Pore transport and ion-pair formation are critical mechanisms for the absorption and distribution of drugs in the body.
Pore transport, also known as convective transport, is a process where small molecules like urea, water, and sugars rapidly cross cell membranes as though there were channels or pores in the membrane. Although direct microscopic evidence is limited but the concept of pores or channels is widely accepted based on physiological evidence. Despite the lack of direct microscopic evidence of such pores, this model explains renal drug excretion and hepatic drug uptake. Transport proteins may form an open channel across the cell's lipid membrane, facilitating faster diffusion of small molecules, including drugs, compared to other parts of the membrane.
On the other hand, ion-pair formation involves the binding of oppositely charged ions to form a neutral complex. The strong electrolyte drugs maintain their charge at all physiological pH values and penetrate membranes poorly. However, when paired with an oppositely charged ion, a neutral ion pair forms, which diffuses more easily across the membrane. Examples include propranolol, which forms an ion pair with oleic acid, and quinine, which pairs with hexyl salicylate.
The use of ion pairs has intriguing applications, such as the complexation of amphotericin B and DSPG in certain amphotericin B/liposomal products. Here, ion pairing can transiently alter distribution, reduce high plasma-free drug concentration, and decrease renal toxicity.
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.
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