Sustainability of forestomach hyperplasia in rats treated with ethyl acrylate for 13 weeks and regression after cessation of dosing.

In a chronic study conducted by the National Toxicology Program (NTP), gavage administration of 100 or 200 mg ethyl acrylate (EA)/kg/day, 5 days/week, to F344 rats and B6C3F1 mice resulted in a significant dose-dependent increase in the incidence of squamous cell papillomas and carcinomas of the forestomach of both sexes of rats and mice. No increase in the incidence of tumors was observed at any other site in these rats. Chemically-induced cell proliferation is currently thought to play a role in the development and progression of chemically-induced neoplasia. Therefore, a stop-study was initiated where 100 or 200 mg EA/kg (in corn oil) was administered daily, 5 days/week, for 13 weeks. Rats sacrificed at the end of the treatment regimen had severe epithelial hyperplasia of the forestomach. No lesions were observed in the glandular stomach or liver of EA-treated rats. Forestomach hyperplasia induced by EA included upward and downward cell proliferation. However, forestomachs of rats treated for 13 weeks and sacrificed 8 weeks after the last EA dose exhibited a significant decline in the incidence and severity of forestomach mucosal hyperplasia. Histopathologic evaluation of forestomachs of EA-treated rats (13 weeks) which were allowed a 19-month-recovery (with no exposure to EA) showed further decline in the incidence and severity of mucosal cell hyperplasia. These results indicate that gavage administration of EA to rats results in extensive and sustained forestomach mucosal hyperplasia. The sustainability of forestomach hyperplasia is apparently dependent on the continued exposure of rats to ethyl acrylate, and regressed after cessation of dosing. Furthermore, although enough post-treatment time was allowed for tumors to develop after cessation of EA administration, forestomachs exhibited a nearly complete recovery with no increased incidence of papillomas or carcinomas. It, therefore, remains to be determined what duration of exposure or other factors are critical for reversibility or progression of EA-induced forestomach mucosal hyperplasia to neoplasia.