Benomyl-- a novel type of base analogue mutagen?

Genotoxicity assay of five pesticides and their mixtures in Saccharomyces cerevisiae D7

Four organophosphorus pesticides (azinphos-methyl, diazinone, dimethoate, and pirimiphos-methyl), and one carbamate (benomyl) were tested for cytotoxicity, reverse mutation and gene conversion in Saccharomyces cerevisiae D7, with and without the S9 metabolic system. Furthermore, two mixtures of the above compounds, namely benomyl + pirimiphos-methyl (6/1 ratio) and dimethoate + diazinone + azinphos-methyl (10/4/6 ratio) were tested in the same experimental model. Azinphos-methyl, benomyl, and pirimiphos-methyl alone did not induce any genotoxic effect, whereas azinphos-methyl and diazinone were active in inducing reversion and gene conversion. The benomyl + pirimiphos-methyl mixture did not show any genotoxic activity. The dimethoate + diazinone + azimphos-methyl mixture was genotoxic, although an antagonistic effect between the components was observed. The addition of S9 post-mitochondrial liver fraction decreased the activity of both single and mixed genotoxic agents.

Evaluation of carbendazim for gene mutations in the Salmonella/Ames plate-incorporation assay: the role of aminophenazine impurities

Benomyl (methyl [1-[(butylamino)carbonyl]-1H-benzimidazol-2- yl]carbamate) and its major metabolite carbendazim (methyl 2-benzimidazolecarbamate) are major agricultural systemic fungicides. These compounds inhibit fungal microtubular function and thereby cause nondisjunction of chromosomes at cell division. Several investigators have proposed that these compounds can also cause gene mutations (base-pair substitutions). In this laboratory, no mutagenic activity was observed with either benomyl (analytical grade) or Benlate (samples tested up to 500 and 1200 micrograms/plate, respectively, the limit of cytotoxicity) in the Salmonella/Ames plate-incorporation test in either base-pair substitution (TA100 and TA1535) or frameshift-sensitive (TA98 and TA1537) strains with or without S9 metabolic activation. However, some carbendazim preparations caused mutations in frameshift-sensitive strains at very high concentrations (> or = 5000 micrograms/plate) with metabolic activation. The mutagenic activity was not due to the major carbendazim metabolite, methyl (5-hydroxy-1H-benzimidazol-2-yl)carbamate (5-OH MBC), since 5-OH MBC was not mutagenic with (up to 20,000 micrograms/plate) or without (up to 16,000 micrograms/plate) activation. Subsequently, two highly mutagenic contaminants, 2,3-diaminophenazine (DAP) and 2-amino-3-hydroxyphenazine (AHP) were detected in mutagenic carbendazim samples. In those samples, DAP and AHP contaminant levels ranged as high as 46.5 and 11.6 ppm, respectively. No evidence of mutagenicity could be detected in preparations in which the DAP content was < 1.8 ppm. The mutagenic activity of these two contaminants was further investigated in strain TA98. Without activation, DAP and AHP were positive at test concentrations as low as 5 and 10 micrograms/plate, respectively. In the presence of S9, mutations were detected at much lower concentrations (beginning at 0.025 and 0.05 microgram/plate, respectively). These results indicate that carbendazim samples containing DAP or AHP at levels as low as 5 or 10 ppm, respectively, would be positive in the Salmonella/Ames test with activation when tested at 5000 micrograms/plate. Purified carbendazim is not mutagenic.

Cytogenetic effects of benzimidazoles in mouse bone marrow

The cytogenetic effects of three benzimidazoles, i.e., benomyl, methyl thiophanate and methyl 2-benzimidazolecarbamate (MBC), were studied in mouse bone marrow cells by analyzing three genetic endpoints: micronuclei, structural chromosome aberrations plus or minus gaps, and aneugenic effects (hyperdiploidy or polyploidy). In general, the effects were small, but it was observed that benomyl and MBC significantly induced micronuclei as well as aneugenic effects, hyperdiploidy (no metaphases with more than one or two extra chromosomes, 2n + 1 or 2n + 2, were observed) and polyploidy (4n). The induction of chromosome gaps and breaks was less evident. Methyl thiophanate significantly induced micronuclei, but it was less effective than benomyl and MBC. Our results showed that micronuclei are a good indicator of aneugenic effects in mouse bone marrow cells. A curvilinear trend test has been devised to fit the curves originating from the time-dependent responses.

A genotoxicological study of the new local anaesthetic carbamate derivatives carbisocaine, heptacaine and pentacaine

The potential mutagenic activity of three carbamate derivatives with local anaesthetic activity was investigated. Genotoxic activity was observed after application of Carbisocaine on Euglena gracilis, whereas no activity was detected by Carbisocaine, Heptacaine and Pentacaine on Salmonella typhimurium, Escherichia coli and Drosophila melanogaster.

