Carrier-Mediated Transport

Carrier-mediated transport facilitates movement of lipid-insoluble drugs through membrane-spanning transporters, enabling absorption of drugs that cannot cross membranes passively. Unlike passive diffusion, this process involves specific protein carriers and can occur via facilitated diffusion (along concentration gradients without energy) or active transport (against gradients using ATP). This mechanism is essential for drugs like methyldopa and gabapentin.

Absorption rates plateau because carrier proteins become saturated as drug concentration increases. Once all available transporters are occupied, the system reaches maximum capacity and cannot transport additional drug molecules, regardless of further concentration increases. This saturation point represents the transporter's maximum velocity and is a key characteristic distinguishing carrier-mediated from passive transport mechanisms.

Uptake transporters move drugs into the blood across intestinal epithelial cells, increasing absorption. Located at brush borders and basolateral membranes, these transporters facilitate movement of amino acids, peptides, and other substances. The intestinal oligopeptide transporter PepT1 exemplifies this function, enabling absorption of di- and tripeptides formed from digested oral proteins, thereby enhancing peptide drug bioavailability.

Efflux transporters such as P-glycoprotein (MDR1) pump drugs back into the gastrointestinal lumen, decreasing intestinal absorption. These ATP-binding cassette proteins are strategically positioned to protect the body from undesirable compounds and xenobiotics. P-glycoprotein's widespread expression across the GI tract and other tissues, particularly the adrenal medulla, enables its defensive role in reducing permeability of lipophilic and cytotoxic drugs from lumen to blood.

Facilitated diffusion moves drugs along their concentration gradient without energy expenditure, relying solely on transporter proteins to facilitate movement. Active transport, conversely, requires ATP to transport drugs against their concentration gradient, enabling absorption of drugs that would otherwise accumulate outside cells. Both are carrier-mediated processes but differ fundamentally in energy requirements and directional capability.

Organic anion transporters driven by α-ketoglutarate facilitate movement of drugs like pravastatin and atorvastatin into the blood, enhancing their absorption. These uptake transporters recognize specific drug structures and actively transport them across intestinal epithelial cells. Their presence demonstrates how diverse transporter types work together to optimize absorption of lipid-insoluble drugs with different chemical properties.

P-glycoprotein contributes to treatment resistance by pumping drugs out of cells, reducing intracellular drug concentration and therapeutic efficacy. This efflux transporter protects vital organs and cells from xenobiotics and cytotoxic compounds through its widespread expression across tissues. In cancer cells, elevated P-glycoprotein expression is a major mechanism of multidrug resistance, making it a critical target for overcoming treatment failure.