Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics Indian Medical PG Practice Questions and MCQs

Question 431: What is the mechanism of action of aprepitant?

A. RANK ligand inhibitor
B. NMDA antagonist
C. NK 1 receptor antagonist (Correct Answer)
D. 5-HT3 antagonist

Explanation: ***NK 1 receptor antagonist*** - Aprepitant selectively blocks the **neurokinin-1 (NK1) receptor**, which is activated by **substance P**. - This mechanism is effective in preventing both acute and delayed **chemotherapy-induced nausea and vomiting (CINV)** by inhibiting the CNS vomiting reflex. *RANK ligand inhibitor* - **RANK ligand inhibitors** such as **denosumab** target bone resorption in conditions like osteoporosis and bone metastases. - This mechanism is not related to the antiemetic properties of aprepitant. *NMDA antagonist* - **NMDA receptor antagonists** like **ketamine** are primarily used for their anesthetic or analgesic effects. - They work by blocking the N-methyl-D-aspartate receptor, which is distinct from the NK1 receptor. *5-HT3 antagonist* - **5-HT3 antagonists** like **ondansetron** block serotonin receptors, primarily in the gastrointestinal tract and chemoreceptor trigger zone. - While also antiemetics, their mechanism is different from aprepitant, and they are often used in combination for CINV.

Question 432: What is the role of oxidation in biotransformation?

A. Functionalization reaction (Correct Answer)
B. Conjugation reaction
C. Synthetic reaction
D. Felson reaction

Explanation: ***Functionalization reaction*** - Oxidation is a **Phase I biotransformation reaction**, which primarily involves introducing or exposing a functional group (like hydroxyl, carboxyl, amine) on the parent compound. - This makes the molecule more **polar** and often more reactive, preparing it for subsequent Phase II (conjugation) reactions. *Conjugation reaction* - Conjugation reactions are **Phase II biotransformation reactions**, which involve the covalent attachment of a large, highly polar, endogenous molecule (e.g., glucuronic acid, sulfate, glutathione) to the functional group created during Phase I. - Their main role is to further increase **water solubility** and facilitate **excretion**, rather than introducing functional groups. *Synthetic reaction* - The term "synthetic reaction" is a general chemical term and not a specific, commonly used classification for biotransformation phases in pharmacology. - While biotransformation involves chemical synthesis within the body, this term does not accurately describe the specific role of oxidation in drug metabolism. *Felson reaction* - The "Felson reaction" is **not a recognized term** in the context of biotransformation or drug metabolism. - This option is a distracter and does not correspond to any known biochemical process.

Question 433: What is the primary reason for the short half-life of adenosine in the bloodstream?

A. Uptake in subcutaneous tissue of adenosine
B. Rapid uptake by red blood cells and endothelial cells (Correct Answer)
C. Renal excretion of adenosine
D. Spontaneous hydrolysis of adenosine

Explanation: ***Rapid uptake by red blood cells and endothelial cells*** - **Adenosine** is rapidly transported into cells, particularly **red blood cells** and **vascular endothelial cells**, via specific **nucleoside transporters**. - Once inside these cells, adenosine is quickly metabolized by **adenosine deaminase** into inosine or phosphorylated by **adenosine kinase** into AMP, limiting its systemic half-life to a few seconds. *Uptake in subcutaneous tissue of adenosine* - While some uptake might occur in various tissues, **subcutaneous tissue** is not the primary site responsible for the rapid clearance and extremely short half-life of adenosine in the bloodstream. - The rapid action and metabolism of adenosine primarily occur in the **vascular compartment** and circulating blood cells. *Renal excretion of adenosine* - **Renal excretion** plays a minor role in the elimination of intact adenosine from the bloodstream due to its rapid cellular uptake and metabolism. - The majority of adenosine is metabolized intracellularly before it can be filtered by the kidneys. *Spontaneous hydrolysis of adenosine* - **Spontaneous hydrolysis** is not a significant mechanism contributing to the rapid breakdown and short half-life of adenosine in the human body. - Enzymatic degradation by **adenosine deaminase** is the primary catabolic pathway.

Question 434: What is the mechanism of elimination for Atracurium?

