Journal of Controlled Release 199 (2015) 45–52
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In vitro dry powder inhaler formulation performance considerations Susanne Ziffels a,b, Norman L. Bemelmans c, Phillip G. Durham c, Anthony J. Hickey a,c,⁎ a b c
Division of Molecular Pharmaceutics, Eshelman School of Pharmacy, University of North Carolina at Chapel Hill, Chapel Hill, NC 25599-7571, USA F. Hoffman-La Roche Ltd, Basel, Switzerland RTI International, 3040 Cornwallis Road, Research Triangle Park, NC 27709, USA
a r t i c l e
i n f o
Article history: Received 10 March 2014 Accepted 17 November 2014 Available online 9 December 2014 Keywords: Aerosols Inhalation Lungs In vitro testing Flow rate Pressure drop Deliver dose Aerodynamic particle size distribution Delivery rate
a b s t r a c t It has long been desired to match airflow conditions during formulation evaluation to those of relevance to lung deposition. In this context several strategies have been adopted involving sampling at different: flow rate (without consideration of flow conditions, e.g. shear, Reynolds number, work function); pressure drop (with and without consideration of flow conditions) and; flow rate and pressure drop. Performance testing has focused on the influence of these sampling conditions on delivered dose uniformity and aerodynamic particle size distribution. However, in order to be physiologically relevant it is also important to know when the drug was delivered with respect to initiation of airflow as variation in this parameter would influence lung deposition. A light obscuration method of detecting the dose delivered from a dry powder inhaler while sampling for aerodynamic particle size distributions (APSD) by inertial impaction has been developed. Four formulations of albuterol sulfate and budesonide in sieved and milled lactose, respectively, were dispersed and their rate of delivery monitored. The differences observed have the potential to impact the site of delivery in the lungs. The rate of delivery of drug is clearly an important companion measurement to delivered dose and APSD if the intent is to predict the similarity of in vivo performance of dry powder inhaler products. © 2014 Published by Elsevier B.V.
1. Introduction In vitro characterization of dry powder inhalers (DPIs) by inertial impaction is the primary approach for evaluating drug formulations to support development of new products for pulmonary disease therapy. The aerodynamic particle size distribution obtained by inertial impaction is known to have a significant impact on regional lung deposition [1]. Furthermore, in vitro studies are essential to demonstrate bioequivalence between different DPIs [2]. In vitro methods have been extensively investigated to fully understand powder dispersion and particle impaction processes. Inspiratory flow rate, pressure drop or device resistance, and powder formulation are known to influence cascade impaction data. Consequently, these parameters are measured in the evaluation of in vitro dry powder inhaler performance. However, it has been observed that despite similarities in comparative DPI in vitro performance, based on inertial impaction data, significant differences in their subsequent in vivo performance did not support their bioequivalence [3]. The time-dependent rate of powder delivery is an important parameter, which is rarely considered in performance evaluation. When the bulk of a powder aerosol is delivered immediately after the inspiratory flow starts, the respirable particles in the formulation are more likely to reach the lower part of the lungs compared with examples where the powder experiences delayed or slow delivery following ⁎ Corresponding author at: RTI International, 3040 Cornwallis Road, Research Triangle Park, NC 27709, USA.
http://dx.doi.org/10.1016/j.jconrel.2014.11.035 0168-3659/© 2014 Published by Elsevier B.V.
initiation of inhalation. A formulation delivering the bulk of respirable particles with a delay, at a defined inspiratory airflow, is more likely to reach only the upper part of the lungs. Therefore, only a small amount of drug would arrive at the desired sites of action, which may result in substantially different in vivo data for various DPIs with the same in vitro performance. Few studies have addressed the time dependency of the inspiratory flow rate and its influence on powder delivery. Moreover, these studies were conducted to investigate the influence of the flow rate effects on the percentage of powder emptying not the rate of delivery [4–7]. 1.1. DPIs in therapy Dry powder inhalers have been employed to deliver drugs to treat asthma, chronic obstructive pulmonary disease and infections associated with cystic fibrosis [8,9]. In addition, they have been evaluated for delivery of insulin to treat diabetes and a variety of antibacterial agents to treat tuberculosis, for example capreomycin [10,11]. There continues to be significant product development activities involving DPIs that result in a desire to develop predictive methods to abbreviate timelines and reduce costs [12]. 1.2. Dry powder formulation Micron sized drug particles are too tightly bound through fundamental forces of interaction to be dispersed from a homogenous powder
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on the inspiratory airflow of a patient [13,14]. It is a common practice to blend these particles with large carrier particles of excipient such as lactose as shown in Fig. 1(a) [15,16]. However, other sugars have been proposed as potential excipients [17]. In some cases a controlled open lattice macroaggregate can be prepared as indicated in Fig. 1(b). Spray drying was introduced to inhalation technology in the 1990s and has gained significance as an alternative to micronization in the last decade [18,19]. It may be considered a destructive method from the perspective that spraying involves the breaking of bulk fluid usually containing drug in solution into smaller droplets, which are subsequently dried. At the solid-state level this may be considered a constructive method since the molecules of drug are brought together to form a solid particle during the drying process. Spray dried powders are frequently amorphous in nature and are susceptible to the deleterious action of moisture on stability. However, barrier packaging and the use of desiccant have been employed to limit the ingress of moisture and its destabilizing effects on spray-dried powders. Fig. 1(c) illustrates schematically the collapsed hollow sphere morphology that is frequently obtained by spray drying. These particles have unique properties. Their large geometric size and low density as predicted by Stokes' Law gives rise to a small aerodynamic size, low forces of interaction and ease of dispersion in response to airflow [20].
