Genetic toxicology of mitotic spindle inhibitors used as anticancer drugs

Induction of chromosome malsegregation by nanomaterials

NMs (nanomaterials), defined as materials with at least one dimension smaller than 100 nm, are able to induce genotoxic effects. One of the hypotheses of the mode-of-action in which they exert their genotoxic potential is to mechanically interfere with subcellular structures, in particular the microtubules. In the present paper, we review studies exploring interactions between NMs and tubulin; therefore a PubMed literature search was performed. From this search 12 studies, applying both acellular and cellular assays, were retrieved and are summarized according to endpoint and particle type. These studies show that there are interactions between different types of NMs and tubulins in both acellular and cellular systems. For several types of NMs, the multi-walled carbon nanotubes, amorphous SiO(2), TiO(2) and CoCr, an induction of aneuploidy was observed in vitro. There is, therefore, a critical need to assess the capacity of NMs to interfere with the cytoskeleton, and in particular the tubulins. This might require definition of relevant dosimetry, adaptations of some testing protocols, possibly development of new methodologies and studies on a larger size-range of NMs.

SFTG international collaborative study on in vitro micronucleus test

This study on the in vitro micronucleus assay, comprising 11 laboratories using human lymphocytes, was coordinated by an organizing committee supported by the SFTG (the French branch of the European Environmental Mutagen Society). Nine coded substances were assessed for their ability to induce micronuclei in human lymphocytes in vitro, mitomycin C being used as a positive control. Cultures were exposed to the test substances for a short (early or late) time or for a long time, followed by a short or long recovery period, in the presence of cytochalasin B. Each chemical was evaluated, generally in two laboratories, using three treatment schedules at least twice. The data were assessed for acceptability, and then classified as negative, positive or equivocal. Two of seven genotoxic compounds, namely colchicine and bleomycin, clearly induced micronuclei. Reproducible results were difficult to obtain for some substances, which tended to be those acting at specific stages of the cell cycle. Cytosine arabinoside, diethylstilboestrol and 5-fluorouracil were classified as equivocal. Urethane and thiabendazole were classified as negative. The two presumed non-genotoxic compounds, mannitol and clofibrate, did not induce micronuclei. Repeat testing, exposing cells at both an early and late time after mitogenic stimulation, was needed to detect substances classified as equivocal. These results show the importance of achieving sufficient inhibition of nuclear division to avoid the possibility of missing an effect. The evaluation of micronuclei in mononucleated as well as binucleated cells was particularly useful to detect aneugens. There were no false positive results using lymphocytes, indicating a high specificity. It is concluded that the clastogenic or aneugenic potential in vitro of the substances tested was correctly identified in this study, but that refining the protocol to take into account factors such as the stages of the cell cycle exposed to the compound, or the duration of recovery would be likely to improve the sensitivity of detection using lymphocytes.

SFTG international collaborative study on in vitro micronucleus test I. General conditions and overall conclusions of the study

This study, coordinated by the SFTG (French branch of European Environmental Mutagen Society), included 38 participants from Europe, Japan and America. Clastogens (bleomycin, urethane), including base and nucleoside analogs (5-fluorouracil and cytosine arabinoside), aneugens and/or polyploidy inducers (colchicine, diethylstilboestrol, griseofulvin and thiabendazole), as well as non-genotoxic compounds (mannitol and clofibrate), were tested. Four cell types were used, i.e. human lymphocytes in the presence of cytochalasin B and CHO, CHL and L5178Y cell lines, in the presence or absence of cytochalasin B, with various treatment-recovery schedules. Mitomycin C was used as a positive control for all cell types. Mannitol and clofibrate were consistently negative in all cell types and with all treatment-recovery conditions. Urethane, known to induce questionable clastogenicity, was not found as positive. Bleomycin and mitomycin C were found positive in all treatment-recovery conditions. The base and nucleoside analogs were less easy to detect, especially 5-fluorouracil due to the interference with cytotoxicity, while cytosine arabinoside was detected in all cell types depending on the treatment-recovery schedule. Aneugens (colchicine, diethylstilboestrol and griseofulvin) were all detected in all cell types. In this study, the optimal detection was ensured when a short treatment followed by a long recovery was associated with a long continuous treatment without recovery. There was no impact of the presence or absence of cytochalasin B on the detection of micronucleated cells on cell lines. Scoring micronucleated cells in both mononucleated and binucleated cells when using cytochalasin B was confirmed to be useful for the detection and the identification of aneugens. In conclusion, these results, together with previously published validation studies, provide a useful contribution to the optimisation of a study protocol for the detection of both clastogens and aneugens in the in vitro micronucleus test.

