Drug Absorption

Inhaled drugs can be absorbed from their deposition site in various parts of the respiratory tract, including the upper airways, mouth, pharynx and lower airways. Due to the large surface area and blood supply, there is considerable interest in delivering drugs systemically via the lung, particularly for drugs such as peptides which are degraded in the gastrointestinal tract. Since the blood from the pulmonary circulation is returns to the heart and is pumped to the tissues, no first-pass metabolism occurs in the liver. The rate-limiting portion of the barrier is thought to be the alveolar membrane itself34. It is believed that this barrier exhibits passive transport characteristics similar to other organs lined with epithelial cells. Lipid soluble drugs are usually absorbed by passive diffusion at rates that correlate with their lipid/water partition coefficients. Anaesthetics and respiratory gases cross the alveolar-capillary membrane quite readily, water crosses easily and in large quantities, but isotonic sodium chloride is only slowly absorbed. The membrane is only slightly permeable to aqueous solutes. Hydrophilic compounds are absorbed by diffusion through aqueous membrane pores, their absorption rate being inversely related to their molecular size.

Compounds such as disodium cromoglycate are at least partly absorbed by a saturable, carrier-type transport mechanism35. Interestingly, high molecular weight amides and alkyl amines pass more readily than their low molecular weight homologues, suggesting that a number of specific transport systems may be present.

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