Pharmacokinetics and Pharmacodynamics Practice Questions

Q: Which of the following is FALSE regarding competitive antagonism?

The efficacy of the agonist decreases. In competitive antagonism, the antagonist competes with the agonist for the same binding site (active site) on the receptor. ### **Why Option B is FALSE (The Correct Answer)** In competitive antagonism, the **efficacy ($E_{max}$)** of the agonist remains **unchanged**. Because the binding is reversible, increasing the concentration of the agonist can "surmount" or displace the antagonist from the receptors. Once the agonist concentration is high enough, it can still achieve the maximum possible response. Therefore, the maximal effect ($E_{max}$) does not decrease. ### **Analysis of Other Options** * **Option A (True):** Because the antagonist competes for the site, a higher concentration of the agonist is required to produce the same effect. This increases the $EC_{50}$ (decreases potency), causing a **parallel rightward shift** of the log dose-response curve. * **Option C (True):** Competitive antagonism is **reversible**. The bond is usually non-covalent (ionic or Van der Waals), allowing the antagonist to dissociate when agonist concentration increases. * **Option D (True):** By definition, competitive antagonists bind to the **orthosteric site** (the same active site where the endogenous ligand or agonist binds). ### **NEET-PG High-Yield Pearls** * **Competitive Antagonism:** $V_{max}$ (or $E_{max}$) is constant; $K_m$ (or $EC_{50}$) increases. (Example: Atropine vs. Acetylcholine). * **Non-Competitive Antagonism:** $V_{max}$ (or $E_{max}$) decreases; $K_m$ (or $EC_{50}$) remains constant. This is usually irreversible or allosteric. (Example: Phenoxybenzamine at alpha receptors). * **Key Visual:** Look for a "parallel shift to the right" on a graph to identify competitive inhibition.

Q: Which of the following drugs primarily binds to albumin?

Warfarin. ### Explanation **Concept of Plasma Protein Binding** In pharmacokinetics, drugs circulate in the blood either bound to plasma proteins or in a free (active) form. The two most important proteins are **Albumin** (which primarily binds **acidic drugs**) and **$\alpha_1$-acid glycoprotein** (which primarily binds **basic drugs**). **Why Warfarin is Correct:** **Warfarin** is a highly acidic drug with a very high affinity for **Albumin** (approx. 99% bound). Because it is so extensively bound, any drug that displaces warfarin from albumin (like sulfonamides or NSAIDs) can significantly increase its free concentration, leading to a high risk of bleeding. This makes it a classic example of clinically significant protein-binding displacement interactions. **Analysis of Incorrect Options:** * **Steroids (Option A):** Endogenous steroids and many synthetic analogs primarily bind to specific globulins, such as **Corticosteroid Binding Globulin (CBG)** or Transcortin, rather than albumin. * **Propranolol (Option B):** This is a **basic drug**. Basic drugs typically bind to **$\alpha_1$-acid glycoprotein (AAG)**. Other examples include lidocaine and quinidine. * **Digoxin (Option C):** Digoxin has minimal plasma protein binding (approx. 25%). It primarily binds to tissue proteins (specifically Na+/K+ ATPase in cardiac and skeletal muscle), which accounts for its large volume of distribution ($V_d$). **High-Yield NEET-PG Pearls:** * **Acidic Drugs (Bind to Albumin):** Warfarin, NSAIDs, Phenytoin, Penicillins, Sulfonamides. * **Basic Drugs (Bind to $\alpha_1$-acid glycoprotein):** Propranolol, Lidocaine, Bupivacaine, Tricyclic Antidepressants (TCAs). * **Clinical Significance:** Only the **free fraction** of a drug is pharmacologically active, metabolized, and excreted. * **Hypoalbuminemia:** In conditions like nephrotic syndrome or cirrhosis, the dose of highly albumin-bound drugs (like Phenytoin) must be reduced to avoid toxicity.

Q: Which of the following drugs has a covalent interaction with its target?

Aspirin. **Explanation:** The correct answer is **Aspirin**. This question tests the concept of **drug-receptor bond types**, specifically irreversible covalent bonding. **1. Why Aspirin is correct:** Aspirin (Acetylsalicylic acid) acts by **irreversibly acetylating** a serine residue in the active site of the **Cyclooxygenase (COX-1 and COX-2)** enzymes. Because a covalent bond is formed, the enzyme is permanently inactivated. In platelets, which lack a nucleus to synthesize new proteins, this effect lasts for the entire lifespan of the platelet (7–10 days), explaining its potent anti-thrombotic effect. **2. Analysis of Incorrect Options:** * **B. Penicillin:** While Penicillin does bind to Penicillin-Binding Proteins (PBPs), it is often categorized under "suicide inhibition" in biochemistry. However, in standard pharmacological classification for NEET-PG, Aspirin is the classic prototype for covalent modification. (Note: Some texts consider Penicillin covalent, but Aspirin remains the most definitive answer in this MCQ context). * **C. Nitric Oxide:** This is a gas that acts as a signaling molecule. It binds **reversibly** to the heme group of soluble guanylyl cyclase to increase cGMP. * **D. Bosentan:** This is a competitive (reversible) antagonist at endothelin (ET-A and ET-B) receptors used in pulmonary hypertension. **High-Yield Clinical Pearls for NEET-PG:** * **Other Covalent/Irreversible Inhibitors:** Omeprazole (H+/K+ ATPase), Organophosphates (AChE), Phenoxybenzamine (Alpha receptors), and Clopidogrel (P2Y12 receptor). * **Aspirin Kinetics:** At low doses, it follows first-order kinetics; at toxic/high doses, it shifts to **zero-order kinetics** (saturation kinetics). * **Key Distinction:** Most drugs form weak, reversible bonds (Ionic, Hydrogen, Van der Waals). Covalent bonds are the strongest and require the synthesis of new receptors/enzymes to overcome the drug effect.

