Generation of Replication-Dependent Double-Strand Breaks by the Novel N2-G-Alkylator S23906-1

S23906-1, a new DNA alkylating agent that reacts with the exocyclic 2-NH2 group of guanine residues yielding monofunctional adducts, is currently under clinical evaluation in phase I trials. To investigate the mechanism of action of S23906-1, we compared parental KB-3-1 cells and KB/S23-500 cells that are 15-fold resistant to S23906-1. Cell death induced by 1 micromol/L S23906-1 in KB-3-1 cells was associated with their irreversible arrest in the G2-M phases of the cell cycle followed by apoptosis, whereas a proportion of the resistant KB/S23-500 cells were able to exit from the G2 arrest and divide, leading to a significantly lower rate of apoptosis. The attenuated apoptotic response was associated with decreased Chk2 protein phosphorylation, indicating that the DNA damage signaling pathways are more potently activated in the sensitive cells. However, similar rates of adduct formation and repair were measured in both cell lines. Exposure to S23906-1 induced a higher formation of DNA breaks, measured by the comet assay, in sensitive cells. In agreement, a histone H2AX phosphorylation assay revealed that S23906-1 induced double-strand breaks (DSB) in a dose- and time-dependent manner and that these were more persistent in the parental cells. These DSBs were found mainly in S-phase cells and inhibited by aphidicolin, suggesting that they are DNA replication-mediated DSBs. These results suggest that secondary DNA lesions play an important role in the cytotoxicity of this compound and make histone H2AX phosphorylation an attractive marker for monitoring the efficacy of S23906-1.

Molecular determinants of topoisomerase I poisoning by lamellarins: comparison with camptothecin and structure-activity relationships

A series of lamellarin derivatives have been studied as topoisomerase I (Top1) inhibitors. Molecular models of the ternary complexes formed between the DNA-Top1 ensemble and lamellarin D (LMD) or camptothecin (CPT) fully intercalated into the duplex DNA have been built and studied by means of nanosecond molecular dynamics simulations in aqueous solution. Our results show that the 20-OH and 8-OH of LMD can participate in hydrogen-bonding interactions with the side chains of Glu356 and Asn722, respectively, the latter being consistent with the finding that CEM/C2 cells, which are resistant to CPT, are cross-resistant to LMD. Our models also account for the observation that LMD stabilizes Top1 cleavage at CG sites in addition to the TG sites observed for CPT and rationalize the structure-activity relationships within the series. The deleterious effect of replacing the 20-OH in LMD with a hydrogen was confirmed using a set of thermodynamic integration free energy simulations.

Targeting DNA with novel diphenylcarbazoles

Double-stranded DNA is a therapeutic target for a variety of anticancer and antimicrobial drugs. Noncovalent interactions of small molecules with DNA usually occur via intercalation of planar compounds between adjacent base pairs or minor-groove recognition by extended crescent-shaped ligands. However, the dynamic and flexibility of the DNA platform provide a variety of conformations that can be targeted by structurally diverse compounds. Here, we propose a novel DNA-binding template for construction of new therapeutic candidates. Four bisphenylcarbazole derivatives, derived from the combined molecular architectures of known antitumor bisphenylbenzimidazoles and anti-infectious dicationic carbazoles, have been designed, and their interaction with DNA has been studied by a combination of biochemical and biophysical methods. The substitutions of the bisphenylcarbazole core with two terminal dimethylaminoalkoxy side chains strongly promote the interaction with DNA, to prevent the heat denaturation of the double helix. The deletion or the replacement of the dimethylamino-terminal groups with hydroxyl groups strongly decreased DNA interaction, and the addition of a third cationic side chain on the carbazole nitrogen reinforced the affinity of the compound for DNA. Although the bi- and tridentate molecules both derive from well-characterized DNA minor-groove binders, the analysis of their binding mode by means of circular and linear dichroism methods suggests that these compounds form intercalation complexes with DNA. Negative-reduced dichroism signals were recorded in the presence of natural DNA and synthetic AT and GC polynucleotides. The intercalation hypothesis was validated by unwinding experiments using topoisomerase I. Prominent gel shifts were observed with the di- and trisubstituted bisphenylcarbazoles but not with the uncharged analogues. These observations, together with the documented stacking properties of such molecules (components for liquid crystals), prompted us to investigate their binding to the human telomeric DNA sequence by means of biosensor surface plasmon resonance. Under conditions favorable to G4 formation, the title compounds showed only a modest interaction with the telomeric quadruplex sequence, comparable to that measured with a double-stranded oligonucleotide. Their sequence preference was explored by DNase I footprinting experiments from which we identified a composite set of binding sequences comprising short AT stretches and a few other mixed AT/GC blocks with no special AT character. The variety of the binding sequences possibly reflects the coexistence of distinct positioning of the chromophore in the intercalation sites. The bisphenylcarbazole unit represents an original pharmacophore for DNA recognition. Its branched structure, with two or three arms suitable to introduce a structural diversity, provides an interesting scaffold to built molecules susceptible to discriminate between the different conformations of nucleic acids.

