Propellant Driven Metered Dose Inhalers

The formulation for a pMDI consists of several key components: propellant(s), drug, cosolvents, and surfactants.

Propellants may be of a number of different types: CFCs, hydrofluoroalkanes (HFAs), or alkanes. The composition impacts on performance. A numerical system is employed to identify fluorinated propellants. The rules governing this numbering system allow the molecular structure to be derived from the numerical descriptor. The rules may be listed as follows:

• The digit on the extreme right, e.g., propellant 114, represents the number of chlorine atoms.

• The second digit from the right, e.g., propellant 114, represents one more than the number of hydrogen atoms.

• The third digit from the right, e.g., propellant 114, represents one less than the number of carbon atoms.

• A subscripted lowercase letter represents the symmetry of the molecule: the earlier in the alphabet, the more symmetrical the molecule being described, e.g., propellant 134a.

Two additional rules have not been required for pharmaceutical purposes but may be included for completeness.

• A fourth number from the right indicates the number of double bonds in the molecule.

• A prefixed lowercase c indicates that the molecule is cyclic.

The vapor pressure and density of propellants are employed to assist in formulation. The vapor pressure dictates the force of emission of the droplets from the metering valve of the inhaler. The force of emission is derived from the difference between the product vapor pressure and atmospheric pressure. The density of the propellant may be matched to

Table 5 Physicochemical Properties of Chlorofluorocarbon and Alternative Hydrofluoroalkane Propellants such as Hydrofluorocarbons

Propellant Molecular Vapor pressure Boiling point Density (g/cm3

Table 5 Physicochemical Properties of Chlorofluorocarbon and Alternative Hydrofluoroalkane Propellants such as Hydrofluorocarbons

Propellant Molecular Vapor pressure Boiling point Density (g/cm3

(chemical formula)

weight (g/mol)

(psia at 25 °C)

(°C at 1 atm)

at 25 °C)

Oil (CC13F)

0 0

Post a comment