Dna binding-independent anti-proliferative action of benzazolo[3,2-α]quinolinium DNA intercalators

The proposed mechanism of action of the antineoplastic drug 3-nitrobenzothiazolo[3,2-& agr;]quinolinium chloride (NBQ-2) involves its interaction with DNA by intercalation and inhibition of topoisomerase II activity by arresting the enzyme in a covalent cleavage complex. In an attempt to identify some structural determinants for activity and develop a molecular structure/cytotoxicity correlation, four new structural analogs of the antitumor NBQ-2 were prepared and their cytotoxic activity and DNA binding properties were investigated. The cytotoxic activity was evaluated against six different human tumor cell lines: U937, K-562, HL-60, HT-29, HeLa, and A431. The results showed that these new drugs elicit pronounced cytotoxic effects against U937, K-562, HL-60 and A431 while HeLa and HT-29 were less sensitive to the new drugs. This apparent selectivity was different to that of m-AMSA, a drug currently used for cancer treatment. Since the interaction of NBQ-2 to DNA by intercalation has been proposed as the initial step leading to its antineoplastic activity, DNA binding and changes in DNA contour length induced by the new NBQ-2 structural analogs were also investigated using calf thymus and human DNA. The drug, 7-(1-propenyl)-3-nitrobenzimidazolo[3,2-& agr;]quinolinium chloride (NBQ-59) was the most cytotoxic agent of the analog series (IC₅₀ = 16 & mgr;M for HL-60 cells), however, it demonstrated the weakest binding to DNA (K_int = 0.9 × 10₅ M^-1 for calf thymus DNA). NBQ-59 was also found to be a poor intercalator into the DNA double helix. Therefore, our results suggest that DNA binding is not the primary mechanism of drug action for this family of compounds. In addition structural determinants important for cytotoxicity of the benzazolo quinolinium chlorides were suggested by our results. In particular, the nitro group in the 3 position does not seem to be necessary for bioactivity, while substitutions in the benzazolo moiety have striking effects on the biological activity of the drugs.     

Synthesis, cytotoxicity, DNA interaction and topoisomerase II inhibition properties of tetrahydropyrrolo[3,4-a]carbazole-1,3-dione and tetrahydropyrido-[3,2-b]pyrrolo[3,4-g]indole-1,3-dione derivatives.

Three tetrahydropyrrolo[3,4-a]carbazole-1,3-diones (6--8) and two tetrahydropyrido[3,2-b]pyrrolo[3,4-g]indole-1,3-diones (11--12) have been synthesized. Their interaction with DNA was probed by absorption and thermal melting studies. Compounds 8 and 12 both equipped with a hydroxyethyl-aminoethyl side-chain demonstrated higher affinities for poly(dA-dT)(2) than compounds 6, 7 and 11 bearing a dimethylaminoethyl side-chain. Circular and electric linear dichroism measurements showed that all five drugs behave as typical DNA intercalating agents. A plasmid cleavage assay was used to evaluate the capacity of the drugs to inhibit human topoisomerase II. Compounds 8 and 12 which bind strongly to DNA were found to stabilize DNA-topoisomerase II covalent complexes but their topoisomerase II inhibitory properties do not correlate with their cytotoxic potential. Compounds 6 and 7 are essentially inactive whereas compounds 8, 11 and 12 exhibit a high toxicity to P388 murine leukemia cells and provoke a marked accumulation in the G2/M phase of the cell cycle. These compounds form a new class of DNA-targeted antitumor agents.     

Absence of mutagenicity of the antitumor drug 3-nitrobenzothiazolo[3,2-a]quinolinium chloride (NBQ) in the germ cells of Drosophila melanogaster males.

The antitumor drug, 3-nitrobenzothiazolo[3,2-a]quinolinium chloride (NBQ) was tested for genotoxicity with the sex-linked recessive lethal test by feeding Drosophila melanogaster males. Although toxic to adults, the drug tested negative at the concentrations studied.     

