A panel of visual bacterial biosensors for the rapid detection of genotoxic and oxidative damage: A proof of concept study.

The emergence of new compounds during the past decade requires a high-throughput screening method for toxicity assay. The stress-responsive whole-cell biosensor is a powerful tool to evaluate direct or indirect damages of biological macromolecules induced by toxic chemicals. In this proof-of-concept study, nine well-characterized stress-responsive promoters were first selected to assemble a set of blue indigoidine-based biosensors. The PuspA-based, PfabA-based, and PgrpE-based biosensors were eliminated due to their high background. A dose-dependent increase of visible blue signal was observed in PrecA-, PkatG-, and PuvrA-based biosensors, responsive to potent mutagens, including mitomycin and nalidixic acid, but not to genotoxic lead and cadmium. The PrecA, PkatG, and Ppgi gene promoters were further fused to a purple deoxyviolacein synthetic enzyme cluster. Although high basal production of deoxyviolacein is unavoidable, an enhanced visible purple signal in response to mitomycin and nalidixic acid was observed as dose-dependent, especially in PkatG-based biosensors. The study shows that a set of stress-responsive biosensors employing visible pigment as a reporter is pre-validating in detecting extensive DNA damage and intense oxidative stress. Unlike widely-used fluorescent and bioluminescent biosensors, the visual pigment-based biosensor can become a novel, low-cost, mini-equipment, and high-throughput colorimetric device for the toxicity assessment of chemicals. However, combining multiple improvements can further improve the biosensing performance in future studies.

A replication-inhibited unsegregated nucleoid at mid-cell blocks Z-ring formation and cell division independently of SOS and the SlmA nucleoid occlusion protein in Escherichia coli.

Chromosome replication and cell division of Escherichia coli are coordinated with growth such that wild-type cells divide once and only once after each replication cycle. To investigate the nature of this coordination, the effects of inhibiting replication on Z-ring formation and cell division were tested in both synchronized and exponentially growing cells with only one replicating chromosome. When replication elongation was blocked by hydroxyurea or nalidixic acid, arrested cells contained one partially replicated, compact nucleoid located mid-cell. Cell division was strongly inhibited at or before the level of Z-ring formation. DNA cross-linking by mitomycin C delayed segregation, and the accumulation of about two chromosome equivalents at mid-cell also blocked Z-ring formation and cell division. Z-ring inhibition occurred independently of SOS, SlmA-mediated nucleoid occlusion, and MinCDE proteins and did not result from a decreased FtsZ protein concentration. We propose that the presence of a compact, incompletely replicated nucleoid or unsegregated chromosome masses at the normal mid-cell division site inhibits Z-ring formation and that the SOS system, SlmA, and MinC are not required for this inhibition.

Microbial genotoxicity bioreporters based on sulA activation.

A bacterial genotoxicity reporter strain was constructed in which the tightly controlled strong promoter of the Escherichia coli SOS response gene sulA was fused to the alkaline phosphatase-coding phoA reporter gene. The bioreporter responded in a dose-dependent manner to three model DNA-damaging agents-hydrogen peroxide, nalidixic acid (NA), and mitomycin C (MMC)-detected 30-60 min after exposure. Detection thresholds were 0.15 µM for MMC, 7.5 µM for nalidixic acid, and approximately 50 µM for hydrogen peroxide. A similar response to NA was observed when the bioreporter was integrated into a specially designed, portable electrochemical detection platform. Reporter sensitivity was further enhanced by single and double knockout mutations that enhanced cell membrane permeability (rfaE) and inhibited DNA damage repair mechanisms (umuD, uvrA). The rfaE mutants displayed a five- and tenfold increase in sensitivity to MMC and NA, respectively, while the uvrA mutation was advantageous in the detection of hydrogen peroxide. A similar sensitivity was displayed by the double rfaE/uvrA mutant when challenged with the pre-genotoxic agents 2-amino-3-methylimidazo[4,5-f]quinoline and 2-aminoanthracene following metabolic activation with an S9 mammalian liver fraction.

Inhibition of photosynthesis by a fluoroquinolone antibiotic.

