Dose-Response ENVR430 Oct 13, 2008 Casarett and Doull, Chapter 2, pp. 18-27(6th Edn) Chapter 2, pp. 19-26 (7th Edn) Timbrell Chapter 2, pp. 7-25 (3rd Edn)

Dose-Response Increasing Response Increasing Dose

The J-shaped curve (hormesis) Increasing Response Increasing Dose

Dose-Response Increasing Response Dose Threshold

Threshold Dose Some Acronyms The basis for regulation No effect level Response at low dose is so low as to be insignificant – A SAFE DOSE NEL, No Effects Level NOEL, No Observed Effects Level NOAEL, No Observed Adverse Effects Level Some Acronyms The basis for regulation

Food Safety, Environmental ADI, Acceptable Daily Intake AWI, AMI etc TDI, Tolerable Daily Intake MCL, Maximum Contaminant Level (SDWA) SF, UF, MF: safety, uncertainty, modifying factors TLV, Threshold Limit Value TLV-C, ceiling level STEL, Short-term exposure limit TWA, Time-Weighted Average (8h day, 40 h week) Occupational exposure limits

Above the threshold Dose-response is linear ? y = ax + b Dose-response is not linear Defining the shape of the dose-response curve Theoretical treatment: Assume Quantal or dichotomous response

Quantal response Dichotomous response Corresponds to “occupied receptor” Examples: Mortality, Tumor incidence Assumes normal (Gaussian) distribution Maximum 100% affected

Basic assumptions (a) The response is causally related to the compound administered (b) The response is a function of the concentration at the site of action (c) The concentration at the site of action is related to the external dose (d) An interaction at the site of action initiates a proportional response (e) The crucial interaction involves reversible formation of a receptor-toxin complex

Receptors Usually proteins Located on outside of cell wall, or inside cell Interact with ligands

Receptors important in Pharmacology Agonists and antagonists of neurotransmitters: cholinergic receptors: acetylcholine; nicotinic receptors: skeletal muscle, autonomic ganglia; muscarinic receptors: smooth muscle, heart, exocrine glands Adrenergic receptors: dopamine, endorphins, enkephalins, histamine Hormone receptors: Insulin, cortisone (glucocorticoids), estrogen, progesterone, testosterone, prostaglandins Drug receptors: Benzodiazepines

Receptors important in Toxicology Ah (TCDD) receptor CAR, PPARα Steroid receptors

Reversible formation of a receptor-toxin complex k1 R + T R-T k-1 R = receptor T = toxic compound R-T = receptor-compound complex k1 and k-1 are rate constants for formation and dissociation (respectively) of the complex R-T

k1 * [R] * [T] = k-1 * [R-T] KT is the dissociation constant of the complex, = k-1/k1 = [R] * [T] / [R-T] Total concentration of receptors [Rt] = [R] + [R-T], or [R] = [Rt] - [R-T] KT = ([Rt] - [R-T])*[T] / [R-T] = ([Rt]*[T] - [R-T]*[T])/[R-T] Proportion of total receptors that are occupied = [R-T]/[Rt] [R-T] = ([Rt] - [R-T])* [T] / KT = [Rt]*[T]/KT - [R-T] * [T]/KT [R-T] + [R-T]*[T]/KT = [Rt] * [T] /KT = [R-T] * (1 + [T]/KT) [R-T] = ([Rt] * [T]/KT) / (1 + [T]/KT) [R-T]/[Rt] = [T] / KT * (1 + [T]/KT) = [T] / (KT + KT*[T]/KT) [R-T]/[Rt] = [T] / (KT + [T]

Response or effect e is proportional to [R-T] Maximum response Emax occurs when all the receptors are occupied e / Emax = [R-T] / [Rt] [R-T]/[Rt] = [T] / (KT + [T]) then e / Emax = [T] / (KT + [T]) and e = Emax * [T] / (KT + [T])

When T = 0, e = 0 When all receptors are occupied: [R-T] = [Rt] e = Emax

Other Considerations Duration of exposure Interaction may not be reversible Repair or removal of complex R-T may be important Response may be multi-step, binding of T to R may not be the rate-limiting step Uniform population - no significant inter-individual differences in response

LD50 Lethal dose LC50 Lethal concentration ED50 Effective dose

Some Acute Oral LD50 Values Category Dose Species Chemical (mg/kg body weight) Practically nontoxic 15 000 Slightly toxic 10 000 Mouse Ethanol 5 000 Moderately toxic 4 900 Rat Glyphosate 750 Rat Atropine 500 Highly toxic 250 Rat Carbaryl 50 Extremely toxic 13 Rat Parathion 5 Supertoxic 3 Rat Warfarin 0.4 Duck Aflatoxin B1

Species differences in the acute toxicity of dioxin* *Dioxin: 2,3,7,8-tetrachlorobenzdioxin: TCDD

Oral LD50 Values (mg/kg) Test Compound: Caffeine

Other toxic effects Acute / chronic Reversible / irreversible Immediate / delayed Idiosyncratic - hypersensitivity Local / systemic Target organs

WHERE DO WE MEASURE THE DOSE ? External dose Internal dose Dose at target tissue Dose of active species at molecular target of action

Do we have to measure the internal dose ? Mathematical Models Kinetics