Paper I
2022 February (2019 Scheme) · 100 marks · 180 min

Question

Define plasma half-life and give two examples of drugs with very short half life. Define therapeutic index and give two examples each for drugs with high and low therapeutic index”. (4+4)4. Classify drugs used to treat Parkinsonism with one example from each group

Q310 marksShort Essays

Answer

Plasma half-life The time required for the plasma concentration of a drug to fall by 50%, for a drug following first-order kinetics. Examples of very short half-life drugs: Insulin, Adrenaline.

Therapeutic index The ratio of the toxic dose (TD50, dose toxic in 50% of subjects) to the effective dose (ED50, dose effective in 50% of subjects) — TI = TD50/ED50. A high TI indicates a wide margin of safety between an effective and a toxic dose; a low TI indicates a narrow margin requiring careful dose titration and monitoring.

  • High therapeutic index: Penicillin, Diazepam.
  • Low therapeutic index: Digoxin, Warfarin (also Lithium, Phenytoin — narrow therapeutic range requiring plasma level monitoring).

Classification of drugs used to treat Parkinsonism, one example from each group

GroupExample
Dopamine precursorLevodopa (with carbidopa)
Dopamine agonistsPramipexole
MAO-B inhibitorsSelegiline
COMT inhibitorsEntacapone
AnticholinergicsTrihexyphenidyl
AmantadineAmantadine (dopamine-release enhancing/NMDA-antagonist mechanism)

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