Clastogenic activity in urine of workers occupationally exposed to pesticides

Genotoxicity in the urine of orchardists occupationally exposed to pesticides was investigated. Urine samples were obtained during pre-spraying and spraying periods from 22 non-smoking orchardists who spray large amounts of pesticides during the fruit growing season. For comparison purposes, urine was collected from 11 non-smoking personnel at an agricultural research station located near the application site and from 21 non-smoking individuals (reference controls) in a non-agricultural area. Organic material was isolated from urine by preparative reversed-phase high-pressure liquid chromatography, and assayed for clastogenic activity using Chinese hamster ovary cells. Urine samples collected during the pre-spraying period showed no significant differences in clastogenic activity compared to that found for the reference control group. However, clastogenic activity of urine specimens collected during the spraying period was significantly elevated (p less than 0.001) for the highly-exposed orchardists, but not for the research station personnel. Clastogenicity of orchardists' urine was observed within 8 h of pesticide application.

Mutagenicity of urine from greenhouse workers

The mutagenicity of urine from spray applicators in 12 greenhouses in 1986 has been evaluated. The workers served as their own controls. Urine samples reflecting pesticide exposure were collected at the end of the day of application and a corresponding control sample was collected 3 d later. Using Salmonella typhimurium bacterial tester strains TA98 and TA100 with and without S9 activation, seven workers showed no significant differences (p less than 0.05) in the mutagenicity of their exposed and control urine. Of the five remaining workers, three, who wore no respirators, showed significantly higher (p less than 0.05) concentrations of mutagens in their exposed urine sample as compared to their respective controls. The mutagenicity of certain of the compounds applied by these latter workers is discussed.

Mutagenicity of methyl 2-benzimidazolecarbamate, diethylstilbestrol and estradiol: structural chromosomal aberrations, sister-chromatid exchanges, C-mitoses, polyploidies and micronuclei

Methyl 2-benzimidazolecarbamate (MBC), diethylstilbestrol (DES) and estradiol were tested with regard to their ability to induce C-mitoses, polyploidies, micronuclei, structural chromosomal aberrations and sister-chromatid exchanges (SCE) in human peripheral lymphocytes in vitro. The compounds did not induce structural chromosomal aberrations either in the presence or absence of metabolic activation. MBC and estradiol were negative in the SCE test. DES induced SCE rates which were not even twice the control level and which were independent of dose and of metabolic activation. All compounds induced C-mitoses, polyploidies and micronuclei. The micronuclei are interpreted as resulting from errors in the anaphase distribution of chromosomes by spindle disturbances rather than from structural chromosomal aberrations.

Genetic effects of methyl benzimidazole-2-yl-carbamate on Saccharomyces cerevisiae

The genetic effects of the mitotic inhibitor methyl benzimidazole-2-yl-carbamate (MBC) have been studied in Saccharomyces cerevisiae. MBC had little or no effect on the frequency of mutation. In some experiments MBC caused an increase in the frequency of mitotic recombination; however, this effect was small and not reproducible. The primary genetic effect of MBC was to induce mitotic chromosome loss at a high frequency. Chromosome loss occurred at equal frequencies for all chromosomes tested (13 of 16). Cells which had lost multiple chromosomes were found more frequently than predicted if individual chromosome loss events were independent. The probability of loss for a particular chromosome increased with length of time cells were incubated with MBC. MBC treatment also increased the frequency at which polyploid cells were found. These results suggested that MBC acted to disrupt the structure or function of the mitotic spindle and cause chromosome nondisjunction.

Genetic effects of methyl benzimidazole-2-yl-carbamate on Saccharomyces cerevisiae

The genetic effects of the mitotic inhibitor methyl benzimidazole-2-yl-carbamate (MBC) have been studied in Saccharomyces cerevisiae. MBC had little or no effect on the frequency of mutation. In some experiments MBC caused an increase in the frequency of mitotic recombination; however, this effect was small and not reproducible. The primary genetic effect of MBC was to induce mitotic chromosome loss at a high frequency. Chromosome loss occurred at equal frequencies for all chromosomes tested (13 of 16). Cells which had lost multiple chromosomes were found more frequently than predicted if individual chromosome loss events were independent. The probability of loss for a particular chromosome increased with length of time cells were incubated with MBC. MBC treatment also increased the frequency at which polyploid cells were found. These results suggested that MBC acted to disrupt the structure or function of the mitotic spindle and cause chromosome nondisjunction.