Exocyclic DNA lesions stimulate DNA cleavage mediated by human topoisomerase II alpha in vitro and in cultured cells

DNA adducts are mutagenic and clastogenic. Because of their harmful nature, lesions are recognized by many proteins involved in DNA repair. However, mounting evidence suggests that lesions also are recognized by proteins with no obvious role in repair processes. One such protein is topoisomerase II, an essential enzyme that removes knots and tangles from the DNA. Because topoisomerase II generates a protein-linked double-stranded DNA break during its catalytic cycle, it has the potential to fragment the genome. Previous studies indicate that abasic sites and other lesions that distort the double helix stimulate topoisomerase II-mediated DNA cleavage. Therefore, to further explore interactions between DNA lesions and the enzyme, the effects of exocyclic adducts on DNA cleavage mediated by human topoisomerase IIalpha were determined. When located within the four-base overhang of a topoisomerase II cleavage site (at the +2 or +3 position 3' relative to the scissile bond), 3,N(4)-ethenodeoxycytidine, 3,N(4)-etheno-2'-ribocytidine, 1,N(2)-ethenodeoxyguanosine, pyrimido[1,2-a]purin-10(3H)-one deoxyribose (M(1)dG), and 1,N(2)-propanodeoxyguanosine increased DNA scission approximately 5-17-fold. Enhanced cleavage did not result from an increased affinity of topoisomerase IIalpha for adducted DNA or a decreased rate of religation. Therefore, it is concluded that these exocyclic lesions act by accelerating the forward rate of enzyme-mediated DNA scission. Finally, treatment of cultured human cells with 2-chloroacetaldehyde, a reactive metabolite of vinyl chloride that generates etheno adducts, increased cellular levels of DNA cleavage by topoisomerase IIalpha. This finding suggests that type II topoisomerases interact with exocyclic DNA lesions in physiological systems.     

Topoisomerase II antagonism and anticancer activity of coordinated derivatives of [RuCl(2)(C(6)H(6))(dmso)

Topoisomerase II poisoning and anticancer activity by the organometallic compound [RuCl(2)(C(6)H(6))(dmso)] was shown by us in an earlier study [Biochemistry 38 (1999) 4382]. Since high concentrations of this complex were required to achieve either effects, we have synthesized four derivatives of this complex in which the dimethyl sulphoxide group on the ruthenium atom was replaced with pyridine, 3-aminopyridine, p-aminobenzoic acid, and aminoguanidine. Three of these molecules showed enhanced potency of topoisomerase II poisoning and consequently also showed higher anticancer activity in breast and colon carcinoma cells in vitro. Detailed analysis of the molecular action of these compounds on topoisomerase II activity was carried out using the classical relaxation and cleavage activity of the enzyme, which revealed that the compounds poison topoisomerase II by freezing the enzyme and enzyme-cleaved DNA in a ternary "cleavage complex". The cleavage complex is implicated in the anti-neoplastic activity of these compounds. DNA interaction studies showed that these compounds interact with DNA in much the same way as [RuCl(2)(C(6)H(6))(dmso)], by external binding of the DNA helix. This is unlike most other topoisomerase II poisons, which predominantly interact with DNA through intercalation with the double helix.     

A mutation in yeast topoisomerase II that confers hypersensitivity to multiple classes of topoisomerase II poisons

