Absorption is the process by which a drug travels from its site of administration into the systemic circulation. For a drug to exert a pharmacological effect, it must first reach the bloodstream at concentrations sufficient to produce a response. The mechanisms by which drugs cross biological membranes are varied and depend on the physicochemical properties of the drug, the nature of the membrane, and the presence of specialized transport systems.
Passive Diffusion
The most common mechanism of drug absorption is passive diffusion, where drug molecules move down a concentration gradient from an area of high concentration to an area of low concentration. This process does not require energy and is driven solely by the concentration difference across the membrane. Most drugs are weak acids or weak bases, and their degree of ionization at a given pH determines their ability to cross lipid membranes. The unionized form is lipid-soluble and can diffuse freely, while the ionized form is water-soluble and trapped. This principle is encapsulated in the pH partition hypothesis, which predicts that acidic drugs are best absorbed in acidic environments like the stomach, while basic drugs are best absorbed in alkaline environments like the small intestine.
The rate of passive diffusion follows Fick’s law, which states that the rate is proportional to the concentration gradient, the surface area, and the permeability coefficient of the drug. The small intestine, with its extensive villi and microvilli, provides a massive surface area for absorption, making it the primary site for most orally administered drugs regardless of their pKa.
Facilitated Diffusion and Active Transport
Some drugs require facilitated diffusion, a carrier-mediated process that does not consume energy but allows movement down a concentration gradient at a faster rate than passive diffusion alone would permit. Carrier proteins bind the drug on one side of the membrane and undergo a conformational change to release it on the other side. This process is saturable and selective, meaning only drugs with the appropriate molecular structure can utilize a given carrier.
Active transport differs in that it moves drugs against a concentration gradient and requires energy, typically in the form of adenosine triphosphate. This mechanism is particularly important for drugs that structurally resemble endogenous substances, such as levodopa, which uses the large neutral amino acid transporter. Active transport systems are saturable, can be competitively inhibited, and exhibit substrate specificity.
Endocytosis and Transcytosis
For very large molecules and macromolecular drugs, endocytosis provides a route across cell membranes. The cell membrane invaginates to engulf the drug molecule, forming a vesicle that is internalized into the cell. The vesicle contents can then be released inside the cell or transported across the cell and released on the opposite side, a process called transcytosis. This mechanism is relevant for certain peptide drugs, vaccines, and targeted drug delivery systems such as nanoparticles.
The Role of Efflux Transporters
An important modulator of drug absorption is the efflux transporter P-glycoprotein, found in the apical membrane of intestinal epithelial cells. P-glycoprotein actively pumps a wide range of drugs back into the intestinal lumen, limiting their absorption. This transporter is also expressed at the blood-brain barrier, where it restricts drug entry into the central nervous system. Many drug-drug interactions occur through inhibition or induction of P-glycoprotein, altering the bioavailability of coadministered medications.
Factors Affecting Absorption
Several factors influence drug absorption beyond membrane transport mechanisms. Gastrointestinal pH varies along the digestive tract and affects drug ionization. Gastric emptying rate determines how quickly a drug reaches its primary absorption site in the small intestine. Food in the stomach can delay gastric emptying, alter pH, or physically bind drugs, reducing absorption. Blood flow to the absorption site also matters, as it maintains the concentration gradient by removing absorbed drug. Disease states affecting gastrointestinal motility or mucosal integrity can significantly alter absorption profiles.
Understanding these mechanisms is essential for predicting drug behavior, designing effective dosage forms, and anticipating interactions that may compromise therapeutic outcomes. The interplay between drug properties and biological transport systems ultimately determines whether a drug achieves adequate systemic exposure.
