Pharmacokinetics and Pharmacodynamics Practice Questions

Q: Phase 1 biotransformation includes

Reduction. ***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.

Q: In the context of drug metabolism, which enzyme system is primarily responsible for the Phase I metabolism of xenobiotics?

Cytochrome P-450. **Cytochrome P-450** - The **Cytochrome P-450 (CYP450) enzyme system** is a superfamily of enzymes primarily located in the liver that are crucial for Phase I **biotransformation** of xenobiotics. - Phase I reactions, which include **oxidation**, reduction, and hydrolysis, typically introduce or expose a polar functional group on the drug molecule, making it more hydrophilic and often less active. *Glutathione S-transferase* - **Glutathione S-transferases (GSTs)** are involved in **Phase II metabolism**, which involves conjugation reactions to highly polar molecules like glutathione, making compounds more water-soluble for excretion. - They play a key role in the **detoxification** of electrophilic compounds and products of oxidative stress, but not Phase I oxidation. *NADPH cytochrome P-450-reductase* - **NADPH cytochrome P-450 reductase** is an essential enzyme that **supplies electrons** to the cytochrome P-450 enzymes. - While critical for the function of CYP450, it is a **cofactor** or electron donor, not the primary enzyme system responsible for the metabolic reaction itself. *Glucuronyl transferase* - **Glucuronyl transferases (UGTs)** are involved in **Phase II metabolism**, specifically **glucuronidation**, which conjugates a drug or metabolite with glucuronic acid. - This process significantly increases the **water solubility** of the compound, facilitating its elimination from the body.

Q: The neurotransmitters noradrenaline, adrenaline, and dopamine act through which of the following receptors?

Seven-pass transmembrane receptor. ***Seven-pass transmembrane receptor*** - Noradrenaline, adrenaline, and dopamine are **catecholamines** that primarily activate **G-protein coupled receptors (GPCRs)**, which are characterized by having seven transmembrane domains. - These receptors mediate a wide range of physiological responses by initiating **intracellular signaling cascades** upon ligand binding. *Single-pass transmembrane receptor* - These receptors, such as **receptor tyrosine kinases**, cross the membrane only once and typically dimerize upon ligand binding to activate their intracellular kinase domains. - While important for growth factors and hormones, they are **not the primary receptors** for catecholamines. *Four-pass transmembrane receptor* - This type of receptor structure is less common for neurotransmitters and is not characteristic of the established receptors for **noradrenaline, adrenaline, and dopamine**. - Examples include some **gap junction proteins** and a subset of ion channels, but not the **adrenergic** or **dopaminergic receptors**. *Ligand-gated ion channel* - These channels directly open in response to ligand binding, allowing ions to pass and rapidly changing the **membrane potential**. - While some neurotransmitters like **acetylcholine (nicotinic receptors)** and **GABA** act this way, catecholamines primarily exert their effects through **GPCRs**, which modulate ion channel activity indirectly.

Q: What percentage of a drug is eliminated after four half-lives in first-order kinetics?

93.75%. ***93.75%*** - In **first-order kinetics**, a constant **fraction** (not amount) of the drug is eliminated per unit time - After 1 half-life: 50% eliminated (50% remains) - After 2 half-lives: 75% eliminated (25% remains) - After 3 half-lives: 87.5% eliminated (12.5% remains) - After 4 half-lives: **93.75% eliminated** (6.25% remains) *84.00%* - This represents approximately 16% remaining drug - Would occur between the 2nd and 3rd half-lives, not at exactly 4 half-lives - Does not correspond to the mathematical progression of first-order kinetics *80.00%* - This represents 20% remaining drug - Does not correspond to any whole number of half-lives - Falls between 2 half-lives (75% eliminated) and 3 half-lives (87.5% eliminated) *5.00%* - This suggests only 5% elimination with 95% drug remaining - Would occur very early in the elimination process, less than 1 half-life - Fundamentally incorrect for 4 half-lives where 93.75% should be eliminated

Q: What is the definition of bioavailability?

Ratio of Area of oral to Area of IV. ***Ratio of Area of oral to Area of IV*** - **Bioavailability (F)** is defined as the **fraction** of an administered dose of unchanged drug that reaches the systemic circulation - It is calculated as: **F = AUC_oral / AUC_IV** (when doses are equal) - This gives a value between **0 and 1** (e.g., F = 0.75 means 75% bioavailable) - This is the **standard pharmacological definition** as per Goodman & Gilman and other authoritative texts *Ratio of Area of oral to Area of IV multiplied by 100* - This expresses bioavailability as a **percentage** rather than a fraction - While bioavailability can be *communicated* as a percentage (e.g., "75% bioavailable"), the formal definition is as a **fraction** - Multiplying by 100 is a conversion for expression, not part of the core definition *Ratio of Area of IV to Area of oral* - This is the **inverse ratio** of bioavailability - It would give values **greater than 1** for drugs with incomplete absorption - Does not represent the fraction of oral dose reaching systemic circulation *Ratio of Area of IV to Area of oral multiplied by 100* - This combines two errors: **inverse ratio** and percentage conversion - Would yield values >100% for drugs with bioavailability <1 - Completely incorrect definition of bioavailability

Q: Which of the following factors does not affect the action of lignocaine?

