Dose-Response Curves

Dose-Response Curves

• Differences exist between individual and population curves.
• Individual dose-response is characterized by a continuous scale of doses leading to an increase in the magnitude of a specific response.
• Population dose-response is "all or none"; individuals are classified as responders or non-responders at any given dose with normal (Gaussian) distribution.
• Threshold signifies an effect or point on the dose-response curve.
• LD50 Lethal dose of a solid in 50% of the population.
  • This is not a measure of hazard on the dose response curve.
• Key acronyms used in reading the dose response curve:
  • LOAEL - Lowest Observed Adverse Effect Level
  • NOAEL - No Observed Adverse Effect Level
  • RfC = NOAEL/(UF x MF)
• Hypersensitive or hypersusceptible individuals show up on the left side of the curve.
• Hyposensitive or resistant individuals show up on the right side of the curve.
• Threshold dose is the minimal effective dose (ED) of any chemical that evokes an "all or none" response.
  • It's the dose at which toxicity is first observed.
• Slopes of dose-response curves:
  • A "flat" curve indicates that a significant change in dose is required before a significant change in response is observed.
  • A "steep" curve indicates that a relatively small change in dose will cause a large change in response.
• Curves can have the same ED50 with different slopes.
• Dose-response curves for essential nutrients may be monotonic.
• Multiple inflection points can exist along the curve, altering its shape.
• Curves are variable and can resemble biphasic "valley" (U-shaped or J-shaped) or an entire hill (inverted U-shaped):
  • This is common for essential nutrients needed for homeostasis.
  • Low doses lead to deficiency, while high doses lead to toxicity.
• Dose-response curves for carcinogens:
  • The shape of the curve varies depending on the mechanism of toxicity.
  • Genotoxic carcinogens (directly modify and damage DNA) are assumed to increase cancer risk even at the lowest possible dose; linear non-threshold extrapolation is used.

Contributions from Paracelsus, Ramazzini, and Potts

• Paracelsus:
  • Articulated that "the dose makes the poison".
  • Anything can be a poison.
  • Known as the "Father of Toxicology".
  • Published on "the miner's sickness and other diseases of miners", acknowledging diseases specific to that occupation.
• Ramazzini:
  • Regarded as the "Father of Occupational Medicine".
  • Published "Discourse on the Diseases of Workers"
  • Outlined a wide range of occupations and diseases.
  • Inquired of patients: "What trade are you in?".
• Potts:
  • Recognized the role of soot in scrotal cancer among chimney sweeps.
  • This marks the first record of PAH carcinogenicity.
  • The first established occupational link to cancer.

Chemical Allergy

• Immunologically mediated adverse reaction to a chemical resulting from a previous exposure to that chemical or a structurally similar one.
• "Hypersensitivity" is often the term used to describe this response.
• Reactions can be dose-related for a given individual, however, dose response curves do not apply to all chemical allergies.
• Mechanism:
  • Most chemicals aren't large enough to be recognized by the immune system as a foreign substance.
  • Therefore, they combine with a protein to form an antigen (hapten).
  • The hapten-protein complex then elicits the formation of antibodies (1-2 weeks).
  • Upon subsequent exposure, the antigen is recognized by the antibody, leading to an allergic reaction (ranging from mild to fatal).
• Once sensitization has occurred, reactions can occur in response to very low doses.

Types of Epidemiology Studies

• Cohort:
  • Prospective studies track exposed vs. non-exposed groups.
  • They often determine the incidence rate of a condition.
• Case-Control:
  • Retrospective in nature.
  • Compares diseased vs. non-diseased individuals.
  • Help determine relative odds.
• Cross-Sectional:
  • Can assess either exposure or disease.
  • Either prospective or retrospective.
  • Determines prevalence.
  • Cannot establish causality.

Free Radicals and Antioxidants

• Free radicals:
  • Molecules or molecular fragments containing one or more unpaired electrons in their outer orbit.
  • They are formed by accepting or losing an electron or through the fission of a covalent bond.
  • Can be naturally occurring or from endogenous sources.
  • Superoxide anion radical (O2•-) is a starting compound in 2-toxication pathways.
• Metals are notorious for changing valence states in the body, a process that generates free radicals.
• Free radicals can initiate apoptosis in cells or cause necrosis. Critical to immune system function, but too many are detrimental.
• Antioxidants counteract or neutralize damaging effects by donating an electron without becoming a radical themselves.

