Pharmacokinetics and Pharmacodynamics Flashcards

A drug has a volume of distribution (Vd) of 700 L in a 70 kg patient. What does this suggest?

Answer: The drug is extensively distributed into tissues

A large Vd (much greater than total body water ~42 L) indicates the drug distributes extensively into peripheral tissues rather than remaining in plasma.

A drug has a half-life of 8 hours. Approximately how long will it take to reach steady-state plasma concentration with regular dosing?

Answer: 40 hours

Steady-state is reached after approximately 4–5 half-lives; 5 × 8 hours = 40 hours.

Which route of administration completely bypasses first-pass hepatic metabolism?

Answer: Intravenous

Intravenous administration delivers the drug directly into systemic circulation, completely bypassing hepatic first-pass metabolism.

The bioavailability of an oral drug is 25%. If the IV dose producing the desired effect is 20 mg, what oral dose is required to achieve the same effect?

Answer: 80 mg

Oral dose = IV dose / bioavailability = 20 mg / 0.25 = 80 mg to achieve equivalent systemic exposure.

Hepatic clearance of a high-extraction drug (extraction ratio > 0.7) is primarily dependent on:

Answer: Hepatic blood flow

High-extraction drugs are efficiently cleared by the liver on first pass, so their clearance is limited by hepatic blood flow, not enzyme capacity.

A drug follows first-order kinetics. Which of the following statements is TRUE?

Answer: The rate of elimination is proportional to drug concentration

In first-order kinetics, a constant fraction (not amount) of drug is eliminated per unit time, meaning the rate is proportional to concentration.

Which pharmacokinetic parameter is most useful when calculating a maintenance dose to maintain a target steady-state plasma concentration?

Answer: Total body clearance (CL)

Maintenance dose rate = CL × target Css; clearance determines how much drug must be given per unit time to replace what is eliminated.

On a dose-response curve, potency refers to:

Answer: The dose required to produce 50% of the maximum effect (ED50)

Potency is indicated by the ED50 — a more potent drug requires a lower dose to produce 50% of maximal effect; it does not describe the magnitude of effect.

A full agonist differs from a partial agonist in that a full agonist:

Answer: Produces maximal receptor activation (efficacy = 1)

A full agonist has intrinsic efficacy of 1, producing the maximum possible response; a partial agonist has intrinsic efficacy between 0 and 1, producing submaximal response even at full receptor occupancy.

The therapeutic index (TI) of a drug is calculated as:

Answer: LD50 / ED50

TI = LD50 / ED50; a higher TI indicates a wider margin between effective and lethal doses, meaning a safer drug.

A competitive antagonist differs from a non-competitive antagonist because a competitive antagonist:

Answer: Can be displaced by increasing agonist concentration, shifting the curve right

Competitive antagonists reversibly bind at the same site as the agonist; adding more agonist can displace them, restoring maximal effect but requiring higher doses.

Drug tolerance is best described as:

Answer: A decreased response to a drug after repeated administration requiring higher doses for the same effect

Tolerance is a pharmacological adaptation where repeated drug exposure causes diminished response, necessitating dose escalation to achieve the original effect.

Which of the following best describes the Emax model in pharmacodynamics?

Answer: Drug effect follows a sigmoidal curve and reaches a maximum ceiling effect

The Emax model describes a sigmoidal concentration-effect relationship where effect increases with concentration but plateaus at Emax when all receptors are occupied.

A patient on warfarin is started on rifampicin. Rifampicin is a potent CYP2C9 inducer. What pharmacokinetic change is expected and what clinical action is required?

Answer: Warfarin metabolism increases; increase warfarin dose to maintain anticoagulation

Rifampicin induces CYP2C9, accelerating warfarin metabolism and lowering plasma levels, which reduces anticoagulant effect and increases thrombosis risk; the warfarin dose must be increased.