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.

Synthesis of Mono- and Bisdihydrodipyridopyrazines and Assessment of Their DNA Binding and Cytotoxic Properties

Aminoalkyl-substituted monomeric and dimeric dihydrodipyridopyrazines have been synthesized and evaluated as antitumor agents. Potent cytotoxic compounds were identified in both series. Biochemical and biophysical studies indicated that all these compounds strongly stabilized the duplex structure of DNA and some of them elicited a selectivity for GC-rich sequences. Sequence recognition by of the dimeric dihydrodipyridopyrazines is reminiscent of that of certain antitumor bisnaphthalimides. Compared to monomers, corresponding dimeric derivatives showed higher affinity for DNA. This property was attributed to a bisintercalative binding to DNA. This assumption was indirectly probed by electric linear dichroism and DNA relaxation experiments. DNA provides a bioreceptor for these dihydrodipyridopyrazine derivatives, but no poisoning of human topoisomerases I or II was detected. Most of the compounds efficiently inhibited the growth of L1210 murine leukemia cells and perturbed the cell cycle progression (with a G2/M block in most cases). A weak but noticeable in vivo antitumor activity was observed with one of the dimeric compounds. This studies identifies monomeric and dimeric dihydrodipyridopyrazines as a new class of DNA-targeted antitumor agents.

Lamellarin D: a novel potent inhibitor of topoisomerase I

We report the identification and characterization of a novel potent inhibitor of DNA topoisomerase I: lamellarin D (LAM-D), initially isolated from a marine mollusk, Lamellaria sp., and subsequently identified from various ascidians. This alkaloid, which displays potent cytotoxic activities against multidrug-resistant tumor cell lines and is highly cytotoxic to prostate cancer cells, bears a 6H-[1]benzopyrano[4',3':4,5]pyrrolo[2,1-a]isoquinolin-one pentacyclic planar chromophore, whereas its synthetic 5,6-dehydro analogue, LAM-501, has a significantly tilted structure. DNA binding measurements by absorbance, fluorescence, and electric linear dichroism spectroscopy show that LAM-D is a weak DNA binder that intercalates between bp of the double helix. In contrast, the nonplanar analogue LAM-501 did not bind to DNA and failed to inhibit topoisomerase I. DNA intercalation may be required for the stabilization of topoisomerase I-DNA complexes by LAM-D. In the DNA relaxation assay, LAM-D strongly promoted the conversion of supercoiled DNA into nicked DNA in the presence of topoisomerase I. The marine product was approximately 5 times less efficient than camptothecin (CPT) at stabilizing topoisomerase I-DNA complexes, but interestingly, the two drugs exhibited slightly distinct sequence specificity profiles. Topoisomerase I-mediated DNA cleavage in the presence of LAM-D occurred at some sites common to CPT, but a few specific sites identified with CPT but not with LAM-D or conversely unique sites cleaved by LAM-D but not by CPT were detected. The distinct specificity profiles suggest that LAM-D and CPT interact differently with the topoisomerase I-DNA interface. A molecular modeling analysis provided structural information on the orientation of LAM-D within the topoisomerase I-DNA covalent complex. The marine alkaloid did not induce DNA cleavage by topoisomerase II. Immunoblotting experiments revealed that endogenous topoisomerase I was efficiently trapped on DNA by LAM-D in P388 and CEM leukemia cells. P388/CPT5 and CEM/C2 cell lines, both resistant to CPT and expressing a mutated top1 gene, were cross-resistant to LAM-D. Collectively, the results identify LAM-D as a novel lead candidate for the development of topoisomerase I-targeted antitumor agents.