1.3. In vitro testing The adoption of in vitro methods that have the potential to guide product development, regulatory guidance and approval while retaining relevance to in vivo disposition is of increasing interest to pharmaceutical aerosol scientists and engineers. APSD and delivered dose uniformity are accepted compendial and regulatory performance measures [22–24]. Aerosol sampling to assess aerodynamic performance is acknowledged to be appropriate for collecting data of relevance to lung delivery. However, the way in which samples are collected should allow assessment of the total dose
delivered, the rate at which the aerosol is delivered and the APSD if lung deposition is to be accurately predicted from in vitro methods. Fig. 2 illustrates the key factors governing the performance of a dry powder drug product consisting of formulation, metering system and inhaler [13,14]. The important performance measures with respect to delivery of the nominal dose are identified as: delivered dose and uniformity; delivery rate and; impactor mass from which the aerodynamic particle size distribution is derived. The performance of the DPI product with respect to airflow has largely been considered in terms of response to different airflow conditions or pressure drops [25,26]. The response is thought to capture the likelihood of flow dependence, which is usually associated with poor dose delivery and/or reproducibility of dosing. However, consideration of the rate of delivery is rarely captured with respect to aerodynamic sizing conditions. In contrast, laser diffraction particle size measurement allows time of sampling to be considered with respect to formulation performance [25,26]. The experiments described here consider the combination of the three important elements used in in vitro product characterization and presents a new method for the determination of delivery rate. The latter is of relevance to presentation of the dose on the inspiratory flow of the patient with consequences for penetration to different regions of the lungs. This study involved development of a tool to assess the timing and rate of powder delivery. Time and rate of powder delivery were investigated for four example formulations to explore the discriminatory potential of the method.
2. Experimental methods 2.1. Drug/lactose powder blend preparation Four powder blends consisting of 1.5% drug, 1.5% fine lactose and 97.0% coarse lactose were prepared by geometrical dilution following sequential mixing. Budesonide (B) (Sigma-Aldrich, Steinheim, Germany),
Fig. 1. Frequently employed dry powder systems: (a) blends with large carrier particles of excipient usually lactose; (b) controlled macroaggregation of easily redispersible particles and; (c) spray dried particles that often form collapsed hollow spheres forming low density powder. Reproduced with permission from [21].
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Fig. 2. Aerosol generation and characterization by inertial impaction illustrating the important in vitro performance parameters of (a) delivered dose and uniformity; (b) delivery rate and (c) aerodynamic particle size distribution.
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through the impactor. The inlet was equipped with a light emitting diode (Osram SFH 4546, 940 nm) and a monolythic photodiode and transimpedance amplifier (Texas Instruments OPT101) and covered with electrical tape to prevent entry of environmental light (Fig. 3). During air flow, reductions in the voltage signal between diode and transistor caused by light obscuration of airborne powder particles were detected time-dependent with an oscilloscope operating in DCmode. In order to ensure accurate delivery time measurements, powder delivery and start of time measurement were synchronized by connecting the oscilloscope with the solenoid valve between impactor and vacuum pump. With this experimental set-up voltage versus time profiles could be recorded. Fig. 4 depicts the transit of the bolus of powder sequentially from (a) initiation of aerosolization under the influence of the airflow generated by the vacuum pump through the ACI. The voltage drop accompanying light obscuration is small as the powder (b) enters and (d) leaves the optical volume and reaches a maximum at the moment the bulk of the plume is (c) present in the optical volume. 2.3. Multivariate statistical analyses
micronized in a jet-mill at 75/55 psig to assure a respirable particle size, and micronized albuterol sulfate (AS) (Pfizer, Kent, UK) were investigated as drugs, while Respitose® SV003 (SV) and Respitose® ML001 (ML) (both DMV-Fonterra, Goch, Germany) were chosen as coarse lactose carriers for powder blends. Added fine lactose particles were produced by jet milling at a pressure ratio of 75/55 psi prior to blending. Blending homogeneity of the powder blends was verified collecting three samples of 10 mg each at different locations in the powder blend and quantifying the drug content of the samples with UV absorbance in a Shimadzu 1700-UV Spectrophotometer at a wavelength of 224.6 nm for SS and 244.0 nm for B. Either double distilled water (for AS) or methanol (for B) was used as solvents. Studies were conducted with the following blends albuterol sulfate and budesonide in sieved (SV-AS and SV-B) and milled (ML-AS and ML-B) lactose. 2.2. Aerodynamic particle size distribution determination APSDs of the four powder formulations were determined with an Andersen, 1ACFM 8-stage non-viable, Cascade Impactor (ACI) operated at 60.0 L/min. Standardized entrainment tubes (SETs) of different diameters and lengths selected to achieve a range of flow conditions with respect to pressure drop, Reynolds number and shear have previously been shown to have utility [27–30]. SETs have the advantage of utilizing a simple geometry, a straight tube, to deliver powder to allow comparisons of formulation effects alone [7]. Commercial inhalers also exhibit different pressure drops and flow profiles that may be employed to characterize formulation performance [30,31]. However, additional geometric features in the flow path of these devices, such as baffles and tortuous channels, were designed to facilitate the delivery of specific drug formulations. When comparisons of formulation performance are required these unique inhaler features contribute to a specific