Mode of Action of Rodent Forestomach Tumours:

Relevance to Humans.

R Cope

Gross anatomy of murine forestomach after NMBA (N-nitrosomethylbenzylamine) treatment.

Zanesi N et al. PNAS 2001;98:10250-10255

©2001 by The National Academy of Sciences

NMBA-induced histopathology of murine forestomach.

Zanesi N et al. PNAS 2001;98:10250-10255

©2001 by The National Academy of Sciences

Introduction

Introduction Forestomach tumors/pre-neoplastic lesions in rats and mice are a

common finding in chronic toxicology studies;

Debate over the human relevance due to: Dose and exposure differences between rodents and humans; Substantial toxicokinetic differences (exposure); Substantial anatomical differences; Substantial physiological/metabolic differences of the

forestomach epithelium; Different mechanisms and tumor types in humans compared with

rodents;

Dose and Exposure Problems

Dose and Exposure Differences Doses used in rodent oral carcinogenesis often far exceed normal human

environmental exposure conditions (possible rare exception is some direct food additives);

Doses that produce forestomach irritation in rodents really should be considered as exceeding the MTD – i.e. poor practice in rodent carcinogenesis studies and not according to GLP/test guidelines;

Gavage can produce forestomach irritation and is not physiological: Large volumes; Damage to the mucosa; Esophageal reflux; Possibly replicates tablets (but not capsules);

Tissue specificity

Tissue Specificity Forestomach carcinogens divisible into at least 3 categories:

Produce forestomach tumors and tumors at other sites when administered by gavage;

Produce only forestomach tumors when administered by gavage; Produce forestomach tumors and tumors when administered by

non-oral routes;

In terms of human relevance, forestomach + tumors at other sites is likely to be more important except in the case of site of first contact carcinogens.

Tissue concordance/anatomical issues

Tissue concordance/anatomical issues Humans do not have a forestomach or a pars esophagea:

Roughly equivalent tissue in terms of histology is the esophagus; Humans do not store food in the esophagus where as rodents

store food in the forestomach; Transit time through the human stomach is lower than transit time

through the rodent stomach (forestomach) difference in tissue exposure;

Chemicals pass quickly through the human esophagus and thus the exposure is very limited compared with chemical exposure of the rodent forestomach.

Tissue concordance/anatomical issues

Tissue concordance/anatomical issues Physiological issues:

Rodent forestomach does not have a protective mucous coating increased tissue exposure to chemicals and more prone to irritant effects;

pH in rodent forestomach is higher than the pH of the human stomach relevant to detoxification (e.g. hexavalent chromium to trivalent chromium in low pH of human stomach);

Potential metabolic differences of rodent forestomach epithelium conversion of 2-butoxy ethanol to 2-butoxyacetic acid in rodent forestomach but not in human stomach;

Tumour types and biology issues

Tumor types and biology issues Rodents

Predominant tumor types are papillomas (non-malignant) and squamous cell (low malignancy – regional metastasis) carcinomas;

Typically located at the limiting ridge;

Possibly have some relevance to human esophageal squamous cell carcinoma BUT chemical exposure of the human esophagus is much lower than in the rodent forestomach due to much lower transit time (no storage in esophagus);

Not relevant to human esophageal adenocarcinoma.

Tumour types and biology issues

Tumor types and biology issues Humans

All human stomach cancers are gastric adenocarcinomas and arise from the glandular epithelium;

Rodent forestomach tumors have a different histiogenesis and are not relevant to the human gastric tumors;

Genotoxicity issues

Genotoxicity issues Forestomach carcinogens are divisible into 3 basic groups:

DNA reactive chemicals (classical in vivo genotoxic carcinogens)Site of first contact carcinogens (generally direct acting

carcinogens and are usually highly reactive chemicals; typically direct acting alkylating agents);

Classical pro-carcinogen DNA reactive chemicals;

Non-DNA reactive chemicals (classical non-genotoxic carcinogens);Typically irritant chemicals or chemicals that produce local

increased cell turnover.

