Biochemical characteristics and physiological significance of major DNA topoisomerases

Atomistic simulation on flavonoids derivatives as potential inhibitors of bacterial gyrase of Staphylococcus aureus

The bacterial DNA gyrase is an attractive target to identify the novel antibacterial agents. The flavonoid derivatives possess various biological activities such as antimicrobial, anti-inflammatory and anticancer activities. The aim of present study is to identify the potential molecule from flavonoid derivatives against Staphylococcus aureus using atomistic simulation namely Molecular Docking, Quantum Chemical and Molecular Dynamics. The molecules Cpd58, Cpd65 and Cpd70 are identified as potential molecules through molecular docking approaches by exploring through the N - H…O hydrogen bonding interactions with Asn31 and Glu35 of Gyrase B. To confirm the intramolecular charge transfer in the flavonoid derivatives, Frontier Molecular Orbital (FMO) calculation was performed at M06/6-31g(d) level in gas phase. The lowest HOMO-LUMO gap was calculated for Cpd58, Cpd65 and Cpd70 among the selected compounds used in this study. Molecular dynamics simulation were carried out for Cpd58 and Cpd70 for a time period of 50 ns and found to be stable throughout the analysis. Therefore, the identified compounds are found to be a potent inhibitor for GyrB of S. aureus that can be validated by experimental studies. Communicated by Ramaswamy H. Sarma.

High Throughput Screen for Inhibitors of Bacterial DNA Topoisomerase I Using the Scintillation Proximity Assay

We report the development of a rapid method to detect binding of supercoiled DNA to Escherichia coli topoisomerase I using the scintillation proximity assay (SPA). Streptavidin-SPA beads were coated with biotinylated topoisomerase I produced in vivo as a chimeric fusion protein. The hybrid biotinyl-fusion protein was overproduced in E. coli and purified in a single step by monomeric avidin affinity chromatography. The assay signal originates from both covalent and noncovalent binding of [3H]DNA to the SPA bead-immobilized enzyme. About 20-30% of the total [3H]DNA bound to the bead-immobilized enzyme remains associated with the bead in the presence of 0.5% SDS. The residual signal arises from the trapping of covalent [3H]DNA-enzyme complexes. The assay was employed in a high throughput screen that identified two general classes of topoisomerase inhibitors: agents that (1) inhibit DNA binding or (2) stabilize a covalent enzyme-DNA intermediate.

Designing a Single-Molecule Biophysics Tool for Characterising DNA Damage for Techniques that Kill Infectious Pathogens Through DNA Damage Effects

Antibiotics such as the quinolones and fluoroquinolones kill bacterial pathogens ultimately through DNA damage. They target the essential type IIA topoisomerases in bacteria by stabilising the normally transient double-strand break state which is created to modify the supercoiling state of the DNA. Here we discuss the development of these antibiotics and their method of action. Existing methods for DNA damage visualisation, such as the comet assay and immunofluorescence imaging can often only be analysed qualitatively and this analysis is subjective. We describe a putative single-molecule fluorescence technique for quantifying DNA damage via the total fluorescence intensity of a DNA origami tile fully saturated with an intercalating dye, along with the optical requirements for how to implement these into a light microscopy imaging system capable of single-molecule millisecond timescale imaging. This system promises significant improvements in reproducibility of the quantification of DNA damage over traditional techniques.

Relationship of cellular topoisomerase IIα inhibition to cytotoxicity and published genotoxicity of fluoroquinolone antibiotics in V79 cells

Fluoroquinolone (FQ) antibiotics are bacteriocidal through inhibition of the bacterial gyrase and at sufficient concentrations in vitro, they can inhibit the homologous eukaryotic topoisomerase (TOPO) II enzyme. FQ exert a variety of genotoxic effects in mammalian systems through mechanisms not yet established, but which are postulated to involve inhibition of TOPO II enzymes. To assess the relationship of inhibition of cell nuclear TOPO II to cytotoxicity and reported genotoxicity, two FQ, clinafloxacin (CLFX) and lomefloxacin (LOFX), having available genotoxicity data showing substantial differences with CLFX being more potent than LOFX, were selected for study. The relative inhibitory activities of these FQ on nuclear TOPO IIα in cultured Chinese hamster lung fibroblasts (V79 cells) over dose ranges and at equimolar concentrations were assessed by measuring nuclear stabilized cleavage complexes of TOPO IIα-DNA. Cytotoxicity was measured by relative cell counts. Both FQ inhibited V79 cell nuclear TOPO IIα. The lowest-observed-adverse-effect levels for TOPO IIα inhibition were 55 μM for CLFX, and 516 μM for LOFX. The no-observed-adverse-effect-levels were 41 μM for CLFX, and 258 μM for LOFX. At equimolar concentrations (175 μM), CLFX was more potent than LOFX. Likewise, CLFX was more cytotoxic than LOFX. Thus, the two FQ, inhibited TOPO IIα in intact V79 cells, differed in their potencies and exhibited no-observed-adverse-effect levels. These findings are in concordance with published genotoxicity data and observed cytotoxicity.

Camptothecin-induced imbalance in intracellular cation homeostasis regulates programmed cell death in unicellular hemoflagellate Leishmania donovani

Leishmania, a unicellular trypanosomatid protozoan parasite, causes a wide range of human diseases ranging from the localized self-healing cutaneous lesions to fatal visceral leishmaniasis. However, it undergoes a process of programmed cell death during treatment with the topoisomerase I poison camptothecin (CPT). The present study shows that CPT-induced formation of reactive oxygen species increases the level of cytosolic calcium through the release of calcium ions from intracellular stores as well as by influx of extracellular calcium. Elevation of cytosolic calcium is responsible for depolarization of mitochondrial membrane potential (DeltaPsim), which is followed by a significant decrease in intracellular pH levels. CPT-induced oxidative stress also causes impairment of the Na+ - K+ -ATPase pump and subsequently decreases the intracellular K+ level in leishmanial cells. A decrease in both intracellular pH and K+ levels propagates the apoptotic process through activation of caspase 3-like proteases by rapid formation of cytochrome c-mediated apoptotic complex. In addition to caspase-like protease activation, a lower level of intracellular K+ also enhances the activation of apoptotic nucleases at the late stage of apoptosis. This suggests that the physiological level of pH and K+ are inhibitory for apoptotic DNA fragmentation and caspase-like protease activation in leishmanial cells. Moreover, unlike mammalian cells, the intracellular ATP level gradually decreases with an increase in the number of apoptotic cells after the loss of DeltaPsim. Taken together, the elucidation of biochemical events, which tightly regulate the process of growth arrest and death of Leishmania donovani promastigotes, allows us to define a more comprehensive view of cell death during treatment with CPT.

