6.2 Toxicokinetics: Absorption, Distribution, Metabolism, and Elimination

Toxicokinetics: Absorption, Distribution, Metabolism, and Elimination

Dose-response curves describe what a dose does. Toxicokinetics describes what the body does to the dose — how much of an airborne concentration actually crosses a biological barrier, where it goes, how it is chemically transformed, and how quickly it leaves. Every biological monitoring decision and every shift-adjustment argument on the exam rests on these four processes.

1. Toxicokinetics: Absorption Routes in the Workplace

Toxicokinetics describes the time course of toxicant movement through the body, broken into Absorption, Distribution, Metabolism, and Excretion (ADME).

   +-------------------------------------------------------------+
   |                  OVERVIEW OF ADME PROCESSES                 |
   +-------------------------------------------------------------+
   |                                                             |
   |  ABSORPTION  --> Inhalation (Pulmonary, Alveolar)           |
   |                  Dermal (Stratum Corneum Diffusion)         |
   |                  Ingestion (GI Tract / Mucociliary Swallowing)|
   |                          |                                  |
   |                          v                                  |
   |  DISTRIBUTION -> Plasma Albumin Binding                     |
   |                  Apparent Volume of Distribution (Vd)       |
   |                  Blood-Brain Barrier / Tissue Partitioning  |
   |                          |                                  |
   |                          v                                  |
   |  METABOLISM   -> Phase I (CYP450 Functionalization)         |
   |                  Phase II (Glucuronidation, GST Conjugation)|
   |                          |                                  |
   |                          v                                  |
   |  EXCRETION   --> Renal (Urine), Biliary (Feces), Pulmonary  |
   |                  First-Order Kinetics: t1/2 = 0.693 / k_el  |
   +-------------------------------------------------------------+

Inhalation Absorption & Particle Deposition

Inhalation is the primary route of occupational chemical exposure. The site and extent of airborne deposition depend on physical state and particle size:

• Gases and Vapors: Absorption is governed by blood-to-air partition coefficients (Ostwald solubility coefficient). Highly water-soluble gases (NH3, SO2) dissolve rapidly in the moist mucosal lining of the upper respiratory tract. Moderately soluble and lipid-soluble vapors (toluene, methyl chloroform) penetrate directly into the alveolar gas-exchange bed and diffuse across the thin (0.2--0.5 µm) alveolar-capillary membrane.
• Particulate Matter (Aerosols): Particle deposition is governed by aerodynamic diameter (dae):
  • Inhalable Fraction (dae ≤ 100 µm, 50% cut at 100 µm): Deposited in the head airways and nasopharyngeal region via inertial impaction.
  • Thoracic Fraction (dae ≤ 28 µm, 50% cut at 10 µm): Penetrates past the larynx into the tracheobronchial region via impaction and turbulent sedimentation.
  • Respirable Fraction (dae ≤ 10 µm, 50% cut at 4.0 µm): Penetrates into unciliated gas-exchange alveolar spaces via gravitational sedimentation and Brownian diffusion (for nanoparticles < 0.5 µm).
• Clearance Mechanisms:
  • Tracheobronchial tree: Mucociliary escalator propels particles upward to the pharynx where they are swallowed into the gastrointestinal tract.
  • Alveoli: Alveolar macrophages engulf insoluble particles via phagocytosis and transport them to the lymphatic drainage or mucociliary boundary.

Dermal Absorption & Stratum Corneum Flux

The stratum corneum (outermost layer of keratinized, non-viable corneocytes embedded in a lipid-rich intercellular matrix) represents the primary rate-limiting barrier to dermal penetration.

Percutaneous absorption follows Fick's First Law of Steady-State Diffusion:

$J = K_p \times \Delta C = \frac{D \times K_{ow} \times \Delta C}{h}$

Where:

• J = Steady-state dermal flux (mg/(cm²·hr))
• Kp = Permeability coefficient (cm/hr)
• D = Diffusion coefficient within stratum corneum (cm²/hr)
• Kow = Octanol-water partition coefficient (measure of lipophilicity)
• h = Stratum corneum membrane thickness (cm)
• Δ C = Concentration gradient across the skin barrier (mg/cm³)

Key Concept: Moderate lipophilicity (Log Kow between 1 and 3) maximizes dermal penetration. Extremely hydrophilic compounds cannot cross the lipid matrix, while extremely lipophilic compounds (Log Kow > 4) dissolve into the stratum corneum but cannot partition out into the aqueous dermal capillary blood flow.

