Drug Distribution and Protein Binding — MCQs
Which of the following drugs is least likely to cross the blood-placental barrier?
What is the mechanism of metabolism for alcohol, aspirin, and phenytoin at high doses?
Choose the correct options regarding the route of administration and bioavailability. A- Intravenous =1 B- 0.75< Oral <1 C-0.75 <IM ≤ 1 D- 0.75<SC ≤ 1 IM - Intramuscular SC- Subcutaneous
A 70 kg man was given a drug with a dose of 100 mg/kg body weight, twice daily. The half-life (t1/2) is 10 hours, the plasma concentration is 1.9 mg/mL, and the clearance is unknown. What is the clearance of this drug?
Major mechanism of transport of drugs across biological membranes is:
Which of the following is the platinum-based chemotherapeutic agent used as first-line treatment for ovarian carcinoma?
Major determinant of loading dose of a drug is:
After IV drug administration, elimination of a drug depends on:
In a patient with nephrotic syndrome and hypoalbuminemia, the protein binding of which drug will NOT be affected?
Low apparent volume of distribution of drug indicates that:
Drug Distribution and Protein Binding Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following drugs is least likely to cross the blood-placental barrier?
- A. Atropine
- B. Physostigmine
- C. Hyoscine hydrobromide
- D. Glycopyrrolate (Correct Answer)
Explanation: ***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 2: What is the mechanism of metabolism for alcohol, aspirin, and phenytoin at high doses?
- A. First pass kinetics
- B. First order kinetics
- C. Zero order kinetics (Correct Answer)
- D. Second order kinetics
Explanation: ***Zero order kinetics*** - This mechanism occurs when the **metabolic enzymes become saturated at high drug concentrations**, leading to a constant amount (not a constant percentage) of drug being eliminated per unit time. - Alcohol, aspirin, and phenytoin are examples of drugs that exhibit **saturable metabolism**, transitioning from first-order to zero-order kinetics at higher doses. *First pass kinetics* - This describes the **metabolism of a drug by the liver or gut wall enzymes before it reaches systemic circulation** after oral administration. - While relevant to the oral bioavailability of these drugs, it does not describe the specific mechanism of elimination at high doses. *First order kinetics* - In this mechanism, a **constant fraction or percentage of the drug is eliminated per unit of time**, meaning the rate of elimination is directly proportional to the drug concentration. - Most drugs follow first-order kinetics at therapeutic doses because metabolizing enzymes are not saturated. *Second order kinetics* - This is a **less common pharmacokinetic model** where the rate of elimination is proportional to the square of the drug concentration or involves two reactants. - It does not typically describe the common elimination patterns of most drugs, including alcohol, aspirin, and phenytoin.
Question 3: Choose the correct options regarding the route of administration and bioavailability. A- Intravenous =1 B- 0.75< Oral <1 C-0.75 <IM ≤ 1 D- 0.75<SC ≤ 1 IM - Intramuscular SC- Subcutaneous
- A. A and D
- B. A and C
- C. A, C, D (Correct Answer)
- D. A, B, D
Explanation: ***A, C, D*** - Intravenous (IV) administration has **100% bioavailability** because the drug enters the systemic circulation directly, bypassing any absorption barriers. - Intramuscular (IM) and subcutaneous (SC) routes generally have **high bioavailability**, often between 75% and 100%, as drugs are absorbed directly into the bloodstream without first-pass metabolism. *A and D* - While options A and D are correct, this choice is incomplete as option C is also a correct statement regarding bioavailability. - IM administration typically results in high systemic bioavailability, similar to SC, making its exclusion here incorrect. *A and C* - While options A and C are correct, this choice is incomplete as option D is also a correct statement regarding bioavailability. - Subcutaneous administration also generally results in high bioavailability, as absorption tends to be complete. *A, B, D* - While options A and D are correct, option B is typically incorrect for oral bioavailability. - Oral bioavailability of many drugs is often less than 0.75 (75%) due to factors like **first-pass metabolism** and incomplete absorption in the gastrointestinal tract.
Question 4: A 70 kg man was given a drug with a dose of 100 mg/kg body weight, twice daily. The half-life (t1/2) is 10 hours, the plasma concentration is 1.9 mg/mL, and the clearance is unknown. What is the clearance of this drug?
