Together with delivered dose uniformity, the Aerodynamic Particle Size Distribution (APSD) is widely recognised as a Critical Quality Attribute (CGA) in the in vitro characterisation of inhaled and nasal products since it is the APSD of an aerosol cloud that defines where the particles in that cloud are deposited following inhalation.

It is generally accepted that to be therapeutically effective, the particles should be in the range of 1 to 5 microns since particles > 5 microns will generally impact in the oropharynx and be swallowed whereas those < 1 micron may remain entrained in the airstream and be exhaled during the next breathing cycle.

The preferred instrument of choice for measuring the APSD of inhaled and nasal products is the cascade impactor because:

• Cascade impactors measure aerodynamic particle size
• Cascade impactors allow measurement of the active pharmaceutical ingredient (drug)
• Cascade impactors measure the entire dose

Between them, the US and European Pharmacopoeias list no less than five different cascade impactors/impingers suitable for the aerodynamic assessment of fine particles.

However, only the Andersen Cascade Impactor (ACI), the Next Generation Impactor (NGI) and the Multi-Stage Liquid Impinger (MSLI) appear in both Pharmacopoeias.

When selecting an impactor, much will depend on the product to be tested, the data that is required, the geographical location where the product is to be marketed and whether the unit is to be used for product development or quality control.

In research applications, in vitro – in vivo correlation and bioequivalence may be important and so detailed particle size data may be required. In routine quality control, where the concern is batch-to-batch variation, a coarser test may be acceptable.

To see an animation of how a cascade impactor works, please click on the following link: How Does a Cascade Impactor Work?

Cascade impactors operate on the principle of inertial impaction. Each stage of the impactor comprises a series of nozzles or jets through which the sample laden air is drawn, directing any airborne towards the surface of the collection plate for that particular stage.

Whether a particular particle impacts on that stage is dependent on its aerodynamic diameter. Particles having sufficient inertia will impact on that particular stage collection plate, whilst smaller particles will remain entrained in the air stream and pass to the next stage where the process is repeated.

The stages are normally assembled in a stack or row in order of decreasing particle size. As the jets get smaller, the air velocity increases such that smaller particles are collected. At the end of the test, the particle mass relating to each stage is recovered using a suitable solvent and then analysed usually using HPLC to determine the amount of drug actually present.

To see an animation of how a cascade impactor works, please click on the following link: How Does a Cascade Impactor Work?

The Andersen Cascade Impactor (ACI) is arguably the most commonly used impactor within the pharmaceutical industry for the testing of inhaled products. The ACI is an 8-stage cascade impactor suitable for measuring the aerodynamic particle size distribution (APSD) of both MDIs and DPIs.

In 1997, a group of prominent pharmaceutical companies came together in the form of a consortium with the express intent of developing a new impactor specifically for inhaled and nasal products.

The result, the Next Generation Impactor (NGI), was launched in 2000 and subsequently accepted into the Pharmacopoeias in 2005. The NGI has seven stages, five of which are in the range 0.5 to 5 microns plus a micro-orifice collector which acts as a final filter and a horizontal planar layout adopted for ease of operation and automation.

The air flow passes through the impactor in a saw tooth pattern. Particle sizing is achieved by successively increasing the velocity of the air stream by forcing it though a series of nozzles containing progressively reducing jet diameters.

The resultant samples from each stage are collected in a series of collection cups. A removable tray holds all the sample collection cups such that all the cups can be removed and/or replaced in one single operation.

The Multi-Stage Liquid Impinger (MSLI) is a versatile five-stage liquid impinger which can be used for determining the particle size (aerodynamic size distribution) of DPIs in the case of USP Chapter <601> and for MDIs, DPIs and nebulisers in the case of Ph.Eur. Chapter 2.9.18.

The MSLI is available in a range of materials: aluminium, 316 stainless steel or titanium. This choice allows flexibility in terms of corrosion resistance, weight and cost. It is fitted with PTFE seals as standard.

The design is such that at a flow rate of 60 L/min, the cut-off diameters of Stages 1, 2, 3 and 4 are 13, 6.8, 3.1 and 1.7 microns. Stage 5 comprises an integral paper filter to capture the remaining fraction of particles less than 1.7 microns.

The MSLI has, by definition, no inter-stage losses and is suitable for use throughout the range 30 – 100 L/min. Unlike the ACI, NGI and MMI, the collection stages of the MSLI are kept moist which helps to reduce the problem of re-entrainment sometimes experienced when using more conventional impactors. It employs the same induction port as other impactors.

The Marple-Miller Cascade Impactor Model 160 is specified as Apparatus 2 in the USP and is a five-stage cascade impactor which has been designed for determining the particle size (aerodynamic size distribution) of Dry Powder Inhalers (DPIs).

The Model 160 has five impaction stages and is calibrated for operation between 60 and 90 L/min. Each stage has a removable collection cup to assist in the quick and simple recovery of the drug particles with low inter-stage losses. A paper filter is incorporated after Stage 5 to ensure total mass balance.

Two other versions of the impactor are available, such that between the three impactors, a flow rate range of 4.9 to 90 L/min is covered.

