Key Principles in Pharmacokinetics: IV Dosing, Bioavailability, and Drug Elimination

Q1. If a dose of morphine is given IV, how would you find the initial concentration in the systemic circulation?

Background

Topic: Pharmacokinetics – Initial Drug Concentration

This question tests your understanding of how to calculate the initial plasma concentration of a drug after intravenous (IV) administration, which is a key concept in pharmacokinetics.

Key Terms and Formulas:

• Volume of Distribution (Vd): The theoretical volume that would be necessary to contain the total amount of an administered drug at the same concentration as it is in the blood plasma.

• Bioavailability (F): The fraction of the administered dose that reaches the systemic circulation (F = 1 for IV drugs).

• Formula:

$C_0 = \frac{\text{Dose}}{V_d}$

Step-by-Step Guidance

1. Recognize that for IV administration, bioavailability (F) is 1, so the entire dose enters the systemic circulation immediately.

2. Recall that the initial concentration ($C_0$) is determined by dividing the dose by the volume of distribution ($V_d$).

3. Set up the equation: $C_0 = \frac{\text{Dose}}{V_d}$.

4. Ensure the units for dose and $V_d$ are compatible (e.g., mg and L).

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Q2. The fraction of dose which reaches the systemic circulation is termed:

Background

Topic: Bioavailability

This question is testing your knowledge of the definition of bioavailability, a fundamental pharmacokinetic parameter.

Key Terms:

• Bioavailability (F): The proportion of a drug that enters the systemic circulation when introduced into the body and is thus available for activity.

Step-by-Step Guidance

1. Recall that bioavailability is a measure of the extent of drug absorption for a given dosage form compared to an IV dose.

2. Consider the definitions of the other terms (rate constant, rate of elimination, volume of distribution, steady state concentration) to ensure you select the correct one.

3. Identify which term directly refers to the fraction of the administered dose reaching systemic circulation.

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Q3. You want to change the route of administration of a drug from IV to oral, but do not want to change the amount of drug that reaches the systemic circulation. Should the oral dose be smaller than, bigger than, or equal to the IV dose?

Background

Topic: Dose Adjustment Based on Bioavailability

This question tests your understanding of how bioavailability affects dosing when switching between IV and oral routes.

Key Terms and Formulas:

• Bioavailability (F): For IV, F = 1; for oral, F < 1 due to first-pass metabolism and incomplete absorption.

• Formula for Equivalent Dosing:

$\text{Oral Dose} = \frac{\text{IV Dose}}{F_{oral}}$

Step-by-Step Guidance

1. Recognize that oral bioavailability is usually less than 1 due to first-pass effect and incomplete absorption.

2. Set up the equation to ensure the same amount of drug reaches systemic circulation: $\text{Oral Dose} \times F_{oral} = \text{IV Dose} \times F_{IV}$.

3. Since $F_{IV} = 1$, rearrange to solve for the oral dose.

4. Consider whether the oral dose should be greater than, less than, or equal to the IV dose based on the value of $F_{oral}$.

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Q4. What is the definition of first-order elimination kinetics?

Background

Topic: Drug Elimination Kinetics

This question tests your understanding of how drugs are eliminated from the body, specifically the concept of first-order kinetics.

Key Terms:

• First-order kinetics: The rate of drug elimination is proportional to the drug concentration.

• Rate constant (k): The proportionality constant in first-order kinetics.

Step-by-Step Guidance

1. Recall that in first-order kinetics, a constant fraction of drug is eliminated per unit time, not a constant amount.

2. Consider how the rate of elimination changes as drug concentration changes.

3. Compare this to zero-order kinetics, where a constant amount is eliminated regardless of concentration.

4. Identify which answer choice best matches the definition of first-order kinetics.

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Q5. A patient takes morphine regularly for a year, and then stops taking it. How long before the medication can be considered to be completely eliminated from the patient's system? (t1/2 = 4 hr, Vd = 245 L, F = 0.35)

Background

Topic: Drug Elimination and Half-life

This question tests your ability to apply the concept of drug half-life to estimate how long it takes for a drug to be eliminated from the body.

Key Terms and Formulas:

• Half-life (t1/2): The time required for the concentration of the drug in the body to decrease by half.

• Rule of thumb: It generally takes about 4–5 half-lives for a drug to be considered eliminated from the body.

Step-by-Step Guidance

1. Recall the definition of half-life and how it relates to drug elimination.

2. Multiply the half-life by 4 or 5 to estimate the time for near-complete elimination.

3. Calculate the total time using the given half-life value.

4. Compare your calculated value to the answer choices provided.

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