Drug A has an intrinsic efficacy of 0.5 at a receptor (it is a partial agonist) and Drug B is a full agonist with intrinsic efficacy of 1.0. If both have equal receptor affinity and both are given together at saturating concentrations, the expected outcome is:

• A Drug A will competitively antagonise Drug B, reducing the maximal effect to an intermediate level between the two intrinsic efficacies ✓
• B Drug B will completely overcome Drug A at all doses because it has greater efficacy
• C The combination will produce greater than additive (synergistic) effects
• D Drug A will act as a full agonist at saturating concentrations by fully occupying receptors

Explanation

When a partial agonist competes with a full agonist for the same receptor at equal affinity and saturating concentrations, the partial agonist occupies receptors but produces submaximal activation. The net effect is a reduction in the overall response, as partial agonist-occupied receptors generate less response than full agonist-occupied ones. Drug A essentially behaves as a partial antagonist in the presence of the full agonist. The maximum response will be intermediate, determined by the partial agonist's intrinsic efficacy.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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