In this impactor, particles are deposited directly onto the bottom of the stage collection cups which can then be easily removed after each test without dismantling the impactor.

The value of the Glass Twin Impinger, particularly with respect to routine quality control, is recognised by its retention as Apparatus A in Ph.Eur. 2.9.18.

Its usage is restricted to the assessment of nebulisers, MDIs and such DPIs where it can be demonstrated that a flow rate of 60 (+/-5) L/min is suitable.

Developed at GSK’s laboratories in Ware, UK, the Glass Twin Impinger is relatively easy to use and assemble. It operates on the principle of liquid impingement to divide the dose emitted from the inhaler into respirable and non-respirable fractions.

The non-respirable dose impacts on the oropharynx and is subsequently swallowed. This is considered as the back of the glass throat and the upper impingement chamber (collectively described as Stage 1). The remaining respirable dose penetrating the lungs is collected in the lower impingement chamber (Stage 2).

The upper impingement chamber is designed such that at a flow rate of 60 L/min through the impinger, the particle cut-off is 6.4 microns. Particles smaller than 6.4 microns pass into the lower impingement chamber.

The coordination required by the patient to synchronise the actuation of the inhaler with inhalation can be a problem when using pressurised metered dose inhalers (pMDIs) particularly in the young, old or chronically ill. Add-on devices, such as spacers and valved holding chambers (VHCs), eliminate this problem by providing a reservoir of aerosolised particles that during treatment, the patient inhales from via a mouthpiece or facemask.

The new USP draft chapter <1602> for testing “Spacers and Valved Holding Chambers used with Inhalation Aerosols” specifies two tests relating to the Aerodynamic Particle Size Distribution (APSD) of the accessories concerned.

Test Part 1A is designed to measure the APSD from the accessory when used in optimal conditions, that is to say, with no delay following the actuation of the inhaler. Test Part 1B is used in worst case conditions with a delay of 2 or more seconds between inhaler actuation and sampling onset.

A minimum of Cascade Impactor (ACI, NGI or MMI), Breath Actuation Controller, Vacuum Pump and Flow Meter are necessary if the requirements of both tests are to be satisfied.

Nasal sprays typically produce droplets in the range 20-200 microns which is outside the effective range of inertial impactors. For this reason, the droplet size distribution of nasal sprays and aerosols is normally determined by means of laser diffraction.

At the same time, however, most sprays deliver a proportion (typically <5%) of fine droplets in the <10 micron range.

It is important to quantify this “fine particle dose” since it can penetrate beyond the nasal tract and into the lower respiratory tract or lungs, which may prove undesirable.

For this reason, the FDA recommends the use of a cascade impactor in conjunction with a high volume expansion chamber to measure the amount of drug in small particles or droplets in respect of nasal sprays and the particle/droplet size distribution in the case of nasal aerosols.

In accordance with the draft guidance, Copley Scientific now offer a range of glass expansion chambers to be used with either the Andersen Cascade Impactor or Next Generation Impactor to meet these requirements.

In 2006, the European Medicines Agency (EMA) issued a new “Guideline on the Pharmaceutical Quality of Inhalation and Nasal Products” in which they included regulatory guidance on the drug aspects of nebulisers on the grounds that the safety and efficacy of nebulisers was dependent on the nebuliser/drug combination and not just on the nebuliser alone.

As a result of the EMA initiative and recognising the lack of suitable test methods for nebulisers, the Pharmacopoeias have in turn introduced a new Chapter on “Preparations for Nebulisation: Characterisation” (see Ph.Eur. Chapter 2.9.44 and USP Chapter <1601>).

The method concerned is based on the Next Generation Impactor (NGI) described on Page 36 of our Inhaler Testing Brochure (see right). The recommended flow rate of 15 L/min employed in the APSD testing of nebulisers is lower than that of other OINDPs in order to better simulate the normal tidal breathing conditions employed in their in vivo use.

For this reason, the archival calibration of the Next Generation Impactor was extended from 30 L/min down to 15 L/min in 2004.

Introduction to Abbreviated Impactor Measurement

The concept of Abbreviated Impactor Measurement (AIM) as typified by the two stage Glass Impinger is not new. However initiatives in recent years have centered around abbreviated versions of the Andersen Cascade Impactor.

AIM is founded on the basis that once the full Aerodynamic Particle Size Distribution (APSD) profile of the product has been established in development using a full-resolution cascade impactor (and the process validated) then for product batch release testing and QC applications, it is possible to use simpler but highly sensitive metrics, solely to determine of the product is fit for purpose. This is known as Efficient Data Analysis (EDA).

Although EDA can be applied to full-resolution impactor testing, its true value comes from combining it with an Abbreviated Impactor since only a reduced number of stages are required, speeding up throughput and reducing operator error.

Abbreviated Impactor Measurement has also been suggested as a possible useful tool in R&D for the fast screening of new formulations in product development.

