Lipid barrier model

Three sets of two-chamber diffusion cell experiments were conducted 1) ethanol/saline in the donor chamber and saline in the receiver chamber, 2) saline in the donor and ethanol/saline in the receiver chamber, and 3) with ethanol/saline in both chambers. The results were shown to deviate enormously from the classical lipid barrier model. A new model, based on diffusion across a binary solvent mixture, was used to analyze the data. A good agreement was observed between experimental data and theoretical results. 

Experimental Results and Comparisons with the Classical Lipid Barrier Model. Some typical experimental data are presented in Figure 1 for the transport of g-estradiol. In each of the experiments a lag-time of 1.5 to 2.5 hours were followed by linear steady state fluxes. The effective permeability coefficient, Peff> was calculated from such data using Equation 1 under sink conditions (i.e., Cj/K Cr/Kr where, Kj is the partition coefficient between membrane and donor phase and Kr the partition coefficient between membrane and receiver phase.)... 

Figure 3 summarizes the results of all the experiments. These data clearly demonstrate the failure of the classical lipid barrier model. Firstly, the upper set of data (A) show that the composition of the receiver solution significantly influences g-estradiol flux the simple model would predict that the P value ought to be constant in this case. Secondly, a simple lipid barrier model would have predicted that the lower two sets of data (B and C) should be superimposed, however the B and C results are significantly different. 

In order to examine the deviations of the data from the predictions of the classical lipid barrier model, one may calculate the theoretical permeability coefficients for the classical lipid barrier model using the expression ... 

Figure 3. Comparison of experimental results with the classical "lipid barrier model". Symbols and dashed curves (A,B,C) represent experimental data. A) saline in the donor compartment, ethanol in respective volume fractions between 0 and 1 in the receiver compartment B) equal compositions of ethanol/saline mixtures in volume fractions between 0 and 1 in donor and receiver compartment. C) saline in the receiver compartment, ethanol/saline in volume fractions between 0 and 1 in donor compartment. A, B and C represent theory based on the "lipid barrier model".

Although there are still notable deviations between experiments and the new theory, the agreement are much better over a wide range of conditions than those obtained with the lipid barrier model (Figure 5). 

One of the key parameters for correlating molecular structure and chemical properties with bioavailability has been transcorneal flux or, alternatively, the corneal permeability coefficient. The epithelium has been modeled as a lipid barrier (possibly with a limited number of aqueous pores that, for this physical model, serve as the equivalent of the extracellular space in a more physiological description) and the stroma as an aqueous barrier (Fig. 11). The endothelium is very thin and porous compared with the epithelium [189] and often has been ignored in the analysis, although mathematically it can be included as part of the lipid barrier. Diffusion through bilayer membranes of various structures has been modeled for some time [202] and adapted to ophthalmic applications more recently [203,204]. For a series of molecules of similar size, it was shown that the permeability increases with octa-nol/water distribution (or partition) coefficient until a plateau is reached. Modeling of this type of data has led to the earlier statement that drugs need to be both... 

Kramer, S. D. Hurley, J. A. Abbott, N. J. Begley, D. J., Lipids in blood-brain barrier models in vitro I TLC and HPLC for the analysis of hpid classes and long polyunsaturated... 

In summary, we may thus conclude that PGLa and GS do not form stable, NMR-observable pores in native membrane as readily as they do in model bilayers. The corresponding tilted and/or inserted states of our two representative MAPs could only be comprehensively characterized in DMPC-based samples, where the peptides could be trapped in a uniform state. In living cells, on the other hand, these states would seem to be only of a transient nature, i.e. at the very moment when the antimicrobial peptide attacks the membrane and passes through the lipid barrier along its concentration gradient towards the cytosol. 

The Sartorius Absorption Model (26), which served as the forerunner to the BCS, simulates concomitant release from the dosage form in the GI tract and absorption of the drug through the lipid barrier. The most important features of Sartorius Absorption Model are the two reservoirs for holding different media at 37°C, a diffusion cell with an artificial lipid barrier of known surface area, and a connecting peristaltic pump which aids the transport of the solution or the media from the reservoir to the compartment of the diffusion cell. The set-up is shown in Figures 7a and b. 

