PULMONARY DELIVERY
Pulmonary delivery has been used for years to treat respiratory diseases by delivering drugs locally to the lung.
However, the latest advances are seeing drugs being delivered via this route for systemic circulation. The delivery of drugs to the bloodstream via the lungs is an attractive hypothesis, offering a non-invasive and rapid-onset alternative to injections.
Inhalation Drug Delivery via Metered-dose Inhalers
Aerosol route of administration targets drugs directly to the lungs and offers advantages of bypassing the first pass elimination, reducing the dosage frequency and minimising the extent of adverse drug reactions.
This is turn, improves the therapeutic effect-toxicity ratio. The metered-dose inhaler (MDI) delivers a precise, reproducible dose of drug accurately to the deep lung. It is also well accepted by patients who depend on MDI’s in their treatment and therefore it is essential to constantly improve this technology.
Patients need to rely on their inhaler to provide them with the required medication which is often at a crucial time when physical strength may be very low. Therapeutic performance of aerosols is affected by various factors such as actuator tube design, orifice diameter, concentration of surfactant in the system, vapour pressure of propellants, efficiency of valve crimping and particle size of the plume emerging from the inhaler.
The metering valve is a critical component of a finished metered-dose inhaler. The valve is a complex assembly of at least seven component parts which are made of various materials and are commonly from different sources.
The valve is undoubtedly the most critical MDI packaging component in terms of the manufacturing quality required to ensure consistent correct performance. The typical metering valve consists of two coupled valves. It is essential that under all conceivable methods of use, during depression of the valve stem the inner seal closes to isolate the container contents before the outer seal opens to allow discharge of the chamber contents. A reversed sequence may allow ‘continuous spraying’, the absence of which must be verified in evaluating valves for inhalation use, because this could cause erratic dosing or even serious overdosage.
The elastomeric sealing components are also particularly critical. Important characteristics of the rubber components include their composition and degree of curing which control the physical properties and swelling characteristics in situ.
The interaction between the elastomeric components and the formulation can significantly affect product performance. A change in formulation, for example implementing a new propellant system, may necessitate a complete redesign of the valve system in order to successfully deliver the drug with the minimum of difficulty.
Measuring Metering Valve Performance
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| Metred Dose Inhaler test Figure 1 [left]: An Inhaler Support attached to a TA.XTplus Texture Analyser Figure 2 [right]: Typical curves obtained for the testing of 3 inhaler systems using the Inhaler Support Rig |
The Inhaler Support Rig (as shown in Figure 1) provides a holding device for the inhaler so that it can be tested with a hemispherical probe which best mimics the action of the finger for drug delivery. The force required to push down on the inhaler to deliver the drug is measured (as shown in typical graphs in Figure 2). This assesses the performance of the metering valve which is a critical aspect of quality control.
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