[PHARM-02] Pharmacokinetics (ADME), Bioavailability, Cytochrome P450 & Elimination Kinetics
PHARM-02 • 2. PharmacologyLearning Objectives
Calculate pH-dependent ionization using the Henderson–Hasselbalch equation and apply ion trapping in poisoning management.; Compute apparent Volume of Distribution (Vd), Hepatic/Renal Clearance (CL), Elimination Half-Life (t1/2), Loading Dose (LD), and Maintenance Dose (MD).; Differentiate first-order (linear) kinetics from zero-order (Michaelis–Menten capacity-limited) kinetics with Indian clinical examples.; Map major CYP450 isoforms (CYP3A4, CYP2D6, CYP2C9, CYP2C19, CYP1A2) and P-glycoprotein interactions.
Chapter Overview & Core Explanation
Pharmacokinetics describes what the body does to the drug across four steps: Absorption (getting into the bloodstream), Distribution (moving into tissues), Metabolism (chemical conversion, mainly in the liver), and Excretion (removal, mainly via kidneys or bile).
Key Exam & Clinical Takeaways
- Urinary alkalinization with IV Sodium Bicarbonate ([10.5]) traps weak acids ([3.39] Aspirin/salicylates, [11.15] Phenobarbitone, [6.20] Methotrexate) in renal tubular fluid and accelerates excretion.
- When co-prescribing a PPI with [3.40] Clopidogrel, prefer [5.2] Pantoprazole over [5.1] Omeprazole/[5.4] Esomeprazole to minimize CYP2C19 inhibition.
- Loading dose depends exclusively on Vd (never changes in renal failure), whereas maintenance dose depends on Clearance (CL) and must be reduced in renal/hepatic impairment.