Transfer, biological membranes absorption

After absorption, a chemical compound enters the circulation, which transfers it to all parts of the body. After this phase, the most important factor affecting the distribution is the passage of the compound through biological membranes. From the point of view of the distribution of a chemical compound, the organism can be divided into three different compartments (1) the plasma compartment (2) the intercellular compartment and (3) the intracellular compartment. In all these compartments, a chemical compound can be bound to biological macromolecules. The proportion of bound and unbound (free) chemical compound depends on the characteristics of both the chemical... 

Lengemann, F.W. 1963 Overall aspects of calcium and strontium absorption. In Wasserman, R.H., ed.. Transfer of Calcium and Strontium Across Biological Membranes. New York, Academic Press 85-96. 

In addition to the passive diffusional processes over lipid membranes or between cells, substances can be transferred through the lipid phase of biological membranes through specialized systems, i.e., active transport and facilitated diffusion. Until recently, the active transport component has been discussed only for nutrients or endogenous substances (e.g., amino acids, sugars, bile acids, small peptides), and seemed not to play any major role in the absorption of pharmaceuticals. However, sufficient evidence has now been gathered to recognize the involvement of transporters in the disposition of pharmaceuticals in the body [50, 127]. 

Transfer of an active substance across a biological membrane is influenced by different physico-chemical drug properties, which are combined in Lipinski s ([7] Rule-of-5 . This rule predicts poor absorption of an orally administered active substance when it ... 

Since transport across the biological membrane of weak bases will be more pronounced in the small intestine (uptake of the unionized form), sufficient precipitation inhibition (polymer) is required upon transfer of the supersaturated solution to the intestine. Therefore, one cannot rely on dissolution studies at constant acidic pH to predict the performance of formulations of weak bases in vivo (Miller et al. 2007). For instance. Six et al. (2005) observed a discrepancy between the results of in vitro dissolution tests in acidic medium and in vivo absorption for four solid dispersions of ITR faster release and increased supersaturation in acidic medium correlated with lower bioavailability. Presumably, this effect can be explained by differences in crystallization rate upon transfer to the small intestine (increased driving force for precipitation in case of higher supersaturation). Thus, it is crucial to simulate the GI pH shift during supersaturation dissolution testing of weak bases to evaluate whether supersaturation is maintained in the small intestine. 

Membrane absorption process is an integrated mass transfer operation in whieh one of the eomponents in a multicomponent gaseous mixture can be selectively removed through a membrane and simultaneously dissolved in an absorbing liquid. It facilitates a bubble free gas-liquid mass transfer, which is most desirable for shear sensitive biological mixtures. Some of the important industrial applications of membrane absorption include removal of COjfrom flue gas, removal of H S from gas streams, recovery of cyanide from waste-water and absorption of NHj (in a polypropylene hollow-fiber coliunn) with dilute HjSO as absorbent. 

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