formulation/device interaction that will compound effects of dispersion mechanisms rendering data interpretation complicated and introducing variables that cannot easily be quantified. In the present study two SETs with definite shear stresses of 1.41 and 4.34 N/m2 were employed and ensured device independent measurements [7]. Pre-coating of Andersen impaction plates with a 1.0% (w/v) solution of silicon oil in hexane prevented particle bouncing and re-entrainment. The amount of drug impacted on the stages was quantified with UV-absorbance according to the method described above. A light obscuration powder inlet instrument was constructed (Carolina Chemistry Electronics Facility, UNCCH) The instrument consisted of a plexiglass tube inlet of 54.10 mm length placed between SET and induction port of the ACI to measure powder delivery time and delivery rate out of the SET (n = 3–5). The inner diameter of the inlet (21.91 mm) was in accordance with the inner diameters of SET and induction port to assure laminar airflow
The experiments were conducted as a 23 full factorial design in which the three factors were drug (albuterol sulfate and budesonide), carrier (sieved and milled lactose) and shear stress (1.41 and 4.34 N/ m2). Data were analyzed using Design Expert® 9.0 (Stat Ease, Minneapolis, MN). The statistical significance of the data can be illustrated by plotting the half-normal probability (y-axis) with respect to a standardized effect (x-axis). The standardized effect represents regression estimates with respect to model coefficients equal to one half of the effects which come from the average response at the high level and the average at the low level adjusted for common error variance. 3. Results Substantial differences in delivery time and rate were observed for the four powder formulations investigated. The blend carrier lactose, the airflow conditions adopted, and the SET used for dispersion influenced the observations. Powder delivery started for all formulations at 40 ms after airflow onset independently of the airflow conditions and the powder formulation. However, the powder blends with sieved lactose as carrier under high shear stress and Reynolds number conditions gave an initially slight increase (within ~20 ms) followed by a constant but low powder delivery rate over the delivery time (Fig. 5). The low delivery rate resulted in total powder delivery lasting about 60 ms. In general formulations with milled lactose as carrier showed an increasing delivery rate over a time period of about 20 ms and a decrease in delivery rate immediately after the maximum rate was reached until the whole amount of powder was delivered. When choosing a SET with
Fig. 3. Diagram of plexiglass inlet for measurement of delivery time and rate linking the standardized entrainment tube to the sampling inlet of the inertial Andersen Cascade Impactor illustrating the dimensions and the light source and detector.
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Fig. 4. Obscuration of the photodetector by the powder upon (a) initial generation; (b) entry to; (c) peak powder density in and; (d) departure from the optical sensing volume.
lower shear stress and Reynolds number the powder delivery increased to a maximum within about 10 ms with sieved lactose as the formulation carrier. Formulations with milled lactose as carrier showed a higher rate of delivery than sieved formulations. Additionally, a substantially higher obscuration was observed due to a larger mass of airborne particles measured at lower shear stress in comparison to lower obscurations observed when measurements were performed under higher shear stress.
(Fig. 5a). The ML-AS (Fig. 5d) appeared to exhibit a more defined bolus at approximately 70 ms in comparison with the SV-AS (Fig. 5c), which was delivered more uniformly. The range of dispersion seen for these formulations suggests that they would deliver the dose at different points in the inspiratory flow of a patient with the implication that lung deposition would differ.
3.1. Albuterol sulfate
The budesonide powders followed a similar pattern to the albuterol sulfate. The powder subjected to low shear gave a large drop in voltage, corresponding to obscuration, of approximately −2.0 V (Fig. 6a and b). The replicates for both SV-B and ML-B gave 50–60 ms troughs. At high shear (Fig. 6c and d) more shallow troughs, corresponding to lower obscuration, occurred with longer duration as seen with the albuterol sulfate to 150 ms (Fig. 6d) compared with 90 ms for both SV-B and ML B, Fig. 6a and b, respectively. It can be concluded that the differences observed in dispersion from these formulations would result in delivery at different point in the inspiratory flow of a patient in clinical use.
The behavior of albuterol sulfate powder shown in Fig. 5 was characterized by significant obscuration at 1.41 N/m2 with a sharply defined trough, −2.5 V, at approximately 60 ms for all replicates for the SV-AS (Fig. 5a) and a broader 50–70 ms trough for the ML-AS (Fig. 5b). At higher shear, 4.34 N/m2 (Fig. 5c and d), the drop in voltage was smaller, approximately −0.7 V, but the duration of obscuration appeared to be 2–3 times that of the lower shear extending to 150 ms for total delivery for ML-AS (Fig. 5d) at 4.34 N/m2 compared to 90 ms SV-AS at 1.41 N/m2
3.2. Budesonide
Fig. 5. Obscuration (V) of the photodetector when albuterol sulfate was delivered at 1.41 N/m2 from (a) sieved and (b) milled lactose and; at 4.34 N/m2 from (c) sieved and (d) milled lactose.
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Fig. 6. Obscuration (V) of the photodetector when budesonide was delivered at 1.41 N/m2 from (a) sieved and (b) milled lactose and, at 4.34 N/m2 from (c) sieved and (d) milled lactose.
3.3. Comparison albuterol sulfate and budesonide An important observation from both the albuterol sulfate and budesonide results is that while there is clearly an effect of blending with sieved or milled lactose, it appears that at low shear stress powder is delivered earlier in the airflow than at high shear stress with the implication that that would be carried further than the material delivered later. Cascade impaction cannot differentiate the significance of this observation since the time of sampling is far longer than the dispersion time but this may not be the case in vivo.