A mutation was constructed in the CAP homology domain of yeast topoisomerase II that resulted in hypersensitivity to the intercalating agent N-[4-(9-acridinylamino)-3-methoxy-phenyl]methanesulfonamide and the fluoroquinolone 6, 8-difluoro-7-(4'-hydroxyphenyl)-1-cyclopropyl-4-quinolone-3-carboxyli c acid, but not to etoposide. This mutation, which changes threonine at position 744 to proline, also confers hypersensitivity to anti-bacterial fluoroquinolones. The purified T744P mutant protein had wild type enzymatic activity in the absence of drugs, and no alteration in drug-independent DNA cleavage. Enhanced DNA cleavage in the presence of N-[4-(9-acridinylamino)-3-methoxy-phenyl]methanesulfonamide and fluoroquinolones was observed, in agreement with the results observed in vivo. DNA cleavage was also seen in the presence of norfloxacin and oxolinic acid, two quinolones that are inactive against eukaryotic topoisomerase II. The hypersensitivity was not associated with heat-stable covalent complexes, as was seen in another drug-hypersensitive mutant. Molecular modeling suggests that the mutation in the CAP homology domain may displace amino acids that play important roles in catalysis by topoisomerase II and may explain the drug-hypersensitive phenotype.     

Effects of antineoplastic drugs on the post-strand-passage DNA cleavage/religation equilibrium of topoisomerase II.

The post-strand-passage DNA cleavage/religation equilibrium of Drosophila melanogaster topoisomerase II was examined. This was accomplished by including adenyl-5'-yl imidodiphosphate, a nonhydrolyzable ATP analogue which supports strand passage but not enzyme turnover, in assays. Levels of post-strand-passage enzyme-mediated DNA breakage were 3-5 times higher than those generated by topoisomerase II prior to the strand-passage event. This finding correlated with a decrease in the apparent first-order rate of topoisomerase II mediated DNA religation in the post-strand-passage cleavage complex. Since previous studies demonstrated that antineoplastic drugs stabilize the pre-strand-passage cleavage complex of topoisomerase II by impairing the enzyme's ability to religate cleaved DNA [Osheroff, N. (1989) Biochemistry 28, 6157-6160; Robinson, M.J., & Osheroff, N. (1990) Biochemistry 29, 2511-2515], the effects of 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA) and etoposide on the enzyme's post-strand-passage DNA cleavage complex were characterized. Both drugs stimulated the ability of topoisomerase II to break double-stranded DNA after strand passage. As determined by two independent assay systems, m-AMSA and etoposide stabilized the enzyme's post-strand-passage DNA cleavage complex primarily by inhibiting DNA religation. These results strongly suggest that both the pre- and post-strand-passage DNA cleavage complexes of topoisomerase II serve as physiological targets for these structurally disparate antineoplastic drugs.     

Rebeccamycin analogues from indolo[2,3-c]carbazole

Glycosylated indolocarbazoles related to the antibiotic rebeccamycin represent an important series of antitumor drugs. In the course of structure-activity relationship studies, we report the synthesis of two new derivatives containing an indolo[2,3-c]carbazole chromophore instead of the conventional indolo[2,3-a]carbazole unit found in the natural metabolites. The N-methylated compound 8 containing one glucose residue behaves as a typical DNA intercalating agent, as judged from circular and electric linear dichroism measurements with purified DNA. In contrast, the bis-glycosylated derivative 7 containing a glucose residue on each indole nitrogen has lost its capacity to form stable complexes with DNA. DNA relaxation experiments reveal that the two drugs 7 and 8 have weak effects on human DNA topoisomerase I. The modified conformation of the indolocarbazole chromophore is detrimental to the stabilization of topoisomerase I-DNA complexes. The lack of potent topoisomerase I inhibition leads to decreased cytotoxicity but, however, we observed that the DNA-intercalating mono-glycosyl derivative 8 is about 5 times more cytoxic than the bis-glycosyl analogue 7. The study suggests that the naturally-occurring indolo[2,3-a]carbazole skeleton should be preserved to maintain the topoisomerase I inhibitory and cytotoxic activities.     

