Embryotoxicity induced by alkylating agents: 6. DNA adduct formation induced by methylnitrosourea in mouse embryos

DNA repair during organogenesis

DNA damage caused by genotoxic agents can impact on virtually any cellular process due to its ability to affect gene expression and subsequent gene products. The importance of repairing damaged DNA is evidenced by the variety of DNA repair pathways that have evolved in all living organisms, and the human syndromes caused by a lack of this repair ability. This review focuses on the expression and activity of DNA repair pathways during mammalian organogenesis, and the role of these pathways in ensuring the stability of the conceptal genome. DNA repair capacity may play a role also in the response of the conceptus to genotoxic agents that may induce malformations; the consequences of exposure to a genotoxic agent during organogenesis depend on the extent of the damage and on the ability of the embryo to respond by repairing DNA or arresting cell division. The four main repair pathways (nucleotide excision repair, base excision repair, mismatch repair, and recombination repair) are expressed to various degrees during organogenesis, as are members of the genotoxic stress-activated cell cycle checkpoint pathways. Developmental-stage-specific alterations in transcript levels, protein levels, as well as activity, indicate that the regulation of DNA repair pathways during development is complex. The importance of DNA repair pathways in endogenous damage control is illustrated by the sensitivity of development to their disruption if some of these genes are mutated. Furthermore, the conceptus has a limited capacity to alter DNA repair responses following exposure to genotoxic agents.

Embryotoxicity induced by alkylating agents: 9. Low dose prenatal-toxic risk estimation of ethylmethanesulfonate based on no-observed-adverse-effect-level risk factor approach, dose-response relationships, and molecular dosimetry

Analogously to an earlier study using methylnitrosourea (MNU) the prenatal-toxic risk of low doses of ethylmethanesulfonate (EMS) was estimated using different procedures, comparatively. First, the risk of low doses was estimated using linear extrapolation to zero. When using the variable "all gross structural abnormalities" the lowest effective dose in the experiment was 150 mg/kg body wt (5.6% incidence), the additional risk over background was calculated to be 5.0%, and the hypothetical incidence 0.1% was associated with the dose 3 mg/kg EMS. When evaluating "gross structural limb abnormalities," which are not observed in controls, the dose associated with the hypothetical incidence 0.1% was 17.4 mg/kg EMS. Furthermore, derived from a dose-response study of teratogenicity extrapolation to the possible risk of low doses was performed using nonlinear mathematical models. In this case, the results obtained are dependent on the dose response variable as well as from the statistical approach which was chosen. As an example, the values obtained from one evaluation are given: all gross structural abnormalities, Weibull transformation, jackknife approach: ED0.1% = 72 mg/kg EMS. For comparison a "virtually safe dose" was calculated by use of the no-observed-adverse-effect-level (NOAEL) risk factor approach. The NOAEL under our experimental conditions was 100 mg per kg body wt. By using an arbitrarily chosen risk factor of 100 a "safe dose" of 1 mg EMS per kg body wt was obtained. In addition, molecular dosimetry of the DNA adduct rate of O6-ethylguanine in the 11-day-old embryos was used. Based on the assumption that a linear correlation exists between this specific adduct rate and the incidence of teratogenic effects, the hypothetical incidence of 0.1% was associated with a dose of 99 mg/kg EMS. This value is quite similar to that obtained by extrapolation using probit analysis which is in contrast to the results obtained with MNU.

Embryotoxicity induced by alkylating agents: 10. Analysis of the combined teratogenic effects of methylnitrosourea and ethylmethanesulfonate in mice

In previous studies the direct-acting alkylating model compounds methylnitrosourea (MNU) and ethylmethanesulfonate (EMS) were investigated with regard to dose-response of teratogenicity as well as DNA adduct formation in mice. In this study the teratogenic effects induced by combined treatment with these substances were analyzed using doses which, following single treatment with either substance, were around the threshold level, i.e., no adverse effect level (NOAEL) and lowest observed adverse effect level (LAOEL). Combined treatment of LAOELs resulted in a threshold-like response, while the combination of the NOAEL of one substance with the LAOEL of the other increased the response rate dramatically to nearly 100%. This phenomenon was further evaluated using biometrical methods. The dose-response surface as well as isobolograms were calculated in order to describe the combination effect. In addition, a dose-response model was fitted to the data. In conclusion, the initially surprising high combination effect revealed to be not so extraordinary when considering the steepness of the dose-response relationships of the single substances.

DNA modification induced by 6-mercaptopurine riboside in murine embryos

The cytostatic drug 6-mercaptopurine riboside (6-MPr) was investigated in mice in order to test the hypothesis that the teratogenicity of this antimetabolite is paralleled by an incorporation into the DNA of the embryos during organogenesis. DNA modification in the embryos was analysed 4 h following s.c. administration of [35S]-labelled 6-MPr to the dams on day 11 of pregnancy. The DNA of the embryos was isolated and hydrolysed to the bases by formic acid. Following separation by cation-exchange HPLC 6-thioguanine was found in the hydrolysate. Quantitation was performed by liquid scintillation counting. Evaluations of 6 doses in the range of 8-25 mg/kg were performed. An incorporation rate of 6-thioguanine from 32-56 pmol per mumol guanine was found in the DNA of the embryos. These findings suggest that, similar to the previously studied alkylating agents, the teratogenicity of 6-MPr may be, at least in part, induced via DNA modification of the embryos.