A. Renal excretion
B. Hepatic elimination
C. Nonenzymatic degradation (Correct Answer)
D. None of the above

Explanation: ***Nonenzymatic degradation*** - Atracurium is primarily metabolized via **Hofmann elimination**, a **nonenzymatic chemical degradation** process that occurs at physiological pH and temperature, and **ester hydrolysis** by plasma esterases. - This mechanism is advantageous in patients with **renal or hepatic impairment** as its elimination is independent of these organ systems. *Renal excretion* - While some metabolites of atracurium are renally excreted, **renal excretion is not the primary mechanism** of its initial breakdown or elimination. - Drugs primarily eliminated by renal excretion would require dose adjustments in patients with **kidney disease**. *Hepatic elimination* - Atracurium does not depend on the liver for its primary metabolism, making it a suitable choice for patients with **hepatic dysfunction**. - Medications primarily undergoing hepatic elimination, often via **cytochrome P450 enzymes**, would be significantly affected by liver health. *None of the above* - This option is incorrect because nonenzymatic degradation (Hofmann elimination and ester hydrolysis) is a well-established and unique mechanism for atracurium's excretion. - The drug's mechanism of action and metabolism are clearly defined in pharmacology.

Question 435: Depot preparations are administered by ?

A. Subcutaneous route
B. Intravenous route
C. Intramuscular route
D. Both subcutaneous and intramuscular route (Correct Answer)

Explanation: ***Both subcutaneous and intramuscular route*** - **Depot preparations** are designed for **sustained release** of medication over an extended period - This is achieved by forming a 'depot' in the tissue, often facilitated by a viscous vehicle or sparingly soluble form of the drug - Both **subcutaneous** and **intramuscular** tissues can sustain depot formulations effectively - **SC depot examples:** Insulin glargine, contraceptive implants (Nexplanon), leuprolide acetate - **IM depot examples:** Haloperidol decanoate, medroxyprogesterone acetate (Depo-Provera), paliperidone palmitate, long-acting risperidone *Subcutaneous route* - While some **depot preparations** are administered **subcutaneously**, it is not the *only* route for all depot formulations - The **subcutaneous tissue** offers relatively low blood flow, suitable for slow absorption - Alone, this option is incomplete as many depot preparations require IM administration *Intramuscular route* - Many **depot preparations** are given **intramuscularly** due to the muscle tissue's vascularity and tissue volume - The **muscle tissue** provides an excellent site for drug reservoir formation - Alone, this option is incomplete as some depot preparations are given subcutaneously *Intravenous route* - **Intravenous administration** is used for immediate and rapid drug delivery directly into the bloodstream - This route is **unsuitable for depot preparations** which require sustained release over time - No 'depot' can be formed with IV route as the drug is immediately diluted and distributed throughout the body

Question 436: Most common enzyme for drug metabolism and detoxification reactions?

A. CYP3A4 (Correct Answer)
B. CYP1A2
C. CYP2A6
D. CYP2B6

Explanation: ***CYP3A4*** - **CYP3A4** is the most abundant and versatile **cytochrome P450 enzyme** in the human liver and intestine, responsible for metabolizing approximately **50% of all clinically used drugs**. - Its broad substrate specificity and high expression levels make it a critical player in **drug detoxification** and metabolism. *CYP1A2* - While important, **CYP1A2** is primarily involved in the metabolism of only about **10% of therapeutic drugs** and specific endogenous compounds. - It plays a significant role in the metabolism of **caffeine** and some **polycyclic aromatic hydrocarbons**, but its overall contribution to drug metabolism is less than that of CYP3A4. *CYP2A6* - **CYP2A6** is a minor **cytochrome P450 enzyme** that metabolizes a limited number of drugs and xenobiotics. - Its primary role is in the metabolism of **nicotine** and certain **tobacco-specific nitrosamines**, making it less globally significant for general drug detoxification compared to CYP3A4. *CYP2B6* - **CYP2B6** metabolizes a relatively small fraction of drugs, around **3% to 5%** of those currently in clinical use. - While it is important for the metabolism of some **antiretrovirals** and **antidepressants**, its contribution to overall drug metabolism and detoxification is considerably less extensive than that of CYP3A4.

Question 437: What is the anti-inflammatory dose of aspirin?

A. 500 mg/d
B. 1 - 2 g/d
C. 3 - 6 g/d (Correct Answer)
D. 6 - 12 g/d

Explanation: ***3 - 6 g/d*** - The anti-inflammatory effect of aspirin is typically achieved at higher doses, ranging from **3 to 6 grams per day**, often divided into multiple doses. - This dosage range is necessary to significantly inhibit prostaglandin synthesis, which mediates inflammation and pain. *500 mg/d* - This dose is generally considered to be in the **analgesic and antipyretic range**, effectively reducing pain and fever. - It is often insufficient to achieve a full anti-inflammatory effect, as it does not fully saturate the prostaglandin synthesis pathway. *1 - 2 g/d* - While higher than common analgesic doses, **1-2 g/d of aspirin** may have *some* anti-inflammatory effects but is typically considered a moderate dose. - It might not be sufficient for treating significant inflammatory conditions, falling short of the fully recognized anti-inflammatory dose. *6 - 12 g/d* - Doses within this range are generally considered to be in the **toxic or potentially toxic range** for aspirin. - This high dosage can lead to severe side effects such as **salicylism**, including tinnitus, nausea, vomiting, metabolic acidosis, and even coma, and is not a standard therapeutic anti-inflammatory dose.

