Gastrointestinal lipophilic drug absorption

In summary, nasal epithelial intercellular junctions are less restrictive compared to the gastrointestinal tract. Such polar pathways will mainly be responsible for the transport of water-soluble compounds, providing a relatively slow, but significant route which is dependent on the molecular weight of the diffusing species. Secondly, transcellular (lipoidal) pathways permit extremely rapid absorption of lipophilic drugs with a rate dependency based on cell membrane partitioning. 

Chapter 6 Enhanced Gastrointestinal Absorption of Lipophilic Drugs.Ill... 

Fig. 4.1 Summary of the biophysical models applied to predict drug absorption from lipophilicity in the gastrointestinal tract.

Permeability and solubility/dissolution are two major determinants of gastrointestinal drug absorption. The prediction of solubility of molecules is more difficult than for lipophilicity. Solubility critically depends on the solid-state properties of compounds. The same compound can exist in amorphous or in several crystalline states and this can result in very different solubility of molecules. The prediction of crystalline properties, represented, for example, by the melting point, is one of the most difficult problems of physical chemistry. Like the octanol-water partition coefficient, water solubility critically depends on the pH and ionization state of molecules. 

Microemulsions are transparent systems of two immiscible fluids, stabilized by an interfacial film of surfactant or a mixture of surfactants, frequently in combination with a cosurfactant. These systems could be classified as water-in-oil, bicontinuous, or oil-in-water type depending on their microstructure, which is influenced by their physicochemical properties and the extent of their ingredients. - SMEDDSs form transparent microemulsions with a droplet size of less than 50 nm. Oil is the most important excipient in SMEDDSs because it can facilitate self-emulsification and increase the fraction of lipophilic drug transported through the intestinal lymphatic system, thereby increasing absorption from the gastrointestinal tract. Long-chain and medium-chain... 

The rate and extent of drug absorption from the gastrointestinal (GI) tract are very complex and affected by many factors. These include physico-chemical factors (solubility, lipophilicity, stabiUty, pK, polar surface area, presence of hydrogen... 

As in gastrointestinal absorption, the lipophillic nonionic form of a drug is more susceptible to reabsorption from the renal tubules by simple diffusion. Therefore,... 

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