Recent microcosm studies have revealed that fluoroquinolone (FQ) antibiotics can have ecotoxicological impacts on photosynthetic organisms, but little is known about the mechanisms of toxicity. We employed a combination of modeling and experimental techniques to explore how FQs may have these unintended secondary toxic effects. Structure-activity analysis revealed that the quinolone ring and secondary amino group typically present in FQ antibiotics may mediate their action as quinone site inhibitors in photosystem II (PS-II), a key enzyme in photosynthetic electron transport. Follow-up molecular simulations involving nalidixic acid (Naldx), a nonfluorinated quinolone with a demonstrated adverse impact on photosynthesis, and ciprofloxacin (Cipro), the most commonly used FQ antibiotic, showed that both may interfere stereochemically with the catalytic activity of reaction center II (RC-II), the pheophytin-quinone-type center present in PS-II. Naldx can occupy the same binding site as the secondary quinone acceptor (Q(B)) in RC-II and interact with amino acid residues required for the enzymatic reduction of Q(B). Cipro binds in a somewhat different manner, suggesting a different mechanism of interference. Fluorescence induction kinetics, a common method of screening for PS-II inhibition, recorded for photoexcited thylakoid membranes isolated from Cipro-exposed spinach chloroplasts, indicated that Cipro interferes with the transfer of energy from excited antenna chlorophyll molecules to the reaction center in RC-II ([Cipro] >or= 5 microM in vitro and >or=10 microM in vivo) and thus delays the kinetics of photoreduction of the primary quinone acceptor (Q(A); [Cipro] >or= 0.6 microM in vitro). Spinach plants exposed to Cipro exhibited severe growth inhibition characterized by a decrease in both the synthesis of leaves and growth of the roots ([Cipro] >or= 0.5 microM in vivo). Our results thus demonstrate that Cipro and related FQ antibiotics may interfere with photosynthetic pathways, in addition to causing morphological deformities in higher plants.

Age-dependent effects on redox status, oxidative stress, mitochondrial activity and toxicity induced by fluoroquinolones on primary cultures of rabbit tendon cells.

The age-related difference in fluoroquinolone-induced tendon toxicity was investigated. In vitro tendon cells from juvenile and young adult rabbits, respectively, were incubated with quinolone (nalidixic acid, NA) or fluoroquinolone (ofloxacin, OFX or pefloxacin, PEF) at 0.01 microM to 1 mM for 72 h. Redox status, glutathione (GSH), reactive oxygen species (ROS), and mitochondrial activity were assessed using intracellular fluorescent probes. Fluorescence signal was detected on living adherent tenocytes in microplates using cold-light cytofluorometry. Tendon toxicity differed significantly between the two cell groups and the difference was greatest with highest dose (1 mM). For 72 h, significant (p < 0.001) differences between immature and young adult primary tenocytes were observed for redox status decrease, GSH decrease, and ROS production increase. Mitochondrial activity remained unaltered in immature tenocytes. We confirm two groups of intrinsic tendon toxicity (OFX/NA vs. PEF) associated to oxidative stress (GSH decrease). Our in vitro experimental model confirms the clinical observations of age dependent tenotoxicity. First group (NA, OFX) showed greater intrinsic tenotoxicity for young adult than immature tenocytes, second group (PEF) was highly toxic for immature and young adult cells. The three quinolones do not altered mitochondrial activity in immature tenocytes whereas alteration was observed in young adult tenocytes.

Liver initiation activity of norfloxacin but not nalidixic acid, pipemidic acid, and ciprofloxacin on in vivo short-term liver initiation assay in rats.

This study aimed to examine the in vivo initiation activity of the quinolone antimicrobials--nalidixic acid (NA), pipemidic acid (PPA), ciprofloxacin (CPFX), and norfloxacin (NFLX)--by using an in vivo short-term liver initiation assay. Rats were subjected to a two-thirds partial hepatectomy on day 0 and 12 h after completion of this procedure were treated once orally with each quinolone or vehicle. Subsequently, they were fed a basal diet for 14 days and a diet containing 0.015% of 2-acetylaminofluorene for the following 10 days. On day 19, a single oral dose of carbon tetrachloride at 0.8 mL/kg body weight was administered. On day 34, they were sacrificed under ether anesthesia, and liver slices were fixed in 10% neutral buffered formalin for immunohistochemical examination of glutathione S-transferase placental form (GST-P) positive foci. Administration of NFLX resulted in a significant increase in the mean number and area of GST-P positive foci; however, administration of the three other quinolones did not produce any increase. These results suggest that only NFLX has an initiation activity in rats under the conditions used in the present study.

[Phototoxicity studies of pazufloxacin mesilate, a novel parenteral quinolone antimicrobial agent--in vitro and in vivo studies].