Gastrointestinal Absorption & First-Pass Hepatic Effect

In workplace settings, GI absorption occurs through hand-to-mouth transfer (eating or smoking with contaminated hands) or swallowing mucociliary-cleared inhaled aerosols. Toxicants absorbed from the stomach and intestines enter the hepatic portal system and pass directly to the liver before reaching systemic circulation—subjecting them to hepatic first-pass metabolism (which may detoxify the substance or bioactivate it into toxic metabolites).

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2. Toxicokinetics: Distribution and Volume of Distribution (Vd)

Once absorbed into systemic circulation, a toxicant distributes to body tissues based on regional organ blood flow, capillary permeability, tissue affinity, and protein binding.

Plasma Protein Binding

Many xenobiotics bind reversibly to plasma proteins (predominantly albumin for acidic/neutral compounds and α1-acid glycoprotein for basic compounds):

$\text{Toxicant (Free)} + \text{Albumin} \rightleftharpoons \text{Toxicant-Albumin Complex}$

• Only the unbound (free) fraction can cross biological membranes, reach cellular target receptors, undergo hepatic biotransformation, or filter through the renal glomerulus.
• Toxicants with high protein binding (> 90%) can be displaced by competing chemicals, causing sharp spikes in free active concentration and unexpected acute toxicity.

Biological Barriers

• Blood-Brain Barrier (BBB): Continuous non-fenestrated capillary endothelial cells joined by tight junctions (zonula occludens), surrounded by astrocyte end-feet. Protects the central nervous system from polar molecules. Lipophilic, non-ionized toxicants (solvents, methylmercury, organotins) cross freely, whereas polar toxicants are excluded unless transported by specific carrier proteins.
• Placental Barrier: Does not prevent the diffusion of most industrial solvents, carbon monoxide, or heavy metals (lead, mercury), allowing significant fetal exposure during critical organogenesis.

Apparent Volume of Distribution (Vd)

Apparent Volume of Distribution (Vd) is the hypothetical volume of fluid into which a total absorbed dose of toxicant would need to be uniformly distributed to achieve the measured plasma concentration at time zero (C0):

$\mathbf{V_d = \frac{\text{Total Dose Administered (mg)}}{C_0\text{ (Plasma Concentration, mg/L)}}}$

$V_d\,(\text{L/kg}) = \frac{V_d\,(\text{L})}{\text{Body Mass (kg)}}$

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3. Toxicokinetics: Metabolism (Biotransformation)

Biotransformation converts lipophilic xenobiotics (which would otherwise accumulate indefinitely in fatty tissues) into hydrophilic, polar metabolites that are readily excreted in urine or bile. Biotransformation proceeds through two sequential enzymatic phases:

   +-------------------------------------------------------------+
   |               TWO-PHASE METABOLIC TRANSFORMATION            |
   +-------------------------------------------------------------+
   |                                                             |
   |  Lipophilic Toxicant (Non-polar, high Kow)                  |
   |       |                                                     |
   |       |  PHASE I: FUNCTIONALIZATION                         |
   |       |  (CYP450 monooxygenases, oxidation, hydrolysis)     |
   |       v                                                     |
   |  Functionalized Metabolite (-OH, -COOH, -NH2, -SH)          |
   |  *May be bioactivated into toxic reactive intermediate      |
   |       |                                                     |
   |       |  PHASE II: CONJUGATION                              |
   |       |  (Glucuronidation, Sulfation, Glutathione-S-Transf) |
   |       v                                                     |
   |  Hydrophilic Conjugate (Polar, water-soluble, ionized)      |
   |       |                                                     |
   |       +---> Rapid Excretion (Urine via kidney, Bile/Feces)  |
   +-------------------------------------------------------------+

Phase I Reactions: Functionalization

• Enzyme Systems: Cytochrome P450 monooxygenase superfamily (CYP450) located primarily in the smooth endoplasmic reticulum (microsomes) of hepatocytes (e.g., CYP1A2, CYP2E1, CYP3A4).
• Mechanism: Catalyzes the insertion of one atom of molecular oxygen into the substrate (R-H + O2 + NADPH + H⁺ → R-OH + H2O + NADP⁺).
• Reaction Types: Hydroxylation, dealkylation, epoxidation, dehalogenation, reduction, and hydrolysis.
• Bioactivation (Toxification): While Phase I generally prepares molecules for detoxification, it frequently bioactivates parent compounds into highly reactive, electrophilic intermediates (e.g., conversion of carbon tetrachloride into trichloromethyl radical ·CCl3 via CYP2E1; conversion of benzene to benzene oxide; epoxidation of benzo[a]pyrene).

