1.1 Toxicokinetics: Absorption, Distribution, Metabolism, and Elimination Alterations in Overdose

Therapeutic pharmacokinetics assumes static, predictable physiological parameters: gut motility remains normal, plasma proteins have abundant unoccupied binding sites, metabolic enzymes operate well below saturation, and renal clearance eliminates a constant percentage of drug per unit time. In moderate-to-severe poisoning, these assumptions collapse. Toxicokinetics examines how supratherapeutic doses alter Absorption, Distribution, Metabolism, and Elimination (ADME), transforming predictable linear pharmacology into volatile, non-linear kinetics.

Applying therapeutic pharmacokinetic parameters to an overdose frequently causes dangerous clinical errors, such as releasing patients prematurely, drawing laboratory levels before peak absorption has occurred, or failing to recognize expanding distribution volumes during acidemia.

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Absorption Alterations in the Poisoned Patient

In therapeutic dosing, gastrointestinal absorption typically follows first-order kinetics, where the rate of drug absorption is proportional to the concentration of dissolved drug at the mucosal surface. In overdose, absorption is frequently delayed, prolonged, or biphasic.

Therapeutic Dose: Rapid Dissolution → Normal Transit → Predictable Peak (1–2 h)
Overdose Ingestion: Concretion/Bezoar → Delayed Emptying → Erratic Prolonged Peaks (12–36 h)

Delayed Gastric Emptying

Many common pharmaceuticals impair gastrointestinal smooth muscle motility through direct pharmacological actions:

• Anticholinergic / Antimuscarinic agents: Diphenhydramine, tricyclic antidepressants (TCAs), hydroxyzine, and scopolamine block muscarinic receptors on gastric smooth muscle, arresting gastric peristalsis and delaying pyloric emptying.
• Opioids: Morphine, methadone, oxycodone, and fentanyl activate μ-opioid receptors in the enteric nervous system, producing bowel hypomotility, pylorospasm, and delayed transit.
• Systemic hypoperfusion and shock: Severe hypotension, sympathetic vasoconstriction, and stress-induced gastroparesis diminish splanchnic blood flow, leaving solid ingestants unabsorbed in the stomach for many hours.

Pharmacobezoars and Concretions

A pharmacobezoar is an aggregated, coalesced mass of unabsorbed tablets, capsules, or vehicle matrix that forms within the stomach or upper small intestine. Bezoars act as slow-release drug reservoirs, causing unpredictable, delayed peak concentrations and secondary surges.

Extended-Release Formulations and Bowel Ischemia

Extended-release (ER, XR, XL, SR) and enteric-coated (EC) formulations are engineered to release active compounds over 12 to 24 hours. In massive ingestion, the volume of intestinal fluid is insufficient to dissolve the binder matrices simultaneously. Enteric coatings remain intact within acidic gastric environments; if pyloric transit is delayed, no absorption occurs until the conglomerate enters the alkaline duodenum, triggering sudden, delayed systemic toxicity.

Furthermore, severe vasoconstrictor overdoses (cocaine, methamphetamine, ergot alkaloids) produce localized mesenteric ischemia. Bowel edema and mucosal sloughing impair consistent vascular uptake, resulting in erratic, discontinuous absorption spikes.

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Distribution Alterations in Overdose

Distribution describes the reversible transfer of drug between the vascular compartment and peripheral tissues. The apparent Volume of Distribution (Vd) quantifies the extent of tissue partitioning relative to plasma concentration:

$V_d = \frac{\text{Total Amount of Drug in Body}}{\text{Plasma Concentration } (C_p)}$

$\text{Physiological model: } V_d = V_p + V_t \cdot \left(\frac{f_u}{f_{ut}}\right)$

Where Vp is plasma volume, Vt is tissue volume, fu is free drug fraction in plasma, and fut is free drug fraction in tissues.

Saturation of Plasma Protein Binding

Under therapeutic conditions, highly protein-bound xenobiotics circulate primarily bound to albumin (acidic drugs like phenytoin, valproic acid, and salicylates) or alpha-1-acid glycoprotein (AAG) (basic drugs like lidocaine, verapamil, and TCAs). The free (unbound) fraction (fu) mediates pharmacological toxicity and clearance.

In acute overdose, molar drug concentrations easily exceed the total binding capacity of circulating proteins. Once binding sites saturate, any additional absorbed xenobiotic remains entirely unbound in the plasma. Consequently, free drug concentrations surge exponentially, even when total serum concentrations appear only modestly elevated.

Clinical Pearl: In massive valproic acid overdose, albumin binding saturates around 100–150 mcg/mL. A total level of 450 mcg/mL may carry a 50–70% free fraction rather than the normal 10%, causing devastating hyperammonemic encephalopathy and cerebral edema.

Impact of Acidemia on Ionization and Tissue Partitioning

According to the Henderson-Hasselbalch principle, only the non-ionized (uncharged) form of a weak acid or weak base readily crosses biological lipid membranes, including the blood-brain barrier (BBB):

$\text{For weak acids: } \text{pH} - \text{p}K_a = \log\left(\frac{[A^-]}{[HA]}\right)$

Salicylic acid is a weak acid with a pKa of approximately 3.0. At physiological blood pH (7.40), the ratio of ionized salicylate (A⁻) to un-ionized salicylic acid (HA) is roughly 25,000:1. If severe salicylate toxicity induces metabolic acidosis (e.g., pH dropping to 7.10), the un-ionized HA fraction roughly doubles (each 0.3-unit fall in pH doubles the un-ionized fraction of a weak acid).

