Experimental study: Assessing AIT performance
The APSD of a pMDI (active ingredient salbutamol) was measured in a collaborative project with Melbourn Scientific (UK) using
an impactor (Next Generation Impactor, Copley Scientific) with both the USP induction port and an AIT (7). Testing was carried
out at a flow rate of 30 L/min, according to pharmacopeial guidance for MDIs, with six replicate measurements conducted in
each case. All collection stages of the NGI and the AIT were coated with silicone oil applied in n-hexane solution (1% w/v),
which is a standard practice that reduces particle bounce (i.e., the re-entrainment of particles impacting at high velocities).
The USP induction port was left uncoated as is done for routine testing.
Table I shows averaged collection data for each component of the test equipment including a combined mass recorded for the throat
and inhaler mouthpiece. Values for fine-particle dose (FPD), fine-particle fraction (FPF), geometric standard-deviation (GSD),
and mass median aerodynamic diameter (MMAD) were determined from these data. Calculations were based on total emitted mass/actuation
and the assumption of a 5-micron upper limit for FPD and FPF.
Table I: Summary collection data for a pressurized metered-dose inhaler (pMDI) tested using a United States Pharmacopeia (USP)
induction port and Alberta Idealized Throat (SD is standard deviation, T/MP is throat/mouthpiece, µg a/ac is µg active/actuation,
MOC is micro orifice collector, FPD is fine particle dose, FPF is fine particle fraction, GSD is geometric standard deviation,
MMAD is mass median aerodynamic diameter).
Deposition data for the throat/mouthpiece shows that the AIT captures more of the dose, thus reducing the mass of drug entering
the cascade impactor. This result is attributed to differences in the geometry of the two interfaces, rather than any coating
Figure 2 shows averaged cumulative APSDs based on the amount of material exiting the mouthpiece/throat. Here, use of the AIT shifts
the APSD to finer sizes across the entire size range, thus suggesting that not all particle sizes are equally retained relative
to the standard induction port. In a parallel experiment with a DPI, the same effect was observed (7). These are early results
and further research is required. The results, however, underline the fact that better representation of mouth/throat deposition
may influence assessments of regional deposition in the lung, which is a function of particle size, as well as estimates of
whole lung deposition. From an in vitro perspective, use of the AIT ensures that only the portion of the aerosol that would actually deposit in the lung is sized
by the cascade impactor, therefore providing more relevant data.
Figure 2: For this pressurized metered-dose inhaler, use of the Alberta Idealized Throat shifts the measured, cumulative,
aerodynamic particle size distribution to finer size distributions relative to the standard United States Pharmacopeia (USP)