The ovarian toxicity of 4-vinylcyclohexene in B6C3F(1) mice: Role of vinylcyclohexene diepoxide

Abstract

4-Vinylcyclohexene (VCH) is an ovarian toxicant in mice that requires bioactivation to epoxides to cause depletion of preantral follicles. Although two monoepoxide metabolites are formed, they are less potent than the diepoxide in terms of follicular depletion. Thus, the hypothesis of this dissertation research was that VCD is the ultimate ovarian toxicant of VCH in mice. To determine the role of the diepoxide in the ovarian toxicity of VCH and related compounds, structure-activity studies were conducted. Following intraperitoneal administration of VCH for 30 days, a significant depletion of ovarian follicles was observed. No reduction in the number of small ovarian follicles occurred following treatment with structural analogues of VCH (vinylcyclohexane, ethylcyclohexene, and cyclohexene) that contain only a single unsaturated site, and thus can only form monoepoxides. Importantly, the monoepoxides of these analogues also were not ovotoxic. Circulating levels of vinylcyclohexene diepoxide (VCD) were observed in mice following administration of a single dose of VCH. In addition, multiple doses of VCH resulted in induction of its metabolism, as evidenced by increased circulating levels of VCH epoxides, including VCD. Elevated levels of hepatic cytochromes P450 (P450 2A, P450 2B, and P450 2E1) were observed following repeated dosing with VCH, which explains the increased bioactivation of VCH. As repetitive doses of VCH are necessary for follicular depletion, autoinduction of VCH metabolism appears to be essential in its ovarian toxicity. Autoinduction would result in higher levels of VCD being delivered to the ovary. Diepoxide metabolites are also critical for butadiene and isoprene-induced follicular loss. Butadiene monoepoxide, butadiene diepoxide, and isoprene were ovotoxic. In contrast, the monoepoxide, epoxybutane, was not ovotoxic. The olefin epoxides were chemically reactive, as demonstrated by their ability to alkylate nicotinamide. The diepoxides (VCD and butadiene diepoxide) exhibited the greatest chemical reactivity, which is consistent with their greater potency as ovarian toxicants. In summary, this research demonstrated that only those olefins which are converted to diepoxides are ovotoxic. The mechanism(s) by which these diepoxides target the ovary remains to be established.