Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics Indian Medical PG Practice Questions and MCQs

Question 521: What does the time for peak plasma concentration (Tmax) indicate?

A. The rate of elimination
B. The rate of absorption (Correct Answer)
C. The duration of effect
D. The intensity of effect

Explanation: ***The rate of absorption*** - **Tmax** is the time at which the **maximum drug concentration** in the plasma is reached [1]. - A shorter **Tmax** indicates a faster rate at which the drug is absorbed from its administration site into the systemic circulation [1]. *The rate of elimination* - The rate of elimination is primarily reflected by the **elimination half-life (t½)** of the drug, which is the time it takes for the plasma concentration to decrease by half. - While Tmax is influenced by both absorption and elimination, it does not directly indicate the elimination rate [1]. *The duration of effect* - The **duration of effect** is related to how long the drug concentration remains above the minimum effective concentration (MEC), which is influenced by both absorption and elimination, but not directly indicated by Tmax alone. - A drug's duration of action is better described by its **elimination half-life** and the **therapeutic window**. *The intensity of effect* - The **intensity of effect** is largely related to the **maximum plasma concentration (Cmax)** and the drug's affinity for its target, and not directly by the time it takes to reach that concentration. - **Cmax** indicates how much drug is in the bloodstream, which often correlates with the intensity of the pharmacological response.

Question 522: Forced diuresis with acidification or alkalinization of urine is a common method for elimination of certain poisons/drugs from the body. The elimination of which of the following drugs is commonly enhanced by alkaline diuresis?

A. Amphetamines
B. Phenobarbitone (Correct Answer)
C. Theophylline
D. Phencyclidine

Explanation: ***Phenobarbitone*** - **Alkaline diuresis** enhances the elimination of weak acids like **phenobarbitone** by keeping them in their ionized form within the renal tubules, preventing reabsorption. - Ionized drugs are more water-soluble and are thus efficiently excreted in the urine. *Amphetamines* - **Amphetamines** are **weak bases**, and their elimination is enhanced by **acidification of urine**, which ionizes them and reduces their reabsorption. - Alkaline diuresis would not be effective, and might even hinder, the elimination of amphetamines. *Theophylline* - Theophylline is primarily metabolized in the liver, and its renal excretion is less influenced by urinary pH manipulation. - While it behaves as a weak acid, forced diuresis is not a primary method for its elimination in overdose situations. *Phencyclidine* - **Phencyclidine (PCP)** is a **weak base**, and its elimination is increased by **acidification of urine**, similar to amphetamines. - Alkaline diuresis would decrease the excretion of phencyclidine.

Question 523: Which of the following insulin preparations has the longest duration of action?

A. Insulin degludec (Correct Answer)
B. Insulin glargine
C. Isophane insulin (NPH)
D. Insulin detemir

Explanation: ***Insulin degludec*** - **Insulin degludec** forms multi-hexamer chains upon subcutaneous injection, leading to a slow and continuous release of monomers and providing the longest duration of action among available insulin preparations, often exceeding **42 hours**. - Its unique mechanism of action allows for once-daily dosing with less variability and a flatter, more stable pharmacokinetic profile compared to other basal insulins. *Insulin glargine* - **Insulin glargine** precipitates in subcutaneous tissue at physiological pH, forming micro-precipitates from which insulin is slowly absorbed, providing a duration of action of approximately **24 hours**. - Although it is a long-acting insulin, its duration is typically less than that of insulin degludec, and it usually requires once-daily administration. *Insulin detemir* - **Insulin detemir** binds reversibly to albumin in the blood, which delays its absorption and degradation, resulting in a duration of action around **12-24 hours**, depending on the dose. - Due to its variable duration, insulin detemir may sometimes require twice-daily dosing in some patients to maintain basal insulin coverage. *Isophane insulin (NPH)* - **Isophane insulin (NPH)** is an intermediate-acting insulin preparation that is formulated with protamine, which delays its absorption, giving it a duration of action of approximately **10-18 hours**. - NPH insulin has a more pronounced peak action compared to newer long-acting analogues, which can lead to a higher risk of nocturnal hypoglycemia.

Question 524: Which of the following statements about drug distribution in tissues is NOT true?