Toxic effects of cyclophosphamide and methylnitrosourea in Japanese quail embryos depend on the route of administration

We studied the toxic effects of cyclophosphamide and methyl-N-nitrosourea in cultured quail embryos when injected into the albumen and the subembryonic liquid, respectively. The data indicate that both agents display a greater toxicity when given into the subembryonic liquid instead of the albumen. This differential toxicity was demonstrated by means of the survival rate, developmental parameters like growth rate and developmental stages, and excreted nucleosides and uric acid. It is reasonable to assume that the lesser toxicity in the albumen is mainly due to the lower concentration reaching the embryo after the dilution of the agent in this compartment. On the basis of the results obtained in these experiments, the cultured avian embryo represents an especially suitable experimental system to study the changes in metabolism induced by toxic agents. This is due to the easy access to the subembryonic compartment and the easy sampling of allantoic liquid under these conditions.

Embryotoxicity induced by alkylating agents: 8. DNA adduct formation induced by ethylmethanesulfonate in mouse embryos

In previous studies using methylating agents a correlation was found between the initial DNA adduct rate (O6-methylguanine) in the embryo and the teratogenic efficiency. This was shown by measuring DNA adduct rates in the teratogenic dose range which exhibited similar adduct rates at the equivalent teratogenic dose levels. A similar approach was performed using the ethylating agent ethylmethanesulfonate (EMS). In the teratogenic dose range (150-250 mg/kg bw) the adduct rates of O6-ethylguanine were similar compared to those of O6-methylguanine which were obtained with methylating agents. We conclude that a correlation between teratogenicity and adduct rate (O6-alkylguanine) exists for both methylating and ethylating agents. Furthermore, DNA adduct formation following doses at and below the no-observed-adverse-effect-level (NOAEL) of teratogenicity was determined. The lowest experimental dose was 45 mg/kg EMS. Substantial DNA adduct rates in the embryos were found. These data will be used for molecular dosimetry in a risk assessment of low doses.

Application of high-performance liquid chromatography for recognition of covalent nucleic acid modification with anticancer drugs

Covalent modification of DNA by antineoplastic agents represents a potent biochemical lesion which can play a major role in drug mechanism of action. The ability to measure levels of DNA covalent modifications in target cells in vivo may, therefore, be seen as the ultimate form of therapeutic drug monitoring. Additionally, elucidation of the structure of critical DNA adducts and definition of their role in tumour cell cytotoxicity will provide more selective targets for rational drug design of new cancer chemotherapeutic agents. High-performance liquid chromatography has contributed significantly to all these areas. In vivo levels of nucleic acid covalent modifications are in the range of 1 in 10(5)-10(8) nucleotides precluding the use of conventional high-performance liquid chromatographic detection methods. Several classes of natural product anticancer drugs have been shown to bond covalently to nucleic acids under optimal laboratory conditions. These have proved more accessible to high-performance liquid chromatographic analysis because of their lipophilicity and strong UV chromophores. However, the majority of experimental evidence to date suggests that with the exception of mitomycin C and morpholino-anthracyclines these compounds do not exert their primary mechanism of action through nucleic acid covalent modification. DNA adducts of alkylating and platinating agents are more difficult to detect by high-performance liquid chromatography and can be chemically unstable. These compounds interact with DNA on the basis of chemical kinetics. Thus, the principle sites of attachment tend to be with the most nucleophilic base (guanine) at its most reactive centre (N-7 position). Limited in vivo high-performance liquid chromatographic studies with all classes of anticancer drugs indicate a much more complex pattern of adductation than would have been anticipated from in vitro studies alone. Some of these differences are probably due to methodological artefacts but these studies stress the need for sensitive detection methods and reliable sample preparation (nucleic acid extraction and digestion techniques) when attempting to determine nucleic acid covalent modifications by anticancer drugs.

Embryotoxicity induced by alkylating agents: 7. Low dose prenatal-toxic risk estimation based on NOAEL risk factor approach, dose-response relationships, and DNA adducts using methylnitrosourea as a model compound

Prenatal-toxic risk estimation for the alkylating model compound methylnitrosourea (MNU) was performed using different procedures. Risk of low doses was estimated using linear extrapolation to zero (estimated ED0.1%: 0.1 mg/kg body wt MNU) as well as extrapolation by probit analysis based on a dose-response study (estimated ED0.1%: 1.6 mg/kg body wt). Furthermore, a "virtually safe dose" was established by means of the NOAEL risk factor approach (e.g., factor 30:0.03 mg MNU per kg body wt). In previous studies in murine embryos using MNU, we combined dose-response data and DNA adduct rate measurements and deduced that O6-methylguanine is a suitable variable for molecular dosimetry. In a tentative approach, we estimated the teratogenic risk of low doses based on the adduct rates of O6-methylguanine in the DNA of the embryos. It is concluded that in the case of steep dose-response relationships, which are typical for the majority of teratogenic effects, the NOAEL risk factor approach is more conservative than extrapolation based on probit analysis. Risk estimation using dosimetry with this model compound yields estimated incidences similar to linear extrapolation.