Question 438: A young child weighing 20 kg is administered a drug at a dose of 100 mg/kg body weight. Given that the plasma concentration of the drug is 2 mg/dL and the clearance is 13,860 mL/hr, calculate the time required to reach steady state plasma concentration.

A. 20 hours
B. 30 hours (Correct Answer)
C. 10 hours
D. 40 hours

Explanation: ***30 hours*** - To reach **steady-state concentration**, approximately **5 half-lives (t½)** are required, at which point the drug reaches ~97% of steady state. - First, calculate the **volume of distribution (Vd)**: Total dose = 100 mg/kg × 20 kg = 2000 mg. Plasma concentration = 2 mg/dL = 20 mg/L. Therefore, **Vd = Dose/Cp = 2000 mg / 20 mg/L = 100 L**. - Next, calculate the **half-life (t½)** using the formula: **t½ = 0.693 × Vd / Cl**. Given **Clearance (Cl)** = 13,860 mL/hr = 13.86 L/hr, we get **t½ = (0.693 × 100 L) / 13.86 L/hr = 5 hours**. - Time to reach steady state = **5 × t½ = 5 × 5 hours = 25 hours**. While the calculated value is 25 hours, **30 hours (6 half-lives)** is the closest option and ensures steady state is definitively reached, as clinical practice often considers 4-6 half-lives as the range for steady state. *20 hours* - This duration represents only **4 half-lives**, which achieves approximately **94% of steady-state** concentration. - While close, this is slightly **premature** compared to the standard 5 half-lives required for steady state, and is further from the calculated 25 hours than the correct answer. *10 hours* - This duration represents only **2 half-lives**, which is **insufficient to reach steady-state** plasma concentration. - At 10 hours, the drug concentration would only be around **75% of steady-state**, not fully accumulated. *40 hours* - This duration represents **8 half-lives**, which is **excessively long** for achieving steady-state. - By this point, the drug would have been at **steady-state for several half-lives**, making this an unnecessarily prolonged estimate.

Question 439: Glucuronidation takes place in?

A. Liver (Correct Answer)
B. RBC
C. Pancreas
D. Thyroid

Explanation: ***Liver*** - The **liver** is the primary organ responsible for **glucuronidation**, a crucial phase II detoxification pathway [1]. - This process conjugates **glucuronic acid** with various endogenous and exogenous substances, making them more water-soluble for excretion [1]. *RBC* - **Red blood cells (RBCs)** primarily function in **oxygen transport** and lack the extensive metabolic machinery for glucuronidation. - While they possess some enzymatic activities, detoxification pathways like glucuronidation are not a significant function. *Pancreas* - The **pancreas** is mainly involved in producing **digestive enzymes** and **hormones** (insulin, glucagon) for blood glucose regulation. - It does not play a direct role in drug or toxin metabolism through processes like glucuronidation. *Thyroid* - The **thyroid gland** is responsible for producing **thyroid hormones** that regulate metabolism. - Its metabolic activity is distinct from the detoxification functions performed by the liver, and it does not perform glucuronidation.

Question 440: Which of the following is the longest acting oral anticoagulant?

A. Bishydroxycoumarin (Correct Answer)
B. Warfarin
C. Acenocoumarol
D. Phenindione

Explanation: ***Bishydroxycoumarin*** - **Bishydroxycoumarin** (dicoumarol) has a very long duration of action, with effects lasting up to 2-10 days after a single dose due to its slow metabolism and excretion. - Its prolonged action and unpredictable anticoagulant response have led to it being largely replaced by other oral anticoagulants like **warfarin** in clinical practice. *Warfarin* - **Warfarin** has a half-life of approximately 36-42 hours, leading to an anticoagulant effect that lasts for about 2-5 days after discontinuation. - It is a widely used oral anticoagulant, but its duration of action is significantly shorter than that of bishydroxycoumarin. *Acenocoumarol* - **Acenocoumarol** has a relatively short half-life of about 8-11 hours, and its anticoagulant effects typically dissipate within 1-2 days after discontinuation. - It is often preferred in situations where a rapid reversal of anticoagulation might be necessary, due to its shorter duration of action compared to warfarin. *Phenindione* - **Phenindione** is an indandione derivative, a class of oral anticoagulants, with a duration of action of approximately 2-4 days. - It is rarely used now due to a higher incidence of adverse effects, including hypersensitivity reactions and hematological toxicities, compared to coumarin derivatives.

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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