3.10 : 孔隙输送和离子对输送

Most drugs cross the membrane barriers through passive transport mechanisms, moving along the concentration gradient.

Many low molecular weight drugs move along with water through small pores in the cell membranes, facilitating faster diffusion. This pore transport is mainly followed during renal drug excretion or hepatic drug uptake.

Some drugs, including quaternary ammonium compounds, are highly ionizable and remain charged at various pH levels. These charged molecules cannot traverse membrane barriers.

Ion-pair transport allows such drugs to be absorbed into the body. In the GI tract, they combine with endogenous ions to form reversible neutral complexes, enabling their membrane permeation and systemic drug absorption.

For example, propranolol forms an ion pair with oleic acid, and quinine pairs with hexyl salicylate, facilitating their movement across the GI membrane barriers.

Notably,  such ion pairs help regulate the free plasma drug levels and decrease the drug's toxic effects on the kidneys.