Camptothecin induced mitochondrial dysfunction leading to programmed cell death in unicellular hemoflagellate Leishmania donovani

The parasites of the order kinetoplastidae including Leishmania spp. emerge from most ancient phylogenic branches of unicellular eukaryotic lineages. In their life cycle, topoisomerase I plays a significant role in carrying out vital cellular processes. Camptothecin (CPT), an inhibitor of DNA topoisomerase I, induces programmed cell death (PCD) both in the amastigotes and promastigotes form of L. donovani parasites. CPT-induced cellular dysfunction in L. donovani promastigotes is characterized by several cytoplasmic and nuclear features of apoptosis. CPT inhibits cellular respiration that results in mitochondrial hyperpolarization taking place by oligomycin-sensitive F0-F1 ATPase-like protein in leishmanial cells. During the early phase of activation, there is an increase in reactive oxygen species (ROS) inside cells, which causes subsequent elevation in the level of lipid peroxidation and decrease in reducing equivalents like GSH. Endogenous ROS formation and lipid peroxidation cause eventual loss of mitochondrial membrane potential. Furthermore, cytochrome c is released into the cytosol in a manner independent of involvement of CED3/CPP32 group of proteases and unlike mammalian cells it is insensitive to cyclosporin A. These events are followed by activation of both CED3/CPP32 and ICE group of proteases in PCD of Leishmania. Taken together, our study indicates that different biochemical events leading to apoptosis in leishmanial cells provide information that could be exploited to develop newer potential therapeutic targets.

A Target-Specific Cellular Assay for Screening of Topoisomerase I Inhibitors

We have developed a cellular, target-specific high-throughput assay for the detection of topoisomerase I inhibitors. Topoisomerase I is a nonessential enzyme involved in controlling DNA topology. Topoisomerase I is the target of anticancer drugs such as camptothecin as well as a candidate target for new antifungal drugs. A wild-type Saccharomyces cerevisiae strain and its isogenic topoisomerase I deletion mutant (DeltatopI) were labeled with S65T and wild-type green fluorescent protein (GFP), respectively. We showed that the growth of such a pair of S. cerevisiae strains labeled with this GFP combination can be independently quantified after both strains were mixed. When growth of the mixture of wild-type and DeltatopI strain was monitored in the presence of compounds, only growth of the wild-type strain was inhibited by the topoisomerase I-specific drug camptothecin. In contrast, amphotericin B, a broad-spectrum antifungal drug, inhibited growth of both strains. The two strains were used to screen compound libraries. While 0.9% of all compounds inhibited growth of both strains, only 0.06% inhibited the wild-type but not the DeltatopI strain. Thus, by using a DeltatopI strain as internal control in the same assay mixture, the number of candidate topoisomerase I inhibitors to be retested could be reduced by more than 90%. Further applications of this type of S. cerevisiae-based cellular high-throughput assays will be discussed.

Design and synthesis of modified quinolones as antitumoral acridones

The bacterial topoisomerase II (DNA gyrase) and the mammalian topoisomerase II represent the cellular targets for quinolone antibacterials and a wide variety of anticancer drugs, respectively. In view of the mechanistic similarities and sequence homologies exhibited by the two enzymes, tentative efforts to selectively shift from an antibacterial to an antitumoral activity was made by synthesizing a series of modified tricyclic quinolones, in which the essential 3-carboxylic function is surrogated by phenolic OH and the classic C-6 fluorine atom is replaced by a NH2 group. The resulting 7-amino-9-acridone derivatives were assayed for their antibacterial as well as cytotoxic activities. No antibacterial activity was found. On the other hand, many derivatives showed significant cytotoxic activity against both HL-60 and P388 leukemias and a wide panel of human and rodent solid tumor cells, derivatives 25 and 26 displaying the best overall antiproliferative activity. Against the LoVo cell line, derivative 25 exhibited higher cytotoxic effects than etoposide.

The role of environmental factors in the regulation of virulence-determinant expression in Staphylococcus aureus 8325-4

Staphylococcus aureus is a major human pathogen, which produces a variety of virulence determinants. To study environmental regulation of virulence-determinant production, several transcriptional reporter gene fusions were constructed. Chromosomal fusions were made with the staphylococcal accessory regulator (sarA), alpha-haemolysin (hla), surface protein A (spa) and toxic-shock syndrome toxin-1 (tst) genes. The effect of many different environmental conditions on the expression of the fusions was examined. Expression of hla, tst and spa was strongly repressed in the presence of sodium chloride (1 M) or sucrose (20 mM), but sarA was relatively unaffected. The global regulator of expression of virulence-determinant genes, agr (accessory gene regulator) was not involved in the salt or sucrose repression. Novobiocin, a DNA gyrase inhibitor, did not significantly increase the expression of tst in wild-type or agr backgrounds and failed to relieve the salt suppression. Expression of tst was strongly stimulated in several low-metal environments, independently of agr, whilst spa levels were significantly reduced by EGTA. The complex, interactive role of environmental factors in the control of expression of the virulence determinants is discussed.

Phase II study of daily oral etoposide in children with recurrent brain tumors and other solid tumors

Pre-clinical data and adult experience suggests that topoisomerase targeted anti-cancer agents may be highly schedule dependent, and efficacy may improve with prolonged exposure. To investigate this hypothesis, 28 children with recurrent brain and solid tumors were enrolled in a phase II study of oral etoposide (ETP). Patients were prescribed ETP at 50 mg/m2/ day for 21 consecutive days. Courses were repeated every 28 days pending bone marrow recovery. Evaluation of response was initially performed after 8 weeks and then every 12 weeks either by CT or MRI. Three of 4 patients with PNET (primitive neuroectodermal tumor)/medulloblastora achieved a partial response (PR). Two of 5 with ependymoma responded, one with a complete response and one with a PR. Toxicity was manageable with only 1 admission for fever and neutropenia in 120 cycles of therapy. Five patients had grade 3 or 4 neutropenia. One had grade 4 thrombocytopenia and one grade 2 mucositis and withdrew as a result. One patient had grade 2 diarrhea. Two patients who achieved a PR had received ETP as part of prior combination chemotherapy regimens. Daily oral etoposide is active in recurrent PNET/medulloblastoma and ependymoma. Toxicity is manageable and rarely requires intervention. Daily oral etoposide in combination with crosslinking agents should be considered in future phase III trials. Determination of activity in glioma and solid tumors is not complete.

