Biomembrane passage

The biomembrane passage of a drug depends primarily on its physicochemical properties and especially on its partition coefficient (Chapters 22 and 34). Thus, the transient attachment of a lipophilic carrier group to an active principle can provide a better bioavailability, mostly by facifitating cell membrane crossing by passive diffusion. Peroral absorption, as well as rectal absorption, ocular drug delivery and dermal drug delivery, are dependent on passive diffusion. Finally, lipophilic carriers can sometimes be useful to reduce first-pass metabolism. ... 

There can be no doubt that one of the most interesting applications of the results from studies of bilayers is to biomembranes and other biostructures, Investigations of the stability and permeability of biomembranes are particularly of great interest. For instance, it is known [425-427] that a possible mechanism of the transfer of permeant (e.g. water dissolved ions, etc.) across a bilayer biomembrane is the passage of the permeant through microholes in the membrane. The statistically distributed holes in the bilayer biomembrane, formed by the mechanism of nucleation described, may thus turn out to be very important for the permeability of such biomembranes. 

For these compounds there is a direct correlation between biological activity and certain physicochemical parameters such as partition coefficient, surface tension or vapor pressure. The accumulation of halogen atoms favors the passage of the biomembranes and the access to the CNS. Flansch has dehned a parameter, vr, representing the... 

Transport passage of ions and certain molecules through biological membranes. Most polar molecules do not pass ely across biomembranes. Exchange of essential metabolites between a cell and its surroundings, or between cytoplasm and organelles, therefore depends on T. mechanisms within the membranes. All T. mechanisms catalysed by biomembranes have three characteristic properties saturation, substrate specificity and specific inhibition. T. may be active or passive. 

With the above-mentioned background, it is informative to describe an application of the N- P equation as applied to the nerve axon, which is surrounded by a plasma membrane. One of the main functions of the plasma membrane is to control the passage of ions and molecules into and out of the cell. For most biomembranes, the intracellular [K+]i greatly exceeds extracellular [K+]o, and the opposite is true for the extracellular [Na+]o and [Cl ]o. These concentration differences are due to the active transport system embedded in the lipid bilayer of the plasma membrane [3]. 

One of the important roles of biomembranes is to moderate the passage of substances between compartments of organisms. In particular, the passage of ions and proteins are actively controlled. The passage of some small nutrients are also regulated. Many small, neutral molecules can passively diffuse (without the assistance of protein channels) across the membrane, however, and this is the principal means of transport of drug molecules into cells. It is this passive transport of small molecules that will be the focus of this paper. We will show how the details of the... 

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