

45

218.

Dexheimer, T. S.; Sun, D.; Hurley, L. H. J. Am. Chem. Soc. 2006, 128, 5404.

219.

Sun, D.; Guo, K.; Rusche, J. J.; Hurley, L. H. Nucleic Acids Res. 2005, 33, 6070.

220.

De Armond, R.; Wood, S.; Sun, D.; Hurley, L. H.; Ebbinghaus, S. W. Biochemistry 2005, 44, 16341.

221.

Qin, Y.; Rezler, E. M.; Gokhale, V.; Sun, D.; Hurley, L. H. Nucleic Acids Res. 2007, 35, 7698.

222.

Qin, Y.; Fortin, J. S.; Tye, D.; Gleason-Guzman, M.; Brooks, T. A.; Hurley, L. H. Biochemistry 2010, 49, 4208.

223.

Palumbo, S. L.; Ebbinghaus, S. W.; Hurley, L. H. J. Am. Chem. Soc. 2009, 131, 10878.

224.

Xu, Y.; Sugiyama, H. Nucleic Acids Symp. Ser. (Oxf). 2005, 49, 177.

225.

Xu, Y.; Sugiyama, H. Nucleic Acids Res. 2006, 34, 949.

226.

Han, H.; Hurley, L. H. Trends Pharmacol. Sci. 2000, 21, 136.

227.

Sun, D.; Hurley, L. H. J. Med. Chem. 2009, 52, 2863.

228.

Schuitmaker, J. J.; Baas, P.; van Leengoed, H. L.; van der Meulen, F. W.; Star, W. M.; van Zandwijk, N. J. Photochem. Photobiol. B. 1996, 34, 3.

229.

Cuesta, J.; Read, M. A.; Neidle, S. Mini Rev. Med. Chem. 2003, 3, 11.

230.

Romera, C.; Bombarde, O.; Bonnet, R.; Gomez, D.; Dumy, P.; Calsou, P.; Gwan, J. F.; Lin, J. H.; Defrancq, E.; Pratviel, G. Biochimie 2011, 93, 1310.

231.

Dixon, I. M.; Lopez, F.; Estève, J. P.; Tejera, A. M.; Blasco, M. A.; Pratviel, G.; Meunier, B. Chembiochem. 2005, 6, 123.

232.

Anantha, N. V.; Azam, M.; Sheardy, R. D. Biochemistry 1998, 37, 2709.

233.

Shi, D. F.; Wheelhouse, R. T.; Sun, D.; Hurley, L. H. J. Med. Chem. 2001, 44, 4509.

234.

Evans, S. E.; Mendez, M. A.; Turner, K. B.; Keating, L. R.; Grimes, R. T.; Melchoir, S.; Szalai, V. A. J. Biol. Inorg. Chem. 2007, 12, 1235.

46

235.

Pan, J.; Zhang, S. J. Biol. Inorg. Chem. 2009, 14, 401.

236.

Wang, P.; Ren, L.; He, H.; Liang, F.; Zhou, X.; Tan, Z. Chembiochem. 2006, 7, 1155.

237.

Dixon, I. M.; Lopez, F.; Tejera, A. M.; Estève, J. P.; Blasco, M. A.; Pratviel, G.; Meunier, B. J. Am. Chem. Soc. 2007, 129, 1502.

238.

Du, Y.; Zhang, D.; Chen, W.; Zhang, M.; Zhou, Y.; Zhou, X. Bioorg. Med. Chem. 2010, 18, 1111.

239.

Goncalves, D. P. N.; Rodriguez, R.; Balasubramanian, S.; Sanders, J. K. M. Chem. Commun. 2006, 4685.

240.

Ren, L.; Zhang, A.; Huang, J.; Wang, P.; Weng, X.; Zhang, L.; Liang, F.; Tan, Z.; Zhou, X. Chembiochem. 2007, 8, 775.

241.

Membrino, A.; Paramasivam, M.; Cogoi, S.; Alzeer, J.; Luedtke, N. W.; Xodo, L. E. Chem. Commun. 2010, 46, 625.

242.

Yaku, H.; Fujimoto, T.; Murashima, T.; Miyoshi, D.; Sugimoto, N. Chem. Commun. 2012, 48, 6203.

