Question

The volume of distribution of two drugs A and B was 40 L and 160 L respectively. Which of the following statements are correct?
1. Dose of A is low compared to B
2. The initial plasma concentration of A is high
3. Dose of B is low compared to A
4. The initial plasma concentration of B is low

• A. \( \text{I only} \)
• B. \( \text{I and II} \)
• C. \( \text{I, II and III} \)
• D. \( \text{I, II and IV} \)

Hint:

Remember the inverse relationship between Volume of Distribution (Vd) and initial plasma concentration (C₀) for a given dose. A larger Vd means the drug is more distributed outside the plasma, resulting in a lower plasma concentration.

Answer 1

The Correct Option is D

The volume of distribution (Vd) relates the amount of drug in the body to the plasma concentration. The formula is: $$ Vd = \frac{\text{Dose}}{\text{Initial Plasma Concentration (C}_0)} $$ Rearranging this, we get: $$ C_0 = \frac{\text{Dose}}{Vd} $$ Given: - Vd of drug A = 40 L - Vd of drug B = 160 L Let's analyze the statements assuming the same dose (for comparison purposes initially): - Statement II: The initial plasma concentration of A is high. If the dose is the same, since Vd of A is smaller, the initial plasma concentration of A will be higher ($C_{0A} = \frac{\text{Dose}}{40}$). This statement is likely correct if we consider a scenario where both drugs are given in similar doses. - Statement IV: The initial plasma concentration of B is low. If the dose is the same, since Vd of B is larger, the initial plasma concentration of B will be lower ($C_{0B} = \frac{\text{Dose}}{160}$). This statement is likely correct under the same assumption. Now consider the doses required to achieve a certain plasma concentration: $$ \text{Dose} = C_0 \times Vd $$ If we want to achieve the same initial plasma concentration for both drugs, then the dose of B would need to be higher because its Vd is larger. This implies: - Statement I: Dose of A is low compared to B. To achieve a similar initial plasma concentration, the dose of A would be lower than the dose of B. This statement is likely correct if the therapeutic target is a certain plasma concentration. - Statement III: Dose of B is low compared to A. This is the opposite of what we just deduced and is likely incorrect if the goal is similar plasma concentrations. Therefore, statements I, II, and IV are consistent with the principles of volume of distribution. A smaller Vd leads to a higher initial plasma concentration for the same dose, implying a lower dose is needed to achieve a certain concentration compared to a drug with a larger Vd. Final Answer: (D) I, II and IV