Phase II Reactions: Conjugation

• Mechanism: Endogenous hydrophilic molecules are covalently coupled to functional groups (-OH, -COOH, -NH2, -SH) introduced during Phase I.
• Key Conjugation Pathways:
  • Glucuronidation: Catalyzed by UDP-glucuronosyltransferases (UGTs); transfers glucuronic acid from UDP-glucuronic acid (UDPGA). Quantitatively the most common clearance pathway.
  • Sulfation: Catalyzed by sulfotransferases (SULTs); transfers sulfate from 3'-phosphoadenosine-5'-phosphosulfate (PAPS). High affinity but low capacity (easily saturated).
  • Glutathione S-Conjugation: Catalyzed by Glutathione S-Transferases (GSTs); couples the tripeptide glutathione (GSH) to reactive electrophilic intermediates, neutralizing cellular attack. Glutathione conjugates are further cleaved in the kidney into mercapturic acids (e.g., S-phenylmercapturic acid from benzene) and excreted in urine.
  • Acetylation: Catalyzed by N-acetyltransferases (NAT1, NAT2); transfers acetyl groups to aromatic amines (e.g., benzidine, 2-naphthylamine). Genetic polymorphisms ("slow acetylators") increase bladder cancer vulnerability.

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4. Toxicokinetics: Elimination Kinetics & Biological Half-Life

First-Order vs. Zero-Order Elimination Kinetics

1. First-Order Kinetics (Linear Elimination):

   • The rate of elimination is directly proportional to the plasma concentration (-dC/dt = kel C).
   • A constant fraction of the toxicant is eliminated per unit time.
   • Clearance pathways are operating below saturation capacity (the typical scenario for most workplace chemical exposures).

2. Zero-Order Kinetics (Saturable / Capacity-Limited Elimination):

   • The rate of elimination is constant and independent of concentration (-dC/dt = k0).
   • A constant absolute amount of toxicant is eliminated per unit time because metabolic or excretory enzyme systems are completely saturated (e.g., Ethanol metabolism via alcohol dehydrogenase at normal intoxicating doses; high-dose Salicylates).

   +-------------------------------------------------------------+
   |             FIRST-ORDER VS. ZERO-ORDER ELIMINATION          |
   +-------------------------------------------------------------+
   |  Concentration (Linear)          Concentration (Semi-Log)   |
   |  |                               |                          |
   |  | \\  First-Order (Exponential) | |\                       |
   |  |   \\                          | | \\ First-Order (Linear)|
   |  |     \\                        | |   \\                   |
   |  | ----- Zero-Order (Linear)     | |    --- Zero-Order      |
   |  0 +--------------------->       0 +----------------------> |
   |  0          Time                 0          Time            |
   +-------------------------------------------------------------+

Mathematical Formulation of First-Order Elimination

For a substance following first-order kinetics:

$\ln(C_t) = \ln(C_0) - k_{el} \cdot t \quad \Longleftrightarrow \quad C_t = C_0 \cdot e^{-k_{el} \cdot t}$

Where:

• Ct = Plasma concentration at time t
• C0 = Initial plasma concentration at time zero
• kel = Elimination rate constant (hr⁻¹ or day⁻¹)
• t = Elapsed time (hours or days)

Elimination Half-Life (t(1/2))

The biological half-life (t(1/2)) is the time required for the systemic concentration of a toxicant to decrease by exactly 50%:

$\mathbf{t_{1/2} = \frac{\ln(2)}{k_{el}} = \frac{0.69315}{k_{el}} \quad \Longleftrightarrow \quad k_{el} = \frac{0.69315}{t_{1/2}}}$

Systemic Clearance (CL)

Total body clearance is the volume of plasma completely cleared of toxicant per unit time:

$CL = k_{el} \times V_d = \frac{0.693 \times V_d}{t_{1/2}}$

The 5 Half-Lives Rule in Industrial Hygiene

During constant occupational exposure (e.g., an 8-hour shift) or post-exposure biological washout, the approach to steady-state (Css) or total elimination follows the exponential rule:

Exam Key Concept: When scheduling biological monitoring samples, substances with t(1/2) < 5 hours reflect same-day shift exposures and must be collected at End of Shift. Substances with t(1/2) ≈ 15--40 hours accumulate over the week and must be collected at End of Workweek. Substances with t(1/2) > 1 year (e.g., Cadmium, Lead) reflect cumulative lifetime burden and can be collected at Discretionary / Any Time.

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5. Worked Step-by-Step Calculation Examples

Worked Example 5.1.1: Elimination Rate Constant and Biological Half-Life Calculation

Problem: A worker accidentally ingests an industrial solvent. Serial venous blood samples indicate a plasma concentration of 80.0 mg/L at 12:00 PM (time zero) and 20.0 mg/L at 8:00 PM (8 hours later). Assuming the solvent follows first-order single-compartment elimination kinetics:

1. Calculate the elimination rate constant (kel).
2. Calculate the biological half-life (t(1/2)).
3. Determine the expected plasma concentration at 12:00 AM (midnight, 12 hours after the initial measurement).

