VPT 311: General and Systemic Veterinary Pharmacology (2+1)

Absorption of drugs after inhalation or topical administration

ABSORPTION OF DRUGS AFTER INHALATION

• The volatile and gaseous anaesthetics are the most important group of drugs administered by inhalation. These substances enter the circulation by diffusion across the alveolar membranes. Since they all have relatively high lipid water partition coefficients and generally are rather small molecules, they equilibrate practically instantaneously with the blood in the alveolar capillaries.
• Particles contained in aerosols may be deposited, depending on the size of the droplets, on the mucosal surface of the bronchi or bronchioles, or even in the alveoli.

Absorption of drugs from topical sites of application

• Drugs may be absorbed through the skin following topical application. The intact skin allows the passage of small lipophilic substances, but efficiently retards the diffusion of water soluble molecules in most cases.
• Highly lipid soluble preparations may be absorbed in considerable proportions encouraging the use of the topical route especially in veterinary practice. Pour on preparations of anthelmintics is a typical example.
• Lipid insoluble drugs generally penetrate the skin slowly in comparison with their rates of absorption through the other body membranes. Absorption of drugs through the skin may be enhanced by inunction or more rarely by iontophoresis if the compound is ionized. Certain solvents like dmethylsulfoxide may facilitate the penetration of drugs through the skin.
• Damaged, inflamed or hyperemic skin allows many drugs to penetrate the dermal barrier much more readily. The same principles that govern the absorption of drugs through the skin also apply to the application of topical preparations on the epithelial surfaces.
• Many drugs traverse the cornea at rates that are related to their degree of ionization and lipid solubility. Thus organic bases such as atropine, ephedrine and pilocarpine often penetrate quite readily, whereas the highly polar aminoglycoside antibiotics generally penetrate cornea poorly.