DNA Sequence Recognition by Bispyrazinonaphthalimides Antitumor Agents

Bifunctional DNA intercalating agents have long attracted considerable attention as anticancer agents. One of the lead compounds in this category is the dimeric antitumor drug elinafide, composed of two tricyclic naphthalimide chromophores separated by an aminoalkyl linker chain optimally designed to permit bisintercalation of the drug into DNA. In an effort to optimize the DNA recognition capacity, different series of elinafide analogues have been prepared by extending the surface of the planar drug chromophore which is important for DNA sequence recognition. We report here a detailed investigation of the DNA sequence preference of three tetracyclic monomeric or dimeric pyrazinonaphthalimide derivatives. Melting temperature measurements and surface plasmon resonance (SPR) studies indicate that the dimerization of the tetracyclic planar chromophore considerably augments the affinity of the drug for DNA, polynucleotides, or hairpin oligonucleotides and promotes selective interaction with G.C sites. The (CH(2))(2)NH(CH(2))(3)NH(CH(2))(2) connector stabilizes the drug-DNA complexes. The methylation of the two nitrogen atoms of this linker chain reduces the binding affinity and increases the dissociation rates of the drug-DNA complexes by a factor of 10. DNase I footprinting experiments were used to investigate the sequence selectivity of the drugs, demonstrating highly preferential binding to G.C-rich sequences. It also served to select a high-affinity site encompassing the sequence 5'-GACGGCCAG which was then introduced into a biotin-labeled hairpin oligonucleotide to accurately measure the binding parameters by SPR. The affinity constant of the unmethylated dimer for this sequence is 500 times higher than that of the monomer compound and approximately 10 times higher than that of the methylated dimer. The DNA groove accessibility was also probed with three related oligonucleotides carrying G --> c(7)G, G --> I, and C --> M substitutions. The level of drug binding to the two hairpin oligonucleotides containing 7-deazaguanine (c(7)G) or 5-methylcytosine (M) residues is unchanged or only slightly reduced compared to that of the unmodified target. In contrast, incorporation of inosine (I) residues considerably decreases the extent of drug binding or even abolishes the interaction as is the case with the monomer. The pyrazinonaphthalimide derivatives are thus much more sensitive to the deletion of the exocyclic guanine 2-amino group exposed in the minor groove of the duplex than to the modification of the major groove elements. The complementary SPR footprinting methodology combining site selection and quantitative DNA affinity analysis constitutes a reliable method for dissecting the DNA sequence selectivity profile of reversible DNA binding small molecules.

Stress-induced apoptosis is impaired in cells with a lysosomal targeting defect but is not affected in cells synthesizing a catalytically inactive cathepsin D

The role of cathepsin D in stress-induced cell death has been investigated by using ovine fibroblasts exhibiting a missense mutation in the active site of cathepsin D. The cathepsin D (lysosomal aspartic protease) deficiency did not protect cells against toxicity induced by doxorubicin and other cytotoxic agents, neither did it protect cells from caspase activation. Moreover, the cathepsin D inhibitor, pepstatin A, did not prevent stress-induced cell death in human fibroblasts or lymphoblasts. The possible role of lysosomal ceramide or sphingosine-mediated activation of cathepsin D in apoptosis was also excluded by using human cells either overexpressing or deficient in acid ceramidase. However, a normal lysosomal function seems to be required for efficient cell death, as indicated by the finding that fibroblasts from patients with mucolipidosis II were partially resistant to staurosporine, sphingosine and TNF-induced apoptosis, suggesting a key role of lysosomes in cell death.