Solution Steps:

1. Calculate the elimination rate constant (kel): $\ln(C_t) = \ln(C_0) - k_{el} \cdot t \implies k_{el} = \frac{\ln(C_0) - \ln(C_t)}{t}$ $k_{el} = \frac{\ln(80.0) - \ln(20.0)}{8\,\text{hr}} = \frac{4.3820 - 2.9957}{8\,\text{hr}} = \frac{1.3863}{8\,\text{hr}} = 0.1733\,\text{hr}^{-1}$

2. Calculate the biological half-life (t(1/2)): $t_{1/2} = \frac{0.69315}{k_{el}} = \frac{0.69315}{0.1733\,\text{hr}^{-1}} = 4.00\,\text{hours}$

3. Calculate concentration at t = 12 hours:

   • Note that 12 hours = 3 half-lives (12 / 4 = 3).
   • After 3 half-lives: C12 = C0 × (0.5)³ = 80.0 × 0.125 = 10.0 mg/L.
   • Using the exponential formula directly: $C_{12} = 80.0 \cdot e^{-(0.1733 \times 12)} = 80.0 \cdot e^{-2.0796} = 80.0 \times 0.1250 = 10.0\,\text{mg/L}$

Result: The elimination rate constant is 0.173 hr⁻¹, the biological half-life is 4.0 hours, and the plasma concentration at midnight will be 10.0 mg/L.

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Worked Example 5.1.2: Reference Dose (RfD) Derivation Using BMDL10 and Uncertainty Factors

Problem: A 90-day subchronic oral toxicity study of a novel industrial plasticizer in Sprague-Dawley rats evaluates renal tubular damage. Benchmark dose modeling determines a BMDL10 of 15.0 mg/kg/day. Toxicologists assign the following uncertainty factors: Interspecies variability (10×), Intraspecies human variability (10×), Subchronic-to-chronic extrapolation (10×), and Database completeness (1×).

1. Calculate the acceptable Human Reference Dose (RfD) in mg/kg/day and µg/kg/day.
2. For an adult worker weighing 70 kg, what is the maximum acceptable daily intake (in milligrams/day)?

Solution Steps:

1. Calculate the Reference Dose (RfD): $\prod \text{UF} = UF_A \times UF_H \times UF_S \times UF_D = 10 \times 10 \times 10 \times 1 = 1,000$ $RfD = \frac{\text{BMDL}_{10}}{\prod \text{UF}} = \frac{15.0\,\text{mg/kg/day}}{1,000} = 0.015\,\text{mg/kg/day} = 15.0\,\mu\text{g/kg/day}$

2. Calculate maximum acceptable daily intake for a 70 kg worker: $\text{Daily Intake} = RfD \times \text{Body Weight} = 0.015\,\text{mg/kg/day} \times 70\,\text{kg} = 1.05\,\text{mg/day}$

Result: The derived human RfD is 0.015 mg/kg/day (15.0 µg/kg/day), corresponding to a maximum allowable daily intake of 1.05 mg/day for a 70 kg adult.

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Worked Example 5.1.3: Apparent Volume of Distribution (Vd) Calculation and Interpretation

Problem: A research worker receives an accidental intravenous injection of 350 mg of a lipid-soluble organic compound. An initial blood sample drawn immediately post-distribution reveals a plasma concentration of 0.050 mg/L. The worker weighs 70.0 kg.

1. Calculate the apparent Volume of Distribution (Vd) in liters (L).
2. Calculate the normalized Vd in L/kg.
3. Interpret what this numerical value indicates regarding tissue distribution.

Solution Steps:

1. Calculate total Vd in Liters: $V_d = \frac{\text{Total Dose (mg)}}{C_0\,(\text{mg/L})} = \frac{350\,\text{mg}}{0.050\,\text{mg/L}} = 7,000\,\text{Liters}$

2. Calculate normalized Vd in L/kg: $V_d\,(\text{L/kg}) = \frac{7,000\,\text{L}}{70.0\,\text{kg}} = 100\,\text{L/kg}$

3. Toxicological Interpretation:

   • Total body water in a 70 kg human is approximately 42 L (0.6 L/kg).
   • A Vd of 7,000 L (100 L/kg) far exceeds total anatomical body volume. This demonstrates that the toxicant is extremely lipophilic and overwhelmingly sequestered in extravascular tissues (such as adipose tissue, liver, or peripheral lipid membranes), leaving only an infinitesimal fraction in circulating plasma.