a rougher particle surfaces compared to the smoother carrier surfaces of formulations consisting of sieved lactose [15,16]. However, similar values for fine particle fractions, emitted doses, mass MMADs and GSDs did not correspond to a similar powder delivery time and rate. Although comparable values were obtained for some powder blends during impaction runs, the shapes of voltage versus time curves were always substantially different. Further experiments are required to verify that this method can be used as a discriminatory in vitro tool. 3.5. Multivariate statistical analyses
3.4. Aerodynamic particle size distribution Shear stress and the formulation carrier lactose influenced the APSD. Measurements with a SET of lower shear stress led, in general, to higher emitted doses while the fine particle fraction decreased concurrently due to a lower dispersion and deagglomeration force during air flow. Higher values for mass median aerodynamic diameter (MMAD) and geometrical standard deviation (GSD) were obtained for powder formulations with milled lactose as carrier compared to lower values for formulations with sieved lactose as carrier (Table 1). Particle deagglomeration from the milled carrier surface was hindered due to
Table 1 Performance of batches of albuterol sulfate (AS) and budesonide (B) prepared in sieved (SV) or milled (ML) lactose and delivered from standardized entrainment tubes (SETs) under defined airflow conditions (N/m2) as measured in terms of delivered dose (% and μg), mass median aerodynamic diameter (MMAD) and geometric standard deviation (GSD). Drug
SET, N/m2
Carrier
DD, %
DD, μg
MMAD, μm
GSD
AS AS AS AS B B B B
1.41 1.41 4.34 4.34 1.41 1.41 4.34 4.34
SV ML SV ML SV ML SV ML
96.4 94.1 97.2 91.6 95.3 84.7 86.6 58.4
103.0 157.5 159.1 179.9 347.5 305.0 279.4 415.2
1.57 1.85 1.84 1.99 2.77 3.31 2.09 3.31
1.78 1.81 1.81 1.80 1.92 2.09 1.67 1.91
Table 2 contains the output data derived from the obscuration study, which was conducted as a 23 full factorial design experiment. All the output variable were assessed in this multivariate statistical design including, from Table 2, onset of obscuration (ms), time of powder to transit (period of obscuration) (ms), time to exit (ms), amplitude of response (V) (ms), area under the response curve (Vms), and from Table 1, delivered dose (%), mass median aerodynamic diameter (MMAD, μm) and geometric standard deviation. Multivariate statistical analyses of the data demonstrated that only four outcomes were affected by the input variables. These were time of transit (onset to exit time), amplitude (V), area under the response curve (Vms) and MMAD. Table 2 Delivery of batches of albuterol sulfate (AS) and budesonide (B) prepared in sieved (SV) or milled (ML) lactose and delivered from standardized entrainment tubes (SETs) under defined airflow conditions (N/m2) as measured in terms of obscuration for which parameters are onset time (ms), transit time (ms), exit time (ms), amplitude (V) and area under the effect curve (AUC, Vms). Drug
SET, N/m2
Carrier
Onset time, ms
Transit time, ms
Exit time, ms
Amplitude, V
AUC, Vms
AS AS AS AS B B B B
1.41 1.41 4.34 4.34 1.41 1.41 4.34 4.34
SV ML SV ML SV ML SV ML
40.3 40.3 43.1 41.7 38.6 41.0 41.0 38.6
48.6 97.2 104.2 163.9 84.1 134.1 111.4 172.8
90.3 136.1 145.8 204.2 125.0 175.0 152.3 211.0
1.95 2.21 0.58 0.74 1.87 2.17 0.56 0.96
−35.8 −59.0 −29.2 −53.0 −33.4 −63.3 −29.3 −52.1
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Time to transit as evaluated by light obscuration was influenced most significantly by carrier and to a lesser, but still significant, extent by the shear stress, as shown in Fig. 7a below. These observations are both consistent with expectations. However, it was clear that time to transit was more rapid at low shear than at high shear which seems counter intuitive. This observation supports the notion that measurement of this phenomenon is important to extrapolation to clinical use if only because it might not otherwise be inferred. Since time of onset was essentially the same for all powders time of exit will not be discussed as it corresponds with time to transit. Amplitude of response, corresponding to extent of obscuration, was also significantly affected by shear stress and to a smaller extent by the carrier employed, which is a reversal of the observation for time to transit as shown in Fig. 7b. One interpretation of this observation in conjunction with that of the transit time is that the increased shear has greater capacity to carry particles of larger sizes or boluses of powder rather than stripping from the surface but the influence of increased turbulence may be give rise to extended residence time in entrainment tube leading to the optical volume. This is a relevant observation for inhaler performance. The area under the response curve observed was influenced by the shear conditions and carrier type as shown in Fig. 7c. This observation may be viewed as a single descriptor of obscuration for the period of transit of the powder. The carrier effect seems to be much larger than the shear effect for this measure and is consistent with, but more significant than, the time to transit observation. Mass median aerodynamic diameter (MMAD) was influenced to a small but significant extent only by the drug as shown in Fig. 7d. Consequently, the performance of the various carrier formulations under the range of shear stresses each drug, taken independently, would be considered equivalent if only based on the conventional measures of APSD and delivered dose. Clearly they would not be considered
equivalent in performance with respect to the time of delivery as indicated by the obscuration measures. 4. Discussion The impact of delivered dose, dose uniformity and APSD on lung deposition has been described thoroughly in the occupational and environmental medicine literature and was established decades ago [32]. The data employed to correlate these parameters with lung disposition were derived from exposure to ambient aerosols generally under steady state conditions. The same models were adopted for pharmaceutical aerosol delivery despite the unique situation in which dry powder inhalers deliver boluses to the patient as a function of inspiratory effort [2]. Consequently, the rate of delivery with respect to inspiratory flow cycle may dictate dose and site of deposition in the lungs in a manner not usually discussed in the context of steady state aerosols. Fig. 8(a) shows two hypothetical mass deliveries as a function of time with respect to inspiratory flow as schematic illustrations. As shown in Fig. 8(b) the potential impact of delivering early in the inspiratory flow is to increase the probability of peripheral lung deposition. However, late delivery may limit the penetration of drug to the lungs since the initial airflow traveling to the periphery cannot convey drug. When the data in Figs. 5 and 6 are considered with respect to the simple illustration in Fig. 8 the importance of rate of delivery and variations that might occur from one formulation to another are clear. Simply knowing the delivered dose uniformity and APSD may not be sufficient to predict lung deposition and by inference other biological performance measures such as pharmacokinetics of disposition or therapeutic effect. The distribution of the receptors of the sympathetic and parasympathetic branches of the autonomic nervous system in the lungs has been measured. These are the receptors that are targeted to achieve bronchodilatation to relieve symptoms in asthma and COPD. It is also
Fig. 7. Half normal probability plots with respect to standardized effect for (a) transit time, (b) amplitude, (c) mass median aerodynamic diameter and, (d) area under the response curve. Points deviating from the line of best fit represent statistically significant effects.