Toxicology

• Toxicology is both an art and a science.
• Its science involves observational and data gathering.
• Its art lies in utilizing data to predict outcomes of exposure in human and animal populations.

Acute vs. Chronic Studies

• acute: <24hrs (single or repeated exposures w/i 24hrs).
• subacute: < 1 month
• subchronic: 1-3 months
• chronic: >3 months
• Acute:
  • Performed on rodents to determine LD50 within 14 days
• Subacute:
  • (Repeated-dose) - performed to obtain info on toxicity of a chemical cafter repeated administration
  • Aims to establish doses for subchronic studies
• Subchronic:
  • Ninety days/13 weeks is the common length.
  • Can establish NOAEL and COAEL.
• Chronic:
  • Six months to two years; maximum tolerable dose (MTB) is usually given.

Systemic vs. Local Effects

• Local:
  • Occurs at the site of first contact between a biological system and toxicant.
  • Very little chemical is absorbed into the bloodstream.
  • Examples include the ingestion of caustics and the inhalation of irritants.
• Systemic:
  • Requires absorption and distribution of a toxicant from its entry point to a distant site, where deleterious effects are produced.
  • Most substances, except highly reactive materials, produce systemic effects.

Routes of Exposure in Occupational Settings

• Inhalation.
• Skin exposure

Combined Effects of Chemicals

• Additive:
• This combined effect equals the sum of the effects of each agent.
• 2 + 3 = 5. This is the most common effect.
• Synergistic:
• This combined effect is much greater than the sum of the effects of each agent given alone.
• 2 + 2 = 20.
• An example is carbon tetrachloride and ethanol, which cause liver damage.
• Potentiation:
  • One substance doesn't have a toxic effect on a certain organ or system.
  • However, it will becomes toxic when added to another chemical that chemical is much more toxic
  • 0 + 2 = 10.
  • Ex. Isopropanol increases the hepatotoxicity of carbon tet.
• Antagonism:
  • Two chemicals when given together interfere with each other's actions, or one interferes with the action of another.
  • Ex is (8) + 4(-4) = 0.
  • Basis of many antidotes.

Factors that Govern Deposition of Particles/Gases in the Respiratory System

• Differs from GI absorption; dissociation of acids/bases and lipid solubility are less important.
• Gases/vapors:
• The mucosa of the nose is covered by an extremely important film (glycoprotein).
• Water-soluble, reactive gases react with cell surface components in glycoprotein.
• The nasal passages act as a scrubber, which protects the lungs
• Non-water-soluble, non-reactive gases reach lung areas at similar inhaled concentrations.
• The blood-gas partition coefficient (solubility ratio) describes the solubility of the inhaled chemical and the concentration in blood vs. gas phase before or at saturation at equilibrium.
• Low solubility ratio means the rate of transfer depends on blood flow through the lungs (perfusion limited).
• High solubility ratio means the rate of transfer is primarily a function of rate and depth of respiration.
• Aerosols/Particles:
  • Size and solubility of the chemical are important.
  • The site of deposition of aerosols depends largely on the size of the particles.
  • Particles ≥5µm (coarse particles) deposit in the nasopharyngeal region and may be dissolved in mucus and carried to the pharynx or absorbed in blood. They could also be trapped in nasal cilia and eventually swallowed.
  • Particles ~2.5µm (fine particles) deposit in the tracheobronchiolar region and are cleared through the mucociliary escalator. They are small enough to escape impaction mechanisms.
• Inhalation of Particles ≤1µm allows them to penetrate to alveolar sacs of the lungs.
• Note that generally, the removal of particles from alveoli is relatively insufficient, and some particles may remain indefinitely.

Biotransformation Purpose

• A highly permeable xenobiotic, if not biotransformed, would be reabsorbed in the kidney/intestine rather than eliminated in urine/feces.
• It usually decreases the lipid solubility of a compound and increases water solubility.
• It may also be called metabolism.
• It is the enzymatic process of chemical modification that changes the physiochemical properties of a xenobiotic from those that favor absorption and distribution to those that favor elimination.

