The insensitivity of FPD to shifts in inhaler APSD is illustrated and underlying causes have been discussed. Data from eight different products are examined. In conclusion, FPD cannot be used alone as a metric for APSD quality control. FPD can be used to control the mass-dimension of the APSD, but must be used in conjunction with an orthogonal metric sensitive to changes in the size-dimension (such as MMAD). Importantly, supplementing FPD with metrics such as ISM or material balance (total mass ex-inhaler), neither of which are sensitive to APSD size changes, is not sufficient.
The liability of fine particle dose (FPD)
The first in a series of articles from the IPAC-RS Cascade Impaction Working Group concerning the limitations of metrics commonly used in the assessment of aerodynamic particle size distributions (APSDs) of orally inhaled products (OIPs). The current article discusses the surprisingly limited utility of the fine particle dose metric in quality control.