In vivo and in vitro studies of 4-vinylcyclohexene diepoxide in wild-caught female brushtail possums (Trichosurus vulpecula) and Norway rats (Rattus norvegicus) and its potential as a fertility control agent

Abstract

In New Zealand (NZ), the brushtail possum (Trichosurus vulpecula) poses a major threat to native flora and fauna and it is the main wildlife vector for bovine tuberculosis. Primary control utilizes lethal means but these methods have limited efficacy long-term and are associated with environmental impact concerns. Fertility control has become more popular as it is potentially more effective, sustainable and humane. The industrial chemical, 4-vinylcyclohexene diepoxide (VCD) may serve as an ideal chemosterilant candidate for investigation. Oral gavage studies have shown that VCD reduces the pool of non-regenerating immature ovarian follicles in rodents, resulting in premature ovarian failure and sterility. The first objective of these studies was to examine the effects of orally administered VCD on female possum ovarian follicle populations. Orally delivered VCD had no effect on the primordial follicles of adult female possums and two formulations aimed at improving VCD uptake and efficacy did not change this outcome. The second objective examined the uptake and metabolism of orally administered VCD in female possums and rats in vivo. VCD concentration in the blood of rats was significantly greater than in possums while concentrations of VCD in the stomach were comparable between species. VCD dosing did not alter pH of stomach contents of possums while that of rats was increased and sustained for 6 hours. VCD-induced reductions in ovarian and liver glutathione levels were observed in the rat with no effects in the possum. It was determined that the highly acidic environment of the stomach of possums poses an initial barrier for orally delivered VCD. Without sufficient quantities of VCD reaching the liver and ovaries of possums, it was not possible to compare species differences in metabolism in vivo. The third objective examined the fate of VCD when exposed to acidic environments and stomach contents and the effects of VCD on liver metabolism in vitro in possums and rats. VCD hydrolysis in stomach contents was slower in possums compared with rats suggesting that possum stomach contents are able to retain VCD longer, thus potentially modulating VCD toxicity. GSH levels in possum liver tissue were less affected when incubated with VCD compared with rats, suggesting an increased detoxifying capacity of possums. Preliminary data on the ability of possum liver microsomes to convert VCD’s parent compound, 4-vinycyclohexene, to VCD corroborated the GSH findings. Collectively, these findings suggest that VCD may not be suitable for possum fertility control when delivered orally in a raw unprotected chemical state. Encapsulation of VCD with an additional active compound, triptolide, is being examined as a novel chemosterilant (ContraPest®, SenesTech®, Flagstaff, Arizona). The fourth objective examined ContraPest® efficacy in wild-caught female Norway rats. The ovarian primordial follicle pools of ContraPest®-consuming rats were reduced compared with controls. The proposed protection of the active components within ContraPest® from stomach acid by encapsulation may provide the first step for its potential use in possums. Further work demonstrating the efficacy of ContraPest® in rodents will be required before further inestigation on possums can be proposed.