Genotoxicity issuse

Genotoxicity issues Site of first contact carcinogens:

Generally require no metabolism to be carcinogenic; Generally will produce tumors at other sites if the route of

administration is different tumor location is the site of contact; Generally only produce forestomach tumors in gavage/dietary

studies because of limited/no systemic bioavailability; Typically alkylating agents; Typically genotoxicants in vitro and in vivo;

Forestomach tumours are potentially human relevant but only at the site of first contact in humans (e.g. dermal exposures)

Genotoxicity issuse

Genotoxicity issues Classical pro-carcinogen DNA reactive chemicals;

Generally pro-carcinogens; Often produce tumours at more than one anatomical site following

oral dosing (at least one systemic site + forestomach); Often other routes of administration also result in tumors; Generally systemically bioavailable;

Human relevance of forestomach tumors depends on: (a) was there evidence of gastric irritation; (b) were the doses excessive (> MTD); (c) were the effects only seen with gavage dosing/diet studies and not with drinking water studies?

• Observation of tumours under different circumstances lends support to the significance of the findings for animal carcinogenicity. Significance is generally increased by the observation of more of the following factors:

•Uncommon tumour types; •Tumours at multiple sites; •Tumours by more than one route of administration; •Tumours in multiple species, strains, or both sexes; •Progression of lesions from preneoplastic to benign to malignant;

•Reduced latency of neoplastic lesions; •Metastases (malignancy, severity of histopath); •Unusual magnitude of tumour response; •Proportion of malignant tumours; •Dose-related increases;•Tumor promulgation following the cessation of exposure.

Benzo(a)pyrene (IARC 1)Parameter

Genotoxicity in vivo that is relevant to humans +

Forestomach cancers following oral dosing +

Not observed in drinking water studies, only observed with gavage/diet studies -

Only observed at doses that irritate the forestomach (> MTD) -

Uncommon tumour types; +

Tumours at multiple sites; +

Tumours by more than one route of administration; +

Tumours in multiple species, strains, or both sexes; +

Progression of lesions from preneoplastic to benign to malignant; +

Reduced latency of neoplastic lesions; +

Metastases (malignancy, severity of histopath); +

Unusual magnitude of tumour response; +

Proportion of malignant tumours; +

Dose-related increases; +

Tumour promulgation following the cessation of exposure. +

Ethyl Acrylate

•Oral gavage: dose related increases in the incidence of squamous-cell papillomas and carcinomas of the forestomach were observed in rats and mice. Exposure caused gastric irritancy;

•Ethyl acrylate was tested by inhalation in the same strains of mice and rats; no treatment-related neoplastic lesions were observed;

•No treatment-related tumour was observed following skin application of ethyl acrylate for lifespan to male mice.

Ethyl Acrylate

Ethyl acrylate (IARC 2B)Parameter

Genotoxicity in vivo that is relevant to humans -

Forestomach cancers following oral dosing +

Not observed in drinking water studies, only observed with gavage/diet studies ?

Only observed at doses that irritate the forestomach (> MTD) +

Uncommon tumour types; -

Tumours at multiple sites; -

Tumours by more than one route of administration; -

Tumours in multiple species, strains, or both sexes; +

Progression of lesions from preneoplastic to benign to malignant; +

Reduced latency of neoplastic lesions; +

Metastases (malignancy, severity of histopath); -

Unusual magnitude of tumour response; -

Proportion of malignant tumours; -

Dose-related increases; -

Tumour promulgation following the cessation of exposure. +

Mercuric chloride (IARC 3)Parameter

Genotoxicity in vivo that is relevant to humans -

Forestomach cancers following oral dosing +

Not observed in drinking water studies, only observed with gavage/diet studies

?

Only observed at doses that irritate the forestomach (> MTD) +

Uncommon tumour types; -

Tumours at multiple sites; -

Tumours by more than one route of administration; (thyroid follicular cell adenomas)

Tumours in multiple species, strains, or both sexes; -

Progression of lesions from preneoplastic to benign to malignant; -

Reduced latency of neoplastic lesions; -

Metastases (malignancy, severity of histopath); -

Unusual magnitude of tumour response; -

Proportion of malignant tumours; -

Dose-related increases; -

Tumour promulgation following the cessation of exposure. -

Thanks!