Cytogenetic Consequences of Vinblastine Treatment in Mouse Bone Marrow

BACKGROUND

Vinblastine (VBL), a vinca alkaloid, has very often been included in different cancer chemotherapeutic treatment regimens. Chemotherapy cures certain cancers and, at least, increases the life expectancy of cancer patients. However, in cancer survivors, a second malignancy frequently occurs after chemotherapy, which warrants detailed genotoxicity testing of the chemotherapeutic agents. The available genotoxicity test reports on VBL are self-contradictory and inconclusive. Thus, following a suitable experimental protocol, it is necessary to test the cytogenetic consequences of VBL treatment in mammals.

METHODS

Swiss mice received 1 of 3 different doses of VBL (0.5, 1.0 and 1.5 mg/kg body weight) as a single intraperitoneal injection. The cytogenetic toxicity of VBL was assessed from the induced aberrant metaphases, chromosomal aberrations (CAs) excluding gaps and the mitotic index (MI) 24 h after treatment, and micronuclei (MN) 30 h after treatment.

RESULTS

All 3 doses of VBL induced statistically significant (p < or = 0.01) percentages of aberrant metaphases and CAs, but there was no significant change in the MI. The induced percentage of aberrant metaphases and CAs were decreased with the increase in the dose of VBL. On the other hand, there was a dose-dependent and significant (p < or = 0.01) increase in MN induction.

CONCLUSIONS

The results of this study indicate the clastogenic potential of VBL in the mouse bone marrow. In the present study, the induction of numerous relatively large-sized MN by VBL is in agreement with the reported aneugenic action of the drug. Although VBL is cytotoxic and is a spindle poison, the mechanism(s) involved in bringing about its clastogenic effects is yet to be elucidated.

Multiple effects of paclitaxel are modulated by a high c-myc amplification level

Paclitaxel affects microtubule stability by binding to beta-tubulin, thus leading to cell accumulation in the G(2)/M phase, polyploidization, and apoptosis. Because both cell proliferation and apoptosis could be somehow regulated by the protooncogene c-myc, in this work we have investigated whether the c-myc amplification level could modulate the multiple effects of paclitaxel. To this aim, paclitaxel was administered to SW613-12A1 and -B3 human colon carcinoma cell lines (which are characterized by a high and low c-myc endogenous amplification level, respectively), and to the B3mycC5 cell line, with an enforced exogenous expression of c-myc copies. In this experimental system, we previously demonstrated that a high endogenous/exogenous level of amplification of c-myc enhances serum deprivation- and DNA damage-induced apoptosis. Accordingly, the present results indicate that a high c-myc amplification level potentiates paclitaxel cytotoxicity, confers a multinucleated phenotype, and promotes apoptosis to a great extent, thus suggesting that c-myc expression level is relevant in modulating the cellular responses to paclitaxel. We have recently shown in HeLa cells that the phosphorylated form of c-Myc accumulates in the nucleus, as distinct nucleolar and extranucleolar spots; here, we demonstrated that, after the treatment with paclitaxel, phosphorylated c-Myc undergoes redistribution, becoming diffused in the nucleoplasm.

Synergistic effects of thearubigin and genistein on human prostate tumor cell (PC-3) growth via cell cycle arrest

There is evidence that habitual consumption of green tea by Japanese men is correlated with a reduction in cancers, including prostate; soybean isoflavones are also associated with increased protection. The present study compared the anti-proliferative effect of black tea (Camellia sinensis) polyphenol, thearubigin (TR), alone or combined with the isoflavone genistein, on human prostate (PC-3) carcinoma cells. TR administered alone did not result in any alteration of cell growth. When combined with genistein, however, TR significantly inhibited cell growth and induced a G2/M phase cell cycle arrest in a dose dependent manner. These findings indicate the potential use of combined phytochemicals to provide protection against prostate cancer.