Differences Between Phase I and Phase II Biotransformation

• Phase I:
• Facilitates the elimination of xenobiotics by increasing TPSA (Topological Polar Surface Area) and decreasing log P.
• The octanol/water partition coefficient also shifts, increasing the hydrophobicity and water solubility of a molecule.
• Changes are relatively modest, resulting in decreased membrane permeability.
• Favors elimination and decreases the likelihood of excreted metabolites being absorbed.
• Products may undergo Phase II after.
• Catalyzed by enzyme systems divided into categories based on the reaction they catalyze: hydrolysis, reduction, oxidation.
• This involves the: addition of -OH, -NH2, SH, -COOH
• Phase II:
• Facilitates elimination by adding an ionizable group (usually acid, negatively charged with pH of 7.4).
• This causes large increase of IPSA, and large decrease in log P.
• Changes are relatively large and can result in decreased membrane permeability.
• Typically mediated using some kind of transporter after biotransformation.
• Involves conjugation of a xenobiotic or Phase I metabolite with:
  • glucuronic acid, sulfuric acid
  • zwiltterion-glutathione amino acids: glycine, taurine, glutamine.
  • acetylation
  • methylation
• Products are then eliminated via bile or urine.
• Decreased partition coefficient leads to decreased lipophilicity and increased hydrophilicity.
• Xenobiotic transformation enzymes are widely distributed throughout the body. Think of them as being associated with the liver.
• CYP450 is a common enzyme.
• Biotransformation is generally positive, but can sometimes increase toxicity (toxication).

Principles that Govern Absorption and Excretion

• Lipophilic compounds are generally hydrophobic.
• Passive Transport:
  • Small hydrophilic molecules permeate membranes through aqueous pores (paracellular).
  • Hydrophobic molecules diffuse across the lipid domain (transcellular), particularly lipid-soluble compounds.
  • In general, as lipophilicity increases, so does the partition coefficient, and therefore also the speed of movement through the membrane.
  • Negative log P = more water-soluble than lipid-soluble.
  • The larger the log P, the larger the amount of that component in the lipid fraction.
• If a molecule is nonionized, it is most likely lipid soluble.
• In general, the nonionized form of weak organic acids and bases is, to some extent, lipid-soluble, and there resulting diffusion across the lipid domain of the membrane.
• With the exception of very small ionized molecules that can pass through membrane pores, only non-ionized forms of most electrolytes are able to cross membranes.
• Ionized forms are generally too large to pass through aqueous pores and are insufficiently lipophilic to be transferred by passive diffusion.
• pKa describes the relative strength of acids and bases.
• For acids, the pKa - pH = log ([nonionized]/[ionized]).
• For bases, the pKa-pH = log ([ionized]/[nonionized]).
• A larger nonionized fraction favors absorption.
• As the diameter of molecules increases, the rate of movement across the membrane decreases.
• Smaller molecules generally have more water soluble
• Brain capillaries have very low aqueous porosity because of the lack of fenestrae.
• In GI, particle absorption increases with decreasing particle diameter.
• The primary mechanism for excretion of xenobiotics is the kidneys.
• Secondary excretion occurs in feces (bile excretion), but they must first be biotransformed to water-soluble compounds.
• Tertiary excretion occurs in the lungs (exhaled gases).

Features of Mammalian Cell Membranes

• Consist of a phospholipid bilayer.
• The outside is more polar and hydrophilic.
• The interior is lipophilic, composed of fatty acid tails.
• Forms a semi-permeable barrier that controls the movement of substances in and out of the cell.
• Has selective permeability.
• Cholesterol gives the structure
• Integral proteins (receptors, transporters) form ion channels and are activated by ligands.
• Lipid-soluble substances can diffuse through the lipid layer.
• Water-soluble substances can't move through the lipid layer.

