Mode of action studies with phthalate acid monoesters: pharmacokinetic and pharmacodynamic factors affecting steroidogenesis Public Deposited

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  • March 21, 2019
  • Clewell, Rebecca Ann
    • Affiliation: Gillings School of Global Public Health, Department of Environmental Sciences and Engineering
  • The use of phthalate esters in plastics and resulting human exposure has led to concern over potential adverse effects in fetal development. This project provided data and quantitative tools to improve phthalate risk assessments. In vivo, in vitro and in silico experiments evaluated pharmacokinetic and pharmacodynamic factors responsible for anti-androgenic effects of phthalate esters. For pharmacokinetics, plasma and tissue metabolite levels were measured in maternal and fetal rats following DBP administration. A physiologically based pharmacokinetic (PBPK) model was developed for DBP distribution in rat gestation, tested against a variety of data across life-stages, doses and exposure routes, and accurately predicted maternal and fetal plasma MBP levels for acute and repeated dosing. The validated model permitted direct correlation of testes phthalate concentrations and testosterone. When extended to DEHP, the model also predicted MEHP kinetics. For pharmacodynamic evaluation, monoester concentrations were measured in the fetal testes after repeated doses of BBP, DEP, DBP, DEHP, and DMP. An in vitro assay tested the effect of inhibition of steroidogenesis directly in the Leydig cell. The differential ability of the monoesters to cause developmental toxicity was found to result from differences in their pharmacodynamic potency. Finally, we attempted to identify the molecular target for the phthalates in the Leydig cell. The phospholipase A2 (PLA2) inhibitor CQ and MEHP had a similar ability to inhibit testosterone production, steroidogenic gene expression and AA release in the LH-stimulated (MA-10) Leydig cell. Both compounds interfered with translocation of fluorescently tagged cPLA2 in human HEK-cells after activation by a calcium ionophore, providing at least indirect evidence that inhibition of AA release by cPLA2 is likely to be involved in phthalate anti-androgenic effects. When CQ was administered to the pregnant rat, fetal testes testosterone levels were reduced in a dose-dependent manner. CQ also down-regulated steroidogenic genes as noted with active phthalate administration. Our results strongly indicate that cPLA2 is a key target of these phthalates in relation to decreased testosterone production. The improved understanding of phthalate dose-response and mechanism of action, together with in vitro derived potencies of phthalates for testosterone inhibition, should greatly improve cumulative risk assessments for the phthalates.
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  • In Copyright
  • Andersen, Melvin E.
Degree granting institution
  • University of North Carolina at Chapel Hill
  • Open access

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