Pharmacokinetics and Pharmacodynamics — MCQs

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: 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.

Pharmacokinetics and Pharmacodynamics — MCQs

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