Activation of TRPA1 by volatile organic chemicals leading to sensory irritation

Abstract

Many volatile organic chemicals (VOCs) have not been tested for sensory pulmonary irritation. Development of in vitro non-animal sensory irritation assay suitable for a large number of chemicals is needed to replace the mouse assay. An adverse outcome pathway (AOP) is designed to provide a clear description of the biochemical and cellular processes leading to toxicological effects or an adverse outcome. The AOP for chemical sensory pulmonary irritation was developed according to the Organization for Economic Co-operation and Development guidance including the Bradford Hill criteria for a weight of evidence to determine the confidence of the AOP. The proposed AOP is based on an in-depth review of the relevant scientific literature to identify the initial molecular event for respiratory irritation. The activation of TRPA1 receptor (transient receptor potential cation channel, subfamily A, member 1) is the molecular initial event (MIE) leading to sensory irritation. A direct measure of TRPA1 activation in vitro should identify chemical sensory irritants and provide an estimate of potency. Fibroblasts expressing TRPA1 are used to determine TRPA1 activation and irritant potency. We report a linear relationship between the in vivo RD₅₀ and the in vitro pEC₅₀ values (R=0.81) to support this hypothesis. We propose that this in vitro assay after additional analysis and validation could serve as a suitable candidate to replace the mouse sensory irritation assay.

Keywords: sensory pulmonary irritation; TRPA1; neurogenic inflammation; TRPV1; non-animal test.

Figure 1.. The temporal sequence of biological events leading the AOP (adverse outcome pathway).

MIE= Molecular initiating event; KE= key events, KER= Key events relationship, AO=Adverse outcome

Figure 2.. Adverse Outcome Pathway for Sensory Irritation.

Some examples of volatile irritant organic chemicals (VOC) are shown. a) acrolein; b) dibenz[b,f,][1,4]oxazepine(CN); c) cinnamaldehyde; d) methyl isocyanate; e) allyl-isothiocyanate; f) chloropicrin; g) 4-oxo-nonenal.

Figure 3.. The relationship between the RD₅₀ values reported for mice exposure to irritants and the reported pEC₅₀ for the in vitro activation of TRPA1.

The vertical linear axis is the RD₅₀ values (ppm) as reported for mice. On the horizontal log axis is pEC₅₀ (M) in for the activation of TRPA1 obtained by measuring the changes in Ca²⁺in fibroblasts expressing hTRPA1 as reported in Liu et al., 2017. All pEC₅₀are from cells expressing human TRPA1 except for formaldehyde obtained with mouse TRPA1 and crotonaldehyde obtained with rat TRPA1. The chemicals and data are listed in Table 1.