Question

Plasma protein bound drugs are:

• A. Pharmacodynamically active
• B. Pharmacokinetically inert
• C. Pharmacokinetically and Pharmacodynamically inert
• D. Pharmacodynamically inert

Answer 1

The Correct Option is D

The correct option is (D): Pharmacodynamically inert.

Answer 2

Only the free, unbound fraction of a drug in plasma is able to cross membranes, reach its receptor, and produce a pharmacological effect; the protein-bound fraction is temporarily held back from doing so. With that distinction, each option can be evaluated:

1. Pharmacodynamically active: An active drug is free to bind its receptor and produce an effect. A drug bound to plasma protein is, by definition, sequestered away from its receptor and cannot exert this action, so it cannot be described as pharmacodynamically active.
2. Pharmacokinetically inert: This would mean the bound drug is untouched by absorption, distribution, metabolism and excretion altogether. In reality, protein binding mainly restricts distribution and delays metabolism/excretion, it does not remove the drug from the pharmacokinetic picture entirely, since bound and free fractions remain in dynamic equilibrium. So calling it pharmacokinetically inert overstates the effect.
3. Pharmacokinetically and Pharmacodynamically inert: Since the drug is not truly pharmacokinetically inert (it is still carried and eventually released for normal handling), labelling it inert on both counts goes too far.
4. Pharmacodynamically inert: Because the bound drug cannot reach or occupy its receptor while attached to plasma protein, it produces no pharmacological action during that time. This matches exactly what "pharmacodynamically inert" describes.

The protein-bound fraction is thus unavailable to act at the receptor even though it remains part of the circulating drug pool.

Therefore, the correct answer is Pharmacodynamically inert.