Factors to consider in choosing a canister for your metered dose inhaler
A typical MDI system includes a metal canister containing medication in the form of a pressurized aerosol. A cap containing the drug metering valve is crimped onto the mouth of the canister, and the entire system is enclosed in a plastic actuator through which the patient inhales the drug. Because canisters,unlike valves, actuators, and dose counters, have no moving parts, developers may take this component for granted. MDI cans, however, have important roles that go beyond storage of the formulation, with the potential to significantly affect product stability and shelf life, patient safety, and dosing consistency.
Since the introduction of the first metered-dose inhaler (MDI) in 1956, manufacturing processes, filling technologies, and drug formulations have evolved considerably to meet changes in industry standards and regulations. In addition, many new MDI components have become available. Although the canister seems to be the simplest component in an MDI system, technological advances have led to the introduction of options that make selection increasingly more challenging.
Choosing the appropriate canister requires consideration of the number of drug doses that the MDI will deliver, the active drug substance, and the physical and chemical properties of the formulation. An inappropriate selection can result in the patient receiving less than the prescribed drug dose, a serious problem when treating life-threatening breathing problems.
Factors involved in canister selection
Canister size depends mostly on dosing requirements and, therefore, the volume of drug formulation.The volume of each dose and the number of doses per canister will determine the canister volume. In addition, the larger the canister, the larger the actuator will have to be, which may affect portability and ergonomics. The dimensions of the canister must also match up with the other MDI components, especially the valve, to prevent leakage.
Canisters come in a range of standard sizes, generally ranging from about 10 ml to about 30 ml.
Most of the world’s canisters are manufactured from aluminium as it is compatible with the majority of formulations. Aluminium does present several challenges; however, manufacturers have developed ways to deal with those issues. For example, aluminium may react with certain formulations, so manufacturers may add a barrier coating on the inside of the can to prevent degradation.
Also, with the switch from CFC propellants to HFA at the end of 2008, cans must now with stand higher pressures than in the past, as much as 5 times higher for HFA at 20 °C than for CFC (Table 1). As a result, aluminium canisters require greater strength, so manufacturers now offer thick-walled aluminium cans. These strengthened aluminium canisters also have a stronger base and neck, making them more robust for high speed filling and crimping.