Phototoxicity of pazufloxacin mesilate (PZFX mesilate), a novel parenteral quinolone antimicrobial agent, were evaluated in vitro and in vivo studies. In vitro, phototoxicity for cultured cells of PZFX, which is active principle of PZFX mesilate, was studied, and stability for long-wavelength ultraviolet (UVA) was examined. In vivo, phototoxicity tests in guinea pigs and rats, and photoallergenicity tests in guinea pigs were conducted. In the phototoxicity test on cultured cells, CHL/IU cells were irradiated UVA of 300-3000 mJ/cm2 in the presence of PZFX, ofloxacin (OFLX), lomefloxacin (LFLX) or sparfloxacin (SPFX) at 10 micrograms/mL. Phototoxic potencies for cultured cells of the quinolones tested were SPFX > LFLX > OFLX > PZFX. In addition, changes in ultraviolet absorption spectrum and residual rate of PZFX, OFLX, LFLX and SPFX were examined after UVA irradiation of 300-3000 mJ/cm2 to each solution. PZFX was stable for UVA compared with OFLX and LFLX. In the phototoxicity test of guinea pigs, each quinolone was administered intraperitoneally daily for 7 days, and UVA of about 11 J/cm2 was irradiated at 30 minutes after the last administration. Dose levels of each quinolone were 65 and 130 mg/kg of PZFX mesilate (dose levels converted to PZFX: 50 and 100 mg/kg), 50 and 100 mg/kg of nalidixic acid (NA), 100 mg/kg of OFLX, enoxacin (ENX), ciprofloxacin (CPFX), LFLX and SPFX. Grade of skin reaction (erythema) at 24 hours after UVA irradiation decreased in the order: SPFX > CPFX > NA > ENX = OFLX > LFLX > PZFX mesilate. Thus, PZFX mesilate was found to have the weakest phototoxicity. In the maximum plasma concentration of quinolones from 0.5 to 2.5 hours after administration, corresponding to the time of UVA irradiation, the concentration of the group administered PZFX mesilate was about 4.1 times higher than that of CPFX group, and about 1.3 times higher than that of SPFX group. The area under the blood concentration-time curve (AUC0.5-2.5) of the group administered PZFX mesilate was the same as that of SPFX group, and about 3.2 times larger than that of CPFX group. These data showed that phototoxicity of PZFX mesilate was also weaker than that of CPFX or SPFX in consideration of AUC0.5-2.5. In the phototoxicity test of rats injected intravenously, no phototoxicity was observed at 130 mg/kg of PZFX mesilate. In the photoallergenicity test of guinea pigs, no photoallergenicity was observed by PZFX mesilate. As mentioned above, from in vitro studies PZFX was found to be stable for UVA irradiation compared with OFLX and LFLX, and phototoxicity for cultured cells of PZFX was weaker than that of SPFX, LFLX or OFLX. In addition, from in vivo studies phototoxicity of PZFX mesilate was found to be weaker than that of NA, OFLX, ENX, CPFX, LFLX or SPFX, and no photoallergenicity was observed. Therefore, photosensitive potency of PZFX mesilate might be less than that of other quinolones.

Toxic waste disposal in Escherichia coli.

About 10% of the nalidixic acid-resistant (Nal(r)) mutants in a transposition-induced library exhibited a growth factor requirement as the result of cysH, icdA, metE, or purB mutation. Resistance in all of these mutants required a functional AcrAB-TolC efflux pump, but the EmrAB-TolC pump played no obvious role. Transcription of acrAB was increased in each type of Nal(r) mutant. In the icdA and purB mutants, each of the known signaling pathways appeared to be used in activating the AcrAB-TolC pump. The metabolites that accumulate upstream of the blocks caused by the mutations are hypothesized to increase the levels of the AcrAB-TolC pump, thereby removing nalidixic acid from the organism.

Green fluorescent protein-based biosensor for detecting SOS-inducing activity of genotoxic compounds.

Increasing levels of environmental pollution demand specific and sensitive methods for detection of genotoxic agents in water, food products and environmental samples. Tests for genotoxicity assessment are often based on biosensor strains that respond to DNA damage induced by chemicals. In the present study, fluorescent reporter Escherichia coli strains have been developed, which contain a plasmid-borne transcriptional fusion between the DNA-damage inducible recA promoter and the green fluorescent protein gene (gfp) or a gene encoding a red-shifted, higher intensity GFP variant (mutant 3). GFP-based biosensors allowed the detection of a dose-dependent response to genotoxic agents such as mitomycin C (MMC), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and nalidixic acid (NA). A reporter strain carrying recA'-gfp mutant 3 fusion gave more dramatic and sensitive response than a strain containing the wild-type gfp. These results indicate that recA'-gfp mutant 3-based biosensor is potentially useful for detection of genotoxins.

