A cross-industry assessment of the flow rate-time profiles of test equipment typically used for dry-powder inhaler (DPI) testing: Part 1 – compendial apparatuses

We report a cross-industry study characterizing flow rate-time profiles of equipment used for testing dry-powder inhalers (DPIs). Nine organizations used the same thermal mass flow sensor to record flow rate-time profiles at the inlet of individual participant compendial DPI test systems including either sample collection tubes (SCT), the Andersen 8-stage non-viable impactor (ACI) or the Next Generation Impactor (NGI™) with and without pre-separator (PS). The plan included some tests with a surrogate DPI consisting of one of three inlet orifices chosen to generate a 4-kPa pressure drop at each of the target final flow rates of 30, 60, and 90 l.min−1, simulating the pressure drop typical of high-, medium-, and low-resistance DPIs. When a particular surrogate DPI was present at the inlet, rise times to 90% of these target flow rates (t90) were shortest at the highest target flow rate, and decreased linearly with decreasing apparatus internal volume, following the order: NGI-PS>NGI>ACI-PS>ACI>SCT. A flow acceleration parameter was also evaluated, expressed as the slope between the rise times when the flow rate attained 20% and 80% of each final steady flow rate (slopet20/80). Flow acceleration was smallest at the lowest target flow rate, decreasing exponentially with increasing internal volume. Measurements were also made without the surrogate DPI, providing a reference condition with no resistance at the inlet to the apparatus. These flow rate-rise time profiles will be useful for those involved in evaluating equipment for characterizing DPIs and in understanding the behavior of these inhalers in development or commercial production.