Pleiotropic over-expression of multidrug-resistance-related genes is correlated to MYCN and max mRNA accumulation during tumour progression in the IGR-N-91 human neuroblastoma model

An experimental model of advanced human neuroblastoma, IGR-N-91, which is able to disseminate in the nude mouse, has been described. The present study was designed to ascertain which cell population from the IGR-N-91 primary tumour actually disseminates throughout the animals. In s.c. IGR-N-91 tumour xenografts, 3 areas, called pearly, vascularized and haemorrhagic, depending on the presence of blood vessels and haemorrhagic suffusions, were consistently observed and independently resected. Molecular analysis of tumour materials revealed a significant increase in MYCN and max gene transcript levels in the haemorrhagic area, as compared with the pearly and vascularized areas. Given the growth kinetics observed both in vitro and in vivo, and the DNA flow-cytometry profiles of tumour cells obtained from the haemorrhagic area, this transcriptional increase did not appear to be associated with enhanced proliferation. In this area of the tumours, multidrug-resistance-related genes, i.e., MDRI, MRP, GST-pi and topoisomerase II alpha were activated concomitantly with MYCN and max genes. The same observations were made, except for the topoisomerase-II alpha gene, when sub-lines derived from metastases were compared with that derived from the primary tumour. These data demonstrate that over-expression of several genes determining the multi-drug-resistance phenotype precedes the metastatic spread of IGR-N-91 NB tumour cells in the nude mouse. Data also suggest that the cell sub-population exhibiting this pleiotropic over-expression within the primary tumour undergoes selection during metastatic dissemination.

Sequence analysis, purification, and study of inhibition by 4-quinolones of the DNA gyrase from Mycobacterium smegmatis

We determined the nucleotide sequence of a 6-kb DNA region harboring the recF, orf192, gyrB, and gyrA genes from Mycobacterium smegmatis mc(2)155. The amino acid sequences deduced from gyrA and gyrB displayed 89 and 86% identity, respectively, with the DNA gyrase from Mycobacterium tuberculosis, and 67 and 65% identity, respectively, with that from Streptomyces coelicolor. An open reading frame encoding the C-terminal region of the M. smegmatis RecF polypeptide was found upstream from gyrB and was 57% identical to the open reading frame encoding the C-terminal region of the S. coelicolor RecF protein. The gene orf192 was identified between recF and gyrB and was 39% identical to orf191 found in S. coelicolor in the recF-gyrB region. The M. smegmatis DNA gyrase, which was purified by affinity chromatography on novobiocin-Sepharose, consisted of two polypeptides with apparent molecular masses of 98 and 80 kDa. Determination of the N-terminal amino acid sequence of the B subunit confirmed GTG as the start codon in gyrB. Analysis of the supercoiling activity of the enzyme indicated that the M. smegmatis DNA gyrase was characterized by a specific activity equivalent to that of the Escherichia coli DNA gyrase. Inhibition of this activity by 4-quinolones was investigated by determining the 50% inhibitory concentrations (IC50S) of nalidixic acid, ofloxacin, and ciprofloxacin. The results indicated that the inhibitory activities of these drugs against the M. smegmatis DNA gyrase were markedly lower than those previously reported for the E. coli DNA gyrase. The results also suggested that the higher levels of activity of ofloxacin and ciprofloxacin against M. smegmatis (MICs, 0.5 to 1 microgram/ml), in contrast to that of nalidixic acid (MIC, 256 micrograms/ml), could be related to the higher inhibitory activities of fluoroquinolones against the DNA gyrase from this species (IC50S, 7 to 14 micrograms/ml) compared with that of nalidixic acid (IC50, 1,400 micrograms/ml).

Trovafloxacin is active against Toxoplasma gondii

Drugs currently used for treatment of toxoplasmosis in pregnant women, congenital infections, immunocompromised patients, and patients with the ocular disease are not always effective or may be dangerous to use; therefore, there is a need for more-effective and less-toxic drugs. Recently, we examined a group of fluoroquinolones for in vitro and in vivo activities against Toxoplasma gondii. Among those examined in vitro (ciprofloxacin, fleroxacin, ofloxacin, temafloxacin, and trovafloxacin), only trovafloxacin significantly inhibited intracellular replication of T. gondii without significant toxicity for host cells. In a murine model of acute toxoplasmosis, 100 or 200 mg of trovafloxacin per kg of body weight per day for 10 days protected 100% of infected mice against death. A dose of 50 mg/kg/day protected 90% of the mice, and a dose of 25 mg/kg/day effected prolongation of time to death. The other fluoroquinolones did not have such in vivo activities. These results indicate that trovafloxacin may be useful for treatment of toxoplasmosis in humans.

In vitro induction of micronuclei and chromosome aberrations by quinolones: possible mechanisms

The bacterial gyrase inhibitors, ciprofloxacin and PD 124816, were tested for clastogenic and aneugenic activity in V79 Chinese hamster lung cells in vitro. Cells were exposed for 3 h, washed free of drug, and subcultured for assessment of various endpoints. For structural chromosomal aberration (SCA) analysis, cells were incubated for 18 h, and treated with Colcemid for 2 h before harvest. For micronucleus (MN) analysis, treated cells were incubated with cytochalasin B (CYB) for 16 h. Aneugenicity was assessed by utilizing antikinetochore antibody to detect kinetochore-containing (K +) MN. Both quinolones induced significant increases in SCAs and MN, indicating clastogenic activity. With both compounds, however, the MN response was apparent at lower doses, and remained much higher throughout the dose range than the SCA response. The induced MN were predominantly K --, indicating that aneugenicity was not playing a major role in their induction. A possible explanation for the chromosome effects is that cross-reactivity of the gyrase inhibitors with mammalian topoisomerase II interferes with the separation of chromatids at anaphase leading to chromosome breaks and MN. Quinolones are known to inhibit resolution of the normally transient topoisomerase II-DNA cleavable complex, which may result in chromosome stickness. Thus, SCAs detected in metaphase cells may be attributed to quinolone-induced inhibition of topoisomerase II prior to mitosis while MN arise in binucleated cells as a result of this effect which interferes with chromatid separation during anaphase.