Action of cholinesterase at the site of injection. ***Action of cholinesterase at the site of injection*** - **Lidocaine** is an **amide-type local anesthetic** and is primarily metabolized by the liver, not by cholinesterases. - Cholinesterases are responsible for metabolizing **ester-type local anesthetics** (e.g., procaine) and acetylcholine. *PH at the site of injection* - The **pH** at the injection site significantly affects lignocaine's action, as its **unionized form** (which is favored in alkaline pH) is essential for penetrating nerve membranes. - In acidic environments (e.g., infected tissue), less unionized lignocaine is available, leading to reduced efficacy. *Blood flow at the site of injection* - **Increased blood flow** at the site of injection can accelerate the systemic absorption of lignocaine, thereby reducing its duration of action at the nerve. - **Decreased blood flow** can prolong its local action, but also increase the risk of systemic toxicity if absorption is very slow. *Vasoconstrictor in the L.A solution* - The addition of a **vasoconstrictor** (e.g., epinephrine) slows the systemic absorption of lignocaine from the injection site. - This **prolongs the duration of action** and reduces the risk of systemic toxicity by keeping the anesthetic concentrated at the target nerves.

Q: Which of the following drugs is least likely to cross the blood-placental barrier?

Glycopyrrolate. ***Glycopyrrolate*** - Glycopyrrolate is a **quaternary ammonium compound**, meaning it is highly ionized and has a low lipid solubility. - Its **polar nature** and **larger molecular weight** restrict its ability to readily cross lipid membranes, including the blood-placental barrier. *Atropine* - **Atropine** is a tertiary amine, making it a **lipid-soluble** compound. - Its lipid solubility allows it to **easily cross the blood-placental barrier** and affect the fetus. *Physostigmine* - Physostigmine is also a **tertiary amine** and is relatively **lipid-soluble**. - This property enables it to **readily cross lipid barriers** such as the blood-placental barrier and the blood-brain barrier. *Hyoscine hydrobromide* - **Hyoscine hydrobromide** (scopolamine) is a **tertiary amine** and highly **lipid-soluble**. - Its high lipid solubility allows for its **easy passage across the blood-placental barrier** and into the fetal circulation.

Question 1

Phase 1 biotransformation includes

Accepted Answer

Reduction

Answer

Methylation

Answer

Acetylation

Answer

Sulfate conjugation

Question 2

In the context of drug metabolism, which enzyme system is primarily responsible for the Phase I metabolism of xenobiotics?

Accepted Answer

Cytochrome P-450

Answer

Glutathione S-transferase

Answer

NADPH cytochrome P-450-reductase

Answer

Glucuronyl transferase

Question 3

In isolated intestinal smooth muscle preparations, drug X binds to cholinergic receptors and causes relaxation. In the absence of drug X, acetylcholine binds to the same receptors and causes contraction. At low concentrations of drug X, the efficacy (Emax) of acetylcholine is decreased, but its potency (EC50) remains unchanged. At high concentrations of drug X, acetylcholine has no effect. Which of the following statements about drug X is true?

Accepted Answer

Non-competitive antagonist

Answer

A chemical antagonist

Answer

A physiologic antagonist

Answer

Competitive antagonist

Question 4

The neurotransmitters noradrenaline, adrenaline, and dopamine act through which of the following receptors?

Accepted Answer

Seven-pass transmembrane receptor

Answer

Single-pass transmembrane receptor

Answer

Four-pass transmembrane receptor

Answer

Ligand-gated ion channel

Question 5

What percentage of a drug is eliminated after four half-lives in first-order kinetics?

Accepted Answer

93.75%

Answer

84.00%

Answer

80.00%

Answer

5.00%

Question 6

What is the definition of bioavailability?

Accepted Answer

Ratio of Area of oral to Area of IV

Answer

Ratio of Area of IV to Area of oral

Answer

Ratio of Area of IV to Area of oral multiplied by 100

Answer

Ratio of Area of oral to Area of IV multiplied by 100

Question 7

Which of the following factors does not affect the action of lignocaine?

Accepted Answer

Action of cholinesterase at the site of injection

Answer

PH at the site of injection

Answer

Vasoconstrictor in the L.A solution

Answer

Blood flow at the site of injection

Question 8

What is the mechanism by which standard doses of lidocaine can lead to cardiac or central nervous system toxicity in patients with circulatory failure?

Accepted Answer

Lidocaine concentration can increase in well-perfused tissues such as the brain and heart during circulatory failure.

Answer

Histamine receptors in the brain and heart are activated during circulatory failure.

Answer

There is a sudden release of catecholamines in the brain and heart during circulatory failure.

Answer

Lidocaine is metabolized in the liver, but this does not lead to a toxic metabolite.

Question 9

Which of the following drugs is least likely to cross the blood-placental barrier?

Accepted Answer

Glycopyrrolate

Answer

Atropine

Answer

Physostigmine

Answer

Hyoscine hydrobromide

Question 10

All of the following statements regarding the bioavailability of a drug are true except which one?

Accepted Answer

Low oral availability always and necessarily means poor absorption

Answer

It is a fraction of administered drug that reaches the systemic circulation in unchanged form

Answer

Bioavailability can be determined from plasma concentration or urinary excretion data.

Answer

Bioavailability of an orally administered drug can be calculated by comparing the Area Under Curve after oral and intravenous administration

Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics — MCQs

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