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formulations should be considered when bioequivalence of dry powder inhalers is an objective. In vitro testing is an important element of quality and performance testing of DPIs. The current desire to adopt predictive methods for in vitro aerosol testing that might be sufficient to allow reduced or selective clinical studies presents both an opportunity and a challenge [31]. The opportunity is to identify the key parameters required for predictive modeling and, in the most desirable outcome allow in vitro in vivo correlation. The challenge is that great control can be exerted over the product and test methodologies but the range of biological variation occurring as a result of age, gender and disease state reduce the accuracy and precision of predictions of estimated lung dose and deposition characteristics. The rewards for success in terms of cost, time and resource savings in product development and regulatory oversight are sufficient to justify continued efforts to grasp the opportunities and overcome the challenges. Acknowledgments
Fig. 8. (a) Differences in point in inspiratory flow on which the aerosol is delivered (A. early, B. late) give rise to, (b) differences in lung deposition (A. good peripheral, whole lung, B. poor peripheral, central airways). Solid shading indicates high deposition, hatched shading low, or no, deposition. Reproduced with permission from [18].
acknowledged that they are not uniformly distributed. Thus, the site of deposition of agonists or antagonists will play a significant role in viable therapy. β-adrenergic receptors are in their highest density in the periphery of the lungs [33,34] whereas cholinergic receptors have their highest density in the upper and central airways [35–38]. The latter are essentially absent in the periphery [34,37]. Differences in deposition of drugs intended to target these receptors, i.e. short or long acting β2adrenergic agonists (e.g. albuterol or salmeterol) or anticholinergic (ipratropium or tiotropium) drugs might be expected to result in different therapeutic outcomes depending on the capacity to exceed a dose threshold for local therapeutic effect. For those drugs that require systemic delivery, such as insulin, it should be noted that surface area for absorption, blood supply into which drug will be transported from the airways, metabolizing enzymes and the cells in which they reside, and clearance mechanisms all differ throughout the lungs [39,40]. The point at which the aerosol particles are delivered on the inspiratory flow and its effect on regional deposition of drugs may influence the capacity to employ or circumvent particular mechanisms or pathways to gain entry to the systemic circulation. The experimental observations from the in vitro studies described may have important therapeutic implications. However, a coordinated evaluation of in vitro DPI performance with human lung deposition, pharmacokinetic and pharmacodynamic studies is required to probe the impact of variation in delivery time on drug deposition, clearance and efficacy under controlled conditions.
5. Conclusions A new tool for the determination of delivery time and rate of dry powder inhaler formulations has been developed. Preliminary results show a significant influence of powder formulation on powder delivery time, time-dependent delivery rate and aerodynamic particle size distribution. However, powder formulations with similar aerodynamic particle size distributions can exhibit substantially different powder delivery times and rates, which may influence the area of impaction in the lungs. Therefore, delivery time and delivery rate of dry powder
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[16] A.J. Hickey, H.M. Mansour, M.J. Telko, Z. Xu, H.D. Smyth, T. Mulder, R. McLean, J. Langridge, D. Papadopoulos, Physical characterization of component particles included in dry powder inhalers. II. Dynamic characteristics, J. Pharm. Sci. 96 (2007) 1302–1319, http://dx.doi.org/10.1002/jps.20943. [17] H. Steckel, N. Bolzen, Alternative sugars as potential carriers for dry powder inhalations, Int. J. Pharm. 270 (2004) 297–306. [18] M.M. van Oort, M. Sacchetti, Spray drying and supercritical fluid particle generation techniques in inhalation aerosols, in: A.J. Hickey (Ed.), Physical and Biological Basis for Therapy, Second editionInforma Healthcare, New York, 2007, pp. 307–346. [19] R. Vehring, Pharmaceutical particle engineering via spray drying, Pharm. Res. 25 (2007) 1000–1022. [20] T.M. Crowder, J.A. Rosati, J.D. Schroeter, A.J. Hickey, T.B. Martonen, Fundamental effects of particle morphology on lung delivery: predictions of Stokes' law and the particular relevance to dry powder inhaler formulation and development, Pharm. Res. 19 (2002) 239–245. [21] A.J. Hickey, Dry Powders for Inhalation, AAPS Formulation, Design and Development, Newsletter, January 2014. [22] United States Pharmacopoeia, General Chapter b601N. [23] US Food and Drug Administration, Draft Guidance for the Industry, Metered Dose inhaler (MDI) and Dry Powder Inhaler (DPI) Chemistry, Manufacturing and Controls Documentation, November 1998. [24] US Food and Drug Administration, Draft Guidance on Fluticasone Propionate; Salmeterol Xinafoate, September 2013. [25] S. Jaffari, B. Forbes, E. Collins, D.J. Barlow, G.P. Martin, D. Mumane, Rapid characterisation of the inherent dispersibility of respirable powders using dry dispersion laser diffraction, Int. J. Pharm. 447 (2013) 124–131. [26] S.R.B. Behara, I.C. Larson, P. Kippax, P.J. Stewart, D.A.V. Morton, Insight into pressure drop dependent efficiencies of dry powder inhalers, Eur. J. Pharm. Sci. 46 (2012) 142–148. [27] Z. Xu, H.M. Mansour, T. Mulder, R. McLean, J. Langridge, A.J. Hickey, Dry powder aerosols generated by standardized entrainment tubes from drug blends with lactose monohydrate: 1. Albuterol sulfate and disodium cromoglycate, J. Pharm. Sci. 99 (2010) 3398–3414, http://dx.doi.org/10.1002/jps.22107. [28] Z. Xu, H.M. Mansour, T. Mulder, R. McLean, J. Langridge, A.J. Hickey, Dry powder aerosols generated by standardized entrainment tubes from drug blends with
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Contents lists available at ScienceDirect
Journal of Controlled Release journal homepage: www.elsevier.com/locate/jconrel
In vitro dry powder inhaler formulation performance considerations Susanne Ziffels a,b, Norman L. Bemelmans c, Phillip G. Durham c, Anthony J. Hickey a,c,⁎ a b c
