Questions
2–3 questions in B.Pharm semester exams
Difficulty
Medium-Hard
Importance
Core high-yield topic for B.Pharm and clinical pharmacy exams
Overview
Elimination kinetics describes the quantitative rate at which drugs are removed from the body, which is critical for determining dosing regimens and predicting drug toxicity. Mastering these models is essential for solving clinical pharmacokinetic problems and understanding how saturation of enzymes affects drug half-life. Students must be able to differentiate between constant amount and constant fraction elimination to succeed in pharmaceutical exams.
Zero-Order Kinetics
In zero-order kinetics, the rate of elimination is constant and independent of the drug concentration in the plasma. This usually occurs when drug elimination pathways, such as enzymatic processes, become saturated, leading to nonlinear pharmacokinetics.
- Rate of elimination is constant (-dC/dt = K0)
- Half-life is not constant; it decreases as concentration decreases
- Linear relationship between concentration and time
- Typical examples include alcohol metabolism and high-dose phenytoin
- Equation: Ct = C0 - K0t
First-Order Kinetics
First-order kinetics describes the elimination of a constant fraction of the drug per unit time, meaning the rate is directly proportional to the plasma concentration. This is the most common model for therapeutic drugs, characterized by a constant half-life regardless of dose.
- Rate of elimination depends on concentration (-dC/dt = Ke*C)
- Half-life is constant (t1/2 = 0.693 / Ke)
- Exponential decay curve in linear coordinates
- Semi-log plot of concentration vs time yields a straight line
- Equation: Ct = C0 * e^(-Ke*t)
Protein Binding
Protein binding refers to the reversible attachment of drugs to plasma proteins like albumin, which significantly impacts the volume of distribution and the elimination rate. Only the 'free' or unbound drug is pharmacologically active and available for filtration or metabolism.
- Albumin binds acidic drugs, alpha-1 acid glycoprotein binds basic drugs
- Only unbound drug crosses membranes and undergoes elimination
- High protein binding decreases the rate of drug elimination
- Equation: Fu = Concentration(free) / Total Concentration
- Displacement from proteins can lead to sudden drug toxicity
Formula Sheet
Zero-order rate: dC/dt = -K0
Zero-order concentration: Ct = C0 - K0t
First-order rate: dC/dt = -Ke * C
First-order concentration: ln(Ct) = ln(C0) - Ke * t
First-order half-life: t1/2 = 0.693 / Ke
Elimination rate constant: Ke = 0.693 / t1/2
Fraction unbound: Fu = Cu / Ct
Exam Tip
Always verify if the drug elimination is concentration-dependent or independent before applying the formula; identifying the kinetic order is 50% of the solution.
Common Mistakes
- Confusing zero-order half-life with first-order half-life, which is only constant for first-order.
- Forgetting to convert units or logs when calculating the elimination rate constant (Ke).
- Assuming total drug concentration represents the active fraction, ignoring the effect of protein binding.
More Revision Notes
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