Abstract
In the current issue of Applied In Vitro Toxicology (AIVT), Landsiedel and colleagues publish the third of their articles concerning the development of an in vitro platform for assessing the toxicity of deiodinase (DIO) inhibitors. For some time now it has been a point of emphasis within European regulatory bodies to push for the validation of multiple methods of in vitro testing. The focus of this effort has been on basing these platform tests on mechanism of action, and this regulatory effort has driven forward the development of novel alternative methods (NAMs). In this three-article series, we can see an example of how to build such a testing system using DIO inhibitors as a target.
Inhibition of DIOs is a reasonable pharmacological target in regulating thyroid hormone signaling. In 2022, the group used liver microsomes as a DIO-containing model system and a known assay, the Sandell-Kolthoff reaction, to establish the reproducibility, reliability, and sensitivity of the assay. This article provided essential data to validate the capacity of the assay system and the enzyme source. In their second article, the group examined 22 compounds in a blinded fashion. A number of these compounds were known DIO inhibitors, while the assay also used a number of compounds with no known activity against DIO. Following experimental completion, the samples were deblinded and the inhibition levels were compared statistically to known literature values. Seven compounds were effective DIO inhibitors by the assay with over 90% reduction in deiodination, whereas 11 compounds were less than 20% effective and were classified as noninhibitors. Meanwhile, two compounds were found to be incompatible with the assay. These results were compatible with known activities for these compounds, and thus, one can consider the assay to be predictive. The authors went on to generate a data interpretation protocol that classified the effectiveness of the approach.
In the final portion of this study, which is published in this edition of AIVT, the group demonstrates an elegant method for translating in vitro-tested doses to relevant in vivo exposures. This is an essential element of any novel in vitro methodology that is being developed with the goal of reducing/replacing the use of animals in toxicological testing. Anyone wishing to develop NAMs is advised to consider this series of articles and reproduce the thoroughness and consideration of the limitations and delimitations of experimental design that have been shown. It is the firm belief of the journal that the future of in vitro toxicology lies in this sort of approach, and we look forward to receiving such submissions in the future.