Distinct mechanisms of guanine-specific DNA photodamage induced by nalidixic acid and fluoroquinolone antibacterials.

Fluoroquinolone antibacterials, which have been used for the treatment of a variety of infectious diseases, are reported to be photocarcinogenic. We investigated the mechanisms of DNA damage by UVA radiation (365 nm) plus fluoroquinolone antibacterials using 32P-labeled DNA fragments obtained from the human c-Ha-ras-1 proto-oncogene and the p53 tumor suppressor gene. Photocarcinogenic nalidixic acid (NA), which is an old member of synthetic quinolone antibacterials, caused DNA damage specifically at 5'-GG-3' sequences, whereas lomefloxacin (LFLX) did not exhibit the site preference for consecutive guanines. LFLX-induced DNA photodamage was inhibited by sodium azide and enhanced in D2O, suggesting that singlet oxygen plays the key role in the DNA damage. LFLX plus UVA induced the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) depending on LFLX concentrations, and 8-oxodG formation was enhanced in single-stranded DNA. In contrast, NA induced larger amounts of 8-oxodG in double-stranded DNA. ESR spin destruction method revealed that NA induced DNA photodamage through electron transfer but LFLX did not. These findings indicate that DNA damage induced by photoactivated LFLX and NA plays an important role in expression of their photocarcinogenicity.

Antimutagenesis of beta-carotene to mutations induced by quinolone on Salmonella typhimurium.

BACKGROUND: Quinolone-induced mutagenesis in the Salmonella typhimurium hisG48 strains suggests that these antibiotics are oxygen free radical generators. The use of beta-carotene as antioxidant was evaluated as an alternative to reduce oxidative cell damage in patients who need therapy with nalidixic acid, norfloxacin, or pipemidic acid. The studied beta-carotene (30%), used by pharmaceutical laboratories as dietary complements, was not toxic or mutagenic for the S. typhimurium TA102 strain at a dose equivalent to 1,500 I.U. At the studied concentrations, the evaluated antimutagen did not modify the minimum inhibitory concentration of nalidixic acid, norfloxacin, or pipemidic acid against uropathogenic Escherichia coli strains. METHODS: The mutagenic effect of nalidixic acid and norfloxacin against hisG48 strains was inhibited with 1500 I.U. of beta-carotene. The antimutagenic effect of beta-carotene against mutations induced by pipemidic acid was observed even with 150 I.U. of beta-carotene. The antimutagenic effect against mutations induced on S. typhimurium TA102 or TA104 strains was observed only when the aroclor 1254 rat-induced liver S9 mixture was used. RESULTS: This mutagenic effect was detected only when the strains were exposed to quinolones and the beta-carotene simultaneously with the S9 mixture, suggesting that quinolones induce oxygen free radicals that may be scavenged by beta-carotene. CONCLUSIONS: The antimutagenic effect of this vitamin A precursor is probably due to the active molecule of vitamin A, a desmutagen with the ability of radical capture. A diet rich in beta-carotene or vitamin A could be a good alternative to reduce genotoxic risk to patients being treated with quinolone.

Effects of nalidixic acid on hamster knee cartilage morphology and synovial fluid composition.

Quinolone-induced changes were studied in the knee joints of 4-wk-old female hamsters given intraperitoneal doses of either nalidixic acid (400 mg/kg body weight) or vehicle on days 0 and 1. After euthanasia on day 4, synovial fluid was collected for cytologic evaluation and for analysis of concentrations of hyaluronan, proteoglycans, total protein, and collagen as hydroxyproline. Slides of formalin-fixed decalcified tissues were stained with hematoxylin-eosin or safranin O for histologic scoring of lesion severity. Nine of 10 hamsters treated with nalidixic acid had fissures within articular cartilage of the femur and reduced safranin O staining of matrix indicative of loss of proteoglycans. Synovial membranes from affected joints, however, were not inflamed. Synovial fluid cell counts and cytomorphology were unaffected by treatment. In synovial fluid from 5 of 10 treated hamsters, proteoglycans were elevated by more than 2 SDs above the control group, and individual animal levels correlated with the histologic severity score (r2 = 0.36; p = 0.02). The hyaluronan content of the synovial fluid from treated hamsters was mildly but significantly elevated (p = 0.005), and the histologic severity score again correlated with individual animal levels (r2 = 0.42; p = 0.01). Hydroxyproline was unaffected by treatment. Although synovial fluid changes and histologic changes were correlated on a group basis, interanimal variability was significant and the magnitude of biochemical changes were far smaller than those that occur during inflammation. Changes in synovial fluid composition are not sufficiently robust to predict cartilage changes in individual animals.