Division of Molecular Pharmaceutics, Eshelman School of Pharmacy, University of North Carolina at Chapel Hill, Chapel Hill, NC 25599-7571, USA F. Hoffman-La Roche Ltd, Basel, Switzerland RTI International, 3040 Cornwallis Road, Research Triangle Park, NC 27709, USA
a r t i c l e
i n f o
Article history: Received 10 March 2014 Accepted 17 November 2014 Available online 9 December 2014 Keywords: Aerosols Inhalation Lungs In vitro testing Flow rate Pressure drop Deliver dose Aerodynamic particle size distribution Delivery rate
a b s t r a c t It has long been desired to match airflow conditions during formulation evaluation to those of relevance to lung deposition. In this context several strategies have been adopted involving sampling at different: flow rate (without consideration of flow conditions, e.g. shear, Reynolds number, work function); pressure drop (with and without consideration of flow conditions) and; flow rate and pressure drop. Performance testing has focused on the influence of these sampling conditions on delivered dose uniformity and aerodynamic particle size distribution. However, in order to be physiologically relevant it is also important to know when the drug was delivered with respect to initiation of airflow as variation in this parameter would influence lung deposition. A light obscuration method of detecting the dose delivered from a dry powder inhaler while sampling for aerodynamic particle size distributions (APSD) by inertial impaction has been developed. Four formulations of albuterol sulfate and budesonide in sieved and milled lactose, respectively, were dispersed and their rate of delivery monitored. The differences observed have the potential to impact the site of delivery in the lungs. The rate of delivery of drug is clearly an important companion measurement to delivered dose and APSD if the intent is to predict the similarity of in vivo performance of dry powder inhaler products. © 2014 Published by Elsevier B.V.
1. Introduction In vitro characterization of dry powder inhalers (DPIs) by inertial impaction is the primary approach for evaluating drug formulations to support development of new products for pulmonary disease therapy. The aerodynamic particle size distribution obtained by inertial impaction is known to have a significant impact on regional lung deposition [1]. Furthermore, in vitro studies are essential to demonstrate bioequivalence between different DPIs [2]. In vitro methods have been extensively investigated to fully understand powder dispersion and particle impaction processes. Inspiratory flow rate, pressure drop or device resistance, and powder formulation are known to influence cascade impaction data. Consequently, these parameters are measured in the evaluation of in vitro dry powder inhaler performance. However, it has been observed that despite similarities in comparative DPI in vitro performance, based on inertial impaction data, significant differences in their subsequent in vivo performance did not support their bioequivalence [3]. The time-dependent rate of powder delivery is an important parameter, which is rarely considered in performance evaluation. When the bulk of a powder aerosol is delivered immediately after the inspiratory flow starts, the respirable particles in the formulation are more likely to reach the lower part of the lungs compared with examples where the powder experiences delayed or slow delivery following ⁎ Corresponding author at: RTI International, 3040 Cornwallis Road, Research Triangle Park, NC 27709, USA.
http://dx.doi.org/10.1016/j.jconrel.2014.11.035 0168-3659/© 2014 Published by Elsevier B.V.
initiation of inhalation. A formulation delivering the bulk of respirable particles with a delay, at a defined inspiratory airflow, is more likely to reach only the upper part of the lungs. Therefore, only a small amount of drug would arrive at the desired sites of action, which may result in substantially different in vivo data for various DPIs with the same in vitro performance. Few studies have addressed the time dependency of the inspiratory flow rate and its influence on powder delivery. Moreover, these studies were conducted to investigate the influence of the flow rate effects on the percentage of powder emptying not the rate of delivery [4–7]. 1.1. DPIs in therapy Dry powder inhalers have been employed to deliver drugs to treat asthma, chronic obstructive pulmonary disease and infections associated with cystic fibrosis [8,9]. In addition, they have been evaluated for delivery of insulin to treat diabetes and a variety of antibacterial agents to treat tuberculosis, for example capreomycin [10,11]. There continues to be significant product development activities involving DPIs that result in a desire to develop predictive methods to abbreviate timelines and reduce costs [12]. 1.2. Dry powder formulation Micron sized drug particles are too tightly bound through fundamental forces of interaction to be dispersed from a homogenous powder
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on the inspiratory airflow of a patient [13,14]. It is a common practice to blend these particles with large carrier particles of excipient such as lactose as shown in Fig. 1(a) [15,16]. However, other sugars have been proposed as potential excipients [17]. In some cases a controlled open lattice macroaggregate can be prepared as indicated in Fig. 1(b). Spray drying was introduced to inhalation technology in the 1990s and has gained significance as an alternative to micronization in the last decade [18,19]. It may be considered a destructive method from the perspective that spraying involves the breaking of bulk fluid usually containing drug in solution into smaller droplets, which are subsequently dried. At the solid-state level this may be considered a constructive method since the molecules of drug are brought together to form a solid particle during the drying process. Spray dried powders are frequently amorphous in nature and are susceptible to the deleterious action of moisture on stability. However, barrier packaging and the use of desiccant have been employed to limit the ingress of moisture and its destabilizing effects on spray-dried powders. Fig. 1(c) illustrates schematically the collapsed hollow sphere morphology that is frequently obtained by spray drying. These particles have unique properties. Their large geometric size and low density as predicted by Stokes' Law gives rise to a small aerodynamic size, low forces of interaction and ease of dispersion in response to airflow [20].