243.

Kim, M. Y.; Vankayalapati, H.; Shin-Ya, K.; Wierzba, K.; Hurley, L. H. J. Am. Chem. Soc. 2002, 124, 2098.

244.

Doi, T.; Yoshida, M.; Shin-ya, K.; Takahashi, T. Org. Lett. 2006, 8, 4165.

245.

Minhas, G. S.; Pilch, D. S.; Kerrigan, J. E.; LaVoie, E. J.; Rice, J. E. Bioorg. Med. Chem. Lett. 2006, 16, 3891.

246.

Barbieri, C. M.; Srinivasan, A. R.; Rzuczek, S. G.; Rice, J. E.; LaVoie, E. J.; Pilch, D. S. Nucleic Acids Res. 2007, 35, 3272.

247.

Rzuczek, S. G.; Pilch, D. S.; LaVoie, E. J.; Rice, J. E. Bioorg. Med. Chem. Lett. 2008, 18, 913.

248.

Iida, K.; Tera, M.; Hirokawa, T.; Shin-ya, K.; Nagasawa, K. Chem. Commun. 2009, 6481.

47

249.

Harrison, R. J.; Gowan, S. M.; Kelland, L. R.; Neidle, S. Bioorg. Med. Chem. Lett. 1999, 9, 2463.

250.

Read, M.; Harrison, R. J.; Romagnoli, B.; Tanious, F. A.; Gowan, S. H.; Reszka, A. P.; Wilson, W. D.; Kelland, L. R.; Neidle, S. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 4844.

251.

Burger, A. M.; Dai, F.; Schultes, C. M.; Reszka, A. P.; Moore, M. J.; Double, J. A.; Neidle, S. Cancer Res. 2005, 65, 1489.

252. Redman, J. E.; Granadino-Roldán, J. M.; Schouten, J. A.; Ladame, S.; Reszka, A. P.; Neidle, S.; Balasubramanian, S. Org. Biomol. Chem. 2009, 7, 76. 253.

Sparapani, S.; Haider, S. M.; Doria, F.; Gunaratnam, M.; Neidle, S. J. Am. Chem. Soc. 2010, 132, 12263.

254.

Sun, D.; Thompson, B.; Cathers, B. E.; Salazar, M.; Kerwin, S. M.; Trent, J. O.; Jenkins, T. C.; Neidle, S.; Hurley, L. H. J. Med. Chem. 1997, 40, 2113.

255.

Perry, P. J.; Gowan, S. M.; Reszka, A. P.; Polucci, P.; Jenkins, T. C.; Kelland, L. R.; Neidle, S. J. Med. Chem. 1998, 41, 3253.

256.

Perry, P. J.; Reszka, A. P.; Wood, A. A.; Read, M. A.; Gowan, S. M.; Dosanjh, H. S.; Trent, J. O.; Jenkins, T. C.; Kelland, L. R.; Neidle, S. J. Med. Chem. 1998, 41, 4873.

257.

Huang, H. S.; Chen, T. C.; Chen, R. H.; Huang, K. F.; Huang, F. C.; Jhan, J. R.; Chen, C. L.; Lee, C. C.; Lo, Y.; Lin, J. J. Bioorg. Med. Chem. 2009, 17, 7418.

258.

Larsen, A. F.; Nielsen, M. C.; Ulven, T. Chemistry 2012, 18, 10892.

259.

Musetti, C.; Lucatello, L.; Bianco, S.; Krapcho, A. P.; Cadamuro, S. A.; Palumbo, M.; Sissi, C. Dalton Trans. 2009, 3657.

260.

Wang, L.; Wen, Y.; Liu, J.; Zhou, J.; Li, C.; Wei, C. Org. Biomol. Chem. 2011, 9, 2648.

48

261.

Wang, J. T.; Zheng, X. H.; Xia, Q.; Mao, Z. W.; Ji, L. N.; Wang, K. Dalton Trans. 2010, 39, 7214.

262.

Yu, Q.; Liu, Y.; Zhang, J.; Yang, F.; Sun, D.; Liu, D.; Zhou, Y.; Liu, J. Metallomics 2013, 5, 222.