The pH dependence of the mutagenicity of methyl benzimidazol-2-yl carbamate (MBC) towards Aspergillus nidulans (Eidam) Winter and Cladosporium cucumerinum Ellis & Arth

The ability of methyl benzimidazol-2-yl carbamate (MBC) to induce point mutations to carboxin and MBC resistance in Aspergillus nidulans (Eidam) Winter and Cladosporium cucumerinum Ellis & Arth. was dependent upon the pH value of the agar medium into which it had been incorporated. The relevance of this in relation to testing chemicals for a possible mutagenic activity with microorganisms is discussed.

Mutagenicity of methyl benzimidazole-2-yl carbamate (MBC) towards Aspergillus nidulans (EIDAM) Winter and Cladosporium cucumerinum Ellis & Arth

The ability of methyl benzimidazol-2-yl carbamate (MBC) to induce point mutations to carboxin and MBC resistance was tested in Aspergillus nidulans (Eidam) Winter and Cladosporium cucumerinum Ellis & Arth. A sub-lethal concentration (E.D.50) of MBC, when incorporated into a complete agar medium, induced MBC resistance in germinating conidia of A. nidulans and carboxin resistance in germinating conidia of C. cucumerinum. The significance of these findings in relation to fungicide resistance in the fields is discussed.

An evaluation of the Escherichia coli WP2 and WP2 uvrA reverse mutation assay

The methodology and status of the Escherichia coli WP2 reverse mutation system as it applies to chemical screening were reviewed using the available published literature. 163 documents were reviewed by the Working Group. These included abstracts, research articles, review articles and publicly available contract and grant final reports. From this group, 115 documents were rejected for critical evaluation by the Working Group. 48 documents were reviewed and the test results summarized. The general conclusion of the Working Group was the the E. coli WP2 reverse mutation system is a valuable tool for mutagenesis research, but that there is no evidence from a review of the literature that this assay will contribute significantly to the results obtainable from careful application of the Ames Salmonella assay. Another review of the role of this system in general screening may be warranted after more research and development with the plasmid-containing WP2 derivatives.

Mitotic arrest by benzimidazole analogs in human lymphocyte cultures

Published reports have suggested the possible association of mutagenic or teratogenic properties of some benzimidazole analogs with induced spindle disruption and consequent mitotic arrest. Studies were conducted to explore this correlation. Seven benzimidazole analogs were evaluated in a human lymphocyte system for ability to block mitosis at metaphase. Confirming the previously reported results, four compounds, mebendazole, parbendazole, cambendazole, and fenbendazole, caused metaphase accumulation, which we found to be dose- and time-related. Minimum effective dose for mebendazole and cambendazole was 10 microgram/ml; for parbendazole, 1 microgram/ml; and for fenbendazole, 100 microgram/ml. The drug-induced mitotic arrest is qualitatively and quantitatively similar to that produced by colcemid. No activity was observed with three other compounds, benzimidazole, thiabendazole, and oxfendazole when tested at 100 microgram/ml. Presence of absence of mitotic effects is correlated with reported teratogenicity with five out of the seven analogs. This suggests some utility of mitotic assessments for predicting teratogenicity.

Mutagenicity evaluation of chemical pesticides

Over the last few decades, the use of chemical pesticides has increased dramatically in the U.S. This relatively sudden increase greatly concerns the U.S. Environmental Protection Agency (EPA), since it has the responsibility for ensuring the safety of all pesticides used in the U.S. In response to this concern, EPA has established a review program, the Rebuttable Presumption Against Registration (RPAR), for periodically reassessing the mutagenic and carcinogenic potential of pesticide compounds. This paper presents a review and evaluation of the data reported in the literature on six chemical pesticides suspect for mutagenic potential. The pesticide chemicals discussed are maleic hydrazide; rotenone; monuron; diallate; triallate, and benomyl.

An investigation of some genetic toxicological effects of the fungicide benomyl

The widely used fungicide benomyl (methyl-1-(butylcarbamoyl)-2-benzimidazole carbamate) and its breakdown product methyl-2-benzimidazole carbamate (MBC) have been reported to have mutagenic activity in some organisms. In experiments with Drosophila melanogaster we found (i) there was no significant increase in recessive lethal frequency after feeding adult flies with Benlate did not increase chromosome breakage or loss significantly; (ii) there was a relatively high incidence of sterility when males of one strain (Oregon R) were fed Benlate or MBC. In experiments in which cultures of human lymphocytes were exposed in vitro to 0.5 mg/ml MBC we observed extreme contraction of the chromosomes but found no evidence of an increase in the number of cells with chromosome aberrations. We conclude that benomyl and MBC are unlikely to be strong mutagens, but more research is needed to exclude the possibility that they are capable of inducing genetic damage in the germ cells of higher organisms.

