Evaluation of inhaler performance using an inhaler support accessory and a hemispherical probe.
The use of metered-dose inhalers (MDIs) for the treatment of diseases, such as asthma, has become popular since their introduction in the late 1950s. MDIs come in the form of nasal or oral inhalers and are much appreciated by patients who depend on them for their treatment. MDIs are designed to deliver precise/accurate and reproducible doses of the drug to the lungs or nose. This is crucial as patients rely on their inhaler to deliver the required dose of medication at often crucial times.
Two vital components of the MDI are the metering valve (important in particle size distribution of the delivered dose), and formulation (crucial in defining the physical stability and performance of the MDI). The clinical efficacy of an MDI therefore depends upon the design, reproducibility, formulation and closure system (comprising the valve and its components). This makes MDIs significantly different from other common conventional drug products.
The compression test assesses the performance of the metering valve using a hemispherical probe that simulates a finger exerting a force on the cylinder containing the metering valve. The force and energy required to expel the dose is measured as the hardness and hardness work done.
Typical TexturePro CT plot:
Figure 1 shows the valve performance of an oral inhaler as a function of time. Four tests have been performed and overlaid on the graph.
Figure 2. This graph shows the load verses distance for the valve performance of the inhaler. A compression distance of 3 mm has been used during which time the valve is compressed by the probe to deliver the dose, before withdrawing from the sample and returning to its starting position above the sample surface (indicated by the negative part of the graph).
When a trigger force of 10 g has been achieved, the hemispherical probe begins to compresses the cylinder containing the metering valve to a defined distance (3 mm used for this test) at a test-speed of 10 mm/s. During this time, an increasing force is generated in the sample. When the specified distance has been attained, the probe withdraws from the sample at a post-test speed of 10 mm/s. The force required for drug delivery is measured as the maximum force (hardness). The area under the curve is then a measure of the hardness work done (the energy required for drug delivery by the patient).
The maximum hardness and hardness work done values for four tests of the oral inhaler give typical values as shown below:
From the table the force required to deliver the dose from the MDI is reproducible as seen by the average hardness and hardness work done values for the four tests. This is an indication of the performance of the metering valve.