Questions
2–4 questions per B.Pharm semester paper
Difficulty
Medium-Hard
Importance
High yield for B.Pharm and competitive exams like NIPER
Overview
Drug absorption, bioavailability, and bioequivalence form the cornerstone of pharmacokinetics, determining how a drug enters the systemic circulation and reaches its target. Mastery of these concepts is essential for understanding dosage forms, clinical efficacy, and the regulatory requirements for generic drug approval.
Factors Affecting Drug Absorption
Absorption is the movement of a drug from the site of administration to the bloodstream, governed by physicochemical properties and physiological barriers. Understanding these variables is critical for predicting the therapeutic failure or success of a specific dosage form.
- Physicochemical factors: Solubility, pKa, lipophilicity, and particle size (Noyes-Whitney equation).
- Physiological factors: Gastric emptying time, intestinal motility, and blood flow.
- Dosage form factors: Excipients, disintegration time, and dissolution rate.
- Fick's First Law of Diffusion governs the rate of passive transport across membranes.
- First-pass metabolism significantly reduces the effective concentration reaching systemic circulation.
Bioavailability Studies
Bioavailability (F) represents the fraction of an administered dose of unchanged drug that reaches systemic circulation. It is calculated by comparing the area under the curve (AUC) of a test formulation against an intravenous reference.
- Absolute Bioavailability (F) = (AUC_oral / AUC_iv) * (Dose_iv / Dose_oral).
- Relative Bioavailability compares a test product to an established non-IV reference.
- AUC measures the total systemic exposure of the drug over time.
- Peak plasma concentration (Cmax) and time to reach peak (Tmax) are crucial markers of rate.
- Influenced by pharmaceutical, physiological, and patient-related variables.
Bioequivalence Testing
Bioequivalence is a regulatory requirement ensuring that two drug products—usually a generic and a brand name—deliver the same amount of active ingredient to the site of action in the same amount of time. It is assessed via pharmacokinetic parameters in crossover clinical trials.
- Statistically defined as 90% confidence interval of the ratio of test/reference being 80-125%.
- Requires randomized, open-label, two-period, two-sequence crossover design.
- Pharmacokinetic parameters used: Cmax, AUC0-t, and AUC0-infinity.
- Dissolution testing is often used as a surrogate for bioequivalence for highly soluble drugs.
- Essential for generic drug substitution and therapeutic interchangeability.
Formula Sheet
Absolute Bioavailability: F = (AUC_oral * Dose_iv) / (AUC_iv * Dose_oral)
Rate of Absorption: dC/dt = K * Cp (First order kinetics)
Fick's Law: dQ/dt = D * A * (Cm - Cb) / h
Exam Tip
Always define bioavailability as the fraction of an unchanged drug reaching systemic circulation, as the 'unchanged' qualifier is the most common point where marks are lost.
Common Mistakes
- Confusing absolute bioavailability with relative bioavailability by using the wrong reference standard.
- Neglecting the impact of gut-wall metabolism in the calculation of total systemic availability.
- Misinterpreting the 80-125% rule as a simple range rather than a 90% confidence interval statistical requirement.
More Revision Notes
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