1.3. In vitro testing The adoption of in vitro methods that have the potential to guide product development, regulatory guidance and approval while retaining relevance to in vivo disposition is of increasing interest to pharmaceutical aerosol scientists and engineers. APSD and delivered dose uniformity are accepted compendial and regulatory performance measures [22–24]. Aerosol sampling to assess aerodynamic performance is acknowledged to be appropriate for collecting data of relevance to lung delivery. However, the way in which samples are collected should allow assessment of the total dose
delivered, the rate at which the aerosol is delivered and the APSD if lung deposition is to be accurately predicted from in vitro methods. Fig. 2 illustrates the key factors governing the performance of a dry powder drug product consisting of formulation, metering system and inhaler [13,14]. The important performance measures with respect to delivery of the nominal dose are identified as: delivered dose and uniformity; delivery rate and; impactor mass from which the aerodynamic particle size distribution is derived. The performance of the DPI product with respect to airflow has largely been considered in terms of response to different airflow conditions or pressure drops [25,26]. The response is thought to capture the likelihood of flow dependence, which is usually associated with poor dose delivery and/or reproducibility of dosing. However, consideration of the rate of delivery is rarely captured with respect to aerodynamic sizing conditions. In contrast, laser diffraction particle size measurement allows time of sampling to be considered with respect to formulation performance [25,26]. The experiments described here consider the combination of the three important elements used in in vitro product characterization and presents a new method for the determination of delivery rate. The latter is of relevance to presentation of the dose on the inspiratory flow of the patient with consequences for penetration to different regions of the lungs. This study involved development of a tool to assess the timing and rate of powder delivery. Time and rate of powder delivery were investigated for four example formulations to explore the discriminatory potential of the method.
2. Experimental methods 2.1. Drug/lactose powder blend preparation Four powder blends consisting of 1.5% drug, 1.5% fine lactose and 97.0% coarse lactose were prepared by geometrical dilution following sequential mixing. Budesonide (B) (Sigma-Aldrich, Steinheim, Germany),
Fig. 1. Frequently employed dry powder systems: (a) blends with large carrier particles of excipient usually lactose; (b) controlled macroaggregation of easily redispersible particles and; (c) spray dried particles that often form collapsed hollow spheres forming low density powder. Reproduced with permission from [21].
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Fig. 2. Aerosol generation and characterization by inertial impaction illustrating the important in vitro performance parameters of (a) delivered dose and uniformity; (b) delivery rate and (c) aerodynamic particle size distribution.
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through the impactor. The inlet was equipped with a light emitting diode (Osram SFH 4546, 940 nm) and a monolythic photodiode and transimpedance amplifier (Texas Instruments OPT101) and covered with electrical tape to prevent entry of environmental light (Fig. 3). During air flow, reductions in the voltage signal between diode and transistor caused by light obscuration of airborne powder particles were detected time-dependent with an oscilloscope operating in DCmode. In order to ensure accurate delivery time measurements, powder delivery and start of time measurement were synchronized by connecting the oscilloscope with the solenoid valve between impactor and vacuum pump. With this experimental set-up voltage versus time profiles could be recorded. Fig. 4 depicts the transit of the bolus of powder sequentially from (a) initiation of aerosolization under the influence of the airflow generated by the vacuum pump through the ACI. The voltage drop accompanying light obscuration is small as the powder (b) enters and (d) leaves the optical volume and reaches a maximum at the moment the bulk of the plume is (c) present in the optical volume. 2.3. Multivariate statistical analyses
micronized in a jet-mill at 75/55 psig to assure a respirable particle size, and micronized albuterol sulfate (AS) (Pfizer, Kent, UK) were investigated as drugs, while Respitose® SV003 (SV) and Respitose® ML001 (ML) (both DMV-Fonterra, Goch, Germany) were chosen as coarse lactose carriers for powder blends. Added fine lactose particles were produced by jet milling at a pressure ratio of 75/55 psi prior to blending. Blending homogeneity of the powder blends was verified collecting three samples of 10 mg each at different locations in the powder blend and quantifying the drug content of the samples with UV absorbance in a Shimadzu 1700-UV Spectrophotometer at a wavelength of 224.6 nm for SS and 244.0 nm for B. Either double distilled water (for AS) or methanol (for B) was used as solvents. Studies were conducted with the following blends albuterol sulfate and budesonide in sieved (SV-AS and SV-B) and milled (ML-AS and ML-B) lactose. 2.2. Aerodynamic particle size distribution determination APSDs of the four powder formulations were determined with an Andersen, 1ACFM 8-stage non-viable, Cascade Impactor (ACI) operated at 60.0 L/min. Standardized entrainment tubes (SETs) of different diameters and lengths selected to achieve a range of flow conditions with respect to pressure drop, Reynolds number and shear have previously been shown to have utility [27–30]. SETs have the advantage of utilizing a simple geometry, a straight tube, to deliver powder to allow comparisons of formulation effects alone [7]. Commercial inhalers also exhibit different pressure drops and flow profiles that may be employed to characterize formulation performance [30,31]. However, additional geometric features in the flow path of these devices, such as baffles and tortuous channels, were designed to facilitate the delivery of specific drug formulations. When comparisons of formulation performance are required these unique inhaler features contribute to a specific formulation/device interaction that will compound effects of dispersion mechanisms rendering data interpretation complicated and introducing variables that cannot easily be quantified. In the present study two SETs with definite shear stresses of 1.41 and 4.34 N/m2 were employed and ensured device independent measurements [7]. Pre-coating of Andersen impaction plates with a 1.0% (w/v) solution of silicon oil in hexane prevented particle bouncing and re-entrainment. The amount of drug impacted on the stages was quantified with UV-absorbance according to the method described above. A light obscuration powder inlet instrument was constructed (Carolina Chemistry Electronics Facility, UNCCH) The instrument consisted of a plexiglass tube inlet of 54.10 mm length placed between SET and induction port of the ACI to measure powder delivery time and delivery rate out of the SET (n = 3–5). The inner diameter of the inlet (21.91 mm) was in accordance with the inner diameters of SET and induction port to assure laminar airflow
