Borghoff S, Wikoff D, Urban JD, Rager JE. A systematic approach to identify plausible mode-of-actions (MOA) for an increased incidence of lung tumors in mice with chronic exposure to 4-methylimidazole (4-MEI). Society of Toxicology Annual Meeting. March 11–15, 2018. San Antonio, TX.
4-MEI may be present in various cooked/heated foods and beverages as a by-product of the thermal browning reaction. In a 2-year cancer bioassay, dietary exposure to 4-MEI increased the incidence of lung adenomas/carcinomas in male and female mice above a high spontaneous control incidence. The objective of this evaluation was to examine plausible MoA for these mouse lung tumors and identify a MoA for further examination. Using a systematic approach, peer-reviewed literature was identified in Pubmed and Embase; validated high throughput screening data from ToxCast/Tox21 database was also considered. Evidence was reviewed for quality, relevance, and activity; data was synthesized using the key characteristics of carcinogens (KCCs) approach. Lines of evidence integrated within and between each KCC were used to identify potential molecular initiating events and key events associated with plausible MoA for the 4-MEI-induced mouse lung tumors. Composite results demonstrated that 4-MEI is not genotoxic, supported by negative findings in endpoints from high quality/relevant in vitro and in vivo studies (e.g., gene mutation chromosome aberrations, micronucleated erythrocytes, lung specific DNA damage). Also, several 4-MEI studies reported no change in gene expression, cell proliferation, or histopathology in mouse lungs following 5, 28, or 90 days of exposure. Thus, 4-MEI is unlikely to operate via a genotoxic, cytotoxic, or mitogenic MoA. The absence of activity within defined KCCs led to further evaluation of contextual data. 4-MEI is structurally similar to histamine and is predicted to mimic the binding of histamine to the active site of carbonic anhydrase (CA), increasing its activity. Since CA is induced in hypoxic tumor tissue, and since mice have a high rate of spontaneous lung tumors, a plausible MoA was identified whereby activation of CA could augment lung tissue microenvironment, conditions hypothesized to promote the progression of lung tumors late in the lifespan of this species. This systematic evaluation of the 4-MEI database demonstrates that well known cancer MoA cannot explain lung tumors reported in mice chronically exposed to 4-MEI. However, a broader consideration of the scientific literature suggests a plausible alternative MoA for 4-MEIinduced mouse lung tumors that can be tested to elucidate its relevance to humans.