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Environmental Health

Environmental Health. Interactions and Mixtures Week 9. Cumulative risks. Which mixtures are important for Public Health? What is the nature, magnitude of cumulative exposures? What is the mechanism of interactions?. Sexton 2007 and supplement. Biological Chemical Physical Psychosocial.

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Environmental Health

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  1. Environmental Health Interactions and Mixtures Week 9

  2. Cumulative risks • Which mixtures are important for Public Health? • What is the nature, magnitude of cumulative exposures? • What is the mechanism of interactions? Sexton 2007 and supplement

  3. Biological Chemical Physical Psychosocial Similar properties Defined (diesel) Coincidental (time, place) Types of mixtures Past and present, all routes, pathways and sources

  4. High priority mixtures • Scope: large number of people exposed • Nature of exposure: magnitude, frequency • Severity of effects: unacceptable risks • Potential for interactions See: Sexton 2007 supplement

  5. Total body burden over time based on exposure frequency and kinetics of a chemical See: Sexton 2007 supplement

  6. Total body burden from two chemicals relative to a health benchmark See: Sexton 2007 supplement

  7. Interactions among what? • Drug-drug • Environmental pollutants • Drug-environmental exposure • Diet-drug • Diet-environmental pollutant • Genes and all the above

  8. When, who, how? • Exposures • Concurrent • Sequential • Order • Physiology differences • Children • Elderly • Pre-existing conditions, overall health • Genetic disposition • Route of exposure • Amount

  9. Types of interactions Dose additivity - if mechanism is similar Response additivity - if acting independently

  10. Putting a number to them

  11. Antagonism of many kinds

  12. So, what interaction is this? Q. A dose of 4 mg of an insecticide causes 20% toxicity whereas the same dose of another insecticide produces 30% toxicity. If 8 mg of a formulation containing both insecticides in equal concentrations causes 50% toxicity, the interaction is known as: Additivity Antagonism Synergism

  13. …and, what about this? Q. Piperonyl butoxide added to pyrethrum insecticide results in a pyrethrum formulation having about 100 times the toxicity of pyrethrum alone. The interaction of this combination is: Additivity Antagonism Synergism

  14. Main types of interactions • Physical prior to absorption • Toxicokinetic interactions • Toxicodynamic interactions

  15. Toxicokinetic Interactions • One chemical affects the kinetic disposition of another: • Absorption • Distribution • Metabolism • Elimination

  16. Performing in vitro CYP450 induction screens, to evaluate potential multi-chemical interactions

  17. “A” increases toxicity of “B” by inhibiting a detox enzyme A B Toxic effect + Enzyme X (Phase I) B metabolite inactive Enzyme Y (Phase II) Excretion

  18. “A” isprotective by inhibiting a metabolic activation reaction A B inactive + Enzyme X (Phase I) B metabolite Toxic effect Enzyme Y (Phase II) Excretion

  19. “A” is protective by inducing a detox enzyme A B Toxic effect + Enzyme X (Phase I) B metabolite inactive Enzyme Y (Phase II) Excretion

  20. “A” increases toxicity of “B” by inducing enzyme of metabolic activation A B inactive + Enzyme X (Phase I) B metabolite Toxic effect Enzyme Y (Phase II) Excretion

  21. Toxicodynamic Interactions • One chemical’s biological activity is related to the biological activity of the other • Changing cell signaling (phosphorylation cascades) • Altering gene expression and genomic repair • Modulating cell communication • Altering cell cycle • Affecting the same biochemical pathway at a different step • Affecting a related biochemical pathway • Systemic level cross-talk interference • Autoimmune effects

  22. First order Michaelis-Menten kinetics of enzyme reaction: Competitive inhibition of a enzyme reaction, transporter activity Uncompetitive inhibition of a enzyme reaction, transporter activity See: Sexton 2007 supplement

  23. Change in slope as indicator of interactions See Gennings 2005

  24. Slopes must be compared at the same effect regions See Gennings 2005

  25. Interaction of piperonyl butoxide with malathion

  26. Non-linear models Binary endpoints and probability of response A - Changing concentrations of one chemical with fixed concentration of the other B - Changing concentrations of both chemicals

  27. Thyroid effects of PCBs Carpenter, 1998

  28. Carpenter, 1998 PTU: propylthiouracil, known to produce hypthyroidism LTP: long term potential, electrophysiological measure indicating cognitive function EPSP: excitatory postsynaptic potential, reflects LTP

  29. Parent compound and metabolite have opposite effects TrCB is also antiestrogenic by virtue of inducing the metabolism of E2 Carpenter, 1998

  30. Increased estrogenic effects of combined PCBs or organochlorines Carpenter, 1998

  31. Carpenter, 1998

  32. Carpenter, 1998

  33. Models of interactions See Groten 2001

  34. Combined effects of UV filter mixtures on ER activation (yeast) B - antagonism Curve shift in-between A - synergism Curve shift to the left Kunz 2006

  35. Most binary mixtures show synergism at EC25, EC50 and EC75 effect levels Kunz 2006

  36. Synergism of mixtures of 4 UV filters at BC10 and NOEC effect levels Kunz 2006

  37. Kunz 2006 Effect of mixture of 4 UV filters at BC10 and NOEC

  38. Effect of mixtures (4 or 8) is stronger at NOEC than at BC10 Kunz 2006

  39. Mixtures of 4 are as potent as the most potent one (BP1) Relative potencies are compared to E2 Kunz 2006

  40. Approaches to toxicity of simple mixtures

  41. Bottom-up approach Systematic studies of binary combinations of chemicals in the mixture based on mechanisms Top-down approach Start from the most complete mixture and continue with subfractions by separating components Simple mixtures toxicity testing

  42. Bottom-up • Kepone’s impairment of liver regeneration will affect the toxicity of liver toxins • CCl4 • 1,1,2,2-Tetrachloroethane • Hexachloro-1,3-butadiene

  43. Kepone + CCl4 • Kepone 10ppm (low environmental level) • CCl4 100ul/kg (injected) - only marginally toxic level • Combination increased lethality by 67-fold

  44. Modeling of CCl4 liver toxicity +/- Kepone CCl4 only CCl4 andKepone

  45. Top-Down

  46. Approaches to toxicity of complex mixtures

  47. Dietary factors • Red meat, processed meat, well-done meat • Vitamins and antioxidants • Trace elements, metals • Alcohol • Phytochemicals (isothiocyanates)

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