3. Active transport is an energy-dependent process that can move drugs against a con-
centration gradient, as in protein-mediated transport systems. Active transport occurs
in only one direction and is saturable. It is usually the mode of transport for drugs that
resemble actively transported endogenous substances such as sugars, amino acids,
and nucleosides. Some transport systems may increase drug transport and the entry
into cells and increase their effects. Other transport systems cause active efflux of drugs
from target cells and decrease their activity, e.g', P-glycoprotein which is also known as
ABCB1 (ABC = ATP-binding cassette) orMDRl (MDR = multi-drug resistance).
4. Filtration is the bulk flow of solvent and solute through channels (pores) in the mem-
brane. Filtration is seen with small molecules (usually with a molecular weight