- A. 20 liter/hr
- B. K is 0.0693
- C. 0.22 L/hr (Correct Answer)
- D. 0.02 L/hr
Explanation: ***0.22 L/hr*** - To calculate clearance at steady state, we use the formula: **Clearance (Cl) = Dose Rate / Css** (steady-state plasma concentration). - **Dose rate calculation**: 100 mg/kg × 70 kg × 2 doses/day = 14,000 mg/day = 583.33 mg/hr - **Converting plasma concentration**: 1.9 mg/mL = 1900 mg/L - **Clearance calculation**: Cl = 583.33 mg/hr ÷ 1900 mg/L = **0.307 L/hr** - **Note**: The calculated value (0.307 L/hr) does not exactly match any option. The marked answer (0.22 L/hr) is the closest approximation among the given choices. This discrepancy may arise from rounding in the original question parameters or implicit assumptions about bioavailability/volume of distribution. *0.02 L/hr* - This value is approximately 15 times lower than the calculated clearance. - Such low clearance would result in much higher plasma concentrations or require significantly lower dosing. *20 liter/hr* - This clearance is approximately 65 times higher than calculated, representing an unrealistically high value for this scenario. - Such high clearance would result in very low plasma concentrations unless extremely high doses were administered. *K is 0.0693* - This represents the **elimination rate constant (k)**, calculated as k = 0.693/t1/2 = 0.693/10 hr = 0.0693 hr⁻¹. - While mathematically correct for k, the question specifically asks for **clearance**, not the elimination rate constant. - Clearance is related to k by: Cl = k × Vd (volume of distribution).
Question 5: 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 6: Which of the following is the platinum-based chemotherapeutic agent used as first-line treatment for ovarian carcinoma?
- A. Cyclophosphamide
- B. Methotrexate
- C. Cisplatin (Correct Answer)
- D. Dacarbazine
Explanation: ***Cisplatin*** - **Cisplatin** is a platinum-based chemotherapy drug that forms **DNA cross-links**, inhibiting DNA synthesis and leading to the death of rapidly dividing cells, making it highly effective against **ovarian carcinoma**. - It is a cornerstone of chemotherapy regimens for ovarian cancer, often used in combination with other agents such as paclitaxel. *Methotrexate* - **Methotrexate** is an **antimetabolite** that inhibits dihydrofolate reductase, thereby interfering with DNA synthesis. - While it is used in various cancers like leukemia, lymphoma, and some solid tumors (e.g., breast cancer, gestational trophoblastic disease), it is **not a primary recommended drug for ovarian carcinoma**. *Cyclophosphamide* - **Cyclophosphamide** is an **alkylating agent** that causes DNA damage, leading to cell death. - It is used in many cancers, including lymphoma, breast cancer, and some leukemias, but it is **not a first-line or primary agent for ovarian carcinoma** in contemporary treatment guidelines. *Dacarbazine* - **Dacarbazine** is an **alkylating agent** primarily used in the treatment of **malignant melanoma** and Hodgkin lymphoma. - It is **not indicated for the treatment of ovarian carcinoma**.
Question 7: Major determinant of loading dose of a drug is:
- A. Half life
- B. Volume of distribution (Correct Answer)
- C. Clearance
- D. Bioavailability
Explanation: **Volume of distribution** - The **loading dose (LD)** of a drug is calculated using the formula: LD = (Target plasma concentration × **Volume of distribution**) / Bioavailability. - The **volume of distribution (Vd)** is the **major determinant** because it directly determines how much drug is needed to achieve the desired plasma concentration throughout all body compartments. - A larger Vd means more drug must be administered to achieve the same plasma concentration, as the drug distributes extensively into tissues. *Half life* - **Half-life** primarily determines the **time to reach steady-state** and the **dosing interval** for maintenance doses. - It does not directly influence the initial amount of drug required to achieve a target concentration in the loading dose calculation. *Clearance* - **Clearance** is the primary determinant of the **maintenance dose rate**, as it dictates how quickly the drug is eliminated from the body. - Formula for maintenance dose: Maintenance dose rate = Clearance × Target concentration. - It is not the major determinant of the initial loading dose, which aims to achieve a therapeutic level quickly. *Bioavailability* - **Bioavailability (F)** is the fraction of an administered drug that reaches systemic circulation unchanged. - While bioavailability is included in the loading dose formula as a correction factor (especially important for oral drugs), the **volume of distribution** remains the **major determinant** as it defines the fundamental space the drug must fill to achieve target concentration.