Comparative genotoxicity of quinolone and quinolonyl-lactam antibacterials in the in vitro micronucleus assay in Chinese hamster ovary cells.

The in vitro micronucleus assay is gaining increased attention as a potential alternative to the standard in vitro metaphase analysis assay. In particular, the in vitro micronucleus assay has been proposed as a useful method for chemicals that induce both structural and numerical chromosome alterations, such as DNA gyrase/topoisomerase inhibitors. In this study, we compared the micronucleus-inducing activity of quinolonyl-lactam antibacterials that inhibit DNA-gyrase and bind to penicillin-binding proteins relative to the activity of structurally related quinolone antibacterials that also inhibit DNA-gyrase. All of the quinolones that were structurally related to the quinolonyl-lactams were cytotoxic and induced large increases in the frequency of micronucleated binucleated cells (MNBC) at concentrations between 0.02 and 0.16 mM. These changes were larger than those seen with the commercial quinolones, ciprofloxacin (cytotoxic at > or = 0.57 mM and MNBC at > or = 0.3 mM) and nalidixic acid (cytotoxic at 1.8 mM and no MNBC up to this dose). In contrast, the quinolonyl-lactams were not cytotoxic up to 1.0 mM concentrations and induced either no MNBC or a low frequency of MNBC at higher concentrations compared to the quinolones. Quinolonyl-lactams appear to be less cytotoxic and genotoxic than structurally related quinolones. These results add to the growing database on the in vitro micronucleus assay in general, and more specifically to the relatively small database for the in vitro micronucleus assay in Chinese hamster ovary cells.

Genotoxic activity of four inhibitors of DNA topoisomerases in larval cells of Drosophila melanogaster as measured in the wing spot assay.

Four inhibitors of DNA topoisomerases namely nalidixic acid, camptothecin, m-amsacrine and etoposide, have been evaluated for genotoxic effects in the wing spot test of Drosophila melanogaster. This assay assesses somatic recombination and mutational events. We studied nalidixic acid as an inhibitor of bacterial DNA gyrase, camptothecin as a topoisomerase I inhibitor, as well as m-amsacrine and etoposide as topoisomerse II inhibitors. The genotoxic effects were determined from the appearance of wing spots in flies trans-heterozygous for the recessive markers multiple wing hairs (mwh) and flare, as well as in flies heterozygous for mwh and the multiply inverted TM3 balancer chromosome. From our results it appears that whilst nalidixic acid and m-amsacrine were compounds that did not increase the incidence of mutant clones, camptothecin and etoposide proved to be significantly genotoxic in this test, being camptothecin more effective than etoposide. A significant proportion of the total spot induction was due to mitotic recombination, confirming previously reported data. On the other hand, the cotreatments of each topoisomerase inhibitor with the alkylating agent ethyl methanesulfonate (EMS) indicate that, while nalidixic acid, m-amsacrine and etoposide show a tendency to an antagonistic interaction, camptothecin shows an additive effect, suggesting mechanistic differences between the activity of the four inhibitors of DNA topoisomerases studied.

Determination of DNA sequences required for regulated Mycobacterium tuberculosis RecA expression in response to DNA-damaging agents suggests that two modes of regulation exist.

