Transfer across membranes drug interactions

Carrier-mediated transport involves interaction of the drug with a specific transporter or carrier, in which drug is transferred across the cell membrane or entire cell and then released from the basal surface of the enterocyte into the circulation. The process is saturable and utilized by small hydrophilic molecules. Drugs that are shown to be transported by this mechanism include p-lactam antibiotics, cephalosporins, and ACE inhibitors. 

In the majority of instances the rates at which absorption, distribution, metabolism and excretion of a drug occur are directly proportional to its concentration in the body. In other words, transfer of drug across a cell membrane or formation of a metabolite is high at high concentrations and falls in direct proportion to be low at low concentrations (an exponential relationship). This is because the processes follow the Law of Mass Action, which states that the rate of reaction is directly proportional to the active masses of reacting substances. In other words, at high concentrations, there are more opportunities for crowded molecules to interact with each other or to cross cell membranes them at low, xmcrowded concentrations. Processes for which rate of reaction is proportional to concentration are called first-order. 

In absorption studies, the appUcation of colloidal systems, which show specific and unspecific interactions with mainly lipophilic substances is of main interest. An obvious application is the study of lipophilic and poorly absorbable drugs that are administered orally or transdermally. Such interactions with surface-active agents may either cause a diminution of the bioavailability by trapping the drug in the micelle or, on the other hand, lead to an improved solubility (prevention of the precipitation of the drug) and facilitated transfer of the solute across lipid membranes (e.g., the intestinal wall in the gastrointestinal tract) and therefore to an improved bioavailability. 

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