Balancing ethanol cosolvent concentration with product performance in 134a-based pressurized metered dose inhalers

The effects of formulation parameters on the product performance characteristics of solution metered dose inhalers (MDIs) were determined using ethanol as the cosolvent and HFA 134a as the propellant. Solubility of beclomethasone dipropionate (BDP) was determined in various blends of 134a and ethanol and was shown to increase with ethanol concentration. Product performance was assessed using the APS Model 3306 Impactor Inlet in conjunction with APS Model 3320 Aerodynamic Particle Sizer (APS). Nine solution formulations containing various BDP and ethanol concentrations were studied. Chemical analysis of the Impactor Inlet was performed in order to determine the "respirable" deposition of the MDI system. With increased ethanol concentration, the throat deposition and plate deposition increased and the respirable deposition decreased. The mass median aerodynamic diameter (MMAD) increased with the increasing drug concentration, but did not show a significant increase with an increase in ethanol concentration. This indicates that the efficiency of solution MDIs decreases with increased ethanol concentration. A Maximum Respirable Mass (MRM) was calculated based on the drug solubility at a particular ethanol concentration and the respirable deposition for a 50mcl valve and QVAR actuator for that ethanol concentration. The MRM represents the maximum amount of a given drug that can be delivered to the lungs theoretically and is very sensitive to the solubility profile of the drug. The MRM increased with the increasing ethanol concentration in the formulation until a plateau was reached at an ethanol concentration of 10-15% w/w. The MRM initially increases with increase in ethanol concentration due to the increase in drug solubility. However, at higher ethanol concentrations the increase in drug solubility was negated by a decrease in the respirable deposition. This study illustrates the importance of considering both formulation properties and product performance characteristics when optimizing a metered dose inhaler drug delivery system.