Covalent binding to glutathione of the DNA-alkylating antitumor agent, S23906-1

The benzoacronycine derivative, S23906-1, was characterized recently as a novel potent antitumor agent through alkylation of the N2 position of guanines in DNA. We show here that its reactivity towards DNA can be modulated by glutathione (GSH). The formation of covalent adducts between GSH and S23906-1 was evidenced by EI-MS, and the use of different GSH derivatives, amino acids and dipeptides revealed that the cysteine thiol group is absolutely required for complex formation because glutathione disulfide (GSSG) and other S-blocked derivatives failed to react covalently with S23906-1. Gel shift assays and fluorescence measurements indicated that the binding of S23906-1 to DNA and to GSH are mutually exclusive. Binding of S23906-1 to an excess of GSH prevents DNA alkylation. Additional EI-MS measurements performed with the mixed diester, S28053-1, showed that the acetate leaving group at the C1 position is the main reactive site in the drug: a reaction scheme common to GSH and guanines is presented. At the cellular level, the presence of GSH slightly reduces the cytotoxic potential of S23906-1 towards KB-3-1 epidermoid carcinoma cells. The GSH-induced threefold reduction of the cytotoxicity of S23906-1 is attributed to the reduced formation of lethal drug-DNA covalent complexes in cells. Treatment of the cells with buthionine sulfoximine, an inhibitor of GSH biosynthesis, facilitates the formation of drug-DNA adducts and promotes the cytotoxic activity. This study identifies GSH as a reactant for the antitumor drug, S23906-1, and illustrates a pathway by which GSH may modulate the cellular sensitivity to this DNA alkylating agent. The results presented here, using GSH as a biological nucleophile, fully support our initial hypothesis that DNA alkylation is the major mechanism of action of the promising anticancer drug S23906-1.

Holocene biomass burning and global dynamics of the carbon cycle

Fire regimes have changed during the Holocene due to changes in climate, vegetation, and in human practices. Here, we hypothesise that changes in fire regime may have affected the global CO2 concentration in the atmosphere through the Holocene. Our data are based on quantitative reconstructions of biomass burning deduced from stratified charcoal records from Europe, and South-, Central- and North America, and Oceania to test the fire-carbon release hypothesis. In Europe the significant increase of fire activity is dated approximately 6000 cal. yr ago. In north-eastern North America burning activity was greatest before 7500 years ago, very low between 7500-3000 years, and has been increasing since 3000 years ago. In tropical America, the pattern is more complex and apparently latitudinally zonal. Maximum burning occurred in the southern Amazon basin and in Central America during the middle Holocene, and during the last 2000 years in the northern Amazon basin. In Oceania, biomass burning has decreased since a maximum 5000 years ago. Biomass burning has broadly increased in the Northern and Southern hemispheres throughout the second half of the Holocene associated with changes in climate and human practices. Global fire indices parallel the increase of atmospheric CO2 concentration recorded in Antarctic ice cores. Future issues on carbon dynamics relatively to biomass burning are discussed to improve the quantitative reconstructions.

Influence of compound structure on affinity, sequence selectivity, and mode of binding to DNA for unfused aromatic dications related to furamidine

In the course of a program aimed at developing sequence-specific gene-regulatory small organic molecules, we have investigated the DNA interactions of a new series of nine diphenylfuran dications related to the antiparasitic drug furamidine (DB75). Two types of structural modifications were tested: the terminal amidine groups of DB75 were shifted from the para to the meta position, and the amidines were replaced with imidazoline or dimethyl-imidazoline groups, to test the importance of both the position and nature of positively charged groups on DNA interactions. The interactions of these compounds with DNA and oligonucleotides were studied by a combination of biochemical and biophysical techniques. Absorption and CD measurements suggested that the drugs bind differently to AT and GC sequences in DNA. The para-para dications, like DB75, bind into the minor groove of poly(dAT)(2) and intercalate between the base pairs of poly(dGC)(2), as revealed by electric linear dichroism experiments. In contrast, the meta-meta compounds exhibit a high tendency to intercalate into DNA whatever the target sequence. The lack of sequence selectivity of the meta-meta compounds containing amidines or dimethyl-imidazoline groups was also evident from DNase I footprinting and surface plasmon resonance (SPR) experiments. Accurate binding measurements using the BIAcore SPR method revealed that all nine compounds bind with similar affinity to an immobilized GC sequence DNA hairpin but exhibit very distinct affinities for the corresponding AT hairpin oligonucleotide. The minor groove-binding para-para compounds have a high specificity for AT sequences. The biophysical data clearly indicate that shifting the cationic substituents from the para to the meta position results in a loss of specificity and change in binding mode. The strong AT selectivity of the para-para compounds was independently confirmed by DNase I footprinting experiments performed with a range of DNA restrictions fragments. In terms of AT selectivity, the compounds rank in the order para-para > para-meta > meta-meta. The para dications bind preferentially to sequences containing four contiguous AT base pairs. Additional footprinting experiments with substrates containing the 16 possible [A.T](4) blocks indicated that the presence of a TpA step within an [A.T] (4) block generally reduces the extent of binding. The diverse methods, from footprinting to SPR to dichroism, provide a consistent model for the interactions of the diphenylfuran dications with DNA of different sequences. Altogether, the results attest unequivocally that the binding mode for unfused aromatic cations can change completely depending on substituent position and DNA sequence. These data provide a rationale to explain the relationships between sequence selectivity and mode of binding to DNA for unfused aromatic dications related to furamidine.

