Demonstration of a Temporal Relationship Between Ethyl Acrylate-Induced Forestomach Cell Proliferation and Carcinogenicity

Carcinogenic mode of action of folpet in mice and evaluation of its relevance to humans

“…The mode of action for folpet-induced forestomach tumors is the same as for the mouse duodenal tumors; reactivity of folpet and thiophosgene with tissue thiol substituents, induction of cytotoxicity (accompanied by inflammation) with consequent increased squamous cell proliferation, hyperplasia (accompanied by hyperkeratosis), and ultimately tumors� The non-neoplastic changes were observed in mice and rats, but the extent of proliferation was adequate for tumor induction only in mice� The temporal relationship of this sequence of events is similar to that seen for a variety of agents, such as ethyl acrylate (Ghanayem et al�, 1994) and butylated hydroxy anisole (BHA) (Clayson et al�, 1990; Ito and Hirose, 1989)� The dose-response shows a somewhat lower dose for the toxicity, inflammatory, and hyperplastic changes compared to the doses required for tumors, also similar to the situation with other agents inducing forestomach tumors (Ghaneyem et al�, 1994;Clayson et al�, 1990;Ito and Hirose, 1989)� It is not surprising that the forestomach is affected by folpet� Squamous epithelial cells contain large amounts of cytokeratins, which notably contain a large percentage of cysteine moieties with available thiol groups� In the forestomach, the pH is generally mildly acidic, but not as acidic as in the lumen of the glandular stomach� To the extent that folpet reacts at higher pH levels (due to an increased production of thiophosgene through hydrolysis), cytotoxicity in the forestomach would be expected to be considerably greater than in the glandular stomach but somewhat less than in the proximal duodenum� The glandular stomach, in contrast to the forestomach, is markedly acidic (pH = 1�0-2�0), which would tend to stabilize folpet from hydrolysis� The half-life of folpet solubilized in blood (average pH is 7�4) is 4�9 s (Gordon et al�, 2001)� By comparison, solubilized thiophosgene has a half-life of 0�6 s (Arndt and Dohn, 2004)� The half-life of folpet in blood is somewhat longer than captan, which has a half life of 0�9 s, but the reverse is true for hydrolytic degradation in mild acid media, such as in the forestomach� Folpet is approximately 7-fold less stable, hydrolytically, at pH 5 than captan� Thus, in the stomach, folpet would be expected to generate thiophosgene faster than captan; this may explain the increased susceptibility of mice to folpet for forestomach tumors compared to captan� In mice, the proliferation in the forestomach induced by folpet occasionally leads to the development of forestomach tumors (see Tables 1 and 2)� Progression to tumors is not seen in the rat, although pathological inflammatory and proliferative findings are similar� The types of lesions of the forestomach and their incidences in one of the rat studies are listed in Table 7� The difference in extent of the tumorigenicity between these species is likely due in part to the exposure levels and possibly due to the degree of generation of thiophosgene occurring between the two species� Because of generalized toxicity, the doses used for the studies in rats are lower than in the mouse� These dose levels in rats, however, attained the MTD, inducing frank toxicity, and were higher than doses producing stomach tumors in mice but folpet did not induce tumors in rats� Differences between the species have been investigated� In mice with duodenal tumors, there was occasionally obst...…”

“…The sequence of events seen with folpet in the forestomach are those commonly seen with non-DNA-reactive, irritative chemicals that produce tumors in the forestomach of rodents (Adams et al�, 2008;Clayson et al�, 1990;Ghanayem et al�, 1994;Grice, 1988;NTP, 2000;Wester and Kroes, 1988;Proctor et al�, 2007)� An irritation process occurs with cytotoxicity, and focal erosion, and occasionally even ulceration occurs� In studies with other chemicals than folpet involving gavage administration rather than dietary or drinking water administration, there is the added toxicity of the instrumentation itself� The toxicity from the chemical and/or the gavage instrument leads to an inflammatory reaction with associated regenerative hyperplasia� As in all of these instances, if the inciting stimulus is removed, the proliferative and inflammatory reaction subsides and the forestomach returns to normal� Eventually, however, once tumors are formed the process becomes irreversible� Considerations of alternative modes of action for folpetinduced forestomach tumors are the same as for the duodenum� Thus, a DNA-reactive mechanism is unlikely for the reasons described above, and there is no evidence of a direct mitogenic effect of folpet or thiophosgene on cells�…”

Chemical Carcinogenesis of the Gastrointestinal Tract in Rodents: An Overview with Emphasis on NTP Carcinogenesis Bioassays

“…Carcinogenic doses of EA administered by gavage for three, six, or twelve months caused a sustained increase in forestomach squamous epithelial hyperplasia for as long as exposure to EA continued; however, in a stop-study, hyperplasia regressed, and no neoplasms developed when animals received EA for three or six months and were allowed to recover. In contrast, rats that were treated for twelve months and allowed to recover developed squamous cell carcinomas and/or papillomas in the forestomach (Ghanayem et al 1993, 1994). This work indicates that cell proliferation, sustained for a sufficient time (twelve months), results in the development of neoplasia despite cessation of chemical administration.…”

Safety of ethyl acrylate to be used as flavouring

“…Ghanayem et al. (, ) studied the time required for sustained forestomach hyperplasia to produce neoplastic lesions in F344 rats administered ethyl acrylate at 200 mg/kg bw per day, 5 days per week, by gavage for 6 or 12 months. The increase in epithelial hyperplasia of the forestomach sustained as long as the exposure continued.…”