Pharmacokinetics and Pharmacodynamics Practice Questions

Q: A 40-year-old patient on oxygen therapy has developed digoxin toxicity. The plasma digoxin level is 4 ng/mL. Renal function is normal, and the plasma half-life of digoxin in this patient is 1.6 days. How long should digoxin be withheld to reach a safer yet probably therapeutic level of 1 ng/mL?

3.2 days. ***3.2 days*** - To reach 1 ng/mL from 4 ng/mL, the digoxin level must undergo two half-lives (4 ng/mL → 2 ng/mL → 1 ng/mL). - Given a half-life of 1.6 days, two half-lives would be 1.6 days * 2 = **3.2 days**. *1.6 days* - This represents only one half-life, which would reduce the digoxin level from 4 ng/mL to 2 ng/mL, not the target of 1 ng/mL. - The patient would still be above the desired therapeutic level, and likely still experiencing toxicity. *2.4 days* - This period is equivalent to 1.5 half-lives (1.6 days * 1.5 = 2.4 days), which would reduce the digoxin level to 1.41 ng/mL. - This level is still higher than the desired 1 ng/mL, and while lower, may not fully resolve the toxicity. *4.8 days* - This period represents three half-lives (1.6 days * 3 = 4.8 days), which would reduce the digoxin level to 0.5 ng/mL. - While safer, this level may be sub-therapeutic and unnecessarily prolong the withholding of the drug, delaying the re-establishment of therapeutic effects.

Q: Zero-order kinetics is seen in all except which of the following drugs?

Methotrexate. ***Methotrexate*** - Methotrexate typically exhibits **first-order kinetics** at therapeutic doses, meaning its elimination rate is proportional to its concentration. - While high doses or impaired renal function could theoretically lead to saturation, its usual elimination follows **first-order principles**. *Phenytoin* - Phenytoin exhibits **zero-order kinetics** at therapeutic concentrations due to saturation of its metabolic enzymes (e.g., CYP2C9, CYP2C19) in the liver. - This means a **constant amount** of the drug is eliminated per unit of time, irrespective of its plasma concentration. *High dose salicylates* - At **high doses**, the metabolic pathways for salicylates (like aspirin) become saturated, leading to **zero-order kinetics**. - This saturation means a **fixed amount** of salicylate is eliminated over time, increasing the risk of toxicity with dose increments. *Ethanol* - Ethanol metabolism, primarily by **alcohol dehydrogenase**, follows **zero-order kinetics** at most concentrations relevant to consumption. - A **constant amount** of ethanol is eliminated per unit of time, regardless of how much is present in the body.

Q: What does the therapeutic index of a drug signify?

Safety margin. ***Safety margin*** - The **therapeutic index (TI)** is a ratio comparing the dose that produces a toxic effect (TD50 or LD50) to the dose that produces a therapeutically desired effect (ED50) [1]. - A higher therapeutic index indicates a **wider safety margin**, meaning there is a greater difference between the effective and toxic doses [1, 2].*Dose which produces maximum effect* - This describes the **efficacy** of a drug at its maximal point, not its therapeutic index. - The therapeutic index is concerned with the range of doses that can be safely given to achieve a therapeutic effect [2].*Efficacy* - **Efficacy** refers to the maximum effect a drug can produce regardless of the dose. - The therapeutic index is a measure of drug safety, not primarily its efficacy [1].*Maximum response that can be elicited by a drug* - This definition also describes the **efficacy** or **maximal effect** of a drug. - The therapeutic index quantifies the **ratio of toxic to effective doses**, providing insight into safety [1].

Q: Which drug is metabolized by glutathionation?

Dapsone. ***Dapsone***- **Dapsone** undergoes hepatic metabolism via **N-hydroxylation** by CYP450 enzymes (particularly CYP2E1 and CYP3A4), forming reactive **hydroxylamine metabolites**.- These reactive metabolites are toxic and can cause **methemoglobinemia** and **hemolysis**.- **Glutathione conjugation (glutathionation)** serves as an important **detoxification pathway** for these reactive dapsone metabolites [1].- Individuals with **glutathione deficiency** (such as G6PD deficiency) are at increased risk of dapsone-induced hemolytic anemia [2].*Fosfomycin*- **Fosfomycin** is primarily eliminated by the kidneys as an **unchanged drug** (up to 90% excreted unchanged in urine).- It undergoes **minimal hepatic metabolism** and does NOT undergo significant glutathionation.- Its primary route of elimination is **renal excretion** via glomerular filtration.*Benzodiazepines*- **Benzodiazepines** are primarily metabolized in the liver via **CYP450 enzymes** (Phase I oxidation) followed by **glucuronidation** (Phase II conjugation).- They do NOT undergo glutathionation as a significant metabolic pathway.*Nicotinic acid*- **Nicotinic acid** (niacin) undergoes conjugation with **glycine** to form nicotinuric acid and **methylation** to form N-methylnicotinamide.- It does NOT undergo glutathione conjugation.

Question 1

Which of the following statements is true about first-order kinetics?

Accepted Answer

The rate of elimination is proportional to the plasma concentration.

Answer

The half-life increases with an increase in dose.

Answer

A constant amount is eliminated in unit time.

Answer

The elimination follows zero-order kinetics at therapeutic doses.

Question 2

A 40-year-old patient on oxygen therapy has developed digoxin toxicity. The plasma digoxin level is 4 ng/mL. Renal function is normal, and the plasma half-life of digoxin in this patient is 1.6 days. How long should digoxin be withheld to reach a safer yet probably therapeutic level of 1 ng/mL?

Accepted Answer

3.2 days

Answer

1.6 days

Answer

2.4 days

Answer

4.8 days

Question 3

What are the primary mechanisms involved in the termination of drug action?

Accepted Answer

Hepatic metabolism and renal excretion are key mechanisms in drug action termination.

Answer

Drugs must be eliminated from the body to terminate their action.

Answer

Metabolism of drugs can either activate or abolish their pharmacologic activity.

Answer

Distribution of a drug from the bloodstream can influence its effects.

Question 4

Zero-order kinetics is seen in all except which of the following drugs?

Accepted Answer

Methotrexate

Answer

Phenytoin

Answer

High dose salicylates

Answer

Ethanol

Question 5

What does the therapeutic index of a drug signify?

Accepted Answer

Safety margin

Answer

Dose which produces maximum effect

Answer

Efficacy

Answer

Maximum response that can be elicited by a drug

Question 6

Which drug is metabolized by glutathionation?

Accepted Answer

Dapsone

Answer

Nicotinic acid

Answer

Fosfomycin

Answer

Benzodiazepines

Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics — MCQs

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