Effect on the concentration-time curve when P-glycoprotein reduces absorption?

Effect on the concentration-time curve when P-glycoprotein reduces absorption?

• A. Shifts the curve upward
• B. Shifts the curve downward
• C. No change to the curve
• D. Increases the apparent half-life

Answer: (B)

P-glycoprotein in the gut acts as an efflux pump that reduces how much drug enters systemic circulation. When its activity lowers absorption, less drug is available in the blood at all time points after dosing. That makes the entire plasma concentration–time profile sit lower, giving a downward shift. The peak concentration and overall exposure (Cmax and AUC) decrease, while the fundamental elimination process remains the same. The curve doesn’t rise or stay unchanged; it simply sits lower. The other possibilities would require higher levels, no effect, or a change in apparent half-life due to altered kinetics, which isn’t the core consequence of reduced absorption here.

P-glycoprotein in the gut acts as an efflux pump that reduces how much drug enters systemic circulation. When its activity lowers absorption, less drug is available in the blood at all time points after dosing. That makes the entire plasma concentration–time profile sit lower, giving a downward shift. The peak concentration and overall exposure (Cmax and AUC) decrease, while the fundamental elimination process remains the same. The curve doesn’t rise or stay unchanged; it simply sits lower. The other possibilities would require higher levels, no effect, or a change in apparent half-life due to altered kinetics, which isn’t the core consequence of reduced absorption here.