Q: What percentage of a substance is eliminated after 3 half-lives in first-order kinetics?

87.50%. ### Explanation In **first-order kinetics**, a constant **fraction** (percentage) of a drug is eliminated per unit of time. This means the half-life ($t_{1/2}$) remains constant regardless of the plasma concentration. **Step-by-Step Calculation:** To determine the percentage eliminated, we first calculate the amount **remaining** after each half-life: * **After 1st half-life:** 50% remains (50% eliminated). * **After 2nd half-life:** 50% of 50% = 25% remains (75% eliminated). * **After 3rd half-life:** 50% of 25% = **12.5% remains**. To find the amount eliminated: $100\% - 12.5\% = \mathbf{87.5\%}$. --- ### Analysis of Options: * **Option A (12.50%):** This represents the amount of substance **remaining** in the body after 3 half-lives, not the amount eliminated. * **Option B (75%):** This is the percentage eliminated after **2 half-lives**. * **Option D (94%):** This is the approximate percentage eliminated after **4 half-lives** ($100 - 6.25 = 93.75\%$). --- ### High-Yield NEET-PG Pearls: 1. **Steady State:** It takes approximately **4 to 5 half-lives** to reach steady-state concentration ($C_{ss}$) and the same amount of time to completely eliminate a drug from the body. 2. **First-order vs. Zero-order:** Most drugs follow first-order kinetics. In **zero-order kinetics** (e.g., Ethanol, Phenytoin, Aspirin at high doses), a constant **amount** is eliminated, and the half-life is not constant. 3. **Formula:** Percentage eliminated = $100 - (100/2^n)$, where $n$ is the number of half-lives. For $n=3$: $100 - (100/8) = 87.5\%$.

Q: All of the following drugs undergo hepatic metabolism before excretion EXCEPT:

Penicillin G. ### Explanation The correct answer is **Penicillin G**. **1. Why Penicillin G is the correct answer:** Most drugs are lipophilic and require hepatic metabolism (Phase I and Phase II reactions) to become water-soluble for excretion. However, **Penicillin G** is a highly polar, organic acid that is already water-soluble. It is excreted **unchanged** primarily via the kidneys. Specifically, about 90% is excreted through **active tubular secretion** via the Organic Anion Transporter (OAT), and 10% via glomerular filtration. This is why Probenecid (which inhibits OAT) is used clinically to prolong the half-life of penicillin. **2. Why the other options are incorrect:** * **Phenytoin:** This is a classic example of a drug that undergoes extensive hepatic metabolism via CYP2C9. It follows **zero-order kinetics** (saturable metabolism) at high therapeutic doses, a high-yield fact for NEET-PG. * **Diazepam:** A long-acting benzodiazepine that undergoes hepatic oxidation (Phase I) to form active metabolites like desmethyldiazepam, which further increases its duration of action. * **Cimetidine:** While a portion is excreted unchanged, it undergoes significant hepatic metabolism. It is clinically significant as a potent **inhibitor of Cytochrome P450 enzymes**, leading to numerous drug-drug interactions. **3. NEET-PG Clinical Pearls:** * **Renal Excretion Rule:** Drugs excreted primarily unchanged in urine (like Penicillin, Aminoglycosides, Digoxin, and Lithium) require strict **dose adjustment in renal failure**. * **Probenecid Interaction:** Probenecid competes with Penicillin for the tubular secretory pathway, increasing Penicillin plasma levels. * **Zero-Order Kinetics Mnemonic:** Remember "**WATT P**" (Warfarin/Whiskey, Aspirin, Tolbutamide, Theophylline, Phenytoin) for drugs that saturate metabolic pathways.

Q: Which of the following statements about non-competitive antagonism is FALSE?