Initiating Factors for Apoptosis and Necrosis

• Damaged cells are eliminated by apoptosis or necrosis.
• Apoptosis-internal triggers ("suicide"):
• Inherent self-destruct sequence built into cells.
• Normally inhibited by trophic (growth) factors, e.g., NGF.
• External triggers ("murder"):
  • This destructive sequence is ordered by other cells (natural killer cells).
  • Employs death ligands, e.g., tumor necrosis factor.
  • Release of cytochrome.
  • Damage to DNA appears to be the central event in triggering apoptosis in injured cells.
• Necrosis is due to a lack of oxygen, Na/K pump and is initiated by damage.

Role of P53 in Apoptosis/Cancer

• It acts as a tumor suppressor and transcription factor.
• DNA damage cell cycle checkpoint which halts cell cycle until DNA is repaired
• When severe damage occurs, it induces series PUMA, NOXA which inhibit BCL-2.
• Cancer is always battling apoptosis.
• Cancer cells usually have suppressed P53 (mutated or underexpressed).

Terms/Concepts

• Presystemic elimination (the "first pass" effect):
• It minimizes distribution to the target site because the toxicant is eliminated prior to reaching systemic circulation.
• Not unusual for chemicals absorbed from GI tract.
• If the liver is the target organ, it may increase toxicity.
• Toxication:
• Biotransformation to harmful products.
• It refers to when biotransformation increases toxicity.
• Detoxication:
  • Biotransformations which eliminate the ultimate toxicant or prevent its formation.
• Disorderly (or messy), Necrosis is a cell death that causes inflammation due to cell membrane bursting. Ends with debris in the extracellular environment; dysregulation of ATP synthesis (which is the power source for Na/k pump therefore, without which cells burst) orPassive cell death.
• Apoptosis: is an orderly (planned) planned membrane death that is an active cell death; cells shrink and break into membrane bound fragments (apoptotic bodies) that are phagocytoseis that avoids dead cells are removed and thus preventing inflammation; .deletion of cells w/ANA damage prevents their to malignant transfomation (Cancer).
• Homeostasis: Steady state.
• Phase I and Phase II biotransformations - See above.
• Toxin: Produced by biological systems (plants, animals, fungi, bacteria).
• Toxicant: Produced by or are a by-product of anthropogenic (human-made) activities.
• Octanol/Water partition coefficient -way to express lipid solubility; larger number = more lipidsoluble.
• Xenobiotic-chemical/substance foreign to an organism.
• Passive Transport:
  • Simple diffusion -passive, require no cell energy, Coes gradient is high to low) is a factor.
• Blood/gas partition coefficient: -Solubility ratio, concentration in blood/concentration in gas phase before or at saturation at-equilibrium.
• **Gases w/ low ratio: **
  • Rate of transfer depends on blood flow through lungs).
• Gases w/ high ratio:
  • Rate of transfer is primarily function of rate and depth of respiration,
• Active transport chemicals removed across electrochemical orconcentration gradients selective:
  • For, Certain structural features,. If for, chemicals and are selective for certain structural features of chemicals and has potential for competing inhibition, or for b/w compounds or substances that transport.
• **Facilitated: **
  • Diffusion carrier-mediated transport that exhibits properties of the substance, or not transported against an electrochemical process where input energy of ATP is to interfere with the prison transport.
• **Fibrosis: **
  • Abnormal collogen deposition and two most common; is An hepatic fibrosis and pulmonary fibrosis; "Overshoot", a repair is is to press forward.
• **Topological polar surface arcce: **
  • Involved in this phase, Phase one and two Bi transformation; and or estimate total that surface molecules like Oxygen And or nitrogen.
• **Types of tolerance: **
  • the tolerance of Decrease of the toxic for Chemical or to structure with a later process.
• **Chemical tolerance, a chemical is:
• • • State of Decreasing that's has a toxic or has a chemica the result for later.
• **Disposition tolerance has: ** -Amount of chemical reaching site. -Action decreases over time.
  • Leading to reduce the responsiveness of the tissue and stimulation is E and G phenobarbital.
• *-Chemical cellular tolerance": **
  • May result in lower availability for, receptors and for drugs.
  • Medators, drugs of abuse and morphine.

Dose-response curves illustrate the relationship between a drug's dosage and its effect. Individual and population curves differ, with the former showing continuous response and the latter showing an all-or-none response. Key metrics include LOAEL and NOAEL.