DNA binding and cleavage properties of certain tetrammine ruthenium(II) complexes of modified 1,10-phenanthrolines--effect of hydrogen-bonding on DNA-binding affinity

A series of ruthenium(II) mixed ligand complexes of the type [Ru(NH(3))(4)(L)](2+), where L=imidazo[4,5-f][1,10]phenanthroline (ip), 2-phenylimidazo[4,5-f][1,10]phenanthroline (pip), 2-(2-hydroxyphenyl)imidazo[4,5-f][1,10]phenanthroline (hpip), 4,7-diphenyl-1,10-phenanthroline (dip), naphtha[2,3-a]dipyrido[3,2-h:2',3'-f]phenazine-5,18-dione (qdppz), 5,18-dihydroxynaphtho[2,3-a]dipyrido[3,2-H:2',3'-f]phenazine (hqdppz), have been isolated and characterized. The interaction of these complexes with calf thymus DNA (CT DNA) has been explored by using absorption, emission, and circular dichroic spectral methods, thermal denaturation studies and viscometry. All these studies suggest the involvement of the modified phenanthroline 'face' rather than the ammonia 'face' of the complexes in DNA binding. An intercalative mode of DNA binding, which involves the insertion of the modified phenanthroline ligands in between the base pairs, is suggested. The results from absorption spectral titration and circular dichroism (CD), thermal denaturation and viscosity experiments indicate that the qdppz and hqdppz complexes (K(b) approximately 10(6) and Delta T(m)=11-13 degrees C) bind more avidly than the ip, pip and hpip complexes (K(b) approximately 10(5), Delta T(m)=6-8 degrees C). Intramolecular hydrogen bonding in the hpip and hqdppz complexes increases the surface area of the intercalating diimines and enhances the DNA binding affinity substantially. The ammonia co-ligands of the complexes are possibly involved in hydrogen bonding with the intrastrand nucleobases to favour intercalation of the extended aromatic ligands. Circular dichroism spectral studies reveal that all the complexes effect certain structural changes on DNA duplex; [Ru(NH(3))(4)(ip)](2+) induces a B to A transition while [Ru(NH(3))(4)(qdppz)](2+) a B to Psi conformational change on CT DNA. Cleavage efficiency of the complexes were determined using pBR322 supercoiled plasmid DNA. All the complexes, except hqdppz complex, promote the cleavage of supercoiled plasmid (form I) to relaxed circular form (form II).     

Synthesis, DNA-binding and photocleavage studies of cobalt(III) mixed-polypyridyl complexes: [Co(phen)(2)(dpta)](3+) and [Co(phen)(2)(amtp)](3+)

Two novel cobalt(III) mixed-polypyridyl complexes [Co(phen)(2)(dpta)](3+) and [Co(phen)(2)(amtp)](3+) (phen=1,10-phenanthroline, dpta=dipyrido-[3,2-a;2',3'-c]- thien-[3,4-c]azine, amtp=3-amino-1,2,4-triazino[5,6-f]1,10-phenanthroline) have been synthesized and characterized. The interaction of these complexes with calf thymus DNA was investigated by spectroscopic, cyclic voltammetry, and viscosity measurements. Results suggest that the two complexes bind to DNA via an intercalative mode. Moreover, these Co(III) complexes have been found to promote the photocleavage of plasmid DNA pBR322 under irradiation at 365nm. The mechanism studies reveal that hydroxyl radical (OH()) is likely to be the reactive species responsible for the cleavage of plasmid DNA by [Co(phen)(2)(dpta)](3+) and superoxide anion radical (O(2)(-)) acts as the key role in the cleavage reaction of plasmid DNA by [Co(phen)(2)(amtp)](3+).     