Mammalian mitochondrial DNA topoisomerase I preferentially relaxes supercoils in plasmids containing specific mitochondrial DNA sequences

Selected regions of mammalian mitochondrial DNA (mtDNA) were inserted into pGEM plasmid vectors and used as substrates in a kinetic analysis of the highly purified bovine mitochondrial type I topoisomerase. Recombinant plasmids containing the bovine mtDNA heavy and light strand origins of replication (pZT-Hori and pZT-Lori, respectively), a major transcription termination region (pZT-Term) and a portion of cytochrome b gene (pZT-Cytb) were prepared. Southern hybridization using probes specific for either control or mtDNA-containing plasmid indicated a relative preference by the mitochondrial topoisomerase I to relax supercoils in pZT-Hori and pZT-Term. Quantitative determination of kinetic parameters derived from double-reciprocal Lineweaver-Burk plots showed that recombinant plasmids containing the heavy and light strand origins and the transcription termination region were preferentially relaxed by the mitochondrial enzyme with Km values 2.3- to 3.3-fold lower than controls. The Km values for pZT-Hori, pZT-Lori and pZT-Term were 21.0 +/- 0.9 microM, 25.2 +/- 1.0 microM and 17.0 +/- 0.8 microM, respectively, while those for control plasmids were 57.5 +/- 2.1 microM and 56.3 +/- 2.3 microM. pZT-Cytb was not preferentially relaxed compared to the control plasmid (Km = 53.4 +/- 2.0 microM vs. 56.3 +/- 2.3 microM, respectively) indicating that mitochondrial topoisomerase I preferentially interacts with certain mtDNA sequences but not others. Identical experiments with the purified nuclear enzyme did not differentiate between control or mtDNA containing plasmids.

Antitrypanosomal activity of camptothecin analogs. Structure-activity correlations

African trypanosomes (Trypanosoma brucei species) are parasitic protozoa that cause lethal diseases in humans and cattle. Previous studies showed that camptothecin, a potent and specific inhibitor of DNA topoisomerase I, is cytotoxic to African trypanosomes and related pathogenic hemoflagellates (Bodley AL and Shapiro TA, Proc Natl Acad Sci USA 92: 3726-3730, 1995). In this study, a series of camptothecin analogs was tested against axenically cultured, bloodstream form, T. brucei. Modifications to the pentacyclic nucleus of camptothecin ablated antiparasitic activity. In contrast, activity could be increased by substituents added to the parent ring system (e.g. 10,11-methylenedioxy or ethylenedioxy groups; alkyl additions to carbon 7; or 9-amino or 9-chloro substituents). Cytotoxicity was correlated with the level of cleavable complexes in trypanosomes, implicating topoisomerase I as the intracellular target for these compounds. To obtain some indication of selective toxicity, ten compounds were also tested against L1210 mouse leukemia cells. The 9-substituted-10,11-methylenedioxy analogs caused a disproportionate increase in antiparasitic activity, compared with mammalian cell toxicity. These findings provide a basis for designing further structural modifications and for selecting camptothecin analogs to test in animal models of trypanosomiasis.

Comparison of aza-anthracenedione-induced DNA damage and cytotoxicity in experimental tumor cells

Aza-anthracenediones are a new class of anti-cancer drugs, which demonstrate promising in vitro and in vivo activity. Our laboratory has synthesized a variety of structural analogs in which we determined previously that the positioning of the nitrogen within the backbone, as well as sidearm modification, results in dramatic differences in the potency of cytotoxicity. We reported previously that although DNA reactivity appears to be a necessary component for mediating cell death, it is not sufficient for predicting cytotoxicity of the aza-anthracenediones. We have chosen three aza-anthracenediones (BBR 2828, BBR 2778 and BBR 2378) to investigate the importance of DNA strand breaks and/or protein-concealed DNA breaks induced by aza-anthracenediones. We determined in the present study that, while all three drugs cause DNA breaks as determined by alkaline and neutral elution, as well as KCl-SDS precipitation, these breaks do not correlate directly with their potency as cytotoxic compounds. Further, we found significant differences in the types of DNA breaks induced by these drugs. Finally, we report that the persistence of protein-DNA complexes induced by all three drugs was similar and, therefore, cannot account for differences in the potency of cytotoxicity of the aza-anthracenediones. Thus, we postulate that, while the total number of drug-induced protein-concealed DNA breaks is an important indicator of drug toxicity, it is possible that the actual nature of the breaks may differ among the aza-anthracenedione congeners, and it is these differences in the actual proteins present in the DNA breaks that differentiate between aza-anthracenediones.

Incidence of various gyrA mutants in 451 Staphylococcus aureus strains isolated in Japan and their susceptibilities to 10 fluoroquinolones

Point mutations in the gyrA genes of 451 clinical strains of Staphylococcus aureus isolated in Japan were detected by a combination of nonradioisotopic single-strand conformation polymorphism analysis and restriction fragment length polymorphism analysis and by direct sequencing. Six types of gyrA mutations were observed in 149 of the 451 strains (33%), and ofloxacin MICs were greater than 6.25 micrograms/ml for 147 of the 149 strains (98.7%). These mutations were localized between codons 84 and 88, and they were associated with fluoroquinolone resistance. Two types of silent mutations were also found. Among these eight types of mutations, three types are novel, i.e., the serine at position 84 (Ser-84)-->Val (TCA-->GTA), Ser-84-->Leu (TCA-->TTA) plus Ile-86 (ATT-->ATC, silent), and Phe-110 (TTT-->TTC, silent). Among GyrA mutants, strains with a Ser-84-->Leu alteration and strains with a Glu-88-->Lys alteration were dominant. In contrast, few strains had Ser-84-->Ala and Glu-88-->Gly alterations. All fluoroquinolones tested showed greater than a fourfold decrease in their activities in terms of their MICs that inhibited 50% of strains tested for each GyrA mutant, in comparison with their MICs that inhibited 50% of strains tested for susceptible strains. Most of the currently available fluoroquinolones, such as norfloxacin, enoxacin, ofloxacin, ciprofloxacin, tosufloxacin, lomefloxacin, sparfloxacin, and fleroxacin, were ineffective against each mutant. Mutants containing a Ser-84-->Leu or Val alteration showed high-level resistance to fluoroquinolones, and one containing a Ser-84-->Ala alteration showed relatively low-level resistance. Double mutations were associated with a higher level of resistance than single mutations.

Characterization of the binding site for cyclothialidine on the B subunit of DNA gyrase

The mechanism of inhibition of DNA gyrase by cyclothialidine, a novel gyrase inhibitor isolated from Streptomyces filipinensis NR0484, has been studied further by using [14C]benzoylcyclothialidine and a reconstituted Escherichia coli gyrase system consisting of the A subunit, the B subunit and relaxed ColE1 DNA. The mechanism of inhibition was also studied with the 43-kDa N-terminal fragment of the B subunit. The [14C]benzoylcyclothialidine could bind to the B subunit alone but not to the A subunit nor to the plasmid DNA alone. Furthermore, the compound also bound to the 43-kDa N-terminal fragment of the B subunit. Scatchard analysis of [14C]benzoylcyclothialidine binding to DNA gyrase showed that the binding affinity of the compound increased, depending on the assembly of the gyrase (A2B2). DNA complex. This suggests that the binding site of cyclothialidine on the B subunit or its vicinity causes a conformational change during the assembly of the gyrase.DNA complex (increase in affinity: B-->A2B2-->A2B2.DNA). Furthermore, displacement curves of [14C]benzoylcyclothialidine binding by nonlabeled cyclothialidine, ATP analogues, and coumarin antibiotics indicated that cyclothialidine, coumarins, and ATP share a common (or overlapping) site of action on the B subunit of DNA gyrase; however, the microenvironment of the binding sites may differ.

