MDIs deliver the drug in the form of an aerosol using a propellant.
When actuated, a metered dose of the drug is released, which the patient inhales.
Components and Mechanism:
Canister: Typically made of metal and houses the formulation.
Valve: Ensures consistent dosing by releasing a fixed volume of formulation per actuation.
Actuator: A plastic mouthpiece where the canister is inserted. It directs the aerosolized drug into the patient's airways.
Counters: Modern MDIs often come with dose counters to inform patients of the number of doses left.
Formulation Challenges:
Propellant Selection: Newer HFA propellants (like HFA 134a and HFA 227) replaced the traditional CFCs. Formulating with HFAs requires consideration of drug solubility and potential for chemical interactions.
Coarse and Fine Particle Fraction: Ensuring the formulation produces a consistent aerosol particle size is critical. The fine particle fraction (typically <5 micrometers) is crucial as it dictates the portion of the dose that can reach the deeper parts of the lungs.
Valve Crystallization: Some drug formulations may lead to crystal formation on the valve components, leading to inconsistent dosing or device malfunction.
Excipients:
Ethanol: Commonly used in HFA-based MDIs as a cosolvent to help dissolve the drug.
Surfactants: (e.g., oleic acid, sorbitan trioleate) might be added to ensure uniform suspension of drug particles.
Antioxidants and Stabilizers: These can be added to prevent drug degradation or to maintain formulation integrity.