Effect of antineoplastic agents on the DNA cleavage/religation reaction of eukaryotic topoisomerase II: inhibition of DNA religation by etoposide

Beyond its essential physiological functions, topoisomerase II is the primary cellular target for a number of clinically relevant antineoplastic drugs. Although the chemotherapeutic efficacies of these drugs correlate with their abilities to stabilize the covalent topoisomerase II-DNA cleavage complex, their molecular mechanism of action has yet to be described. In order to characterize the drug-induced stabilization of this enzyme-DNA complex, the effect of etoposide on the DNA cleavage/religation reaction of Drosophila melanogaster topoisomerase II was studied. Under the conditions employed, etoposide increased levels of enzyme-mediated double-stranded DNA cleavage 5-6-fold and single-stranded cleavage approximately 4-fold. Maximal stimulation was observed at 80-100 microM etoposide with 50% of the maximal effect at approximately 15 microM drug. By employing a topoisomerase II mediated DNA religation assay [Osheroff, N. & Zechiedrich, E.L. (1987) Biochemistry 26, 4303-4309], etoposide was found to stabilize the enzyme-DNA cleavage complex (at least in part) by inhibiting the enzyme's ability to religate cleaved DNA. Moreover, in order for the drug to affect religation, it has to be present at the time of DNA cleavage.     

Synthesis, characterization, DNA-binding and photocleavage of complexes [Ru(phen)2(6-OH-dppz)]2+ and [Ru(phen)2(6-NO2-dppz)]2+

Two new complexes, ([Ru(phen)(2)(6-OH-dppz)](2+)) (1) and ([Ru(phen)(2)(6-NO(2)-dppz)](2+)) (2) (phen=1,10-phenanthroline; 6-OH-dppz=6-hydroxyl-dipyrido[3,2-a:2',3'-c]phenazine; 6-NO(2)-dppz=6-nitro-dipyrido[3,2-a:2',3'-c]phenazine), have been synthesized and characterized by elemental analysis, ES-MS (electrospray mass spectra), (1)H NMR, UV-Vis (UV-visible) and CV (cyclic voltammetry). The DNA-binding behaviors of both complexes have been studied by spectroscopic methods and viscosity measurements. The results indicate that the two complexes all bind to calf thymus DNA (CT-DNA) in an intercalative mode, and the DNA-binding affinity of complex 2 is greater than that of complex 1. In addition, complex 1 can promote photocleavage of pBR322 DNA upon irradiation, whereas complex 2 can promote cleavage of pBR322 DNA both upon irradiation and in the dark, with more efficient cleavage occurring upon irradiation. Theoretical studies for these complexes have been also carried out with the density functional theory (DFT) method. The difference in the DNA-binding behaviors of the two complexes can be reasonably explained by the DFT calculations.     

Topoisomerase II poisoning and antineoplastic action by DNA-nonbinding diacetyl and dicarboxaldoxime derivatives of ferrocene

Topoisomerase II is a major molecular target for a number of DNA-binding anticancer drugs. In the present study, we report topoisomerase II inhibition and anticancer activity by four substituted ferrocene derivatives which do not bind to DNA. The first derivative, acetyl-substituted ferrocene (monoacetylferrocene), showed a minor inhibition of topoisomerase II activity along with a consequent inhibition of cancer cell proliferation. The second derivative (diacetylferrocene) showed a higher potency of action compared to the monosubstituted derivative. The third and fourth derivatives, with mono- and disubstituted carboxaldoxime groups (ferrocenecarboxaldoxime and ferrocenedicarboxaldoxime), showed a higher anticancer action and stronger topoisomerase II inhibition. To understand their molecular mechanism of action, cleavage assays were carried out to monitor the drug-induced, topoisomerase II mediated DNA cleavage. The results show that diacetylferrocene and ferrocenedicarboxaldoxime could form an enzyme-drug-DNA ternary complex, called a "cleavage complex," resulting in DNA cleavage. These results along with those of an immunoprecipitation assay indicate that the two compounds interact with topoisomerase II alone and poison its activity by trapping the enzyme and enzyme-cleaved DNA in the covalently closed cleavage complex. The formation of such a complex has numerous genetic implications, which ultimately results in neoplastic cell death.     

