Membranes, barrier permeability

This book is written for the practicing pharmaceutical scientist involved in absorption-distribution-metabolism-excretion (ADME) measurements who needs to communicate with medicinal chemists persuasively, so that newly synthesized molecules will be more drug-like. ADME is all about a day in the life of a drug molecule (absorption, distribution, metabolism, and excretion). Specifically, this book attempts to describe the state of the art in measurement of ionization constants (p Ka), oil-water partition coefficients (log PI log D), solubility, and permeability (artificial phospholipid membrane barriers). Permeability is covered in considerable detail, based on a newly developed methodology known as parallel artificial membrane permeability assay (PAMPA). 

The evaluation of the apparent ionization constants (i) can indicate in partition experiments the extent to which a charged form of the drug partitions into the octanol or liposome bilayer domains, (ii) can indicate in solubility measurements, the presence of aggregates in saturated solutions and whether the aggregates are ionized or neutral and the extent to which salts of dmgs form, and (iii) can indicate in permeability measurements, whether the aqueous boundary layer adjacent to the membrane barrier, Umits the transport of drugs across artificial phospholipid membranes [parallel artificial membrane permeation assay (PAMPA)] or across monolayers of cultured cells [Caco-2, Madin-Darby canine kidney (MDCK), etc.]. 

Permeability-pH profiles, log Pe - pH curves in arhficial membrane models (log Pjpp - pH in cehular models), generally have sigmoidal shape, similar to that of log Dod - pH cf. Fig. 3.1). However, one feature is unique to permeabihty profiles the upper horizontal part of the sigmoidal curves may be verhcally depressed, due to the drug transport resistance arising from the aqueous boundary layer (ABL) adjacent to the two sides of the membrane barrier. Hence, the true membrane contribution to transport may be obscured when water is the rate-limiting resistance to transport. This is especially true if sparingly soluble molecules are considered and if the solutions on either or both sides of the membrane barrier are poorly stirred (often a problem with 96-well microhter plate formats). 

Using liposomes made from phospholipids as models of membrane barriers, Chakrabarti and Deamer [417] characterized the permeabilities of several amino acids and simple ions. Phosphate, sodium and potassium ions displayed effective permeabilities 0.1-1.0 x 10 12 cm/s. Hydrophilic amino acids permeated membranes with coefficients 5.1-5.7 x 10 12 cm/s. More lipophilic amino acids indicated values of 250 -10 x 10-12 cm/s. The investigators proposed that the extremely low permeability rates observed for the polar molecules must be controlled by bilayer fluctuations and transient defects, rather than normal partitioning behavior and Born energy barriers. More recently, similar magnitude values of permeabilities were measured for a series of enkephalin peptides [418]. 

The equations used to calculate permeability coefficients depend on the design of the in vitro assay to measure the transport of molecules across membrane barriers. It is important to take into account factors such as pH conditions (e.g., pH gradients), buffer capacity, acceptor sink conditions (physical or chemical), any precipitate of the solute in the donor well, presence of cosolvent in the donor compartment, geometry of the compartments, stirring speeds, filter thickness, porosity, pore size, and tortuosity. 

Fig. 7.1. The intestinal permeability of drugs in vivo is the total transport parameter that may be affected by several parallel transport mechanisms in both absorptive and secretory directions. Some of the most important transport proteins that may be involved in the intestinal transport of drugs and their metabolites across intestinal epithelial membrane barriers in humans are displayed.

Several in vitro assays using filter immobilized artificial membranes exist for the estimation of permeability (Parallel artificial membrane permeation assay, PAMPA). In these assays the permeation of a compound is followed directly by estimating the amount of compound on either side of the membrane barrier. The results of these experiments are expressed as permeability values rather than lipophilicity values. 

Coincident with the reduction in brain level of hydroxyl radicals, U-74006F administered at 5 minutes post-injury also acts to reduce post-traumatic opening of the blood-brain barrier (i.e. decreased brain uptake of 14C-albumin) [56]. This effect of U-74006F to close the barrier may be related to the attenuation of hydroxyl-radical levels or an antagonism of the effects of free radicals on the barrier endothelium (i.e. decreased membrane-lipid peroxidation). Indeed, free radicals are known to increase barrier permeability [57]. Consistent with this reduction in post-traumatic opening of the blood-brain barrier which would lead to vasogenic brain edema, U-74006F has been shown to attenuate post-traumatic brain edema in a rat model of fluid percussion head injury [58]. 

Figure 1.2 Schematic representation of the pathway of elementary reaction ij in the traditional energetic coordinates with the activation barrier (a) and in the coordinates of thermodynamic rushes h of reactants (b). in the latter case, the reaction can be represented as a flow of a fluid between two basins separated by a membrane with permeability e-,j the examples are given for the left-to-right and right-to-left reactions (cases 1 and 3, respectively) case 2 illustrates the thermodynamically equilibrium system.

Membranes represent permeability barriers to the movement of ions (and electrons) and this phenomenon is utilised in a number of ways in biological systems as mentioned in the previous section. In terms of generalising and for the sake of briefness, this gradient of ions has two principal uses, the first as an energy store albeit labile and secondly a means of long... 

Using the concept of the one-dimensional series of diffusion resistances, and regarding the principle of resistance additivity [1 0,46-53], the overall mass transfer coefficients or permeability coefficients [58,77] Kp on the feed side and Xr on the strip side are related to the individual films (including membrane barrier) mass transfer coefficients, k ... 

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