263.

Teulade-Fichou, M. P.; Carrasco, C.; Guittat, L.; Bailly, C.; Alberti, P.; Mergny, J. L.; David, A.; Lehn, J. M.; Wilson, W. D. J. Am. Chem. Soc. 2003, 125, 4732.

264.

Huang, F. C.; Chang, C. C.; Lou, P. J.; Kuo, I. C.; Chien, C. W.; Chen, C. T.; Shieh, F. Y.; Chang, T. C.; Lin, J. J. Mol. Cancer Res. 2008, 6, 955.

265.

Dumat, B.; Bordeau, G.; Faurel-Paul, E.; Mahuteau-Betzer, F.; Saettel, N.; Bombled, M.; Metgé, G.; Charra, F.; Fiorini-Debuisschert, C.; Teulade-Fichou, M. P. Biochimie 2011, 93, 1209.

266.

Tsai, Y. L.; Chang, C. C.; Kang, C. C.; Chang, T. C. J. Lumin. 2007, 127, 41.

267.

Dash, J.; Das, R. N.; Hegde, N.; Pantos, G. D.; Shirude, P. S.; Balasubramanian, S. Chemistry 2012, 18, 554.

268.

Dash, J.; Shirude, P. S.; Balasubramanian, S. Chem. Commun. 2008, 3055.

269.

Moorhouse, A. D.; Santos, A. M.; Gunaratnam, M.; Moore, M.; Neidle, S.; Moses, J. E. J. Am. Chem. Soc. 2006, 128, 15972.

270.

Paul, A.; Jain, A. K.; Misra, S. K.; Maji, B.; Muniyappa, K.; Bhattacharya, S. PLoS One 2012, 7, e39467.

271.

Jain, A. K.; Paul, A.; Maji, B.; Muniyappa, K.; Bhattacharya, S. J. Med. Chem. 2012, 55, 2981.

272.

Paul, A.; Maji, B.; Misra, S. K.; Jain, A. K.; Muniyappa, K.; Bhattacharya, S. J. Med. Chem. 2012, 55, 7460.

273.

Bhattacharya, S.; Chaudhuri, P.; Jain, A. K.; Paul, A. Bioconjug. Chem. 2010, 21, 1148.

49

274.

Jain, A. K.; Reddy, V. V.; Paul, A.; Muniyappa, K.; Bhattacharya, S. Biochemistry 2009, 48, 10693.

275.

Li, G.; Huang, J.; Zhang, M.; Zhou, Y.; Zhang, D.; Wu, Z.; Wang, S.; Weng, X.; Zhou, X.; Yang, G. Chem. Commun. 2008, 4564.

276.

Maiti, S.; Chaudhury, N. K.; Chowdhury, S. Biochem. Biophys. Res. Commun. 2003, 310, 505.

277.

Maji, B.; Bhattacharya, S. Chimia 2013, 67, 39.

278.

Gunaratnam, M.; de la Fuente, M.; Hampel, S. M.; Todd, A. K.; Reszka, A. P.; Schätzlein, A.; Neidle, S. Bioorg. Med. Chem. 2011, 19, 7151.

279.

Hampel, S. M.; Sidibe, A.; Gunaratnam, M.; Riou, J. F.; Neidle, S. Bioorg. Med. Chem. Lett. 2010, 20, 6459.

280.

Fedoroff, O. Y.; Salazar, M.; Han, H.; Chemeris, V. V.; Kerwin, S. M.; Hurley, L. H. Biochemistry 1998, 37, 12367.

281.

Neidle, S.; Harrison, R. J.; Reszka, A. P.; Read, M. A. Pharmacol Ther. 2000, 85, 133.

282.

Naasani, I.; Seimiya, H.; Yamori, T.; Tsuruo, T. Cancer Res. 1999, 59, 4004.

283.

Franceschin, M.; Rossetti, L.; D'Ambrosio, A.; Schirripa, S.; Bianco, A.; Ortaggi, G.; Savino, M.; Schultes, C.; Neidle, S. Bioorg. Med. Chem. Lett. 2006, 16, 1707.

284.