Question 8: After IV drug administration, elimination of a drug depends on:
- A. Lipid solubility
- B. Volume of distribution
- C. Clearance (Correct Answer)
- D. All of the options
Explanation: ***Clearance*** - **Clearance (CL)** is the primary and direct determinant of drug elimination after IV administration. - It represents the **volume of plasma cleared of drug per unit time** (e.g., mL/min or L/hr). - The **rate of elimination** is directly calculated as: Rate = CL × Plasma concentration - Clearance integrates the efficiency of all eliminating organs (liver, kidneys) and is the key parameter determining how fast a drug is removed from the body. - Formula: **CL = Rate of elimination / Plasma concentration** *Lipid solubility* - Lipid solubility affects drug **distribution** and **renal reabsorption** but does not directly determine the rate of elimination. - Highly lipid-soluble drugs may be reabsorbed in renal tubules, but the elimination rate is still governed by clearance. - Lipid solubility is more relevant to drug distribution and metabolism pathways than to the rate of elimination itself. *Volume of distribution* - Volume of distribution (Vd) describes how extensively a drug distributes into tissues versus plasma. - While Vd affects the **half-life** (t½ = 0.693 × Vd/CL), it does NOT directly determine the elimination rate. - A large Vd means more drug in tissues, which affects how long elimination takes, but the actual rate of elimination is still determined by clearance. - Vd is a distribution parameter, not an elimination parameter. *All of the options* - This is incorrect because only **clearance** directly determines the rate of drug elimination. - While lipid solubility and volume of distribution can indirectly influence how long a drug remains in the body, they do not determine the elimination rate itself—clearance does.
Question 9: In a patient with nephrotic syndrome and hypoalbuminemia, the protein binding of which drug will NOT be affected?
- A. Diazepam
- B. Valproate
- C. Tolbutamide
- D. Morphine (Correct Answer)
Explanation: ***Morphine*** - Morphine primarily binds to **alpha-1-acid glycoprotein** and has a relatively low affinity for albumin. - In nephrotic syndrome with **hypoalbuminemia**, the binding of drugs that primarily bind to albumin would be affected, whereas morphine's binding would be largely preserved. *Diazepam* - Diazepam is highly protein-bound, primarily to **albumin**. - In conditions of **hypoalbuminemia**, such as nephrotic syndrome, the unbound fraction of diazepam would increase, potentially leading to altered drug effects. *Valproate* - Valproate is extensively protein-bound, predominantly to **albumin**. - **Hypoalbuminemia** in nephrotic syndrome would lead to a significant decrease in protein binding and an increase in the free, active drug concentration. *Tolbutamide* - Tolbutamide is a sulfonylurea that is extensively bound to plasma proteins, mainly **albumin**. - **Hypoalbuminemia** would decrease its protein binding, increasing the free drug concentration and potentially enhancing its hypoglycemic effect.
Question 10: Low apparent volume of distribution of drug indicates that:
- A. Drug has low bioavailability
- B. Drug has low efficacy
- C. Drug is not extensively distributed to tissue (Correct Answer)
- D. Drug has low half life
Explanation: ***Drug is not extensively distributed to tissue*** - A **low apparent volume of distribution (Vd)** suggests that the drug primarily remains in the **vascular compartment**. - This indicates **minimal binding to peripheral tissues** and less distribution into extravascular spaces. *Drug has low bioavailability* - **Bioavailability** refers to the fraction of an administered drug that reaches the **systemic circulation unchanged**. - While related to drug disposition, a low Vd does not directly imply low bioavailability; a drug can have high bioavailability but remain largely in the blood. *Drug has low efficacy* - **Efficacy** is the maximum effect a drug can produce regardless of the dose. - Vd relates to drug distribution, not its pharmacological effect or **intrinsic activity** at its target. *Drug has low half life* - The **half-life** of a drug is determined by its **volume of distribution (Vd)** and **clearance (CL)** (t½ = 0.693 × Vd / CL). - While a low Vd can contribute to a shorter half-life if clearance is high, Vd alone does not solely determine half-life; clearance also plays a significant role.
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