The cell membranes are predominantly a lipid matrix or can be considered a lipid barrier with an average width of a membrane being approximately 75 A. The membrane is described as the fluid mosaic model (Figure 6.2) which consist of (1) a bilayer of phospholipids with hydrocarbons oriented inward (hydrophobic phase), (2) hydrophilic heads oriented outward (hydrophilic phase), and (3) associated intra- and extracellular proteins and transverse the membrane. The ratio of lipid to protein varies from 5 1 for the myelin membrane to 1 5 for the inner structure of the mitochondria. However, 100% of the myelin membrane surface is lipid bilayer, whereas the inner membrane of the mitochondria may have only 40% lipid bilayer surface. In this example the proportion of membrane surface that is lipid will clearly influence distribution of toxicants of varying lipophilicity. 

Another cutoff model, which deals with nonlinearity in biological systems, is one defined by McFarland (191). It attempts to elucidate the dependency of drug transport on hydrophobicity in multicompartment models. McFarland addressed the probability of drug molecules traversing several aqueous lipid barriers from the first aqueous compartment to a distant, final aqueous compartment. The probability of a drug molecule to access the final compartment n of a biological system was used to define the drug concentration in this compartment. 

Several factors that determine antibiotic diffusion into prostatic secretions were delineated from the canine model. Lipid solubility is a major determinant in the ability of drugs to diffuse from plasma across epithelial membranes. The degree of ionization in plasma also affects the diffusion of drugs. Only un-ionized molecules can cross the lipid barrier of prostatic cells, and the drug s pKa directly determines the fraction of unchanged drug. 

The pH gradient across the membrane has an influence on tissue penetration as well. A pH gradient of at least 1 pH unit between separate compartments allows for ion trapping. As the un-ionized drug crosses the epithelial barrier into prostatic fluid, it becomes ionized, allowing less drug to diffuse back across the lipid barrier. In early studies with the canine model, the prostatic pH was reported to be acidic (6.4). More recent studies in humans, however, have reported that the pH of prostatic secretions from an inflamed prostate is actually basic (8.1 to 8.3). ... 

A Key Experiment PAMPA-BBB, a Lipid-Based Model for the Blood-Brain Barrier 357... 

A KEY EXPERIMENT PAMPA-BBB, A LIPID-BASED MODEL FOR THE BLOOD-BRAIN BARRIER... 

Biological membranes provide the essential barrier between cells and the organelles of which cells are composed. Cellular membranes are complicated extensive biomolecular sheetlike structures, mostly fonned by lipid molecules held together by cooperative nonco-valent interactions. A membrane is not a static structure, but rather a complex dynamical two-dimensional liquid crystalline fluid mosaic of oriented proteins and lipids. A number of experimental approaches can be used to investigate and characterize biological membranes. However, the complexity of membranes is such that experimental data remain very difficult to interpret at the microscopic level. In recent years, computational studies of membranes based on detailed atomic models, as summarized in Chapter 21, have greatly increased the ability to interpret experimental data, yielding a much-improved picture of the structure and dynamics of lipid bilayers and the relationship of those properties to membrane function [21]. 

With the adequacy of lipid bilayer membranes as models for the basic structural motif and hence for the ion transport barrier of biological membranes, studies of channel and carrier ion transport mechanisms across such membranes become of central relevance to transport across cell membranes. The fundamental principles derived from these studies, however, have generality beyond the specific model systems. As noted above and as will be treated below, it is found that selective transport... 

The most likely way for pardaxin molecules to insert across the membrane in an antiparallel manner is for them to form antiparallel aggregates on the membrane surface that then insert across the membrane. We developed a "raft"model (data not shown) that is similar to the channel model except that adjacent dimers are related to each other by a linear translation instead of a 60 rotation about a channel axis. All of the large hydrophobic side chains of the C-helices are on one side of the "raft" and all hydrophilic side chains are on the other side. We postulate that these "rafts" displace the lipid molecules on one side of the bilayer. When two or more "rafts" meet they can insert across the membrane to form a channel in a way that never exposes the hydrophilic side chains to the lipid alkyl chains. The conformational change from the "raft" to the channel structure primarily involves a pivoting motion about the "ridge" of side chains formed by Thr-17, Ala-21, Ala-25, and Ser-29. These small side chains present few steric barriers for the postulated conformational change. 

---