The Pharmacokinetics of Inhaled Drugs

The pharmacokinetic (PK) profile of a drug after inhalation may differ quite markedly from that seen after dosing by other routes of administration. Drugs may be administered to the lung to elicit a local action or as a portal for systemic delivery of the drug to its site of action elsewhere in the body. Some knowledge of PK is important for both locally- and systemically-acting drugs. For a systemically-acting drug, the plasma concentration-time profile shares some similarities with drug given by the oral or intravenous routes, since the plasma concentrations (after the distribution phase) will be in equilibrium with concentrations at the site of action. For a locally-acting drug, however, the plasma concentrations reflect its fate after it has been absorbed and removed from the airways, and not what is available to its site of action in the lung. Consequently, those typical PK parameters which are determined from plasma concentration measurements, e.g., area under the curve (AUC), C_max, t_max and post-peak t1/2 may provide information on the deposition and absorption of drugs from the lung; however, the information from these parameters becomes more complicated to decipher for those drugs which are locally-acting in the lung. Additionally, the plasma concentration profile for both locally- and systemically-acting drugs will not only reflect drug absorbed from the lung but also that absorbed from the gastrointestinal (GI) tract from the portion of the dose which is swallowed. This absorption from the GI tract adds a further complication to the interpretation of plasma concentrations, particularly for locally-acting drugs. The influence of physiological and pathological factors needs to be considered in the absorption of some inhaled drugs. The absorption of some hydrophilic drugs is influenced by the inspiratory maneuver used during initial inhalation of the drug, and at later times after deposition. Similarly, the effects of smoking have been shown to increase lung permeability and increase the absorption of certain hydrophilic drugs. The effects of different disease states of the lung have less defined influences on absorption into the systemic circulation.