Questions
2–3 questions in B.Pharm semester exams
Difficulty
Medium-Hard
Importance
Core topic for Pharmaceutics and Clinical Pharmacokinetics.
Overview
Dosage regimen design is the systematic process of determining the optimal drug dose and dosing interval to maintain plasma concentrations within the therapeutic window. It is critical for avoiding toxicity and sub-therapeutic effects, serving as a pillar of clinical pharmacokinetics in B.Pharm curricula. Mastering this allows you to solve both steady-state numericals and clinical management problems.
IV Bolus and Infusion Kinetics
IV bolus provides immediate systemic circulation, while constant rate infusion is used to maintain steady-state levels. For bolus, the concentration declines exponentially, whereas infusion kinetics involve reaching a plateau where the rate of drug entry equals the rate of elimination.
- IV Bolus: C = (Dose / Vd) * e^(-kt)
- Steady-state concentration (Css) = R0 / (Vd * k)
- Rate of infusion (R0) = Css * Cl
- Time to reach steady state depends solely on elimination half-life
- Typically 5 half-lives are required to reach 97% of steady state
Extravascular Administration Kinetics
When a drug is administered extravascularly, its plasma concentration is influenced by the rate of absorption (ka) and the rate of elimination (k). The concentration-time profile follows a biexponential equation representing the rise and fall of the drug in the systemic circulation.
- C = (F * Ka * Dose) / (Vd * (Ka - k)) * (e^(-kt) - e^(-kat))
- Tmax = ln(Ka / k) / (Ka - k)
- Cmax is reached when the rate of absorption equals the rate of elimination
- F represents the bioavailability fraction of the drug
- Absorption phase is typically characterized by a steeper slope
Multiple Dosing Regimens
Multiple dosing involves periodic administration of drugs to achieve therapeutic accumulation. The objective is to maintain drug levels between the Minimum Effective Concentration (MEC) and the Minimum Toxic Concentration (MTC).
- Accumulation factor (AF) = 1 / (1 - e^(-kt))
- Average steady-state concentration (Css_avg) = (F * Dose) / (Cl * Tau)
- Fluctuation = (Cmax_ss - Cmin_ss) / Css_avg
- Loading dose (DL) = (Css * Vd) / F
- Maintenance dose (DM) = (Css * Cl * Tau) / F
- Tau (dosing interval) determines the degree of fluctuation
Formula Sheet
Css = (F * Dose) / (Cl * Tau)
DL = (Css * Vd) / F
Tmax = ln(Ka/k) / (Ka-k)
C = (Dose / Vd) * e^(-kt)
Exam Tip
Always verify if the question requires steady-state calculations (where accumulation factor applies) or a single-dose profile to avoid choosing the wrong formula set.
Common Mistakes
- Confusing the elimination rate constant (k) with the absorption rate constant (ka) in extravascular equations.
- Neglecting the bioavailability factor (F) when calculating the maintenance dose for non-intravenous routes.
- Using the wrong units for clearance (Cl) and volume of distribution (Vd), leading to significant numerical errors.
More Revision Notes
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