Bacterial breakdown of benomyl. II. Mixed cultures

Experiments with mixed bacterial cultures grown in liquid media which contained the benzimidazole fungicide benomyl, with or without Na-lactate, as source of carbon provided circumstantial evidence for cleavage of the benzimidazole heterocyclic ring. Yet, neither 2-aminobenzimidazole (2-AB) nor benzimidazole, as sole source of carbon, supported any bacterial growth. "Total 14C-balance analysis" experiments conclusively showed production of 14CO2 from [2-14C]methyl benzimidazol-2-yl carbamate (MBC), and thus cleavage of the benzimidazole nucleus: bioassays, however, showed that the actual rate of benomyl and MBC breakdown was only small, the parent compound benomyl being still recovered in substantial quantities after up to 80 days of incubation. Therefore, cleavage of the benzimidazole ring is probably a matter of cometabolism, n-butylamine which originates from the butylcarbamoyl side chain serving as the proper source of carbon. Besides radiolabelled 2-AB and CO2, an unknown metabolite was isolated which showed characteristics of a 2-AB-nucleotide. Probably, 2-AB was incorporated into bacterial DNA, which upon lysis of the bacterial cells gave rise to the nucleotide in question. Therefore, 2-AB might exert its inhibitory action by interfering with the normal functioning of DNA.

On the mechanisms of induced somatic recombination by certain fungicides in Aspergillus nidulans

Four fungicides interfered with the segregation of chromosomes at mitosis of Aspergillus nidulans by increasing the somatic recombination, shown as colour sectors in green colonies, in a strain heterozygous for spore colour mutations. In an attempt to discover the mechanisms by which these fungicides increased the somatic recombination, a prototrophic diploid strain, heterozygous for colour and several other appropriate markers in all chromosomes, was used which enabled the detection and classification of all colour recombinants to be made by genetic analysis. The fungicides investigated were: benomyl (methyl-1-(butylcarbamoyl)-2-benzimidazole carbamate) a benzimidazole derivative, botran (2,6-dichloro-4-nitroaniline) and chloroneb (1,4-dichloro-2,5-dimethoxybenzene) of the aromatic hydrocarbon group of fungicides, and the antibiotic actinomycin D. At least three different mechanisms, non-disjunction, mitotic crossing-over and breakage-deletion, were found to be responsible for the recombinogenic activity of the compounds studied.

Mutagenicity testing of benomyl, methyl-2-benzimidazole carbamate, streptozotocin and N-methyl-N'-nitro-N-nitrosoguanidine in Salmonella typhimurium in vitro and in rodent host-mediated assays

The fungicide benomyl and its commercial preparations Fundazol 50WP and Benlate 50WP and the benomyl metabolite methyl-2-benzimidazole carbamate and its commercial preparation MBC 50WP were tested for mutagenicity in in vitro spot tests, in microsomal plate assay, in liquid-culture treatments, or in rodent host-mediated assay. The base-pair substitution Salmonella typhimurium mutant hisG46 and the hisG46-bearing uvrB excision-repair-deficient mutants TA100, TA1530, TA1535 or TA1950 were used as test organisms. Complete genotypic information of these mutants is given in Ames et al. [2]. Captain 50WP, streptozotocin (SZN), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), 2-aminopurine and N-acetylaminofluorene were used as positive control compounds. In nonoverlay spot tests Benlate 50WP was not mutagenic over a dose range of 50-5000 microgram/spot in hisG46 and TA1535. In overlay spot tests 50 or 100 microgram/spot Benomyl, MBC, Fundazol 50WP, Benlate 50WP and MBC 50WP were tested in hisG46, TA1530 or TA1950. Only a non-commercial MBC sample at 100 microgram/spot showed weak mutagenic activity in hisG46. In microsomal activation plate assay MBC, benomyl, Fundazol 50WP and Benlate 50WP were tested in TA100 over a dose range of 50-2000 microgram/plate. None of the compounds showed mutagenicity. In a 20-h liquid-culture treatment 10, 100, 1000 and 10 000 microgram/ml Fundazol 50WP were not mutagenic in TA 30. In 1-h liquid-culture treatments benomyl, Benlate 50WP or Fundazol 50WP failed to induce mutations in hisG46, TA100 or TA1950 over a dose range of 0.25-1000 microgram/ml. Appropriate positive controls were mutagenic in each experiment. The consistently negative results in this study with commercial MBC and benomyl preparations are contrary to positive results reported earlier with similar methods and similar commercial preparations. Possible reasons to explain the different results are presented. The alkylating agents SZN and MNNG induced fewer mutations in TA1530 and TA1950 uvrB excision-repair-deficient strains than in the hisG46 excision-proficient strain, indicating that with these mutagens excision-repair is also a mutation-prone process. In rodent host-mediated assays with Fundazol 50WP in mice 3 consecutive subcutaneous hourly doses of 500 mg/kg in hisG46 and TA1950 and in rats or mice an oral dose of 4000 mg/kg in TA1950 were not mutagenic. The positive control SZN was mutagenic.