Human immunodeficiency virus type 1 reverse transcriptase: enhancement of activity by interaction with cellular topoisomerase I

A number of studies have suggested that topoisomerase I (topo I) activity may be important in human immunodeficiency virus type 1 (HIV-1) replication. Specifically it has been reported that purified virus particles have topo I activity and that inhibitors of this enzyme can inhibit virus replication in vitro. We have investigated a possible association of HIV-1 gag proteins with topo I activity. We found that whereas the gag-encoded proteins by themselves do not have activity, the nucleocapsid protein p15 can interact with and enhance the activity of cellular topo I. Furthermore it could be demonstrated that topo I markedly enhanced HIV-1 reverse transcriptase activity in vitro and that this could be inhibited by the topo I-specific inhibitor camptothecin. The findings suggest that cellular topo I plays an important role in the reverse transcription of HIV-1 RNA and that the recruitment of this enzyme may be an important step in virus replication.

Molecular and cytotoxic effects of camptothecin, a topoisomerase I inhibitor, on trypanosomes and Leishmania

Parasites pose a threat to the health and lives of many millions of human beings. Among the pathogenic protozoa, Trypanosoma brucei, Trypanosoma cruzi, and Leishmania donovani are hemoflagellates that cause particularly serious diseases (sleeping sickness, Chagas disease, and leishmaniasis, respectively). The drugs currently available to treat these infections are limited by marginal efficacy, severe toxicity, and spreading drug resistance. Camptothecin is an established antitumor drug and a well-characterized inhibitor of eukaryotic DNA topoisomerase I. When trypanosomes or leishmania are treated with camptothecin and then lysed with SDS, both nuclear and mitochondrial DNA are cleaved and covalently linked to protein. This is consistent with the existence of drug-sensitive topoisomerase I activity in both compartments. Camptothecin also inhibits the incorporation of [3H]thymidine in these parasites. These molecular effects are cytotoxic to cells in vitro, with EC50 values for T. brucei, T. cruzi, and L. donovani, of 1.5, 1.6, and 3.2 microM, respectively. For these parasites, camptothecin is an important lead for much-needed new chemotherapy, as well as a valuable tool for studying topoisomerase I activity.

In vivo inhibition of trypanosome mitochondrial topoisomerase II: effects on kinetoplast DNA maxicircles

Kinetoplast DNA, the mitochondrial DNA of trypanosomes, is a topologically complex structure composed of interlocked minicircles and maxicircles. We previously reported that etoposide, a potent inhibitor of topoisomerase II, promotes the cleavage of about 20% of network minicircle DNA (T. A. Shapiro, V. A. Klein, and P. T. Englund, J. Biol. Chem. 264:4173-4178, 1989). We now find that virtually all maxicircles are released from kinetoplast DNA networks after trypanosomes are treated with etoposide. As expected for a topoisomerase II cleavage product, the linearized maxicircles have protein bound to both 5' ends. After etoposide treatment, the residual minicircle catenanes have a sedimentation coefficient which is only 70% that of controls, and by electron microscopy the networks are less compact. Double-size networks, the characteristic dumbbell-shape forms that normally arise in the final stages of network replication, are replaced by aberrant unit-size forms.

Mechanism of inhibition of DNA gyrase by cyclothialidine, a novel DNA gyrase inhibitor

We investigated how cyclothialidine (Ro 09-1437), a novel DNA gyrase inhibitor belonging to a new chemical class of compounds, acts to inhibit Escherichia coli DNA gyrase. Cyclothialidine up to 100 micrograms/ml showed no effect on DNA gyrase when linear DNA was used as a substrate. Under the same conditions, quinolones, which inhibit the resealing reaction of DNA gyrase, caused a decrease in the amount of linear DNA used. No effect of cyclothialidine was observed on the accumulation of the covalent complex of DNA and the A subunit of DNA gyrase induced by ofloxacin in the absence of ATP. The effect of cyclothialidine on the DNA supercoiling reaction was antagonized by ATP, reducing the inhibitory activity 11-fold as the ATP concentration was increased from 0.5 to 5 mM. Cyclothialidine competitively inhibited the ATPase activity of DNA gyrase (Ki = 6 nM). The binding of [14C]benzoyl-cyclothialidine to E. coli gyrase was inhibited by ATP and novobiocin, but not by ofloxacin. These results suggest that cyclothialidine acts by interfering with the ATPase activity of the B subunit of DNA gyrase. Cyclothialidine was active against a DNA gyrase resistant to novobiocin, suggesting that its precise site of action might be different from that of novobiocin.

In vivo inhibition of trypanosome mitochondrial topoisomerase II: effects on kinetoplast DNA maxicircles

Kinetoplast DNA, the mitochondrial DNA of trypanosomes, is a topologically complex structure composed of interlocked minicircles and maxicircles. We previously reported that etoposide, a potent inhibitor of topoisomerase II, promotes the cleavage of about 20% of network minicircle DNA (T. A. Shapiro, V. A. Klein, and P. T. Englund, J. Biol. Chem. 264:4173-4178, 1989). We now find that virtually all maxicircles are released from kinetoplast DNA networks after trypanosomes are treated with etoposide. As expected for a topoisomerase II cleavage product, the linearized maxicircles have protein bound to both 5' ends. After etoposide treatment, the residual minicircle catenanes have a sedimentation coefficient which is only 70% that of controls, and by electron microscopy the networks are less compact. Double-size networks, the characteristic dumbbell-shape forms that normally arise in the final stages of network replication, are replaced by aberrant unit-size forms.