The recA gene of Mycobacterium tuberculosis has previously been cloned and sequenced (E. O. Davis, S. G. Sedgwick, and M. J. Colston, J. Bacteriol. 173:5653-5662, 1991). In this study, the expression of this gene was shown to be inducible in response to various DNA-damaging agents by using a transcriptional fusion to the reporter gene encoding chloramphenicol acetyltransferase. A segment of DNA around 300 bp upstream of the coding region was shown to be required for expression. However, primer extension analysis indicated that the transcriptional start sites were 47 and 93 bp upstream of the translation initiation codon. Sequence motifs with homology to two families of Escherichia coli promoters but also with significant differences were located near these proposed transcription start sites. The differences from the E. coli consensus patterns would explain the previously described lack of expression of the M. tuberculosis recA gene from its own promoter in E. coli. In addition, the M. tuberculosis LexA protein was shown to bind specifically to a sequence, GAAC-N4-GTTC, overlapping one of these putative promoters and homologous to the Bacillus subtilis Cheo box involved in the regulation of SOS genes. The region of DNA 300 bp upstream of the recA gene was shown not to contain a promoter, suggesting that it functions as an upstream activator sequence.

Fluoroquinolone antibacterials enhance UVA-induced skin tumors.

Fluoroquinolone antibacterials are known to be phototoxic, both in vivo and in vitro. The action spectrum for the phototoxicity of the quinolones lies mainly in the UVA region. During studies of systemic drug phototoxicity, Johnson et al. (Dundee) induced dose-dependent phototoxicity in Swiss albino mice, and severe phototoxic reactions were followed by the development of skin tumors. The present study was designed to compare the ability of several quinolones to produce photobiologic effects following chronic, subphototoxic UVA radiation. To compare the activities of different quinolones (lomefloxacin, fleroxacin, ciprofloxacin, ofloxacin and nalidixic acid), doses that result in similar plasma and skin levels of drug were administered by gavage to slightly pigmented Skh-1 hairless mice for up to 78 weeks. 8-Methoxypsoralen (8-MOP) was used as a positive control, and unirradiated, drug-treated and irradiated and unirradiated drug-free controls were also used. No signs of phototoxicity were seen, except for minimal-to-slight erythema and swelling of the skin in animals of the lomefloxacin-UVA group. Skin tumors (1 mm in diameter or larger) were observed in all the irradiated groups and the incidence was increased in all the groups treated with the test articles. The cumulative tumor prevalence was accelerated, the median latent periods were shortened and tumor onset was significantly enhanced by 8-MOP plus UVA, lomefloxacin plus UVA and fleroxacin plus UVA, as compared with vehicle plus UVA-exposed animals. The majority of skin tumors (with the exception of lomefloxacin and 8-MOP) were benign. The majority of squamous cell carcinomas in the lomefloxacin group were of a histologic type different from those previously reported in UVA-exposed animals. Thus, all the fluoroquinolone antibiotics studied have the capability of enhancing UVA-induced phototumorigenesis, but only lomefloxacin caused the development of cystic squamous cell carcinomas in the majority of treated animals.

Quinolone antibacterials: a new class of photochemical carcinogens.

Hairless mice were exposed orally to antibiotics of the fluoroquinolone group alone and in combination with irradiation with UVA over an extended period of time to determine the possible skin carcinogenicity in comparison with that with 8-methoxypsoralen, i.e. a known photochemical skin carcinogen. Animals exposed to UVA and fleroxacin, ciprofloxacin, nalidixic acid and ofloxacin exhibited an increase in the number of benign skin tumors when compared with animals exposed to UVA alone. Animals exposed to lomefloxacin and UVA exhibited a specific type of neoplastic progression. In addition to benign papillomas and solar keratoses, a number of cystic squamous cell carcinomas were observed. In the positive control group, which was given 8-methoxypsoralen and UVA, a number of papillomas and superficial squamous cell carcinomas were found. In animals exposed to UVA alone, only a few benign tumors were seen; in unexposed animals, no cutaneous neoplasms were observed. It is concluded that fluoroquinolones warrant further study, because they have potential photocarcinogenic properties.

Sister-chromatid exchanges (SCE) induction by inhibitors of DNA topoisomerases in cultured human lymphocytes.

The induction of sister-chromatid exchanges (SCE) in cultured human lymphocytes by four inhibitors of DNA topoisomerases: m-amsacrine, camptothecin, etoposide and nalidixic acid has been evaluated. Although the four compounds apparently increase the frequency of SCE, the effect of nalidixic acid is weak because only a statistically significant positive response was found in one donor at the highest concentration (500 microM). The other compounds tested act as SCE inducers in both donors, camptothecin being the most effective. In addition, the influence of these four topoisomerase inhibitors on the SCE frequency induced by MMC was also analysed. The results reveal that less than additive SCE effect was induced by the combined treatments which could suggest that the process leading to SCE induction by MMC and the four inhibitors of DNA topoisomerases are not totally independent.