Q40 — Pharmacokinetics: Absorption, Distribution, Metabolism, Excretion
Pharmacokinetics studies drug movement in the body, guiding dosage for desired effects based on biochemical properties. Table 1 shows pharmacokinetic properties that regulate the duration of a drug’s therapeutic effect.
| Property | Description |
|---|---|
| Absorption | — |
| Distribution | Influenced by ionization and other biochemical properties of the compound, as well as polarity, transport-protein binding, and blood flow. The volume of distribution (V) describes a drug’s ability to disperse within body fluids — a V close to the total body volume indicates the best distribution. |
| Metabolism | Primarily occurs in the liver. First-pass metabolism happens when the liver metabolizes a drug before it enters systemic circulation. A drug’s half-life is the time for its blood concentration to halve. A prodrug is a drug that becomes active only after being metabolized (typically in the liver). |
| Excretion | Kidneys typically eliminate hydrophilic substances, while hydrophobic ones exit via bile. Only about 5% of bile salts (and drugs) are excreted; the rest is reabsorbed, transported to the liver, and re-secreted with bile — known as enterohepatic circulation. |
Table 1. Pharmacokinetic properties regulating the duration of a drug’s therapeutic effect.
On your answer sheet, indicate “T” for true statements and “F” for false ones.
A. Oral administration is generally more suitable for prodrugs with high first-pass metabolism than for those with low first-pass metabolism.
B. Drugs that bind strongly to plasma proteins have a small volume of distribution.
C. Lipophilic drugs that do not bind to plasma proteins are expected to have a shorter half-life compared to lipophilic drugs that can bind to plasma proteins.
D. Enterohepatic circulation of oestrogen-containing oral contraceptives prolongs their activity.
Question reproduced from IBO 2024, Theoretical Exam Part B, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF