SELECTION AND VALIDATION OF CASCADE IMPACTOR TEST METHODS

Respiratory Drug Delivery IX, 2004 Byron et al. SELECTION AND VALIDATION OF CASCADE IMPACTOR TEST METHODS Peter R. Byron, 1 R. Harris Cummings, 2 Steven C. Nichols, 3 Guirag Poochikian, 4 Michael J. Smurthwaite, 5 Stephen W. Stein, 6 and Keith G. Truman 7 1 School of Pharmacy, Virginia Commonwealth University, Richmond, VA; 2 Cardinal Health, RTP, NC; 3Aventis, Holmes Chapel, UK; 4 Food and Drug Administration, Rockville, MD; 5 Westech Instrument Services, Peterborough, UK; 6 3M Drug Delivery Systems, St. Paul, MN; 7 GlaxoSmithKline, Ware, UK KEYWORDS: cascade impaction, inhaler testing, pharmacopeial tests, particle size distribution, PSD profiles, validation SUMMARY Regulatory insistence on improved methods to determine in vitro aerodynamic particle size distribution (PSD) profiles for the aerosol drug doses delivered from inhalers has stimulated considerable debate. For product QC purposes, segregation of the entire delivered dose of drug into different aerodynamic size ranges must now be accomplished by cascade impaction routinely, and shown to conform to agreed specifications. Analysts are confronted by the need to choose a type of cascade impactor, qualify a number of these impactors for use, then develop and validate test methods during the drug development process. ese methods must be sufficiently reproducible to discriminate between good and bad product batches. is article seeks to describe the issues and to stimulate debate; to assure the availability of fully accredited instruments and test methods with which these may be (a) qualified (and re-qualified); and (b) used to determine PSD profiles for batch release of inhalers prior to marketing. INTRODUCTION In the late 1980s in the USA, as inhaler and aerosol drug patents expired, industry and FDA recommended the inclusion of several innovator-developed impingement or impaction devices into the pharmacopeias (1). ese simple devices, like the twin stage impinger (1), were designed to cut an aerosol cloud into two or three fractions on the basis of both the aerodynamic diameter of its particles and droplets, and the plume or cloud velocity as it left the inhaler. As such, they were used to assure the quality of a given product for release into the marketplace, and to assess product performance through its shelf life. For a single product, these simple devices were often adequate for routine QC purposes. However, once alerted to the difficulties of assessing the equivalence of emitted size distributions from innovator and generic inhalers, FDA, and many in the industry, became convinced that improved cascade impaction techniques were needed with greater discriminating power; both to those seeking to control the quality of inhalers in the 169

170 Selection and Validation of Cascade Impactor Test Methods Byron et al. marketplace, and those seeking to show that a generic inhaler or line extension produced a similar size distribution to the devices used in the clinical tests performed during drug development. As a result, we now have a menu of impactors in both USP and EP (2,3) which are considerably better than the early impingers which held sway through the late 1970s and 1980s. Even though these new devices are presented with apparently standard methods in the compendia (Table 1; the NGI is presently proposed for inclusion in the pharmacopeias), there is sufficient latitude and a series of unresolved problems which create problems for the analyst. He or she must select an impactor rationally; then the chosen impactor(s) must be qualified for use and then used to develop and validate a final method capable of reliably determining the PSD of the delivered drug dose from a specific inhaler. In addition, critical dimensions of the impactor should be re-measured periodically to ensure that impactors remain within specification and fit for purpose. is article seeks to cast a broad perspective on these issues, as well as stimulate debate on some of the major problems which require general resolution. Table 1 Cascade impactors and test methods a described in USP (2). Inhaler Impactor Flow rates (+5%) Size range b # of bins c MDI Andersen Mk II 28.3 L/min 9 0.4 8 Next generation Pharmaceutical Impactor (NGI) 30 L/min 8.1 0.54 7 e DPI Marple-Miller Model 160 30 100 L/min 14.2 0.48 5 Andersen Mk II with preseparator d as defined during Debatable d 6 d Multi stage liquid impinger delivered dose 18.4 1.32 4 NGI with preseparator uniformity testing 14.9 0.24 8 e a Aerosol entrance conditions into each impactor have been standardized and harmonized across the pharmacopeias b Range of effective cutoff diameters (ECD values in um); topmost to bottom impaction stage; DPI values are at 30 and 100 L/min (see text for calibration ranges) c Number of clearly definable size categories; note that deposition has an undefined upper size limit unless instrument is used with a calibrated preseparator with a known ECD value d Andersen is also available commercially with additional stages (-0 and -1) for DPI testing at higher flows; these are not recommended in USP where multiflow calibration data has not been presented, even for the regular stages. It is also necessary to bore out the Andersen exit to the vacuum pump and omit stages 6 and 7 at flow rates > 60 L/min). e The NGI has an additional collection surface for use with or without a final filter. This is the Micro Orifice Collector (MOC) PURPOSE OF CASCADE IMPACTION TESTING One aspect of cascade impactor testing that is often the cause of substantial disagreement relates to the philosophical question, What is the purpose of the test, once selected? Is the purpose to enable the deposition of the aerosolized drug to be estimated in the patient? If so, we must ensure that in vivo/in vitro correlations (IVIVC) are established which are sound. Alternately, we may ask Is the test strictly a quality control tool to assure aerosol formation and delivery consistency? Clearly, the same test may be required to provide both types of information and thus the answers to these questions have significant implications in the discussion of the way cascade impaction is practiced. Consistency, for example, can sometimes be assessed using tests with flow rates which bear no relationship to those used by patients. If in vivo relevance is sought however, decisions must often be made concerning the additional questions: Which inlet is most appropriate? How many actuations should be used? What airflow rate should be used? ese and other factors influence the PSD profiles which are collected and they require standardization. While in vivo relevance is certainly important, the following discussion will focus solely on the use of cascade impaction as a quality control tool, whereby the analyst seeks to assure the reliability of an inhaler through its shelf-life.

Respiratory Drug Delivery IX, 2004 Byron et al. 171 SELECTING AN IMPACTOR Table 2 shows a series of frequently asked questions which have been presented to one of the authors (PRB) at different venues. Some of these should certainly be answered before finally committing to the purchase and use of a single type of cascade impactor. e four major instruments proposed for USP (Table 1), along with their minor method variations for MDIs and DPIs, have been included in the official compendium because of the need to offer standard test methods for use during the development and quality control of inhalers. Table 2 Some FAQs which may be important during the cascade impactor selection process. a Is it accurate? - How easy is it to (a) mensurate, and (b) calibrate? Are commercial calibrator companies available for hire? Is there a standard aerosol product I can test it with (from USP)? Will the second and third impactors provide comparable results to the first; what is the variability? How many size groupings do I need to separate my aerosol into? Can I omit or pool drug from different stages? Will each impactor suit my inhaler product; which is best for DPI, MDI, nebulizer, etc? Will it discriminate between good and bad product reliably? Will it be acceptable to regulators? How variable are results (a) between instruments, (b) beween minor method variations, and (c) between analysts? Is total variability likely to be less than that between inhalers? How easy is it to recover 100% of my drug dose (mass balance)? How many doses should I collect for each test? How easy is it to deal with interstage drug deposition (wall losses)? How easy is it to use: (a) if I do not coat the stages, or (b) if I do coat the stages? What will it cost (a) to buy, and (b) to keep going? Is the impactor manufactured and tested to acceptable quality standards? Will I be able to perform periodic stage mensuration? Will the manufacturer be able to supply well-qualified instruments on demand, over the long haul? How many impactions per day will each analyst be able to perform? Can the impactor be automated? a Answering many of these questions requires an accurate, precise, and robust drug assay and a clear understanding of the test inhaler characteristics Accuracy From the perspective of accuracy, even a perfectly calibrated cascade impaction device can only claim to provide an estimate of a drug product s aerodynamic size distribution (the true value is never determined). Moreover, this estimate is heavily influenced by a number of factors which continue to cast doubt on the validity of the reported results. Among these factors are: (a) dynamic changes in particle size and momentum of aerosol clouds during cloud segregation (especially for MDI clouds and droplet aerosols); (b) discontinuous flow profiles used during the testing of dry powder inhalers (instrument calibrations assume steady state flow); and (c) the assumption that each stage can be represented by an absolute aerodynamic cutoff diameter (collection efficiency of any impaction stage is a probabilistic function of aerodynamic diameter and the sharpness of cutoff is dependent on stage design). Nevertheless, the NGI (2) and all of the instruments in the present USP (3), with the exception of Andersen Mk II, were accepted only after the manufacturer or designer had provided full steady state calibration and mensuration details to the pharmacopeia. Unlike many other cascade impactors which have been described in the literature and offered for sale, the MMI, MSLI, and NGI were calibrated using state-of-the-art methods with calibrationquality, charge-neutralized, monodisperse non-volatile aerosols. e collection efficiency of each

172 Selection and Validation of Cascade Impactor Test Methods Byron et al. stage of these impactors was quantified over a range of aerosol sizes and steady-state air flow rates over the 30 90 (MMI and MSLI) and 30 100 (NGI) L/min ranges. us, the MSLI and NGI have reliable public mensuration data (critical dimensions) available to support their USP published calibrations (Effective cutoff diameter (ECD) values for charge-neutralized aerosols) within these ranges (Table 1). e MMI is an instrument with proprietary mensuration specifications and a publicly available calibration over the same range as MSLI; the instrument s engineering specifications are supported by a single manufacturer r that provides essential mensuration details or services on request. us, in these three cases, correctly mensurated devices may be confidently employed as accurate aerosol measurement devices for aerosols which do not display dynamic size changes with time, and which can be tested during steady-state flow; that is, they are as accurate as we can expect any cascade impactors to be, given the circumstances associated with testing pharmaceutical inhalers. e issue facing the prospective purchaser is to decide which supplier can best be relied upon to provide instruments conforming to pharmacopeial specifications. In the case of Andersen, however, the device was recommended by FDA for use in the early 1990s so as to provide draft guidance information to generic companies wishing to develop off-patent drugs. Since much of the available data on drug products was generated using the Andersen, the instrument has been grandfathered into USP despite the fact that it has been shown to suffer from manufacturing variability leading to interlaboratory differences in results (4). e Andersen has also been criticized for its high interstage ( wall ) losses, poor design for pharmaceutical testing purposes (difficult assembly and tendency to suffer from inter-stage gas leakage), as well as low impaction stage Reynold s numbers and overly small values for the ratio of (nozzle-to-plate distance)/(nozzle diameter), leading to overlapping stage collection efficiency curves (5). Most importantly however, Andersen suffers from non-standard, ill-defined mensuration parameters (6). e instrument is also now produced to different specifications by several manufacturers, leading to the question When is an Andersen not an Andersen? All of these issues have been blamed to differing degrees for their production of results claimed to distort and add to the variability of PSD profiles. How Many Stages Do I Need? While there is usually no good scientific reason for needing more than the four or five aerosol size subdivisions offered by MSLI and MMI (7), FDA expressed preference for more, and in some cases more appropriate stages, because (a) more size bins are most likely to segment the major mode(s) of different PSDs from different products, and (b) more such bins should offer a greater ability to discriminate between products. In general, and across products, it is difficult to refute such an argument in vitro. In vivo, of course, there is no biologic or clinical literature which implies that drug mass in the very small aerosol size bins (created by Andersen or NGI s various stages) is segregated differently by the human lung. Nevertheless, MMI and MSLI can be rejected unreasonably by companies and their analysts, when their stage cutoffs at the selected flow rate provide good classification and subdivision of the aerosol size distribution of a chosen product, in spite of their possessing some excellent technical characteristics. Not surprisingly therefore, the industry s commission of a new impactor design to replace Andersen has produced the NGI; this has a similar number of stages and cut points to Andersen, but is believed to overcome many of that impactor s difficulties. (5, 8-10). In general, an impactor should be chosen which subdivides the delivered dose into at least five major size categories. Furthermore, the selection of an instrument which, by virtue of its cutoff diameters fails to segment the test product s PSD into an adequate number of categories, is likely to be viewed as inadvisable.

Respiratory Drug Delivery IX, 2004 Byron et al. 173 NGI, Andersen Mk II, or Another Multi-Staged Cascade Impactor? For the reasons above, and most analysts unwillingness to reinvent the wheel, companies are presently confronted with a choice between Andersen or NGI. Both offer a surfeit of stages, but only NGI presently offers a state-of-the-art calibration and published mensuration details. It is obvious, however, from the recent data emerging from industry and academe (9-11), in which these two instruments have been compared, that much of the industry s dissatisfaction (4) with the high variability of Andersen data has now been resolved, principally by improved instrument manufacturing and the use of strictly controlled test methods. Figure 1 shows typical MDI cascade impactor data in which newly commissioned Andersen and NGI impactors were (a) used to validate the analytical methods and (b) compare the apparent PSDs from single puffs of the same marketed MDI. Irrespective of the improved and demonstrated calibration of NGI, and its claimed design features (5,8), it is clear from this figure that at 28.3 L/min, the Andersen produced a PSD profile that effectively overlaid that from the NGI, operated at 30 L/min. is data, and that of others in which different aerosol products have been tested (9-11), can be looked at in one of two ways. Assuming for a moment, that both instruments are equally easy to qualify for service in the QC laboratory (e.g., mensurate or calibrate), then it is either possible to switch to the analyst-preferred instrument (by providing regulators with bridging data to justify minor changes in product specifications) or there is no need to change instruments at all, because both instruments provide comparable data with similar variability (Figure 1). erefore, the major issues appear to relate to the ease with which these devices can be qualified into service, the reproducibility impactor to impactor, and their ease of use (e.g., potential for automation) during routine QC testing. % Cumulative Mass under Size 100 Data : mean ± SD (n=6) NGI; coated ACI; coated 50 0 0.1 1 10 100 Aerodynamic Diameter ( m) Figure 1. Typical drug size distributions (cumulative % undersize ± sample standard deviations) for beclomethasone dipropionate reaching the Andersen Mk II or the NGI impactor following USP testing and data treatments. PSDs are shown for single actuations into silicone-oil stagecoated impactors in both cases. Interpolated data at identical aerodynamic diameters showed overlapping error bars. Data plotted from reference 9. CASCADE IMPACTOR MENSURATION AND METHOD VALIDATION Table 2 shows some of the important questions to be asked during the process of instrument selection. Many of these are related to the issues in Table 3 which are grouped to facilitate discussion of the validation process. Overall, the key to method validation for cascade impaction of marketed products is to ensure that method variability is much smaller than that due to the product itself. To do this, it is logical to reduce predictable method variability as much as possible.

174 Selection and Validation of Cascade Impactor Test Methods Byron et al. Table 3 Method validation (reducing test method variability). Mensuration - Select a well-specified and manufactured instrument Ensure its critical dimensions are in agreement with specifications a Re-mensurate regularly b Chemical analysis - Validate your assay with LOQ small enough to reliably quantify drug deposited on each stage Assure that your drug can be quantitatively recovered from the stages (and walls) of the instrument c Understand how many impactions can be performed before wall losses and thorough washing becomes essential Data processing - Define the variables to be measured (mass on each stage?) Method performance - Ensure that instrument set up is without problems (avoid leaks, assure volumetric flow rate, stage order, etc.) Ensure that likely inter-laboratory changes have minimal effects or are standardized (temperature, humidity) Assess the effects of stage coating (type and thickness should be considered) if it matters, do it! Assess the effects of multiple actuations and duration of flow into the impactor Assess the impact of flow profile variability (particularly important for DPIs) Assess the effects of different analysts and instruments Reference Standard - Periodically evaluating a known low-variability product (with a similar PSD to yours) is worth considering a Compendial specifications or those defined within your own company and NDA b The effects of routine use and wear, as well as cleaning, should be determined. Some products may have abrasive effects; others may contain compounds that block nozzles and be difficult to remove. c Solvent choice and duration are often important. Spiking, both with drug and formulation is appropriate; assess recovery with and without stage coating material(s). Good drug recovery data and proof of mass balance is essential (12). Mensuration is now firmly established as the preferred method for admitting or qualifying cascade impactors for use in the laboratory. e difficulties of producing monodisperse aerosols reliably, and then using these aerosols to calibrate the individual stages of a cascade impactor are well known. e description of this process, as it relates to NGI, is an excellent example of the present state-of-the-art, and its complexity (8). In the authors opinion, as a means of assuring reproducible results, calibration of all of these devices is much less precise than mensuration. us, similarly mensurated impactors (those with the same critical dimensions), when used at the same flow rates under well-standardized conditions (12), can be expected to produce comparable, low variability PSD profiles, when they are used to test low variability inhalers (e.g., Figure 1). It should be obvious from the first three points in Table 3 that mensuration requires both a method, and knowledge of the acceptable ranges for the instrument s critical dimensions. Stein and Olson (4) used microscopy to measure chords across the apparent center of a sample of theoretically circular nozzles, on each stage of their Andersen impactors. ey converted their average measurements into nozzle diameters and compared these to the manufacturer s specifications. Nichols (6), again using sampling, reported values from Graseby-Andersen themselves (when that company still existed), who created a video image and enlarged hole display for each stage. Measurements were taken using an unspecified method to generate the quoted nozzle diameters (6). Other companies are known to be employing Mitutoyo computerized optical measurement technology (13) to measure all l of the nozzles on all l of the plates at specified intervals to reduce method variance. Clearly, there is no consensus on exactly how the process of mensuration can be accomplished although it is to be hoped that the compendia can assist with the standardization of techniques for those instruments in Table 1. It must be remembered, of course, that when defining nozzle diameter specifications (or perhaps nozzle area specifications), engineering tolerances need to be considered as well as taking into account the accuracy of the optical measurement system used during the mensuration process. In the case of Andersen, no compendial mensuration specifications presently exist and these must presently be defined within your own company and NDA if you are to ensure that you avoid the problems due to instrument variability reported by Stein and Olson (4). Table 4 is a partial listing of some of Andersen s specifications, which USP and EP hope to be able to complete and add as features of the standard Andersen Mk II in the future.

Respiratory Drug Delivery IX, 2004 Byron et al. 175 Table 4 Proposed engineering specifications for Andersen Mk II cascade impactor. Stage # Number of jets Target Diameter (mm) a Tolerance (mm) b Distance to plate (mm) c 0 96 2.55 0.024 1.02 1 96 1.89 0.012 1.02 2 400 0.914 0.042 2.17 3 400 0.711 0.156 2.17 4 400 0.533 0.027 2.17 5 400 0.343 0.015 2.17 6 400 0.254 0.003 2.17 7 201 0.254 0.003 2.17 a Original Andersen Mk II specifications were drill diameters with tolerances, in inches (e.g., Stage 0 was 0.0994 to 0.1014 inches; reference 4). Values in table are metric equivalents based on Nichols (6). US manufactured instruments may still be produced with non-metric drills. Impactors purchased from Andersen between about 1992 and 1998 were produced in the UK using metric tools. Note that there is presently no consensus on these dimensional qualifications. b Tolerances are best estimates based on (3 x standard deviation) values noted by Nichols for a group of acceptably calibrated Andersen Mk II cascade impactors. Values are experimental estimates (based on sampling and measurement) that need to be debated and agreed between impactor manufacturers, users and the compendia. c Values require debate; tolerances need to be defined (15). Finally, mensuration should be performed using a standard technique routinely, so as to determine the effects of routine use and wear, as well as cleaning; this because some products may have abrasive effects and others contain compounds which block nozzles and are difficult to remove. For example, during mensuration, some nozzles may be observed to be partially blocked. When this occurs, it may be necessary to assess whether the impactor has been operating in a fit for purpose condition since its last mensuration. For the Andersen Mk II impactor, blockage is most prevalent for stages 6 and 7 which have the smallest hole diameters, although stage 5 can have similar problems (blockages rarely occur on the other stages due to the prevalence of larger nozzles on those stages). Blocked nozzles lead to an increase in air velocity through the remaining nozzles on the stage and cause the stage cutpoint to decrease slightly. However, the magnitude of this change can be quite small. For example, having 5 of the 400 nozzles on stage 6 or 5 of the 201 nozzles on stage 7 of the Mk II impactor blocked, causes the cutpoint to decrease by approximately 0.7% (0.005 µm) and 1.5% (0.006 µm), respectively. ese differences are much smaller than the stage cutpoint differences observed by Stein and Olson (4) and may be considered negligible in most circumstances. Reducing test method variability: Following successful mensuration, analytical methods must be developed and decisions made concerning the variables to be assessed. Common practices in chemical analysis (Table 3) are well known and will not be discussed here. Worldwide, the PSD variables which are measured and used during QC testing are a subject of much debate (7). FDA tend to require assessment of individual and/or grouped stage depositions known to be typical of the batches tested during clinical trials; specifications are then chosen for the drug mass in each apparent size category, and its permissible variance. Overall, the product developer usually needs to ensure that the method is sufficiently robust in development to meet regulatory expectations prior to Phase 3 testing. While choice of common data transformations as product specifications (e.g., mass median aerodynamic diameter (MMAD), drug mass <5 µm aerodynamic diameter determined by interpolation), are often frowned upon by regulators as unnecessary additions to the actual measurements made during the test itself, it is often helpful when assessing (and attempting to minimize) method variance to select as small a number of variables as possible. For validation purposes therefore, transformations which capture all of the PSD profile and its mass better than numerous individual stage depositions provide a more understandable view of what really adds and subtracts

176 Selection and Validation of Cascade Impactor Test Methods Byron et al. from the overall variance due to the method. For example, the primary author recommends the choice of MMAD, (84.1% diameter/15.9% diameter) ratio, especially for monomodal PSD profiles which show a reasonable adherence to log-normality. In such a case, the geometric standard deviation (GSD = (84.1/15.9% diameter ratio) 1/2 and MMAD can best be determined by linear-linear interpolation of data such as that shown in Figure 1, and used alongside the total mass recovered from the impactor as three suitable variables. However, there are sufficient instances where these variables are unsuitable, making generalizations impossible. For DPIs in particular, one factor that can influence measurement accuracy and reproducibility concerns the establishment of the flow profile (through the inhaler and the impactor) to be used during testing. Pharmacopeial methods for DPIs require the use of complex switching valves to establish airflow through the impactor during the measurement (2, 3). It is a fact that many aerosols are actually being sized, at least as they pass the upper stages of the impactor, and while the steady-state flow rate is still being established. is inherently results in inaccurate particle size measurements. It is essential therefore that not only the steady-state airflow for a given test be well controlled (and measured during set-up), but also that the experimental set-up ensures that every time a test is performed (irrespective of which impactor, pump, or flow diverter is employed), the flow profile is reproducible. Currently within the pharmacopeias, there are no specific controls to ensure that this is assured. Method validation must eventually be performed by assessing the variance of the parameters which will ultimately be used for QC purposes. Comparison of the variance associated with the chosen parameters in accord with the effect of the issues described under method performance (Table 3), generally enables improvement in method precision to be brought about by standardizing the control features of the test method. Furthermore, variability problems associated with cascade impaction are usually to do with poorly described and executed methodology. e Mass Balance Group of the Product Quality Research Institute recently published their recommendations for Best Practices to be used during the performance of cascade impactor tests (12). ese should be fol- lowed carefully because the lacking standardization of an important control maneuver is often the reason for unwanted high variance results. Reference standards: QC analysts reading this advocacy article will likely have asked on several occasions why method variance cannot be determined using a Reference Standard Aerosol. Indeed, a series of reference aerosol products which are polydisperse with respect to their aerodynamic particle size distributions would be most helpful as a means of assuring both accuracy and precision during the validation process. In practice these days, pressurized MDI formulations can be manufactured which have apparent particle size distributions which are quite reproducible (e.g., Figure 1 and reference 7). Different formulations and actuators may potentially be chosen to cover the range of impactor deposition profiles illustrated by Lee (14). USP could potentially solicit the creation of such standards and test and issue them as tools to be used in the method validation package. is process requires considerable investment by USP however, in order to (a) commission and manufacture, and (b) test and assure the quality of these MDIs, so that (c) such standards could be provided publicly along with certificates to accredit their PSD profiles (by several different standard methods). In the absence of publicly available reference standard products, the analyst must develop an alternative approach to ensure that the cascade impactor is working correctly. However, one possibility when developing cascade impaction methods for large in vitro variance DPIs, could involve the periodic use of a commercially available, low-variance MDI product, with an appropriately selected PSD profile, as a means of assuring that the instrument and some aspects of the method remain under control as providers of valid data.

Respiratory Drug Delivery IX, 2004 Byron et al. 177 ACKNOWLEDGEMENTS e authors are grateful to Bo Olsson, Michael Hindle, Aki Kamiya, and Masahiro Sakagami for their advice during the early stages of manuscript preparation. AK and MS also assisted with the preparation of Figure 1 which was abstracted from a poster presentation described in reference 9. e Medical College of Virginia Foundation supported the research and preparation of this manuscript. REFERENCES 1. Chowan, Z.T., Casey, D.L., Byron, P.R., Kelly, E.L., Chaudry, I., Lovering, E.G., Vadas, E., iel, C.G., Barnstein, C.H., and Srinivasan, V. (1991), Report and recommendations of the USP Advisory Panel on Aerosols on the USP general chapter [601] on aerosols, Pharmacopeial Forum, 17, 1703-1713, and subsequent USP editions. 2. USP Aerosol Expert Committee (2003), Aerosols, nasal sprays, metered dose inhalers and dry powder inhalers, Pharmacopeial Forum, 29, 1176-1210. 3. United States Pharmacopiea, 25 (2002), Aerosols, Metered Dose Inhalers and Dry Powder Inhalers, General Chapter <601>. United States Pharmacopeial Convention, Rockville, MD, pp. 1964-1978 and European Pharmacopiea, Preparations for Inhalation Inhalanda. See also: Pharmacopeial Forum (2002), 28, 1470-1491. 4. Stein, S.W. and Olson, B.A. (1997), Variability in size distribution measurements obtained using multiple Andersen Mark II Cascade Impactors, Pharmaceutical Research, 14, 1718-1725. 5. Marple, V.A, Roberts, D.L., Romay, F.J., Miller, N.C., Truman, K.G., Van Oort, M., Olsson, B., Holroyd, M.J., Mitchell, J.P., and Hochrainer, D. (2003), Next generation pharmaceutical impactor (a new impactor for pharmaceutical inhaler testing). Part I: Design, J. Aerosol Med.,. 16, 283-299. 6. Nichols, S.C. (2000), Andersen cascade impactor: Calibration and mensuration issues for the standard and modified impactor, Pharmeuropa, 12, 584-588. 7. Byron, P.R. (2002), In vitro bioequivalence of particle size distributions: A commentary on FDA s draft guidance, Respiratory Drug Delivery VIII,, Vol 1, pp145-154. Dalby, R.N., Byron, P.R., Peart, J., and Farr, S.J. (eds.), Davis-Horwood International, Raleigh, NC. Available at www.rddonline.org. 8. Marple, V.A., Olson, B.A., Santhanakrishnan, K., Mitchell, J.P., Murray, S.C., and Hudson- Curtis, B.L. (2003), Next generation pharmaceutical impactor (a new impactor for pharmaceutical inhaler testing). Part II: Archival calibration, J. Aerosol Med., 16, 301-324. 9. Kamiya, A., Sakagami, M., Hindle, M., and Byron, P.R. (2003), Particle sizing with the next generation pharmaceutical impactor; a study of Vanceril metered dose inhaler, J. Aerosol Med., 16, 216.

178 Selection and Validation of Cascade Impactor Test Methods Byron et al. 10. Mitchell, J.P., Nagel, M.W., Wiersema, K.J., and Doyle, C.C. (2003), Aerodynamic particle size analysis of aerosols from pressurized metered dose inhalers: Comparison of Andersen 8 stage cascade impactor, next generation pharmaceutical impactor and Model 3321aerodynamic particle sizer aerosol spectrometer, AAPS PharmSciTech, 4, Article 54. Available at http://www.aapspharmscitech.org. 11. Asking, L. and Nichols, S. (2003), Next generation pharmaceutical impactor (NGI) EPAG collaborative study, presentation at Drug Delivery to the Lungs, 14, 33-36, e Aerosol Society, London, UK. 12. e PQRI Particle Size Distribution Working Group: Christopher, D. et al. (2003), Considerations for the development and practice of cascade impaction testing, including a mass balance failure investigation tree, J. Aerosol Med.,. 16, 235-247. 13. Vision Systems information, available at www.mitutoyo.com. 14. Lee, D. (2004), Searching for the holy grail of a single PSD profile comparator: Technical challenges with the chi-square approach, in Respiratory Drug Delivery IX,, Dalby, R.N., Byron, P.R., Peart, J., Suman, J.D., and Farr, S.J. (eds.), Davis Healthcare International, River Grove, IL, pp161-168. Available at www.rddonline.org. 15. Vaughan, N.P. (1989), e Andersen impactor: Calibration, wall losses and numerical simulation, J. Aerosol Sci., 20, 67-90.