Absorption of Small Molecules

D. E. Leahy, J. Lynch, D. C. Taylor. Mechanisms of absorption of small molecules. In eds. L. F. Prescott and W. S. Nimmo, Novel Drug Delivery and its Therapeutic Application. Wiley, New York 1989, 33-44. 

Poor absorption of small-molecule drugs across such external barriers has been presumed to be due to the drugs physio-chemical properties, which can be systematically modified by medicinal chemists, and the nature of the physiological barrier, which can sometimes be transiently altered. To overcome physiological barriers, scientists have studied a series of synthetic and natural compounds exhibiting absorption enhancement properties. These molecules, included in the dosage formulation to enhance absorption, are... 

In recent years study of the absorption of small molecules on well characterised single crystal surfaces has attracted many research workers. Here, however, we will only be concerned with relatively large molecules such as, for example, long chain fatty acids. In 1946 Bigelow et al. [16] showed that a carboxylic acid dissolved in a non-polar solvent will adsorb on to a hydrophilic surface immersed in this solvent and that,... 

All the above transport mechanisms are only applicable to the absorption of small molecules, less than approximately 500 Da. There is evidence that larger molecules can be absorbed with low efficiency due to endocy tosis. Endocy tosis is defined as the internalization of plasma membrane with concomitant engulfment of extracellular material and extracellular fluid. The process can be divided into two types, pinocytosis and phagocytosis. 

Digestive secretions Absorption of small molecules and water... 

The absorption of small molecules (molecular radii less than 4 A) through water-filled pores of biological membrane is referred to as convective absorption. The rate of absorption because of this mechanism is equated to the product of a sieving coefficient, the rate of fluid or water absorption, and the concentration of solute in the luminal content. The sieving coefficient is indirectly related to the relative sizes of the pores and the molecules. 

The parenteral route represents the only efficient entry of a polymer into the body. This can be accomplished, e.g. by intraperitoneal, subcutaneous, intramuscular or intravascular injection of a polymer solution. However, these ways are by no means equivalent. The absorption of polymers from the cavities, such as the peritoneal or pleural, is hindered by the serose, lining the inner surface of cavities. Therefore, especially large molecules may remain there for several days The absorption is slow and takes place mainly in the lymphatic capillaries. Therefore, a rapid increase of the polymer concentration is observed in the lymphatic system, particularly in the regional lymph nodes while an increase in the concentration in the central compartment and consequently rapid distribution throughout the body is protracted and delayed i59> Owing to this behavior, which is quite different from the absorption of small molecules, a lowering in the systemic toxicity and excretion rate of basic antibiotics bound to the polyanions was observed... 

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