Efficacy remains the same.. In **non-competitive antagonism**, the antagonist binds to an allosteric site (different from the agonist’s binding site) [2] or binds irreversibly to the active site [1]. This prevents the agonist from producing a maximal effect, regardless of how much agonist concentration is increased [1]. In non-competitive antagonism, the **efficacy (Emax) decreases** [1]. Because the antagonist effectively "removes" functional receptors from the system or prevents the agonist from activating them, the maximum possible response of the drug is reduced [1]. Therefore, the statement "Efficacy remains the same" is false. As the maximal response (Emax) decreases, the height of the log dose-response curve (LDRC) drops, leading to a characteristic "flattening" of the curve [1]. While not all non-competitive antagonists are irreversible, most irreversible antagonists (like Phenoxybenzamine) function non-competitively because they cannot be displaced by increasing agonist concentrations [1]. In pure non-competitive antagonism, the **EC50 (potency)** typically remains unchanged because the remaining functional receptors still have the same affinity for the agonist [1].

Q: A steady state plasma concentration is achieved in how many hours if the half-life of a drug is 40 hours?

180 hours. ### Explanation **1. Understanding Steady State Concentration (Css)** Steady state is reached when the rate of drug administration equals the rate of drug elimination. In clinical pharmacokinetics, it is a standard rule that it takes approximately **4 to 5 half-lives ($t_{1/2}$)** to reach steady state [1]. * After 1 $t_{1/2}$: 50% of Css is reached. * After 2 $t_{1/2}$: 75% of Css is reached. * After 3 $t_{1/2}$: 87.5% of Css is reached. * After 4 $t_{1/2}$: 93.75% of Css is reached [2]. * **After 4.5 to 5 $t_{1/2}$:** >95% of Css is reached (clinically considered steady state) [1]. **Calculation:** Given $t_{1/2} = 40$ hours. Steady state $\approx 4.5 \times 40 = \mathbf{180 \text{ hours}}$. **2. Analysis of Incorrect Options** * **Option A (140 hours):** This represents only 3.5 half-lives. At this point, the drug has not yet reached the therapeutic plateau. * **Option B (160 hours):** This represents exactly 4 half-lives. While close, 4.5 to 5 half-lives is the more accurate physiological benchmark for complete steady state in competitive exams. * **Option D (200 hours):** This represents 5 half-lives. While 5 half-lives is also a valid point for steady state, in many standardized questions (including this one), the calculation based on 4.5 half-lives (180 hours) is the preferred specific answer choice. **3. NEET-PG High-Yield Pearls** * **Independence from Dose:** The time to reach steady state depends **only** on the half-life, not on the dose or the frequency of administration. * **Loading Dose:** To achieve therapeutic levels rapidly without waiting for 4-5 half-lives, a **Loading Dose** is administered. * **Elimination:** Similarly, it takes 4-5 half-lives for a drug to be completely eliminated from the body after stopping the infusion [1]. * **First-Order Kinetics:** This rule applies only to drugs following first-order kinetics (where a constant fraction of drug is eliminated per unit time) [1].

Question 1

Which of the following is FALSE regarding competitive antagonism?

Accepted Answer

The efficacy of the agonist decreases

Answer

The dose-response curve shifts to the right

Answer

The antagonism is reversible

Answer

The antagonist binds to the active site of the enzyme

Question 2

Which of the following drugs primarily binds to albumin?

Accepted Answer

Warfarin

Answer

Steroids

Answer

Propranolol

Answer

Digoxin

Question 3

A new drug is found to be highly lipid soluble. It has a metabolism rate of 10% per hour. On intravenous injection, it produces general anaesthesia that lasts only for 15 minutes. What is the likely reason for the short duration of anaesthesia?

Accepted Answer

Redistribution of the drug

Answer

Metabolism of drug in the liver

Answer

High plasma protein binding of the drug

Answer

Excretion of drug by the kidney

Question 4

High hepatic first-pass metabolism is seen in all EXCEPT?

Accepted Answer

Insulin

Answer

Propranolol

Answer

Lignocaine

Answer

Nitroglycerin

Question 5

Phenytoin follows zero-order kinetics at high doses and first-order kinetics at low doses. If 400 mg of phenytoin is present in the body and its rate of elimination at this dose is 200 mg/hr, how much will remain in plasma after 2 hours?

Accepted Answer

Cannot be predicted from this information

Answer

200 mg

Answer

100 mg

Answer

0 mg

Question 6

Which of the following drugs has a covalent interaction with its target?

Accepted Answer

Aspirin

Answer

Penicillin

Answer

Nitric oxide

Answer

Bosentan

Question 7

What percentage of a substance is eliminated after 3 half-lives in first-order kinetics?

Accepted Answer

87.50%

Answer

12.50%

Answer

75%

Answer

94%

Question 8

All of the following drugs undergo hepatic metabolism before excretion EXCEPT:

Accepted Answer

Penicillin G

Answer

Phenytoin

Answer

Diazepam

Answer

Cimetidine

Question 9

Which of the following statements about non-competitive antagonism is FALSE?

Accepted Answer

Efficacy remains the same.

Answer

The dose-response curve flattens.

Answer

The antagonism is irreversible.

Answer

Potency remains the same.

Question 10

A steady state plasma concentration is achieved in how many hours if the half-life of a drug is 40 hours?

Accepted Answer

180 hours

Answer

140 hours

Answer

160 hours

Answer

200 hours

Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics — MCQs

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