Substituted benz[a]acridines and benz[c]acridines as mammalian topoisomerase poisons

Coralyne and several other synthetic benzo[a,g]quinolizium derivatives related to protoberberine alkaloids have exhibited activity as topoisomerase poisons. These compounds are characterized by the presence of a positively charged iminium group, which has been postulated to be associated with their pharmacological properties. The objective of the present study was to devise stable noncharged bioisosteres of these compounds. Several similarly substituted benz[a]acridine and benz[c]acridine derivatives were synthesized and their relative activity as topoisomerase poisons was determined. While the benz[c]acridine derivatives evaluated as part of this study were devoid of topoisomerase poisoning activity, several dihydrobenz[a]acridines were able to enhance DNA cleavage in the presence of topo I. In contrast to certain protoberberine derivatives that did exhibit activity as topo II poisons, none of the benz[a]acridines derivatives enhanced DNA cleavage in the presence of topo II. Among the benz[a]acridines studied, 5,6-dihydro-3,4-methylenedioxy-9,10-dimethoxybenz[a]acridine, 13e, was the most potent topo I poison, with comparable potency to coralyne. These data suggest that heterocyclic compounds structurally related to coralyne can exhibit potent topo I poisoning activity despite the absence of an iminium cation within their structure. In comparison to coralyne or other protoberberine derivatives, these benz[a]acridine derivatives possess distinctly different physicochemical properties and represent a novel series of topo I poisons.     

Base mutation analysis of topoisomerase II-idarubicin-DNA ternary complex formation. Evidence for enzyme subunit cooperativity in DNA cleavage.

Antitumor drugs, such as anthracyclines, interfere with mammalian DNA topoisomerase II by forming a ternary complex, DNA-drug-enzyme, in which DNA strands are cleaved and covalently linked to the enzyme. In this work, a synthetic 36-bp DNA oligomer derived from SV40 and mutated variants were used to determine the effects of base mutations on DNA cleavage levels produced by murine topoisomerase II with and without idarubicin. Although site competition could affect cleavage levels, mutation effects were rather similar among several cleavage sites. The major sequence determinants of topoisomerase II DNA cleavage without drugs are up to five base pairs apart from the strand cut, suggesting that DNA protein contacts involving these bases are particularly critical for DNA site recognition. Cleavage sites with adenines at positions -1 were detected without idarubicin only under conditions favouring enzyme binding to DNA, showing that these sites are low affinity sites for topoisomerase II DNA cleavage and/or binding. Moreover, the results indicated that the sequence 5'-(A)TA/(A)-3' (the slash indicates the cleaved bond, parenthesis indicate conditioned preference) from -3 to +1 positions constitutes the complete base sequence preferred by anthracyclines. An important finding was that mutations that improve the fit to the above consensus on one strand can also increase cleavage on the opposite strand, suggesting that a drug molecule may effectively interact with one enzyme subunit only and trap the whole dimeric enzyme. These findings documented that DNA recognition by topoisomerase II may occur at one or the other strand, and not necessarily at both of them, and that the two subunits can act cooperatively to cleave a double helix.     

Drug interactions with Bacillus anthracis topoisomerase IV: biochemical basis for quinolone action and resistance

