Dose-Response Relationships — MCQs
What does ED50 measure in pharmacology?
Which of the following statements is true regarding competitive reversible antagonism?
What does the therapeutic index of a drug signify?
The therapeutic index of a drug is defined as the ratio between the toxic dose and the effective dose.
A shift to the right in the biological activity dose- response curve for a hormone with no accompanying change in the maximal response indicates:
What would happen to the half-life and plasma concentration of a drug which follows first-order kinetics, if the dose is doubled?
In which phase of clinical trials is drug dosing typically determined?
Which of the following best demonstrates the variability in drug responsiveness among individuals?
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?
In treatment of Parkinsonism, L-Dopa is combined with carbidopa mainly to:
Dose-Response Relationships Indian Medical PG Practice Questions and MCQs
Question 1: 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 2: Which of the following statements is true regarding competitive reversible antagonism?
- A. ED50 remains unchanged in competitive reversible antagonism.
- B. Efficacy and Vmax remain unchanged. (Correct Answer)
- C. Potency remains unchanged in the presence of a competitive antagonist.
- D. Affinity (Kd) remains unchanged in competitive reversible antagonism.
Explanation: ***Efficacy and Vmax remain unchanged.*** - In competitive reversible antagonism, the antagonist binds to the same receptor site as the agonist but can be overcome by increasing the agonist concentration [2]. This means the **maximum effect (efficacy or Vmax)** of the agonist can still be achieved, although a higher dose is needed [2]. - The antagonist does not alter the intrinsic ability of the agonist to produce a full response, only its **apparent affinity** for the receptor. - This is the hallmark of competitive reversible antagonism: **rightward shift of the dose-response curve with no change in maximum response** [2]. *Potency remains unchanged in the presence of a competitive antagonist.* - **Potency** is a measure of the amount of drug needed to produce a given effect (often defined by EC50 or ED50) [3]. - A competitive antagonist requires a **higher concentration of agonist** to achieve the same effect, thus **decreasing the apparent potency** of the agonist [4]. - The dose-response curve shifts to the right (parallel shift) [4]. *ED50 remains unchanged in competitive reversible antagonism.* - **ED50 (effective dose 50)** is the dose that produces 50% of the maximum effect. - Because competitive antagonists shift the dose-response curve to the right, a **higher ED50** is required to achieve 50% of the maximum effect in the presence of an antagonist [4]. *Affinity (Kd) remains unchanged in competitive reversible antagonism.* - The **dissociation constant (Kd)** represents the affinity of a drug for its receptor [1]. - In competitive reversible antagonism, the antagonist increases the **apparent Kd** of the agonist (reduces apparent affinity), requiring more agonist to achieve receptor occupancy. - The **intrinsic Kd** of the agonist doesn't change, but its apparent affinity is reduced due to competition with the antagonist.
Question 3: What does the therapeutic index of a drug signify?
- A. Dose which produces maximum effect
- B. Safety margin (Correct Answer)
- C. Efficacy
- D. Maximum response that can be elicited by a drug
Explanation: ***Safety margin*** - The **therapeutic index (TI)** is a ratio comparing the dose that produces a toxic effect (TD50 or LD50) to the dose that produces a therapeutically desired effect (ED50) [1]. - A higher therapeutic index indicates a **wider safety margin**, meaning there is a greater difference between the effective and toxic doses [1, 2].*Dose which produces maximum effect* - This describes the **efficacy** of a drug at its maximal point, not its therapeutic index. - The therapeutic index is concerned with the range of doses that can be safely given to achieve a therapeutic effect [2].*Efficacy* - **Efficacy** refers to the maximum effect a drug can produce regardless of the dose. - The therapeutic index is a measure of drug safety, not primarily its efficacy [1].*Maximum response that can be elicited by a drug* - This definition also describes the **efficacy** or **maximal effect** of a drug. - The therapeutic index quantifies the **ratio of toxic to effective doses**, providing insight into safety [1].
Question 4: 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 5: A shift to the right in the biological activity dose- response curve for a hormone with no accompanying change in the maximal response indicates:
- A. Increased sensitivity and decreased responsiveness
- B. Decreased sensitivity (Correct Answer)
- C. Increased responsiveness
- D. Decreased responsiveness and decreased sensitivity
Explanation: ***Decreased sensitivity*** - A **right shift** in the dose-response curve means a **higher concentration** of the hormone is required to achieve the same effect, indicating reduced sensitivity. - No change in the **maximal response** implies the system can still reach the same peak effect, but it needs more hormone. *Increased sensitivity and decreased responsiveness* - **Increased sensitivity** would be represented by a **left shift** in the dose-response curve, meaning less hormone is needed for a given effect. - **Decreased responsiveness** implies a reduction in the **maximal effect**, which is stated as unchanged in the question. *Increased responsiveness* - **Increased responsiveness** would mean a **higher maximal effect** can be achieved or a steeper slope in the dose-response curve, neither of which is described. - A right shift is related to the dose required for an effect, not the magnitude of the maximal effect. *Decreased responsiveness and decreased sensitivity* - While **decreased sensitivity** is correct, **decreased responsiveness** is incorrect because the question specifies "no accompanying change in the maximal response." - **Decreased responsiveness** would be indicated by a **lower maximal effect (Emax)**, which is not the case here.
Question 6: What would happen to the half-life and plasma concentration of a drug which follows first-order kinetics, if the dose is doubled?
