How can drug transporters influence pharmacokinetics?

• A. They have no effect on PK.
• B. They only affect distribution, not absorption or excretion.
• C. They always increase clearance linearly with dose.
• D. They can alter absorption and excretion via active transport; saturable and can cause nonlinear PK.

Answer: (D)

Drug transporters actively move drugs across membranes and can be saturated, so their impact on pharmacokinetics goes beyond simple distribution. In the gut, transporters can limit absorption by pumping drug back into the lumen, or enhance entry with uptake transporters. In the kidney and liver, they govern secretion into urine and bile, respectively, shaping clearance. When transporter-mediated processes become saturated at higher drug concentrations, the rate of absorption or excretion no longer increases proportionally with dose, leading to nonlinear PK. Because transporters can influence both absorption and excretion and their effects can be saturable, they can cause nonlinear changes in drug exposure. This is why the correct statement is that they can alter absorption and excretion via active transport, are saturable, and can cause nonlinear PK.