Bacillus anthracis, the causative agent of anthrax, is considered a serious threat as a bioweapon. The drugs most commonly used to treat anthrax are quinolones, which act by increasing the levels of DNA cleavage mediated by topoisomerase IV and gyrase. Quinolone resistance most often is associated with specific serine mutations in these enzymes. Therefore, to determine the basis for quinolone action and resistance, we characterized wild-type B. anthracis topoisomerase IV, the GrlA(S81F) and GrlA(S81Y) quinolone-resistant mutants, and the effects of quinolones and a related quinazolinedione on these enzymes. Ser81 is believed to anchor a water-Mg(2+) bridge that coordinates quinolones to the enzyme through the C3/C4 keto acid. Consistent with this hypothesized bridge, ciprofloxacin required increased Mg(2+) concentrations to support DNA cleavage by GrlA(S81F) topoisomerase IV. The three enzymes displayed similar catalytic activities in the absence of drugs. However, the resistance mutations decreased the affinity of topoisomerase IV for ciprofloxacin and other quinolones, diminished quinolone-induced inhibition of DNA religation, and reduced the stability of the enzyme-quinolone-DNA ternary complex. Wild-type DNA cleavage levels were generated by mutant enzymes at high quinolone concentrations, suggesting that increased drug potency could overcome resistance. 8-Methyl-quinazoline-2,4-dione, which lacks the quinolone keto acid (and presumably does not require the water-Mg(2+) bridge to mediate protein interactions), was more potent than quinolones against wild-type topoisomerase IV and was equally efficacious. Moreover, it maintained high potency and efficacy against the mutant enzymes, effectively inhibited DNA religation, and formed stable ternary complexes. Our findings provide an underlying biochemical basis for the ability of quinazolinediones to overcome clinically relevant quinolone resistance mutations in bacterial type II topoisomerases.     

Site-specific DNA cleavage by Chlorella virus topoisomerase II

The DNA cleavage reaction of topoisomerase II is central to the catalytic activity of the enzyme and is the target for a number of important anticancer drugs. Unfortunately, efforts to characterize this fundamental reaction have been limited by the low levels of DNA breaks normally generated by the enzyme. Recently, however, a type II topoisomerase with an extraordinarily high intrinsic DNA cleavage activity was isolated from Chlorella virus PBCV-1. To further our understanding of this enzyme, the present study characterized the site-specific DNA cleavage reaction of PBCV-1 topoisomerase II. Results indicate that the viral enzyme cleaves DNA at a limited number of sites. The DNA cleavage site utilization of PBCV-1 topoisomerase II is remarkably similar to that of human topoisomerase IIalpha, but the viral enzyme cleaves these sites to a far greater extent. Finally, PBCV-1 topoisomerase II displays a modest sensitivity to anticancer drugs and DNA damage in a site-specific manner. These findings suggest that PBCV-1 topoisomerase II represents a unique model with which to dissect the DNA cleavage reaction of eukaryotic type II topoisomerases.     

Topoisomerase I-mediated DNA cleavage as a guide to the development of antitumor agents derived from the marine alkaloid lamellarin D: triester derivatives incorporating amino acid residues

The marine alkaloid lamellarin D (LAM-D) has been recently characterized as a potent poison of human topoisomerase I endowed with remarkable cytotoxic activities against tumor cells. We report here the first structure-activity relationship study in the LAM-D series. Two groups of triester compounds incorporating various substituents on the three phenolic OH at positions 8, 14 and 20 of 6H-[1]benzopyrano[4',3':4,5]pyrrolo[2,1-a]isoquinolin-6-one pentacyclic planar chromophore typical of the parent alkaloid were tested as topoisomerase I inhibitors. The non-amino compounds in group A showed no activity against topoisomerase I and were essentially non cytotoxic. In sharp contrast, compounds in group B incorporating amino acid residues strongly promoted DNA cleavage by human topoisomerase I. LAM-D derivatives tri-substituted with leucine, valine, proline, phenylalanine or alanine residues, or a related amino side chain, stabilize topoisomerase I-DNA complexes. The DNA cleavage sites detected at T downward arrow G or C downward arrow G dinucleotides with these molecules were identical to that of LAM-D but slightly different from those seen with camptothecin which stimulates topoisomerase I-mediated cleavage at T downward arrow G only. In the DNA relaxation and cleavage assays, the corresponding Boc-protected compounds and the analogues of the non-planar LAM-501 derivative lacking the 5-6 double bond in the quinoline B-ring showed no effect on topoisomerase I and were considerably less cytotoxic than the corresponding cationic compounds in the LAM-D series. The presence of positive charges on the molecules enhances DNA interaction but melting temperature studies indicate that DNA binding is not correlated with topoisomerase I inhibition or cytotoxicity. Cell growth inhibition by the 41 lamellarin derivatives was evaluated with a panel of tumor cells lines. With prostate (DU-145 and LN-CaP), ovarian (IGROV and IGROV-ET resistant to ecteinascidin-743) and colon (LoVo and LoVo-Dox cells resistant to doxorubicin) cancer cells (but not with HT29 colon carcinoma cells), the most cytotoxic compounds correspond to the most potent topoisomerase I poisons. The observed correlation between cytotoxicity and topoisomerase I inhibition strongly suggests that topoisomerase I-mediated DNA cleavage assays can be used as a guide to the development of superior analogues in this series. LAM-D is the lead compound of a new promising family of antitumor agents targeting topoisomerase I and the amino acid derivatives appear to be excellent candidates for a preclinical development.     

