Lipophilic special

The dipping solutions described in Part II of this book are usually less concentrated than the corresponding spray solutions. The solvents employed are specially chosen for their suitability to the special requirements of dipping solutions. Water, which on the one hand, can sit on the surface of RP plates and not penetrate them and, on the other hand, can cause disintegration of water-incompatible layers is usually replaced by alcohol or other lipophilic solvents. 

The quantitative descriptor of lipophilicity, the partition coefficient P, is defined as the ratio of the concentrations of a neutral compound in organic and aqueous phases of a two-compartment system under equilibrium conditions. It is commonly used in its logarithmic form, logP. Whereas 1-octanol serves as the standard organic phase for experimental determination, other solvents are applied to better mimic special permeation conditions such as the cyclohexane-water system for BBB permeation. Measurement of log P is described in Chapters 12 and 13 as well as in Ref [22]. 

A CRO may also allow for the in-house introduction of specialized lipophilic scales by transferring routine measurements. While the octanol-water scale is widely applied, it may be advantageous to utilize alternative scales for specific QSAR models. Solvent systems such as alkane or chloroform and biomimetic stationary phases on HPLC columns have both been advocated. Seydel [65] recently reviewed the suitabihty of various systems to describe partitioning into membranes. Through several examples, he concludes that drug-membrane interaction as it relates to transport, distribution and efficacy cannot be well characterized by partition coefficients in bulk solvents alone, including octanol. However, octanol-water partition coefficients will persist in valuable databases and decades of QSAR studies. 

Note that the distribution coefficient depends only on pH, pKa values, and P (not on concentration of sample species). Equation (4.7) is applicable to all lipophilicity calculations. Special cases, such as eq. 4.9, have been tabulated [275],... 

Sugano et al. [561,562] explored the lipid model containing several different phospholipids, closely resembling the mixture found in reconstituted brush border lipids [433,566] and demonstrated dramatically improved property predictions. The best-performing lipid composition consisted of a 3% wt/vol lipid solution in 1,7-octadiene (lipid consisting of 33% wt/wt cholesterol, 27% PC, 27% PE, 7% PS, 7% PI). The donor and acceptor compartments were adjusted in the pH interval between 5.0 and 7.4 [562]. With such a mixture, membrane retention is expected to be extensive when lipophilic drugs are assayed. The use of 1,7-octadiene in the assay was noted to require special safety precautions. 

Fig. 10.2 The host ranges of species that utilize hosts that produce ( Present ) or do not produce ( Absent ) lipophilic chemistry (Poore et al., unpublished data). The more generalized amphipods and fishes have a broader host range when incorporating chemically rich seaweeds in their diet, while the host range of the more specialized slugs does not display any effect of secondary metabolites...

Special attention shall be drawn to the preparation of the excised skin prior to the experiment. Long lag-times encountered with hydrophilic substances, as well as an unfavorable partition of lipophilic compounds into viable skin layers, may require the further separation of the skin into its individual layers [80], Furthermore, the absence of dermal blood flow in vitro may build up a significant hindrance to diffusion [81]. Reducing the membrane thickness will generally reduce experiment times and thus minimize the risk of bacterial contamination. 

Chemicals are distributed throughout the body via the bloodstream (or the lymph in special cases). Lipophilic compounds are to a large extent bound to proteins in the blood instead of just dissolved in water. A more lipophilic compound may remove a less lipophUic substance from the binding site and thus severely increase the concentration of unbound compound available for toxicological effect. This situation is well known for medical dmgs administered simultaneously (Feron et al. 1995c). 

Each hormone is the center of a hormonal regulation system. Specialized glandular cells synthesize the hormone from precursors, store it in many cases, and release it into the bloodstream when needed (biosynthesis). For transport, the poorly water-soluble lipophilic hormones are bound to plasma proteins known as hormone carriers. To stop the effects of the hormone again, it is inactivated by enzymatic reactions, most of which take place in the liver (metabolism). Finally, the hormone and its metabolites are expelled via the excretory system, usually in the kidney (excretion). All of these processes affect the concentration of the hormone and thus contribute to regulation of the hormonal signal. 

There is thus a complicated interplay of the thermodynamics and kinetics of complexation, lipophilicity, diffusion and interface phenomena. Synthetic ligands are of special interest since, in principle, stepwise variations in the different parameters may be brought about via structural modification of the carrier. An efficient and selective carrier should have the following features (see also 16, 134) ... 

A further important issue is the balance between the perfluoroalkane and the alkane part in the RFRH molecules and the branching ratio in both these compartments. This is of special importance if penetration processes into tissues must be taken into consideration. Especially low molecular weight species or partial fluorinated liquids with their distinctly higher lipophilic potential as PFCLs can easily penetrate tissues. 

A special group of lipids that possess both hydrophilic and hydrophobic (lipophilic) parts are termed as amphiphiles or amphipathics and are also referred to as surfactants. They adsorb at surfaces or interfaces and change the interfacial free energy associated with the building of an interface. A surfactant molecule consists of two distinct chemical groups (i) the head which is hydrophilic (water-loving) and (ii) the tail which is hydrophobic (water-fearing). 

The availability of a chemical to the cells is affected by where it is stored. First, lipophilic chemicals tend to get absorbed by and retained in fat cells, from which they are released slowly back into the bloodstream. Second, some chemicals are strongly bound to plasma proteins and are released to the cells more slowly over time. For example, acetaminophen (Tylenol ) does not bind strongly to plasma proteins, while diazepam (Valium ) does. Thus, diazepam will persist in the body for longer periods of time than will acetaminophen. Finally, some elements, such as fluorine, lead, and strontium, are bound up in bone for long periods of time. As bone slowly renews itself or is broken down under special circumstances such as pregnancy, the chemicals are released and can affect the mother and fetus. 

Courtship behavior of Dysdera crocata, a specialized feeder on woodlice (Pso-coptera), is released by contact with nest silk or the cuticle of a live or even dead female. Washing with ether rendered these substrates inactive, suggesting the presence of a lipophilic contact pheromone (Pollard et al., 1987). 

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