Complexing of amino acids to DNA by chromate in intact cells

Using o-pthaldialdehyde (OPT) fluorescence, the amino acids associated with DNA were studied following exposure of intact Chinese hamster ovary cells to chromate. Rigorous extraction with EDTA, acid, or base was required to release the amino acids cross-linked to the DNA isolated from control or chromate-treated cells by standard procedures (i.e., proteinase K, phenol, etc.). Amino acids resisting extraction from DNA were not studied since analysis was limited to those that could be released by these procedures. There was a chromate dose-dependent increase in amino acids complexed with the DNA that could be released by EDTA, acid, and base, and these amino acids were separated by HPLC and identified. Substantial increases in cysteine, glutamine, glutamic acid, histidine, threonine, and tyrosine were found as a function of increasing concentrations of chromate. There was also a time-dependent increase in complexing of these amino acids to the DNA by chromate. The amino acids found complexed to DNA in intact cells by chromate were thought to originate from reactions of free amino acids or small peptides with the DNA rather than being proteolytic products derived from larger proteins that were cross-linked to the DNA. This was supported by a number of experiments: a) free amino acids or bovine serum albumin (BSA) were cross-linked by chromium to DNA in vitro and the DNA was isolated by standard procedures.(ABSTRACT TRUNCATED AT 250 WORDS)

New chemotherapeutic agents for breast cancer

The drug discovery programs of the National Cancer Institute and the pharmaceutical industry recently have provided oncologists with a wide array of new chemotherapeutic agents that have considerable potential for breast cancer treatment. Foremost among these new agents are the taxanes, of which paclitaxel and docetaxel, the only members of this class currently in clinical use, have been associated with impressive response rates in patients with metastatic disease. Importantly, they display some evidence clinically of not being cross-resistant with the anthracyclines. Efforts now are being directed toward optimizing dose and schedule in the metastatic setting while integrating these agents into standard adjuvant regimens. Other agents that have undergone Phase II testing in breast cancer include vinorelbine, edatrexate, and losoxantrone. It remains to be determined, however, whether these drugs possess substantial advantages over other members of their class. Newer compounds, such as pyrazoloacridine, ICI D1694, topoisomerase-I inhibitors, temozolomide, penclomidine, fumagillin (TNP-470), and differentiators like the retinoids, hold substantial promise because of their unique mechanisms of action; however, Phase II testing of these agents is just beginning. Although alternative approaches to treatment, such as gene therapies, monoclonal antibodies, and growth factor inhibitors, are likely to have a positive impact, it is probable that progress will best be made by combining these strategies with chemotherapy. Therefore, continuation of the search for more effective chemotherapeutic agents should remain a high priority.

Inhibition of topoisomerase II by a novel antitumor cyclic depsipeptide, BE-22179

BE-22179, a novel cyclic depsipeptide antibiotic having two 3-hydroxyquinoline moieties, inhibited the DNA-relaxing activity of L1210 topoisomerase II completely at 0.08 microM. This effect was far stronger than that of VP-16. However, it did not show any marked effect on topoisomerase II-mediated DNA cleavage. BE-22179 was ineffective in inhibiting the DNA relaxation by topoisomerase I at concentrations up to 10 microM, but showed DNA-intercalating ability (DNA unwinding) at 30 microM. The structure of BE-22179 is quite novel for a topoisomerase II inhibitor. Echinomycin, a quinoxaline antibiotic structurally related to BE-22179, interfered with DNA relaxation by topoisomerase II, though the effect was not due to inhibition of the catalytic activity of topoisomerase II but to conformational change of DNA based on its intercalation into DNA. Therefore, the potent inhibitory activity on topoisomerase II might not be a common activity of quinoxaline antibiotics, but might rather be specific to BE-22179. BE-22179 prevented DNA synthesis as well as RNA synthesis in L1210 cells and inhibited the growth of the cells. However, it remains unclear to what extent the topoisomerase II inhibition was responsible for the cytotoxicity of BE-22179.

Biological characterization of cyclothialidine, a new DNA gyrase inhibitor

Cyclothialidine is a new DNA gyrase inhibitor isolated from Streptomyces filipinensis NR0484. Structurally, it belongs to a new class of natural products containing a unique 12-membered lactone ring that is partly integrated into a pentapeptide chain. Cyclothialidine was found to be one of the most active of all the DNA gyrase inhibitors tested in the DNA supercoiling reaction of Escherichia coli DNA gyrase; 50% inhibitory concentrations (in micrograms per milliliter) of 0.03 (cyclothialidine), 0.06 (novobiocin), 0.06 (coumermycin A1), 0.66 (norfloxacin), 0.88 (ciprofloxacin), and 26 (nalidixic acid) were found. In addition, DNA gyrases from gram-positive species were inhibited equally as well as DNA gyrase from E. coli. Cyclothialidine also inhibited the in vitro DNA replication directed from oriC of E. coli. Among the bacterial species tested, only Eubacterium spp. were inhibited by cyclothialidine, suggesting that it can enter the cells of Eubacterium and exert antibacterial activity through interference with the DNA gyrase within the cells, although its penetration into most bacterial cells appears to be poor. These results provide a basis for cyclothialidine to be a lead structure for novel antibacterial agents with DNA gyrase inhibitory activities.

Drug features that contribute to the activity of quinolones against mammalian topoisomerase II and cultured cells: correlation between enhancement of enzyme-mediated DNA cleavage in vitro and cytotoxic potential

CP-115,953 [6,8-difluoro-7-(4'-hydroxyphenyl)-1-cyclopropyl-4- quinolone-3-carboxylic acid] is a novel quinolone that is highly active against topoisomerase II in vitro and in mammalian cells in culture (M. J. Robinson, B. A. Martin, T. D. Gootz, P. R. McGuirk, M. Moynihan, J. A. Sutcliffe, and N. Osheroff, J. Biol. Chem. 266:14585-14592, 1991). However, the features of the drug that contribute to its activity towards mammalian systems have not been characterized. Therefore, CP-115,953 and a series of related quinolones were examined for their activity against calf thymus topoisomerase II and cultured mammalian cells. CP-115,953 stimulated DNA cleavage mediated by the type II enzyme with a potency that was approximately 600-fold greater than that of the antimicrobial quinolone ciprofloxacin and approximately 50-fold greater than that of the antineoplastic drug etoposide. As determined by the ability to enhance enzyme-mediated DNA cleavage, quinolone activity towards calf thymus topoisomerase II was enhanced by the presence of a cyclopropyl group at the N-1 ring position and by the presence of a fluorine at C-8. Furthermore, the 4'-hydroxyphenyl substituent at the C-7 position was critical for the potency of CP-115,953 towards the mammalian type II enzyme. In this regard, the aromatic nature of the C-7 ring as well as the presence and the position of the 4'-hydroxyl group contributed greatly to drug activity. Finally, the cytotoxicity of quinolones in the CP-115,953 series towards mammalian cells paralleled the in vitro stimulation of DNA cleavage by topoisomerase II rather than the inhibition of enzyme-catalyzed DNA relaxation. This correlation strongly suggests that these quinolones promote cell death by converting topoisomerase II to a cellular poison.