A. Minocycline - adipose tissue
B. Digoxin - skeletal muscle
C. Chloroquine - eye
D. Ephedrine - bone (Correct Answer)

Explanation: ***Ephedrine - bone*** - This statement is **NOT true**. **Ephedrine** is a sympathomimetic amine that does not have clinically significant accumulation in **bone tissue**. - While ephedrine distributes to various tissues due to its physicochemical properties, there is **no established evidence** of preferential or significant bone accumulation that is clinically relevant. - Unlike the other drug-tissue pairs listed, this pairing lacks strong pharmacokinetic evidence. *Chloroquine - eye* - This statement is **true**. **Chloroquine** (and hydroxychloroquine) accumulates significantly in the **retina** and **cornea**, leading to **retinal toxicity** (e.g., bull's eye maculopathy) with chronic use. - Ocular side effects are a significant concern with long-term chloroquine therapy, necessitating regular **ophthalmological monitoring**. *Minocycline - adipose tissue* - This statement is **true**. **Minocycline** is a highly **lipophilic tetracycline** that accumulates significantly in **adipose tissue**. - Its high lipid solubility allows extensive tissue distribution, including fat stores, contributing to its **prolonged elimination half-life** (15-23 hours). - This lipophilicity also enables excellent CNS penetration and wide tissue distribution. *Digoxin - skeletal muscle* - This statement is **true**. **Digoxin** has a large volume of distribution (5-7 L/kg) primarily due to its significant binding to **skeletal muscle** and **cardiac muscle**, where it exerts its therapeutic effects. - Its accumulation in skeletal muscle contributes to its prolonged **elimination half-life** and wide distribution pattern.

Question 525: What does ED50 measure in pharmacology?

A. Toxicity
B. Safety
C. Potency (Correct Answer)
D. Efficacy

Explanation: ***Potency*** - **ED50** (Effective Dose 50%) is the dose of a drug that produces a **therapeutic effect** in 50% of the population or the maximum effect in 50% of subjects [3, 4]. - It is a key measure of a drug's **potency**: a lower ED50 indicates higher potency [1, 3]. *Toxicity* - **Toxicity** is primarily measured by **LD50** (Lethal Dose 50%), which indicates the dose at which 50% of the population would die [3, 4]. - While related to drug safety, ED50 does not directly quantify toxicity. *Safety* - **Safety** is evaluated using the **therapeutic index**, which ideally compares the **TD50** (Toxic Dose 50%) or **LD50** to the ED50 [3, 4]. - ED50 alone measures effectiveness, not the margin between effective and toxic doses. *Efficacy* - **Efficacy** refers to the **maximal response** a drug can produce, regardless of the dose [1]. - A drug with high efficacy might have a high or low ED50, as efficacy is about the *height* of the effect, not the *dose* at which it occurs [1].

Question 526: The therapeutic index of a drug is defined as the ratio between the toxic dose and the effective dose.

A. Margin of safety
B. Ratio of toxic dose to effective dose (Correct Answer)
C. Efficacy of the drug
D. Drug potency

Explanation: ***Ratio of toxic dose to effective dose***- The **therapeutic index (TI)** is quantitatively defined as the ratio of the toxic dose (TD50 or LD50) to the effective dose (ED50) [1, 2].- This ratio provides a measure of **drug safety**, indicating the range between the therapeutic and toxic concentrations [1, 3].*Margin of safety*- While related to safety, the **margin of safety** is a different concept, often calculated as (TD1 - ED99) / ED99, focusing on the overlap between very few people experiencing toxicity and almost everyone receiving benefit [2].- The therapeutic index is a broader, simpler ratio that doesn't explicitly guarantee overlap safety but indicates overall drug risk.*Efficacy of the drug*- **Efficacy** refers to the maximal effect a drug can produce regardless of the dose, and it is independent of the therapeutic index [2].- A drug can have high efficacy but a narrow therapeutic index, meaning it is very effective but also very toxic at doses slightly above the therapeutic range.*Drug potency*- **Potency** is the amount of drug needed to produce a given effect (e.g., ED50), reflecting its affinity for receptors and efficiency of action [2].- It is distinct from the therapeutic index, which assesses the separation between desired and undesired effects, not the concentration required to achieve a therapeutic effect.

Question 527: Major mechanism of transport of drugs across biological membranes is:

A. Passive diffusion (Correct Answer)
B. Facilitated diffusion
C. Active transport
D. Endocytosis

Explanation: ***Passive diffusion*** - This is the **most common mechanism** for drug transport across biological membranes, especially for **lipid-soluble** drugs. - It occurs down a **concentration gradient** and does not require energy or carrier proteins. *Facilitated diffusion* - This process requires **carrier proteins** to move drugs across membranes, but it still occurs down a **concentration gradient** and does not consume energy directly. - It handles substances that are **too large or too polar** to cross by passive diffusion, but it is not the primary mechanism for most drugs. *Active transport* - This mechanism uses **carrier proteins** and **expends energy (ATP)** to move drugs against their **concentration gradient**. - It is important for the transport of specific drugs, but it is not the predominant mode for the majority of drug molecules. *Endocytosis* - This involves the **engulfment of large molecules** or particles by the cell membrane, forming vesicles. - It is a less common mechanism for drug absorption, primarily used for **very large molecules** like proteins or nanoparticles.

Question 528: In the metabolism of xenobiotics, which of the following reactions does not occur in phase one?