Synthesis and DNA interaction of a mixed proflavine-phenanthroline Tröger base

We report the synthesis of an asymmetric Tröger base containing the two well characterised DNA binding chromophores, proflavine and phenanthroline. The mode of interaction of the hybrid molecule was investigated by circular and linear dichroism experiments and a biochemical assay using DNA topoisomerase I. The data are compatible with a model in which the proflavine moiety intercalates between DNA base pairs and the phenanthroline ring occupies the DNA groove. DNase I cleavage experiments were carried out to investigate the sequence preference of the hybrid ligand and a well resolved footprint was detected at a site encompassing two adjacent 5'-GTC.5-GAC triplets. The sequence preference of the asymmetric molecule is compared to that of the symmetric analogues.

Recognition of ATGA sequences by the unfused aromatic dication DB293 forming stacked dimers in the DNA minor groove

Furamidine and related diamidines represent a promising series of drugs active against widespread parasites, in particular the Pneumocystic carinii pathogen. In this series, the phenylfuranbenzimidazole diamidine derivative DB293 was recently identified as the first unfused aromatic dication capable of forming stacked dimers in the DNA minor groove of GC-containing sequences. Here we present a detailed biochemical and biophysical characterization of the DNA sequence recognition properties of DB293. Three complementary footprinting techniques using DNase I, Fe(II)-EDTA, and an anthraquinone photonuclease were employed to locate binding sites for DB293 in different DNA restriction fragments. Two categories of sites were identified by DNase I footprinting: (i) 4/5 bp sequences containing contiguous A.T pairs, such as 5'-AAAA and 5'-ATTA; and (ii) sequences including the motif 5'-ATGA.5'-TCAT. In particular, a 13-bp sequence including two contiguous ATGA motifs provided a highly preferential recognition site for DB293. Quantitative footprinting analysis revealed better occupancy of the 5'-ATGA site compared to the AT-rich sites. Preferential binding of DB293 to ATGA sites was also observed with other DNA fragments and was confirmed independently by means of hydroxyl radical footprinting generated by the Fe(II)-EDTA system, as well as by a photofootprinting approach using the probe anthraquinone-2-sulfonate (AQS). In addition, this photosensitive reagent revealed the presence of sites of enhanced cutting specific to DB293. This molecule, but not other minor groove binders such as netropsin, induces specific local structural changes in DNA near certain binding sites, as independently shown by DNase I and the AQS probe. Recognition of the ATGA sequence by DB293 was investigated further using melting temperature experiments and surface plasmon resonance (SPR). The use of different hairpin oligonucleotides showed that DB293 can interact with AT sites via the formation of 1:1 drug-DNA complexes but binds much more strongly, and cooperatively, to ATGA-containing sequences to form 2:1 drug-DNA complexes. DB293 binds strongly to ATGA sequences with no significant context dependence but is highly sensitive to the orientation of the target sequence. The formation of 2:1 DB293/DNA complexes is abolished by reversing the sequence 5'-ATGA-->3'-ATGA, indicating that directionality plays an important role in the drug-DNA recognition process. Similarly, a single mutation in the A[T-->G]GA sequence is very detrimental to the dimer interactions of DB293. From the complementary footprinting and SPR data, the 5'-ATGA sequence is identified as being a highly favored dimer binding site for DB293. The data provide clues for delineating a recognition code for diamidine-type minor groove binding agents, and ultimately to guide the rational design of gene regulatory molecules targeted to specific sites of the genetic material.