A topoisomerase II-like protein is part of an inducible DNA-binding protein complex that binds 5' of an immunoglobulin promoter

We previously identified a B cell-specific protein-DNA complex 5' of an immunoglobulin mu heavy chain promoter. The sequences to which this protein complex bound were required for induction of immunoglobulin mRNA levels with interleukin-5 + antigen. Further studies identified a second sequence 5' of these regulatory sequences that bound to both the nuclear matrix and to a similar interleukin-5 + antigen inducible protein complex. Therefore, we sought to identify the putative regulatory proteins that comprised this DNA-binding complex. In this study, we have used anti-topoisomerase II antibodies to demonstrate that one of the proteins found in the interleukin-5 + antigen inducible complexes is serologically related to topoisomerase II. To our knowledge, this is the first report where a topoisomerase II related protein participates in an inducible mobility shifted protein-DNA complex. These data suggest a model in which the enhanced immunoglobulin gene transcription observed after treatment with interleukin-5 + antigen might be explained by the induction of a protein complex that acts to relieve torsional stress along the gene.

Synthesis and biological evaluation of tetrademethyl isocolchicine derivatives as inhibitors of DNA topoisomerase action in vitro

Four tetrademethyl isocolchicine analogs were prepared and evaluated as inhibitors of mammalian DNA topoisomerase in vitro. All compounds inhibited topoisomerase II-dependent DNA unknotting by a mechanism which did not involve "cleavable-complex" formation. N-Deacetylation as well as N-substitution with the (3',4',5'-trihydroxybenzoyl)-group afforded compounds which were less selective, based on their added ability to inhibit topoisomerase I-mediated DNA relaxation.

Novel gyrA point mutation in a strain of Escherichia coli resistant to fluoroquinolones but not to nalidixic acid

We have previously described a clinical isolate of Escherichia coli (Q2) that is highly resistant to fluoroquinolones (MIC of ciprofloxacin, 16 micrograms/ml) but susceptible to nalidixic acid (MIC of nalidixic acid, 4 micrograms/ml) (N. Moniot-Ville, J. Guibert, N. Moreau, J.F. Acar, E. Collatz, and L. Gutmann, Antimicrob. Agents Chemother. 35:519-523, 1991). Transformation of strain Q2 with a plasmid carrying the wild-type gyrA gene from E. coli K-12(pAFF801) resulted in a 32-fold decrease in the MIC of ciprofloxacin, suggesting that at least one mutation in gyrA was involved in the resistance of Q2. Intragenic gyrA fragments of 668 and 2,500 bp from strain Q2 were amplified by the polymerase chain reaction. We sequenced the 668-bp fragment and identified a single novel point mutation (transition from G to A at position 242), leading to an amino acid substitution (Gly-81 to Asp) in the gyrase A subunit. We constructed hybrid plasmids by substituting either the 668-bp fragment or the 2,500-bp fragment from Q2 DNA, both of which contained the gyrA point mutation, for the corresponding fragments in wild-type gyrA (2,625 bp) of E. coli K-12. When introduced into E. coli KNK453 (gyrA temperature sensitive), both plasmids conferred an eightfold increase in the MIC of ciprofloxacin, but only a twofold increase in the MIC of nalidixic acid. When introduced into E. coli Q2, neither plasmid conferred any change in the MICs of ciprofloxacin or nalidixic acid, suggesting that only the point mutation found in gyrA was involved in the resistance that we observed.

Novel 1-8-bridged chiral quinolones with activity against topoisomerase II: stereospecificity of the eukaryotic enzyme

A series of novel C-7 quinolyl-substituted enantiomers of ofloxacin were used to determine the stereospecificity of topoisomerase II for the C-11 methyl group in tricyclic quinolones. In all cases, the S isomer was the most active compound against the eukaryotic enzyme. It was approximately 2.2-fold more potent than the R isomer at inhibiting the overall catalytic activity of topoisomerase II (as monitored by DNA relaxation assays). A markedly greater difference in quinolone activity was observed in enzyme-mediated DNA cleavage reactions. While the S enantiomer stimulated nucleic acid breakage approximately 3.5-fold, the R compound did not enhance and, in fact, decreased initial DNA cleavage levels by approximately 50%. The activity of the racemic mixture more closely resembled that of the R enantiomer. In competition experiments, the DNA cleavage-enhancing effects of the S isomer were attenuated by the R compound. Taken together, these latter results indicate that the R enantiomer is an antagonist of S isomer-promoted topoisomerase II-mediated DNA cleavage. Finally, the cytotoxic potential of quinolyl-substituted ofloxacin analogs correlated with the ability to stimulate enzyme-mediated DNA cleavage. Thus, stereochemistry appears to be a governing factor for the potential development of tricyclic quinolones as topoisomerase II-targeted drugs with antineoplastic activity.

Protein kinase C modulates the catalytic activity of topoisomerase II by enhancing the rate of ATP hydrolysis: evidence for a common mechanism of regulation by phosphorylation

The catalytic activity of topoisomerase II is stimulated approximately 2-3-fold following phosphorylation by either casein kinase II or protein kinase C. A previous study [Corbett, A. H., DeVore, R. F., & Osheroff, N. (1992) J. Biol. Chem. 267, 20513-20518] demonstrated that casein kinase II regulates the activity of topoisomerase II by specifically enhancing the ability of the enzyme to hydrolyze its ATP cofactor. To determine whether other protein kinases use a similar mechanism to activate the enzyme, the effects of protein kinase C mediated phosphorylation on the individual steps of the topoisomerase II catalytic cycle were assessed. Modification stimulated rates of enzyme-mediated ATP hydrolysis approximately 2.7-fold, but had no effect on any reaction that preceded this step, including enzyme.DNA binding, pre- or poststrand passage DNA cleavage/religation, or the double-stranded DNA strand passage event. Furthermore, the activation of ATP hydrolysis was reversed following treatment of phosphorylated topoisomerase II with alkaline phosphatase. As determined by partial proteolytic mapping, the site(s) of protein kinase C modification was (were) localized to the 350 amino acid C-terminal regulatory domain of topoisomerase II within approximately 50 amino acids of the site(s) phosphorylated by casein kinase II. Finally, while protein kinase C and casein kinase II were able to modify the enzyme simultaneously, rates of ATP hydrolysis for doubly-modified topoisomerase II were comparable to those observed for the enzyme following phosphorylation by either individual kinase.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of resistance to quinolones