- A. Half - life and plasma concentration remains the same
- B. Half - life doubles and plasma concentration remains the same
- C. Half - life remains the same and plasma concentration doubles (Correct Answer)
- D. Half - life and plasma concentration doubles
Explanation: ***Half - life remains the same and plasma concentration doubles*** - In **first-order kinetics**, drug elimination is proportional to the **plasma concentration**, meaning a constant *fraction* of the drug is eliminated per unit of time. - Doubling the dose will **double the initial plasma concentration**, but the **half-life** (time taken for plasma concentration to halve) remains constant because the *rate of elimination proportionally increases* with concentration. *Half - life and plasma concentration remains the same* - This would only be true if the dose was not changed, or if the drug followed **zero-order kinetics** and the elimination system was already saturated, which is not the case here. - If the plasma concentration remained the same after doubling the dose, it would imply either no absorption or extremely rapid elimination, contradicting typical first-order drug behavior. *Half - life doubles and plasma concentration remains the same* - For **half-life to double**, there would need to be a **decrease in drug clearance** or an *increase in volume of distribution*, not simply a dose increase. - If plasma concentration remained the same despite a doubled dose, it would suggest a **major increase in clearance** or volume of distribution, which is not stated. *Half life and plasma concentration doubles* - While plasma concentration doubles with a doubled dose in **first-order kinetics**, the **half-life remains constant**. - Half-life is an **intrinsic pharmacokinetic parameter** determined by clearance and volume of distribution, not by the administered dose in first-order kinetics.
Question 7: In which phase of clinical trials is drug dosing typically determined?
- A. Phase 1 (Correct Answer)
- B. Phase 2
- C. Phase 3
- D. Phase 4
- E. Phase 0
Explanation: ***Phase 1*** - This phase involves a small group of **healthy volunteers** to assess the drug's safety, **pharmacokinetics (PK)**, and establish an initial dosing range. - The primary goal is to determine a **safe dosage level**, establish the **maximum tolerated dose (MTD)**, and identify potential side effects. - This is where drug dosing is **typically determined**. *Phase 0* - This is an exploratory phase involving **microdosing** studies with subtherapeutic doses. - The goal is to gather preliminary PK/PD data, but **not to determine therapeutic dosing**. *Phase 2* - This phase involves a larger group of **patients** with the condition to be treated. - The main goal is to evaluate the drug's **effectiveness** and further assess safety, but not primarily to determine initial dosing. *Phase 3* - This phase involves a large number of patients across multiple sites to confirm the drug's **efficacy** and monitor side effects in a broader population. - Dosing strategies have generally been established in earlier phases, and this phase primarily validates them. *Phase 4* - This phase occurs **after a drug has been approved** and marketed. - It involves ongoing surveillance to monitor long-term effects, collect additional information on safety, and identify new uses, but not initial dose determination.
Question 8: Which of the following best demonstrates the variability in drug responsiveness among individuals?
- A. Potency
- B. Quantal Dose Response Curve (Correct Answer)
- C. Efficacy
- D. Graded Dose Response Curve
Explanation: ***Quantal Dose Response Curve*** - A **quantal dose-response curve** plots the percentage of individuals exhibiting a discrete, all-or-none effect against the log dose of a drug. - This curve directly illustrates the **variability in drug responsiveness** within a population by showing the range of doses required to produce a specific effect in different individuals. *Efficacy* - **Efficacy** refers to the maximum effect a drug can produce, regardless of the dose. - While efficacy is an important pharmacological parameter, it describes the drug's overall therapeutic potential, not the **individual variability** in response. *Potency* - **Potency** is a measure of the amount of drug needed to produce an effect of given intensity. - It relates to the absolute dose required for a particular effect but does not directly demonstrate the **inter-individual differences** in biological response. *Graded Dose Response Curve* - A **graded dose-response curve** depicts the relationship between the dose of a drug and the **magnitude of the effect** in a **single biological unit** (e.g., an individual, a tissue, or a cell). - This curve reflects the relationship between drug concentration and effect intensity, but not the **variability in response among different individuals** in a population.
Question 9: 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 10: In treatment of Parkinsonism, L-Dopa is combined with carbidopa mainly to:
- A. To decrease dose requirement of L–Dopa
- B. To decrease side effects of L–Dopa (Correct Answer)
- C. To decrease effectiveness of L–Dopa
- D. To increase crossing of L–Dopa through BBB
Explanation: ***To decrease side effects of L–Dopa*** - This is the **primary/main reason** for combining carbidopa with L-Dopa. - Carbidopa is a **peripheral DOPA decarboxylase inhibitor**, preventing the conversion of L-Dopa to dopamine in the periphery. - This reduces side effects like **nausea, vomiting, and cardiac arrhythmias**, which are caused by peripheral dopamine. - Without carbidopa, peripheral side effects make L-Dopa therapy **intolerable at therapeutic doses**. *To decrease dose requirement of L–Dopa* - This is an **important secondary benefit** but not the main reason. - Carbidopa does allow for **75-80% reduction in L-Dopa dose** (from ~5-6g to ~1g daily) by preventing peripheral metabolism. - However, this dose reduction is a **consequence** of preventing peripheral conversion, not the primary therapeutic goal. - The main goal is making L-Dopa therapy **tolerable and safe**, with dose reduction being a beneficial side effect. *To decrease effectiveness of L–Dopa* - Carbidopa **increases the effectiveness** of L-Dopa by ensuring more of it reaches the central nervous system to be converted into dopamine. - By preventing premature peripheral metabolism, carbidopa allows for a greater therapeutic effect on Parkinson's symptoms. *To increase crossing of L–Dopa through BBB* - Carbidopa itself **does not cross the blood-brain barrier (BBB)** and therefore does not directly affect the transport of L-Dopa into the brain. - L-Dopa uses an **active transport system** (large neutral amino acid transporter) to cross the BBB, and carbidopa's role is to prevent its peripheral breakdown before it can utilize this system. - While more L-Dopa reaches the BBB due to reduced peripheral metabolism, carbidopa does not enhance the actual crossing mechanism.
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