Which drug is described as saturating P-gp and increasing absorption of other drugs?

• A. Rifampicin
• B. Verapamil
• C. Loperamide
• D. Digoxin

Answer: (B)

P-glycoprotein acts as an efflux gate in the intestinal lining, pumping many drugs back into the gut and limiting their absorption. The transporter has a finite capacity, so it can be saturated: when a drug binds strongly enough, it occupies the transporter and reduces its ability to efflux other co-administered drugs. A drug that does this effectively will raise the oral bioavailability of other drugs that are P-gp substrates.

Verapamil is a well-known P-gp inhibitor. at sufficient concentrations it can occupy and block the transporter, so other drugs that are substrates of P-gp experience less efflux and are absorbed more readily. This clash with P-gp’s normal function explains why coadministration can increase the systemic exposure of those substrates. In contrast, rifampicin induces P-gp expression, which typically reduces absorption of substrates; loperamide and digoxin are substrates themselves and are affected by P-gp, but they do not describe the saturating inhibitor effect that increases absorption of other drugs.