DNA mediated resonance energy transfer from 4',6-diamidino-2-phenylindole to [Ru(1,10-phenanthroline)2L]2+

The binding site of Delta- and Lambda-[Ru(phenanthroline)2L]2+ (L being phenanthroline (phen), dipyrido[3,2-a:2'3'-c]phenazine (DPPZ), and benzodipyrido[3,2-a:2'3'-c]phenazine (benzoDPPZ)), bound to poly[d(A-T)2] in the presence and absence of 4',6-diamidino-2-phenylindole (DAPI) was investigated by circular dichroism and fluorescence techniques. DAPI binds at the minor groove of poly[d(A-T)2] and blocks the groove. The circular dichroism spectrum of all Ru(II) complexes are essentially unaffected whether the minor groove of poly[d(A-T)2] is blocked by DAPI or not, indicating that the Ru(II) complexes are intercalated from the major groove. When DAPI and Ru(II) complexes simultaneously bound to poly[d(A-T)2], the fluorescence intensity of DAPI decreases upon increasing Ru(II) complex concentrations. The energy of DAPI at excited state transfers to Ru(II) complexes across the DNA via the F?rster type resonance energy transfer. The efficiency of the energy transfer is similar for both [Ru(phen)2DPPZ]2+ and [Ru(phen)2benzoDPPZ]2+ complexes, whereas that of [Ru(phen)3]2+ is significantly lower. The distance between DAPI and [Ru(phen)3]2+ is estimated as 0.38 and 0.64 F?rster distance, respectively, for the Delta- and Lambda-isomer.     

Intercalative antitumor drugs interfere with the breakage-reunion reaction of mammalian DNA topoisomerase II

Many intercalative antitumor drugs have been shown to induce reversible protein-linked DNA breaks in cultured mammalian cells. Using purified mammalian DNA topoisomerase II, we have demonstrated that the antitumor drugs ellipticine and 2-methyl-9-hydroxyellipticine (2-Me-9-OH-E+) can produce reversible protein-linked DNA breaks in vitro. 2-Me-9-OH-E+ which is more cytotoxic toward L1210 cells and more active against experimental tumors than ellipticine is also more effective in stimulating DNA cleavage in vitro. Similar to the effect of 4'-(9-acridinylamino)-methanesulfon-m-anisidide (m-AMSA) on topoisomerase II in vitro, the mechanism of DNA breakage induced by ellipticines is most likely due to the drug stabilization of a cleavable complex formed between topoisomerase II and DNA. Protein denaturant treatment of the cleavable complex results in DNA breakage and covalent linking of one topoisomerase II subunit to each 5'-end of the cleaved DNA. Cleavage sites on pBR322 DNA produced by ellipticine or 2-Me-9-OH-E+ treatment mapped at the same positions. However, many of these cleavage sites are distinctly different from those produced by the antitumor drug m-AMSA which also targets at topoisomerase II. Our results thus suggest that although mammalian DNA topoisomerase II may be a common target of these antitumor drugs, drug-DNA-topoisomerase interactions for different antitumor drugs may be different.