A. Reduction
B. Hydrolysis
C. Oxidation
D. Conjugation (Correct Answer)

Explanation: ***Correct Answer: Conjugation*** - **Conjugation** reactions are characteristic of **Phase II metabolism**, NOT Phase I - In Phase II, a polar molecule (glucuronide, sulfate, acetyl, or glutathione) is added to the xenobiotic to increase water solubility and facilitate excretion - This process typically renders the xenobiotic inactive and more readily eliminated by the kidneys or bile - Common conjugation reactions include glucuronidation, sulfation, acetylation, and glutathione conjugation *Incorrect: Oxidation* - **Oxidation** is a primary **Phase I reaction**, primarily involving the cytochrome P450 (CYP450) enzyme system - Phase I oxidation introduces or exposes polar functional groups (-OH, -COOH, -NH2) - This makes the xenobiotic more reactive and prepares it for Phase II conjugation - Examples include hydroxylation, N-dealkylation, and O-dealkylation *Incorrect: Reduction* - **Reduction** reactions are also common in **Phase I metabolism** - Particularly important for compounds containing nitro groups, carbonyl groups, or azo compounds - These reactions can occur in various tissues, including the liver - Catalyzed by reductases such as cytochrome P450 reductase and other enzyme systems *Incorrect: Hydrolysis* - **Hydrolysis** is another key **Phase I reaction** that breaks down xenobiotics by adding water - Especially important for esters, amides, and other compounds with hydrolyzable bonds - Enzymes like esterases, amidases, and peptidases catalyze these reactions - Results in more polar metabolites that can undergo Phase II conjugation

Question 529: Phase 1 biotransformation includes

A. Reduction (Correct Answer)
B. Methylation
C. Acetylation
D. Sulfate conjugation

Explanation: ***Reduction*** - **Phase 1 biotransformation reactions** are non-synthetic reactions that introduce or expose polar functional groups (e.g., -OH, -NH2, -SH) on xenobiotics to make them more water-soluble. - The three main Phase 1 reactions are **oxidation**, **reduction**, and **hydrolysis**. - These reactions typically involve **cytochrome P450 enzymes** and prepare drugs for excretion or Phase 2 conjugation. *Acetylation* - **Acetylation** is a **Phase 2 (conjugation) reaction**, not Phase 1. - Involves transfer of an acetyl group to amine-containing drugs via **N-acetyltransferase**. - Increases water solubility and facilitates excretion. *Sulfate conjugation* - **Sulfate conjugation** is a **Phase 2 (conjugation) reaction**, not Phase 1. - Involves addition of a sulfate group via **sulfotransferase enzymes**. - Significantly increases hydrophilicity for renal excretion. *Methylation* - **Methylation** is a **Phase 2 (conjugation) reaction**, not Phase 1. - Involves addition of a methyl group via **methyltransferase enzymes**. - Unlike most Phase 2 reactions, methylation may sometimes **decrease** water solubility but is still classified as conjugation.

Question 530: A factor that is likely to increase the duration of action of a drug that is partially metabolized by CYP3A4 in the liver is:

A. Chronic administration of phenobarbital with the drug
B. Displacement from tissue binding sites by another drug
C. Chronic administration of rifampicin
D. Chronic administration of cimetidine with the drug (Correct Answer)

Explanation: ***Chronic administration of cimetidine with the drug*** - **Cimetidine** is a potent inhibitor of various **cytochrome P450 (CYP450) enzymes**, including **CYP3A4**. - By inhibiting the metabolism of a drug predominantly metabolized by **CYP3A4**, cimetidine will increase its plasma concentration and extend its **duration of action**. *Chronic administration of phenobarbital with the drug* - **Phenobarbital** is a strong **inducer of CYP450 enzymes**, including **CYP3A4**. - Induction would accelerate the metabolism of the drug, thus **decreasing its duration of action**, not increasing it. *Displacement from tissue binding sites by another drug* - Displacement from tissue binding sites would primarily increase the **free fraction of the drug in the plasma**, leading to a more rapid distribution to eliminating organs and potentially **shorter duration of action** if elimination is extraction-limited. - This mechanism does not directly impact the **metabolic rate** unless clearance is significantly altered through increased availability for metabolism. *Chronic administration of rifampicin* - **Rifampicin** is a potent **inducer of CYP3A4** and other CYP enzymes. - Its administration would lead to **increased metabolism** of the co-administered drug, thereby **reducing its duration of action**.

Practice by Chapter

Absorption and Bioavailability

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Drug Distribution and Protein Binding

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Biotransformation and Metabolism Pathways

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Renal and Non-renal Excretion

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Compartment Models

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Dose-Response Relationships

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Drug Efficacy and Potency

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Drug Tolerance and Tachyphylaxis

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Population Pharmacokinetics

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Pharmacokinetic Variability

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