Pharmacokinetics, Biotransformation, and Drug Elimination

Posted on Jul 23, 2026 in Biotechnology

Biotransformation (Drug Metabolism)

Biotransformation is the process by which a drug is chemically altered in the body. It changes active drugs into less active or inactive compounds (to be excreted) or transforms inactive “prodrugs” into active forms.

Phase I Reactions (Non-Synthetic)

This phase alters the drug’s chemical structure by adding or exposing a functional group.

  • Reactions: Oxidation (adding oxygen or removing hydrogen, often via Cytochrome P450 enzymes), Reduction, and Hydrolysis.

Phase II Reactions (Synthetic/Conjugation)

These reactions attach a small, naturally occurring, water-soluble molecule (like a sugar or an amino acid) to the drug or its Phase I metabolite. This makes the drug highly water-soluble so it can be easily excreted.

  • Reactions: Glucuronidation, Sulfation, and Acetylation.

Factors Affecting Biotransformation

  • Enzyme Induction: Some drugs (e.g., rifampicin) stimulate the liver to produce more metabolizing enzymes, making other drugs break down faster.
  • Enzyme Inhibition: Certain drugs or foods (e.g., grapefruit juice) block metabolizing enzymes, causing other drugs to build up in the body and become toxic.
  • Age: Metabolism is often slower in newborns and the elderly.
  • Genetics: Genetic differences in enzymes cause people to metabolize drugs at different rates (e.g., fast metabolizers vs. slow metabolizers).

Renal Excretion and Factors Affecting It

Renal excretion is the removal of drugs and their waste products from the body via the urine. It involves three primary steps:

  1. Glomerular Filtration: Small, unbound (free) drug molecules pass from the blood into the urine through the kidney filters.
  2. Active Tubular Secretion: Carrier proteins actively pump drugs from the blood into the urine.
  3. Tubular Reabsorption: As urine moves down the kidney, lipid-soluble drugs are reabsorbed back into the blood.

Factors Affecting Renal Excretion

  • Urine pH: The pH of urine changes based on diet and drugs. Weak acids are excreted faster in alkaline urine because they become ionized and cannot be reabsorbed. Weak bases are excreted faster in acidic urine.
  • Protein Binding: Only free (unbound) drugs are filtered in the glomerulus. If a drug is highly bound to proteins, it cannot be filtered easily, increasing its time in the body.
  • Renal Blood Flow: Decreased blood flow (due to heart failure or kidney disease) reduces how fast the kidneys can filter and remove drugs.
  • Age: Kidney function naturally declines with age, requiring lower drug doses for older patients to prevent toxicity.

Pharmacokinetic and Compartment Models

  • Pharmacokinetic Model: A mathematical framework using equations to quantitatively predict how a drug will be absorbed, distributed, and eliminated over time.
  • Compartment Model: The most common modeling approach, which simplifies the complex human body into hypothetical units or “compartments” that have similar drug concentrations.
    • One-Compartment Model: Treats the body as a single, well-mixed unit; drugs distribute instantly and uniformly throughout.
    • Multi-Compartment Model: Treats the body as having multiple zones (e.g., a central compartment like blood/organs, and a peripheral compartment like muscle/fat tissue), with the drug moving between them at different rates.

Pharmacokinetic Parameters

Pharmacokinetic (PK) parameters are mathematical values used to measure and describe how a drug moves through the body (Absorption, Distribution, Metabolism, and Excretion). Key parameters include:

  • Bioavailability (F): The fraction of an administered drug dose that reaches the systemic circulation in an unchanged form.
  • Volume of Distribution (Vd): A theoretical volume that relates the amount of drug in the body to the concentration of the drug in the plasma.
  • Clearance (Cl): The volume of plasma from which a drug is completely removed per unit of time.
  • Half-life (t1/2): The time required for the drug concentration in the body to decrease by 50%.

One Compartment Open Model (Continuous IV Infusion)

Continuous IV infusion administers a drug at a constant rate (R0) to maintain stable, therapeutic drug levels in the blood.

  • Characteristics: The body acts as a single, uniform compartment where the drug distributes instantly and is eliminated. The drug input is considered zero-order (constant rate), while elimination is first-order (rate is proportional to concentration).
  • Steady-State: The drug reaches a plateau (steady-state concentration, Css) when the rate of drug administration equals the rate of elimination.
  • Loading Dose: Because steady-state takes time (approx. 4–5 half-lives) to reach, a rapid “loading dose” is often given first to quickly achieve the desired therapeutic concentration. [11, 13, 14]

IVIVC and In-Vitro Drug Dissolution Model

  • In-Vitro Dissolution Model: A laboratory test that measures how quickly a drug dissolves (releases its active ingredient) from its dosage form (e.g., tablet or capsule) in a controlled fluid and temperature mimicking the gastrointestinal tract. Standard devices include USP Dissolution Apparatus I (Basket) and Apparatus II (Paddle).
  • IVIVC (In Vitro-In Vivo Correlation): A predictive mathematical model that connects laboratory dissolution data to the actual in vivo (in the body) performance (such as drug plasma concentration) of the drug.
    • Level A Correlation: The most common and recommended FDA standard, establishing a direct, point-to-point relationship between the in vitro dissolution curve and the in vivo input (absorption) curve.

Kinetics of Multiple Dosing

When a patient takes medication repeatedly over time (e.g., a pill every 8 hours), the drug builds up in the body before being fully eliminated.

  • Accumulation: With each successive dose, drug concentration increases until the rate of drug administration balances the rate of elimination.
  • Steady-State: This equilibrium is called “steady-state,” which is typically achieved after roughly 4 to 5 half-lives of the drug.
  • Loading vs. Maintenance Dose: A loading dose (larger initial amount) is used to jump-start the body to target levels immediately, while maintenance doses are regular, smaller doses meant to keep the drug concentration within the therapeutic window.
  • Fluctuation: The difference between the highest (Peak) and lowest (Trough) drug concentration during one dosing interval.