Novel dications with unfused aromatic systems: trithiophene and trifuran derivatives of furimidazoline

We report the synthesis, interaction with DNA, topoisomerase II inhibition, and cytotoxicity of two novel unfused aromatic dications derived from the antimicrobial agent furimidazoline. The central diphenylfuran core of furimidazoline has been replaced with a trithiophene (DB358) or a trifuran (DB669) unit and the terminal imidazoline groups were preserved. The strength and mode of binding of the drugs to nucleic acids were investigated by complementary spectroscopic techniques including spectrophotometric, surface plasmon resonance, circular and linear dichroism measurements. The trifuran derivative forms intercalation complexes with double-stranded DNA, whereas the mode of binding of the trithiophene derivative varies depending on the drug/DNA ratio, as independently confirmed by NMR spectroscopic studies performed with (A-T)7 and (G-C)7 oligomers. Two-dimensional NMR data provided a molecular model for the binding of DB358 within the minor groove of the AATT sequence of the decanucleotide d(GCGAATTCGC)(2). DNase I footprinting experiments confirmed the sequence-dependent binding of DB358 to DNA. The trithiophene derivative interacts preferentially with AT-rich sequences at low concentrations, but can accomodate GC sites at higher concentrations. DNA relaxation assays revealed that DB358 stimulated DNA cleavage by topoisomerase II, in contrast to DB669. The substitution of N-alkylamidines for the imidazoline terminal groups abolished the capacity of the drug to poison topoisomerase II. At the cellular level, flow cytometry analysis indicated that DB358, which is about six times more cytotoxic than the trifuran analogue, induced a significant accumulation of HL-60 human leukemia cells in the G2/M phase. The incorporation of thiophene heterocycles appears as a convenient procedure to limit the strict AT selectivity of dications containing an extended unfused aromatic system and to design cytotoxic DNA intercalating agents acting as poisons for human topoisomerase II.

Sequence-dependent binding of bis-amidine carbazole dications to DNA

The conventional wisdom argues that DNA intercalators possess a condensed polyaromatic ring whereas DNA minor groove binders generally contain unfused aromatic heterocycles, frequently separated by amide bonds. Recently, this view has been challenged with the discovery of powerful intercalating agents formed by unfused aromatic molecules and groove binders containing a polyaromatic nucleus. Bis-amidinocarbazoles belong to this later category of drugs having a planar chromophore and capable of reading the genetic information accessible within the minor groove of AT-rich sequences [Tanious, F.A., Ding, D., Patrick, D.A., Bailly, C., Tidwell, R.R. & Wilson, W.D. (2000) Biochemistry 39, 12091-12101]. But in addition to the tight binding to AT sites, we show here that bis-amidinocarbazoles can also interact with GC sites. The extent and mode of binding of 2,7 and 3,6 substituted amidinocarbazoles to AT and GC sequences were investigated by complementary biochemical and biophysical methods. Absorption, fluorescence, melting temperature and surface plasmon resonance (SPR) measurements indicate that the position of the two amidine groups on the carbazole ring influences significantly the drug-DNA interaction. SPR and DNase I footprinting data confirm the AT-preference of the compounds and provide useful information on their additional interaction with GC sequences. The 3,6-carbazole binds approximately twice as strongly to the GC-containing hairpin oligomer than the 2,7-regioisomer. The high tendency of the 3,6 compound to intercalate into different types of DNA containing G.C base pairs is shown by electric linear dichroism. This work completes our understanding of the sequence-dependent DNA binding properties of carbazole dications.

[Study of flecainide in the treatment of ventricular arrhythmia]

The effect of flécaïnide (new class I antiarrhythmic) on ventricular rhythm troubles was studied in 24 patients using Holter's method. The following results were obtained. For ventricular tachycardia, two cases of suppression, three cases of distinct reduction in the number and duration of attack and one failure were recorded. For single ventricular extrasystoles, doublets and triplets, nine very good results (80 to 100 per cent reduction of arrhythmia) and four 50 to 80 per cent reductions in ventricular prematurity were obtained. Results were uncertain in two patients as the disappearance of extra-systoles was not followed by relapse after termination of treatment. There were three failures. A twenty-fifth patient was treated with flécaïnide IV followed by oral administration with uncertain results. Undesirable effects are infrequent and are often characterized by sinusal bradycardia. Blurred vision and paraesthesia were encountered. The principal biological parameters remained unchanged.