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Pharmacology 3% exam weight

Pharmacology Topic 3

Part of the INI CET (AIIMS PG) study roadmap. Pharmacology topic pharma-003 of Pharmacology.

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Pharmacology Topic 3

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Pharmacology Topic 3 covers pharmacodynamics — what the drug does to the body at receptors and downstream signaling. The two most tested formulas are Therapeutic Index (TI = TD50 / ED50) and Clark’s occupancy (Fraction occupied = [D] / ([D] + Kd)).

  • Agonist: binds receptor and produces a response (intrinsic activity α = 1).
  • Antagonist: binds without intrinsic activity (α = 0); competitive shifts the curve right, non-competitive reduces Emax.
  • Potency = EC50 (left-right position); Efficacy = Emax (height of curve).
  • Spare receptors let a full agonist reach Emax at <100% occupancy.

INI CET usually asks 1–2 MCQs on curve interpretation or TI calculation. Memorize that partial agonists antagonize full agonists.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Receptor families and binding forces

Four receptor superfamilies dominate INI CET questions: ionotropic (ion-channel), G-protein coupled (GPCR), enzyme-linked (e.g., tyrosine kinase), and nuclear/intracellular receptors. Binding between drug and receptor uses ionic, hydrogen, van der Waals, hydrophobic, and covalent interactions. Covalent bonds (aspirin on COX, phenoxybenzamine on α-receptors) cause irreversible antagonism.

Agonist spectrum

TypeIntrinsic activity (α)Behaviour
Full agonist1Produces Emax
Partial agonist0 < α < 1Lower Emax; antagonizes a full agonist
Inverse agonistNegativeProduces effect opposite to agonist (requires constitutive receptor activity)
Competitive antagonist0Parallel right shift, Emax preserved
Non-competitive antagonist0Reduces Emax, curve flattened

Dose–response curves

  • Graded curves: continuous response vs log-dose, used to derive EC50 and Emax.
  • Quantal curves: cumulative % population responding vs log-dose, used to derive ED50 and TD50/LD50.

Therapeutic Index — worked relationship

TI = TD50 / ED50. A larger TI indicates a wider safety margin. Example: a drug with ED50 = 10 mg/kg and TD50 = 100 mg/kg has TI = 10; doubling dose still lies below toxicity for most patients.

Common traps

  • Confusing potency (EC50) with efficacy (Emax).
  • Assuming a higher TI guarantees clinical safety — slope and patient variability still matter.
  • Treating partial agonists as weak full agonists — clinically important (buprenorphine vs morphine).

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Signal transduction pathways

GPCRs couple to Gs (↑cAMP, e.g., β1, β2), Gi (↓cAMP, e.g., M2, α2), Gq (↑IP3/DAG → ↑Ca²⁺, e.g., M1, M3, α1). Enzyme-linked receptors include receptor tyrosine kinases (insulin, growth factors) and serine/threonine kinases (TGF-β). Nuclear receptors (steroids, thyroid) regulate gene transcription with delayed onset (hours).

Receptor regulation

Desensitization (seconds–minutes) involves phosphorylation of GPCRs by GRKs and β-arrestin binding. Tachyphylaxis is rapid tolerance within a single dose interval (e.g., nitrates, tyramine). Downregulation (hours–days) reduces receptor number via internalization; upregulation explains rebound phenomena after abrupt withdrawal (clonidine, β-blockers).

Edge cases and Hill equation

The Hill equation E/Emax = [L]ⁿ / ([L]ⁿ + Kd) describes cooperative binding; n > 1 = positive cooperativity, n < 1 = negative. Spare receptors mean full agonists reach Emax at low fractional occupancy — explaining why potency (EC50) and receptor affinity (Kd) can diverge.

Exam strategy

For INI CET, draw the curve, label axes, mark EC50 and Emax, and identify the shift type. For TI problems, always state both ED50 and TD50 values before computing the ratio.

  1. A drug shows ED50 = 5 mg/kg and LD50 = 500 mg/kg. Compute the Selectivity Index and comment on its safety margin relative to TI = 50 for another drug.
  2. On a graded dose–response curve, a parallel right-shift with unchanged Emax after adding an antagonist indicates which antagonism type, and what parameter changes numerically?

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