Fast Screening Andersen (FSA)

The Fast Screening Andersen (FSA) is an AIM version of the standard full-resolution Andersen Cascade Impactor (ACI) suitably modified to provide a reduced stack plus filter (F) suitable for either:

• Quality Control (FSA-QC) or
• Product Development (FSA-HRT)

In the FSA-QC, Stages 0 (or -1, or 2A) and F are used in conjunction with a Stage X, having a cut-off diameter as close as possible to the Mass Median Aerodynamic Diameter (MMAD) of the aerosol, as determined during full-resolution cascade impactor testing.

In the FSA-HRT, stages with cut-off diameters are available at 5.0 and 1.0 microns for MDI applications at 28.3 L/min. Stages having more traditional cut-off points of 4.7 and 1.1 microns are also available for varying flow rates, primarily for DPI applications.

Reduced Next Generation Impactor (rNGI)

The drive for greater efficiency continues to stimulate debate as to whether full-resolution, multiple-stage cascade impaction still needs to be applied to the extent that it is currently.

The Reduced Next Generation Impactor (rNGI) is an abbreviated method for utilising the NGI in both AIM-QC and AIM-HRT applications.

In the same way as with the Fast Screening Andersen (FSA), depending on the flow rate to be used, a stage of the NGI can be selected having a cut-off diameter close to the product MMAD (in the case of an rNGI-QC application) or close to 5 microns (in the case of a rNGI-HRT application).

A paper filter held in place with a special rNGI Filter Holder Assembly is now placed in the top of the stage immediately after that of the one selected.

On operating the rNGI, particles smaller than the cut-off diameter of the stage preceding the filter will be captured on the filter, whilst particles larger than that stage will impact as normal in the collection cups of those stages upstream.

Fast Screening Impactor (FSI)

Based on proven NGI Preseparator technology, the Fast Screening Impactor (FSI) represents a purpose-made approach to AIM that segregates the dose into Coarse Particle Mass (CPM) and Fine Particle Mass (FPM) making it suitable for AIM-HRT applications (i.e. FSI-HRT) for MDIs, DPIs and nasal sprays.

A range of inserts is available, generating a 5 micron cut-off diameter within the flow rate range of 30-100 L/min at 5L/min intervals, making it ideal for DPIs tested at a flow rate that equates to a 4 kPa pressure drop over the inhaler.

The FSI uses the same Induction Port as the NGI. It employs a two-stage separation process in which first large non-inhalable boluses are captured in a liquid trap followed by a fine-cut impaction stage at 5 microns.

Bespoke inserts are available on request with a range of cut-off diameter/flow rate combinations allowing for a FSI-QC version.

Introduction to Improved IVIVC

Better In Vitro – In Vivo Correlation (IVIVC) has long been an industry aim. This is particularly difficult in the case of inhaled and nasal products because of the complications involved in precisely correlating drug deposition behaviour with clinical efficacy, the impact of patient-to-patient variability and the complex interaction between formulation and device.

One strategy for improving the significance of cascade impaction data is to modify the test set-up in order to mimic the in vivo drug delivery process more closely.

Two factors that have been identified as critical to this process are:

• 1. Replacing the existing Ph.Eur./USP Induction Port with a mouth/throat model having a more realistic human-like geometry.
• 2. Replacing the existing constant flow rate conditions employed in testing with breathing profiles more representative of conditions in vivo.

The Alberta Idealised Throat (AIT) and the Mixing Inlet are two devices designed to address these particular problems.

Alberta Idealised Throat (AIT)

For more than a decade, researchers at the Aerosol Research Laboratory, University of Alberta, Canada have been working to develop a more suitable representation of the mouth-throat for routine cascade impactor testing, aiming to produce an interface that is both easy to manufacture and reflective of in vivo behaviour, a solution that lies part way between the human throat cast and the pharmacopoeial induction port.

The Alberta Idealised Throat (AIT) was developed as a result of extensive research into typical patient populations including information provided by CT and MRI scans, direct visual observation of living subjects and data in the archival literature.

The throat has a standardised, highly reproducible, human like geometry offering robust performance independent of flow rate.

Its smooth, more uniform internal geometry has been specifically designed to make drug recovery quick and simple in comparison with a human throat cast. Quick release clips make for easy internal access.

The Alberta Idealised Throat (AIT) is based on adult geometry, but child and infant geometries are available on request.

Mixing Inlet

The cut-off diameters generated by each stage of any cascade impactor are dependent on a steady, fixed flow of air passing through it.

In contrast, the in vivo inhalation profiles of breathing cycles generated by patients produce a continually varying flow rate far removed from the fixed, steady-state flow rates employed in in vitro testing.

For this reason, there have been various attempts to link cascade impactors directly to breath simulators in order to reproduce actual clinical conditions more closely. Any such system must be capable of varying the flow rate through the inhaler whilst ensuring that the aerosol generated is sampled at a fixed rate through the impactor.

The Mixing Inlet fits between the USP Induction Port (or Alberta Idealised Throat) and the inlet of the impactor used to carry out the test.

It is designed to permit the cascade impactor to be operated at a constant flow rate (e.g. 100 L/min), whilst allowing a lower fixed or variable rate, such as a breathing pattern generated by a breath simulator, to pass through the inhaler itself.