Samuel M. Cohen, M.D., Ph.D. Food Safety Case Study: Arsenic. ILSI North America Southampon, Bermuda January 21, 2014

Transcription

1 Samuel M. Cohen, M.D., Ph.D. Department of Pathology & Microbiology University of Nebraska Medical Center Omaha, NE Food Safety Case Study: Arsenic ILSI North America Southampon, Bermuda January 21, 2014

2 Food Safety Case Study: Arsenic Welcome: Drs. Lee and Cohen Sources and Issues: Sam Cohen, University of Nebraska Medical Center National Research Council Update: Jim MacDonald, Chrysalis Pharma Consulting Risk/Benefit Analysis: Joyce Tsuji, Exponent Nutritional Influences: Megan Hall, Columbia University Regulatory Perspective: Suzie Fitzpatrick, FDA Panel Session: Speakers plus Lynn Frewer, University of Wageningen, and Mike Grusak, Baylor Children s Nutrition Research Center.

3 Arsenic: Occurrence Arsenic is a ubiquitous element, widely distributed on earth The average concentration in earth crust is 2-5 ppm Concentrations as high as 8000 ppm have been reported in soils, due to the nature of the rock present in the soil (in the US range of arsenic in soil is < μg/g, with an average of 7.2 ppm). High levels of arsenic are found in ground water of many countries in Asia and Latin America, due to the nature of soil and rock in these areas Highest well concentration in: Taiwan: 690 μg/l US: 2 μg/l average Low levels of arsenic are found in most foods resulting from occurrence in water.

4 Inorganic Arsenic Toxicity Cancer Bladder Lung Skin Non-Cancer Skin arseniasis Ischemic heart disease Diabetes mellitus Others

5 Sources of Arsenic Drinking Water: Current USA limit 10ppb inorganic arsenic Total (ng/g) Inorganic (ng/g) Food: Juices: Rice: Seafood Fresh Water: Salt Water: Beef: 50 < 1 Chicken: 90 < 1 Tomato: 10 < 1 Schoof et al., Food Chem. Toxicol., 37:839, 1999.

6 Forms of Arsenic Inorganic: Arsenate Arsenite Inorganic: Methylated (mono, di, tri) Arsenosugars Arsenolipids Arsenobetaine Arsenocholine

7 Classic Pathway for Arsenic Metabolism Putative Pathway for Arsenic Metabolism OH O = As OH 2e - + O 2e - OH As OH As OH = OH OH OH As V As III MMA V As OH + O = As OH 2e - As OH + OH MMA III DMA V DMA III O = As 2e - As Le, et al., Anal. Chem. 72: , 2000 TMAO V TMA III

8 Web of arsenic metabolism Aggregate exposure to arsenic should include all sources of arsenic Inorganic As in food and water Complex organic arsenicals in food Arsenosugars Arsenofatty acids Arsenobetaine How does chemical form affect metabolism and toxicity? Complex organic arsenicals and inorganic arsenic share common metabolites U.S. Environmental Protection Agency

9 Inorganic Arsenic and Cancer Epithelial: urinary bladder, skin, lung Possible Modes of Action: 1. DNA Reactive: direct binding to DNA 2. Indirect Genotoxicity: micronuclei, DNA repair, oxidative damage, mitotic spindle (tubulin) 3. Non-genotoxic: cytotoxicity and regeneration vs. direct mitogenicity Only DNA reactive MOA can be linear or non-threshold - a MOA that is not relevant to arsenic. Nesnow et al. (2002)

10 Mode of Action of Inorganic Arsenic Intracytoplasmic Inclusions of Urothelium Mouse Human These inclusions were mistaken for micronuelei:

11 Mode of Action of DMA V DMA III Metabolite Sustained Urothelial Toxicity Regenerative Proliferation BrdU Labeling Urinary bladder from a female F344 rat treated with 100 ppm DMA V EPA (Dellarco) Hyperplasia Urinary Bladder Tumors

12 Urothelial Cytotoxicity and Proliferation Induced By Inorganic Arsenic

13 In Vivo Cytotoxicity of Inorganic Arsenic Threshold is 1 ppm elemental Rats and mice Males and females Arsenite and arsenate Detection method: Histopathology Labeling index Scanning electron microscopy Genomics

14 In Vitro Cytotoxicity of Inorganic Arsenic Threshold approximately 0.2 µm Urothelial cells (rats and humans) Bronchial epithelial cells (humans) Keratinocytes (humans and mice) Less than 0.1 µm Adaptive µm Cytotoxicity Greater than 10 µm Lethal

15 Implications for Risk Assessment Trivalent arsenicals + Threshold Non-cancer Biological Effects Skin arseniasis Bronchial toxicity Urothelial toxicity Cancer Sulfhydryl groups Other non-cancer toxicities

16 Mode of Action for Inorganic Arsenic: Working Hypothesis Cancer Non-Cancer Ingestion of Arsenic Conversion to Trivalent Arsenicals (ias III, MMA III, DMA III ) Ingestion of Arsenic Conversion to Trivalent Arsenicals (ias III, MMA III, DMA III ) Interaction with Critical Cellular Sulfhydryl Groups Threshold Cytotoxicity Cell death Threshold Interaction with Critical Cellular Sulfhydryl Groups Cytotoxicity Non-cancer effects Regenerative Proliferation Carcinoma

17 Key Question Regarding Toxicity: Threshold or Non-Threshold Based on the mode of action (MOA) of a chemical, toxicity may have a threshold: Effect Effect Exposure No threshold, linear (EPA s default) Exposure Threshold (Depending on MOA)

18 Conclusions The MOA for InAs carcinogenicity is non-linear with a threshold. (The only possible linear, non-threshold MOA is DNA reactivity, which has been excluded) Cancer mode of action is cytotoxicity and regeneration Cancer and non-cancer mode of action involve conversion to trivalent metabolites that interact with sulfhydryl groups of critical proteins in target cells