Epichlorohydrin was used as stabilizer for 1,3-D, but was replaced with epoxidized soybean oil (ESO) since 1985. As epichlorohydrin is a known mutagen and carcinogen, it is a potential confounder for older in vitro and in vivo studies. How should the studies that were confounded by the presence of epichlorohydrin be handled?

From the limited metabolism studies, it appears that epoxidation of 1-3D, although a minor pathway does occur when metabolic activation is present – perhaps at several percent yield. Since some epoxide will be formed. It seems unreasonable to discard results with 1-3D containing around 1% epichlorohydrin. One could estimate the contribution of say, 1% epichlorohydrin to the genotoxicity of purified 1,3-D by comparing the genotoxic effects of the pure compounds (e.g. if epichlorohydrin is 100x more potent than 1,3-D could it account all of the observed genotoxic effects?). In practice this might be difficult (but maybe not impossible) as conditions for assaying the pure compounds would have to be very similar. Without this information there is no way to know if contaminated samples should be discarded. However, in cases where no metabolic activation is present (e.g., Ames test without S-9 fraction), results containing contaminated 1,3-D are not reliable.

(Please add the following to the Comments Section. I can’t figure out to do that)

Consider on a case-by-case basis focused on an evaluation of the likelihood that the amount of epichlorohydrin used in the study contributed to the tumors observed.

Yes, at least for the in vitro studies where epichlorohydrin is a clear genotoxic agent. However, for the in vivo genotoxicity studies the presence of epichlorohydrin as stabilizer for 1,3-D does could not represent a confounder factor since it has been shown in a number of cytogenetic studies a negative outcome in mice and rats at dose-levels up to 200 mg/kg bw (Rossi et al., 1983; Tsuchimoto and Matter, 1981; Kirkhat, 1981; Salamone et al., 1981; Terada et al., 1992; Asita et al., 1992; Dabney et al., 1979; Sram et al., 1981).

This is a difficult question. If there was a choice saying 'case by case' then I would have checked that box. I belive that the White paper and Zeiger make this sound a bit too simplistic - known mutagen and carcinogen - yes, but in my view the question to ask is its relevance towards the outcome of these studies. Did check for epichlorohydrin data and found, for example, a 2018 NTP micronucleus study that was clearly negative (tested up to 200 mg/kg. Even if it was positive at such high doses - how could a presence of a 1% impurity (i.e., 1% of the respective top dose tested in these assays) explain the positive results in the described studies?