Zhang, W. J.; Ou, T. M.; Lu, Y. J.; Huang, Y. Y.; Wu, W. B.; Huang, Z. S.; Zhou, J. L.; Wong, K. Y.; Gu, L. Q. Bioorg. Med. Chem. 2007, 15, 5493.

285.

Ma, Y.; Ou, T. M.; Tan, J. H.; Hou, J. Q.; Huang, S. L.; Gu, L. Q.; Huang, Z. S. Bioorg. Med. Chem. Lett. 2009, 19, 3414.

286.

Zhou, J. M.; Zhu, X. F.; Lu, Y. J.; Deng, R.; Huang, Z. S.; Mei, Y. P.; Wang, Y.; Huang, W. L.; Liu, Z. C.; Gu, L. Q.; Zeng, Y. X. Oncogene 2006, 25, 503.

50

287.

Zhou, J. L.; Lu, Y. J.; Ou, T. M.; Zhou, J. M.; Huang, Z. S.; Zhu, X. F.; Du, C. J.; Bu, X. Z.; Ma, L.; Gu, L. Q.; Li, Y. M.; Chan, A. S. J. Med. Chem. 2005, 48, 7315.

288.

Drygin, D.; Siddiqui-Jain, A.; O'Brien, S.; Schwaebe, M.; Lin, A.; Bliesath, J.; Ho, C. B.; Proffitt, C.; Trent, K.; Whitten, J. P.; Lim, J. K.; Von Hoff, D.; Anderes, K.; Rice, W. G. Cancer Res. 2009, 69, 7653.

289.

Datta, A.; Bellon, M.; Sinha-Datta, U.; Bazarbachi, A.; Lepelletier, Y.; Canioni, D.; Waldmann, T. A.; Hermine, O.; Nicot, C. Blood 2006, 108, 1021.

290.

Strahl, C.; Blackburn, E. H. Mol. Cell Biol. 1996, 16, 53.

291.

Strahl, C.; Blackburn, E. H. Nucleic Acids Res. 1994, 22, 893.

292.

Gomez, D. E.; Armando, R. G.; Alonso, D. F. Front. Oncol. 2012, 2, article 113.

293.

Leonetti, C.; Scarsella, M.; Riggio, G.; Rizzo, A.; Salvati, E.; D'Incalci, M.; Staszewsky, L.;

Frapolli, R.; Stevens, M. F.; Stoppacciaro, A.; Mottolese, M.;

Antoniani, B.; Gilson, E.; Zupi, G.; Biroccio, A. Clin. Cancer Res. 2008, 14, 7284. 294.

Biroccio, A.; Porru, M.; Rizzo, A.; Salvati, E.; D'Angelo, C.; Orlandi, A.; Passeri, D.; Franceschin, M.; Stevens, M. F.; Gilson, E.; Beretta, G.; Zupi, G.; Pisano, C.; Zunino, F.; Leonetti, C. Clin. Cancer Res. 2011, 17, 2227.

295.

Salvati, E.; Scarsella, M.; Porru, M.; Rizzo, A.; Iachettini, S.; Tentori, L.; Graziani, G.; D'Incalci, M.; Stevens, M. F.; Orlandi, A.; Passeri, D.; Gilson, E.; Zupi, G.; Leonetti, C.; Biroccio, A. Oncogene 2010, 29, 6280.

296.

McLuckie, K. I.; Di Antonio, M.; Zecchini, H.; Xian, J.; Caldas, C.; Krippendorff, B. F.; Tannahill, D.; Lowe, C.; Balasubramanian, S. J. Am. Chem. Soc. 2013, 135, 9640.

297.

Aggarwal, M.; Sommers, J. A.; Shoemaker, R. H.; Brosh, R. M. Jr. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 1525.

298.

Schilsky, R. L. Clin. Cancer Res. 2002, 8, 935.

299.

Arrowsmith, J. Nat. Rev. Drug Discov. 2011, 10, 328.

51

300.

Zhang, Y.; Chen, T. Int. J. Nanomedicine 2012, 7, 5283.

301.

Lin, J. H.; Lu, A. Y. Pharmacol. Rev. 1997, 49, 403.

52

53