Un-ionized molecules immediately cross the blood-brain barrier into neuronal cells, where intracellular trapping triggers central neurotoxicity, cerebral edema, hyperpyrexia, and death. Conversely, therapeutic systemic urinary and blood alkalinization (maintaining blood pH 7.45–7.55 and urine pH 7.5–8.0) shifts the equilibrium toward ionized salicylate (A⁻), trapping it in the vascular space and tubular lumen, preventing central nervous system penetration and promoting urinary excretion.

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Metabolism Alterations in Overdose

In therapeutic dosing, lipophilic xenobiotics undergo predictable Phase I functionalization (CYP450) and Phase II conjugation (glucuronidation, sulfation) to generate water-soluble excretion products.

Phase II Exhaustion and Toxifying Shunts

Phase II conjugation systems possess high affinity but limited substrate capacity. In severe overdose, these physiological defense pathways saturate, and essential endogenous co-factors are exhausted:

• Acetaminophen (APAP): Therapeutically, 85–90% is eliminated via non-toxic glucuronidation (UGT) and sulfation (SULT). In massive overdose, 3'-phosphoadenosine-5'-phosphosulfate (PAPS) and UDP-glucuronic acid (UDPGA) are rapidly depleted. Excess substrate shunts into CYP2E1, 1A2, and 3A4, generating large quantities of the electrophilic cytotoxin N-acetyl-p-benzoquinone imine (NAPQI).
• Glutathione (GSH) Depletion: Hepatocellular GSH conjugates and detoxifies NAPQI. When intracellular GSH stores are depleted past a critical threshold of 70–80% (leaving <20–30% of normal reserve), free NAPQI binds covalently to cysteinyl sulfhydryl groups on hepatocellular proteins, destroying mitochondrial function.

Acute Substrate Competition

When multiple xenobiotics competing for the same enzymatic active site are ingested simultaneously, clearance of both compounds is delayed. Alternatively, deliberate competitive substrate administration forms the basis of antidotal therapy: administering ethanol or fomepizole competitively inhibits alcohol dehydrogenase (ADH), preventing the biotransformation of methanol and ethylene glycol into formic and glycolic/oxalic acids.

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Elimination Kinetics: First-Order vs. Zero-Order

Understanding the mathematical differences between first-order, zero-order, and Michaelis-Menten elimination is essential for calculating clearance and predicting drug exposure duration.

First-Order Elimination: Constant FRACTION eliminated per hour (t1/2 is constant)
Zero-Order Elimination:  Constant AMOUNT eliminated per hour (t1/2 increases with dose)

First-Order (Linear) Kinetics

In first-order kinetics, the rate of drug elimination is directly proportional to plasma concentration (C):

$-\frac{dC}{dt} = k_e \cdot C$

• A constant fraction of drug is cleared per unit of time (e.g., 20% per hour).
• The elimination half-life (t1/2) is fixed and independent of dose:

$t_{1/2} = \frac{0.693}{k_e}$

• Clearance (Cl = ke · Vd) remains constant regardless of concentration.

Zero-Order (Saturation) Kinetics

When plasma drug concentrations saturate all available metabolic enzymes or active renal tubular transport carriers, the elimination rate reaches its maximal velocity (Vmax):

$-\frac{dC}{dt} = k_0$

• A constant absolute amount (mass) is eliminated per unit time (e.g., 100 mg/hour or 15–20 mg/dL/hour for ethanol).
• The rate of elimination is independent of concentration.
• The apparent half-life is dose-dependent: as concentration rises, the time required to eliminate half the remaining drug increases dramatically.

Michaelis-Menten Kinetics

Most biological elimination systems follow Michaelis-Menten kinetics, described by the differential equation:

$-\frac{dC}{dt} = \frac{V_{\max} \cdot C}{K_m + C}$

Where Vmax is the maximum rate of elimination and Km is the substrate concentration at which elimination occurs at half-maximal velocity (0.5 · Vmax).

1. When C ≪ Km: The denominator approximates Km, simplifying to -dC/dt = (Vmax/Km) · C. This represents first-order kinetics.
2. When C ≈ Km: Clearance is mixed-order; small dosage increments cause disproportionately large increases in serum concentration.
3. When C ≫ Km: The denominator approximates C, simplifying to -dC/dt = Vmax. This represents zero-order saturation kinetics.

Classic xenobiotics displaying Michaelis-Menten saturation in overdose include salicylates, ethanol, phenytoin, and theophylline.

Half-Life Prolongation in Severe Overdose

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Clinical Poison Center Case Scenario: The Aspirin Concretion Trap

A 22-year-old patient presents to the emergency department after ingesting approximately 45 tablets of 325 mg enteric-coated aspirin (total dose ~14.6 g, or ~208 mg/kg) 4 hours prior. The initial triage assessment notes mild nausea, normal respiratory rate (16 breaths/min), and an initial serum salicylate level of 26 mg/dL (therapeutic range: 15–30 mg/dL). An inexperienced resident proposes discharging the patient with outpatient referral, assuming the exposure is non-toxic.

Toxicokinetic Consultation Points

1. Enteric-Coating Latency: Enteric coating resists gastric acid dissolution. Coalescence with gastric mucus forms a cohesive pharmacobezoar, delaying significant duodenal dissolution for 8 to 16 hours.
2. Serial Level Mandate: A single salicylate concentration drawn within 4–6 hours of an enteric-coated ingestion is clinically uninterpretable. Levels must be measured every 2 to 4 hours until at least two consecutive determinations demonstrate a clear, consistent downward trajectory.
3. Outcome: Serial monitoring is initiated. At 8 hours, the level rises to 48 mg/dL; by 14 hours, the patient develops tachypnea, tinnitus, and a level of 72 mg/dL with a mixed respiratory alkalosis and high anion gap metabolic acidosis. Timely IV sodium bicarbonate infusion and urinary alkalinization prevent severe neurotoxicity.

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Kinetic Alterations Summary: Therapeutic vs. Toxic States