Isoflucypram: Combining in vivo and NAMs data in a weight of evidence approach to demonstrate the human non-relevance of the mode of action leading to the subtle thyroid effects observed in the rat

Abstract

Isoflucypram (ISY) is a new cereal fungicide with an overall favorable toxicity profile. As the thyroid was identified as a target organ only in the rat, following repeat dosing; short term in vivo (rat) and in vitro mechanistic studies were conducted to substantiate the thyroid changes as being secondary to liver enzyme induction via PXR/CAR activation and to determine the human non-relevance of the thyroid effects. The in vivo studies established ISY as a weak prototypical hepatic PXR/CAR enzyme inducer (P450 and T4-UDP-glucuronosyltransferase (UDPGT-T4) activities), with the induction being associated with increased liver weight/hepatocellular hypertrophy/proliferation. Thyroid effects (minimal follicular cell hypertrophy/proliferation, slight, statistically significantly increased thyroid stimulating hormone) occurred at doses where liver stimulation was already established. Direct thyroid effects (in vitro thyroid peroxidase and sodium iodide symporter inhibition) were excluded. Marked quantitative species differences were identified when comparing rat and human hepatic enzyme activities in vitro, particularly for UDPGT-T4. Specifically, basal UDPGT-T4 was 4-fold lower in human compared to rat hepatocytes. In addition, UDPGT-T4 was induced in vitro in rat but not in human hepatocytes following ISY treatment. Overall, the weight of evidence supports a liver mediated mode of action for the isoflucypram-induced slight thyroid changes in the rat as well as the human non-relevance of these findings.