Frequency of minisatellite repeat number changes at the MS205 locus in human sperm before and after cancer chemotherapy

To determine whether the measurement of repeat number mutations at a minisatellite locus could detect human germline mutations induced by chemotherapy, we performed a longitudinal study of the mutation frequencies in sperm from 10 patients treated for Hodgkin's disease. Polymerase chain reaction on small pools of DNA equivalent to 100 sperm and Southern blotting were used to screen at least 7900 sperm in each sample to quantify the mutation frequency at the minisatellite MS205 locus. Pretreatment and posttreatment semen samples were obtained at least 2 months after completion of therapy from 4 patients treated with a regimen (Novantrone, Oncovin, vinblastine and prednisone [NOVP]) that lacks alkylating agents and from three patients treated with regimens (Cytoxan, vinblastine, procarbazine and prednisone/Adriamycin, bleomycin, dacarbazine, lomustine, and prednisone [CVPP/ABDIC] or mechlorethamine, Oncovin, procarbazine and prednisone [MOPP]) containing alkylating agents. There were no effects of NOVP or CVPP/ABDIC on the mutation frequencies. In the 1 patient treated with MOPP, the treatment with the highest dose of gonadotoxic alkylating agents, there was a statistically significant increase in mutation frequency from 0.79% pretreatment to 1.14% posttreatment, indicating induction of mutations in stem spermatogonia. During-treatment semen samples obtained from 2 patients treated with ABVD, which does not contain gonadotoxic alkylating agents, and 1 with NOVP also did not show any increases above the baseline mutation frequencies, indicating no increase in the minisatellite mutation frequency in spermatocytes. Thus, measurement of repeat number changes at minisatellite MS205 appears to be able to detect induced germline mutations in human sperm. However, most chemotherapy regimens do not significantly increase this class of mutations.

Chromosomal aberrations in vitro induced by aneugens

Various aneugens were reported to induce structural chromosomal aberrations beside their influence on cell division and their aneugenic potential To asses, whether a relationship between disturbance of cell division and clastogenic potential exists, CHO cells were treated with the well-known aneugens colcemid, colchicine and vincristine and investigated for the induction of structural chromosomal aberrations, polyploid cells and alterations in mitotic index. At low and intermediate concentration, all compounds induced polyploidy and an increase in mitotic index, but no structural aberrations at all. However, at high concentrations, colcemid and colchicine both induced numerous structural chromosomal aberrations in diploid cells. Colchicine was also clastogenic in tetraploid cells. Vincristine did not induce structural chromosomal aberrations in diploid cells, but in tetraploid cells. The clastogenic effects showed a clear-cut threshold with all three compounds. Furthermore, it was found that the tetraploid condition in CHO cells is generally accompanied by an increase in structural chromosomal aberrations, in vehicle controls as well as in cultures treated with the aneugens. Nevertheless, this study demonstrates that for the three aneugenic compounds tested, no direct relationship between compound induced disturbance of cell cycle and compound induced structural chromosomal aberration incidence exists.

The clinical use of mutagenic anticancer drugs

Cytotoxic chemotherapy is routinely used in the treatment of cancer, and has been an important factor in increasing 5-year survival rates for some types of this disease. A range of drugs are currently available, with differing modes of action. As well as causing some direct toxic effects, most if not all of these drugs are both mutagenic and carcinogenic. Although comparative information on these properties is generally available for anticancer drugs which alkylate DNA, it has been less readily accessible for other drug classes. This special issue contains seven reviews on the mutagenic properties of the major classes of cytotoxic drugs in clinical use, as well as one on a class of drugs that is under development. Some carcinogenicity data are also summarised, where available. Additionally, there are four more general papers, including one on the use of genetic activity profiles for comparing mutagenicity of the drugs, two on germ-cell effects, and one on biomonitoring for exposure to genotoxic anticancer drugs.