Pharmacology: Pharmacokinetics, Pharmacodynamics and Drug Classes
Pharmacology in GPAT tests pharmacokinetics, pharmacodynamics, autonomic and CNS drugs, antimicrobials and adverse effects. Many items are single-step calculations or mechanistic distinctions.
Pharmacokinetics
- For first-order elimination, t½ = 0.693 × Vd/CL; steady state is reached in about 4–5 half-lives.
- Vd = dose/C0 after IV bolus; loading dose = (Vd × target Cp)/F.
- Zero-order kinetics (e.g., phenytoin, ethanol at high dose) give a constant amount eliminated per time, not a constant fraction.
Pharmacodynamics
- Competitive antagonists shift the agonist log-dose–response curve right without lowering Emax; non-competitive antagonists depress Emax.
- Partial agonists have intrinsic activity between 0 and 1; inverse agonists reduce constitutive activity.
Worked example
A 500 mg IV bolus gives an extrapolated C0 of 25 mg/L. Vd = 500/25 = 20 L. To reach 2 mg/L immediately with F = 1, loading dose = 20 L × 2 mg/L = 40 mg. If F were 0.8, the dose would be 40/0.8 = 50 mg.
Traps
Treating Vd as a real anatomical volume; assuming half-life is independent of Vd when clearance is fixed; ignoring that enzyme inducers lower plasma concentration over days, not immediately; mixing potency with efficacy.