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In vitro developmental toxicity of five direct-acting alkylating agents in rodent embryos: structure-activity
E M Faustman1, Z Kirby, D Gage
1Department of Environmental Health, University of Washington, Seattle 98195.
Abstract:
Five direct-acting alkylating agents were examined qualitatively and quantitatively for their ability to produce developmental toxicity in rodent postimplantation embryos. These agents were structurally related and were capable of donating either a methyl (methylnitrosourea, MNU; methylnitronitrosoguanidine, MNNG; methyl methanesulfonate, MMS) or ethyl (ethylnitrosourea, ENU; ethyl methanesulfonate, EMS) group to nucleophiles. These agents' reactivities were known to differ. In day 10 rat embryos in vitro a single, 2-hour exposure was shown to be sufficient to elicit dose-dependent increases in embryo lethality and malformations. Qualitatively, the patterns of embryo malformations reported in treated embryos paralleled those observed in in vivo studies, especially in regard to adverse effects on central nervous system and craniofacial systems. Quantitatively, the order of potency of these agents in vitro was: MNNG greater than MNU greater than ENU greater than MMS greater than EMS. In vivo studies reported a different order of potency. In vitro, methylating agents were consistently more potent than ethylating agents. Other chemical properties such as nucleophilic reactivity or half-life under physiological conditions could not explain observed potency relationships. Future investigation of other chemical properties of these agents such as specific alkylation and carbamylation reactivities may expand these initial structure-activity observations.
Insights
Five alkylating agents caused developmental toxicity in rodent embryos. Methylating agents like MNNG were more potent in vitro than ethylating agents, impacting the central nervous system and craniofacial development.
Area of Science:
- Toxicology
- Developmental Biology
- Organic Chemistry
Background:
- Direct-acting alkylating agents are structurally related compounds.
- These agents can donate methyl or ethyl groups to nucleophiles.
- Their differing reactivities are known.
Purpose of the Study:
- To qualitatively and quantitatively assess developmental toxicity of five direct-acting alkylating agents in rodent postimplantation embryos.
- To compare in vitro and in vivo potency of these agents.
- To investigate structure-activity relationships in developmental toxicity.
Main Methods:
- In vitro exposure of day 10 rat embryos to five alkylating agents (MNU, MNNG, MMS, ENU, EMS).
- Dose-dependent assessment of embryo lethality and malformations.
- Qualitative analysis of malformation patterns, particularly central nervous system and craniofacial effects.
- Quantitative comparison of agent potency.
Main Results:
- A single 2-hour exposure induced dose-dependent embryo lethality and malformations.
- In vitro malformation patterns mirrored in vivo observations.
- In vitro potency order: MNNG > MNU > ENU > MMS > EMS.
- Methylating agents were consistently more potent than ethylating agents in vitro.
- In vivo potency order differed from in vitro findings.
Conclusions:
- In vitro studies demonstrate significant developmental toxicity of alkylating agents.
- Methylating agents show higher in vitro potency compared to ethylating agents.
- Chemical properties like nucleophilic reactivity or half-life do not fully explain potency differences.
- Further research into specific alkylation and carbamylation reactivities is warranted.