Pore Transport and Ion-Pair Transport

Pore transport, also called convective transport, enables small molecules like drugs, water, and urea to rapidly cross cell membranes through channels or pores. These transport proteins form open channels across the lipid membrane, facilitating faster diffusion of small molecules compared to other membrane regions. Pore transport is particularly important during renal drug excretion and hepatic drug uptake.

Highly ionizable drugs like quaternary ammonium compounds remain charged at various pH levels and cannot traverse membrane barriers on their own. Ion-pair formation allows these charged molecules to combine with oppositely charged endogenous ions in the GI tract, creating reversible neutral complexes that diffuse easily across membranes and enable systemic drug absorption.

Propranolol forms an ion pair with oleic acid, while quinine pairs with hexyl salicylate. These ion-pair complexes facilitate drug movement across GI membrane barriers. Additionally, amphotericin B complexes with DSPG in liposomal products, demonstrating how ion pairing can alter drug distribution and reduce renal toxicity.

Ion pairs help regulate free plasma drug levels by forming neutral complexes that alter drug distribution in the body. By reducing high plasma-free drug concentrations, ion pairing decreases the drug's toxic effects on the kidneys and other organs. This mechanism is particularly valuable for drugs that would otherwise accumulate to harmful levels.

Passive transport is the general mechanism where drugs move along concentration gradients without energy. Pore transport is a specific type of passive transport where small molecular weight drugs move through water-filled channels or pores in membranes, enabling faster diffusion than simple diffusion across the lipid bilayer itself.

Although direct microscopic evidence of membrane pores is limited, the pore transport model is widely accepted based on strong physiological evidence. This model effectively explains renal drug excretion and hepatic drug uptake, and the concept of transport proteins forming open channels across the lipid membrane is supported by functional observations of rapid small molecule diffusion.

In the GI tract, charged drugs combine with endogenous ions present in the environment to form reversible neutral complexes through ion-pair formation. These neutral complexes can then permeate across membrane barriers, enabling absorption into the systemic circulation. The process is reversible, allowing the drug to dissociate once absorbed.