Mechanisms of resistance to the quinolones have been described for several bacterial species, but mainly for Escherichia coli and Staphylococcus aureus. Two principal mechanisms have been described: 1) alteration of the DNA gyrase, which is the target site of the quinolones; and 2) diminished accumulation in the cell as a result of either decreased uptake or increased efflux. Alteration of DNA gyrase is usually the result of a mutation in the gyrA, or more rarely, the gyrB gene. All substitutions in subunit A of the gyrase are located in the 67 to 106 amino-acid domain and are clustered around Ser-83 in E. coli and Ser-84 in S. aureus. A decrease in uptake has been described for Gram-negative bacteria such as Enterobacteriaceae and Pseudomonas aeruginosa. It has almost always been correlated with a modified electrophoretic profile of outer membrane proteins of the quinolone-resistant mutants. In E. coli, a decrease in OmpF seemed to be linked to the activation of the micF operon in most of the mutants described. These mutants were cross-resistant to unrelated antibiotics, such as trimethoprim, chloramphenicol, tetracycline, and some beta-lactams. In all these mutants the normal or enhanced efflux of quinolones increased the level of resistance. Enhanced efflux has been described as the second mechanism of resistance in S. aureus. Acquired resistance to the quinolones was thought, until recently, to result from chromosomal mutation. Plasmid-mediated resistance associated with an enhanced efflux has been described in S. aureus, but this needs to be confirmed. When a high level of resistance is observed, 2 or 3 mechanisms may be involved.(ABSTRACT TRUNCATED AT 250 WORDS)

Development and utilization of a new simple assay for DNA-protein crosslinks as a biomarker of exposure to welding fumes

A new method for DNA-protein crosslinks involving selective precipitation of DNA containing crosslinked proteins by K+ sodium dodecyl sulfate was utilized in the peripheral white blood cells of 21 male metal arc welders and in 26 male controls of similar age and racial characteristics who were not exposed to welding fumes. DNA was quantitated by Hoescht fluorescence. Although the concentration of nickel and chromium in the peripheral blood was low and did not differ between subjects in the two groups, one-fourth of the welders had levels of DNA-protein crosslinks that were above the upper limit of the controls. Mean crosslink values were 1.85% (+/- 1.14) among the welders and 1.17% (+/- 0.46) among the controls, a 58%, statistically significant difference (P = 0.01). Thus, many welders appeared to be burdened with an excess of DNA-protein crosslinks suggesting exposure to crosslinking agents and, possibly, a detectable biologic effect of potential genotoxic consequences.

Coumermycin A1 inhibits growth and induces relaxation of supercoiled plasmids in Borrelia burgdorferi, the Lyme disease agent

Coumermycin A1 is an inhibitor of DNA gyrase, an enzyme that catalyzes supercoiling of DNA and is required for bacterial DNA replication. We have investigated the activity of this coumarin antibiotic on Borrelia burgdorferi, a spirochete and the causative agent of Lyme disease. B. burgdorferi was more susceptible than many other eubacteria to coumermycin as well as novobiocin, another coumarin antibiotic; this contrasted with its relative resistance to the DNA gyrase inhibitors nalidixic acid, oxolinic acid, and ciprofloxacin. Coumermycin at 0.2 micrograms/ml inhibited the growth of B. burgdorferi B31 in BSK II medium. A 100-fold-lower concentration induced the relaxation of two negatively supercoiled circular plasmids within 2 h. Plasmid supercoiling was restored within 2 h of removal of coumermycin. These results suggest that B. burgdorferi has a DNA gyrase and that this enzyme's activity is required for growth. Furthermore, structural analogs of coumermycin may be considered as treatments for Lyme disease.

Conditional growth of Escherichia coli caused by expression of vaccinia virus DNA topoisomerase I

Active vaccinia virus topoisomerase I is expressed in Escherichia coli containing plasmid p1940 (S. Shuman, M. Golder, and B. Moss, J. Biol. Chem. 263:16401-16407, 1988). Growth curves showed a decline of 2 to 3 logs in the number of viable cells at 42 degrees C after shift from 30 degrees C because of increased vaccinia virus topoisomerase I level. Mutations in the gyrA and gyrB genes allowed cells to grow equally well at 42 and 30 degrees C. The presence of gyrase inhibitor also improved growth at 42 degrees C.

Induction of chromosomal aberrations by camptothecin in root-tip cells of Vicia faba

When root-tip cells of Vicia faba were exposed during early and middle interphase to camptothecin (Cpt), the aberrations obtained were exclusively of the chromatid type and tended to be localized in late replicating heterochromatic regions of the chromosomes. In these respects the clastogenic effect of Cpt resembles that of agents that act by an S-phase-dependent mechanism. In contrast to typical S-phase-dependent agents, Cpt produced lesions capable of giving rise to aberrations only in S-phase cells that were synthesizing DNA at the time of treatment. The dependence on ongoing DNA synthesis was suggested in autoradiographic experiments and by the fact that the clastogenic effect of Cpt was strongly suppressed by hydroxyurea, an inhibitor of DNA synthesis. After Cpt treatments, there were many more cells with 3-12 aberrations and far fewer cells with 0, 1 or 2 aberrations than expected on the basis of a Poisson distribution. Cpt further differed from typical S-phase-dependent agents by being capable of inducing lesions in the G2 phase that give rise to chromosomal aberrations in the first mitosis after treatment. This effect was obtained at Cpt concentrations around 10 microM, whereas only 0.03 microM Cpt was required to produce chromatid aberrations in the S phase. Results of G2-phase experiments with Cpt and 5-fluorodeoxyuridine, an inhibitor of DNA synthesis, suggest that DNA synthesis is required for the clastogenic effect of Cpt not only during the S phase, but also during the G2 phase, although the DNA syntheses involved are both quantitatively and qualitatively different.

In vitro antibacterial activity of Q-35, a new fluoroquinolone

The in vitro activity of Q-35, an 8-methoxy fluoroquinolone, was compared with those of ofloxacin, ciprofloxacin, tosufloxacin, lomefloxacin, and sparfloxacin. The MICs of Q-35 for 90% of strains tested (MIC90s) of Staphylococcus aureus, methicillin-resistant S. aureus, Staphylococcus epidermidis, Streptococcus pneumoniae, and Streptococcus pyogenes were 0.2, 6.25, 0.2, 0.39, and 0.39 micrograms/ml, respectively. The activity of Q-35 was 4- to 16-fold greater than those of ofloxacin, ciprofloxacin and lomefloxacin but equal to those of tosufloxacin and sparfloxacin against these organisms. For 82 ciprofloxacin-resistant staphylococci (MIC90 = 100 micrograms/ml), Q-35 was the most active of the new quinolones tested (MIC90 = 6.25 micrograms/ml). The MIC90s of Q-35 against Escherichia coli, Enterobacter aerogenes, and Pseudomonas aeruginosa were 0.2, 0.78, and 12.5 micrograms/ml, respectively, and Q-35 was 2- to 16-fold less active than the other quinolones tested. Q-35 showed potent bactericidal activity and inhibited the supercoiling activity of DNA gyrase of S. aureus, E. coli, and P. aeruginosa.