The experiments were conducted as a 23 full factorial design in which the three factors were drug (albuterol sulfate and budesonide), carrier (sieved and milled lactose) and shear stress (1.41 and 4.34 N/ m2). Data were analyzed using Design Expert® 9.0 (Stat Ease, Minneapolis, MN). The statistical significance of the data can be illustrated by plotting the half-normal probability (y-axis) with respect to a standardized effect (x-axis). The standardized effect represents regression estimates with respect to model coefficients equal to one half of the effects which come from the average response at the high level and the average at the low level adjusted for common error variance. 3. Results Substantial differences in delivery time and rate were observed for the four powder formulations investigated. The blend carrier lactose, the airflow conditions adopted, and the SET used for dispersion influenced the observations. Powder delivery started for all formulations at 40 ms after airflow onset independently of the airflow conditions and the powder formulation. However, the powder blends with sieved lactose as carrier under high shear stress and Reynolds number conditions gave an initially slight increase (within ~20 ms) followed by a constant but low powder delivery rate over the delivery time (Fig. 5). The low delivery rate resulted in total powder delivery lasting about 60 ms. In general formulations with milled lactose as carrier showed an increasing delivery rate over a time period of about 20 ms and a decrease in delivery rate immediately after the maximum rate was reached until the whole amount of powder was delivered. When choosing a SET with
Fig. 3. Diagram of plexiglass inlet for measurement of delivery time and rate linking the standardized entrainment tube to the sampling inlet of the inertial Andersen Cascade Impactor illustrating the dimensions and the light source and detector.
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Fig. 4. Obscuration of the photodetector by the powder upon (a) initial generation; (b) entry to; (c) peak powder density in and; (d) departure from the optical sensing volume.
lower shear stress and Reynolds number the powder delivery increased to a maximum within about 10 ms with sieved lactose as the formulation carrier. Formulations with milled lactose as carrier showed a higher rate of delivery than sieved formulations. Additionally, a substantially higher obscuration was observed due to a larger mass of airborne particles measured at lower shear stress in comparison to lower obscurations observed when measurements were performed under higher shear stress.
(Fig. 5a). The ML-AS (Fig. 5d) appeared to exhibit a more defined bolus at approximately 70 ms in comparison with the SV-AS (Fig. 5c), which was delivered more uniformly. The range of dispersion seen for these formulations suggests that they would deliver the dose at different points in the inspiratory flow of a patient with the implication that lung deposition would differ.
3.1. Albuterol sulfate
The budesonide powders followed a similar pattern to the albuterol sulfate. The powder subjected to low shear gave a large drop in voltage, corresponding to obscuration, of approximately −2.0 V (Fig. 6a and b). The replicates for both SV-B and ML-B gave 50–60 ms troughs. At high shear (Fig. 6c and d) more shallow troughs, corresponding to lower obscuration, occurred with longer duration as seen with the albuterol sulfate to 150 ms (Fig. 6d) compared with 90 ms for both SV-B and ML B, Fig. 6a and b, respectively. It can be concluded that the differences observed in dispersion from these formulations would result in delivery at different point in the inspiratory flow of a patient in clinical use.
The behavior of albuterol sulfate powder shown in Fig. 5 was characterized by significant obscuration at 1.41 N/m2 with a sharply defined trough, −2.5 V, at approximately 60 ms for all replicates for the SV-AS (Fig. 5a) and a broader 50–70 ms trough for the ML-AS (Fig. 5b). At higher shear, 4.34 N/m2 (Fig. 5c and d), the drop in voltage was smaller, approximately −0.7 V, but the duration of obscuration appeared to be 2–3 times that of the lower shear extending to 150 ms for total delivery for ML-AS (Fig. 5d) at 4.34 N/m2 compared to 90 ms SV-AS at 1.41 N/m2
3.2. Budesonide
Fig. 5. Obscuration (V) of the photodetector when albuterol sulfate was delivered at 1.41 N/m2 from (a) sieved and (b) milled lactose and; at 4.34 N/m2 from (c) sieved and (d) milled lactose.
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Fig. 6. Obscuration (V) of the photodetector when budesonide was delivered at 1.41 N/m2 from (a) sieved and (b) milled lactose and, at 4.34 N/m2 from (c) sieved and (d) milled lactose.
3.3. Comparison albuterol sulfate and budesonide An important observation from both the albuterol sulfate and budesonide results is that while there is clearly an effect of blending with sieved or milled lactose, it appears that at low shear stress powder is delivered earlier in the airflow than at high shear stress with the implication that that would be carried further than the material delivered later. Cascade impaction cannot differentiate the significance of this observation since the time of sampling is far longer than the dispersion time but this may not be the case in vivo.
a rougher particle surfaces compared to the smoother carrier surfaces of formulations consisting of sieved lactose [15,16]. However, similar values for fine particle fractions, emitted doses, mass MMADs and GSDs did not correspond to a similar powder delivery time and rate. Although comparable values were obtained for some powder blends during impaction runs, the shapes of voltage versus time curves were always substantially different. Further experiments are required to verify that this method can be used as a discriminatory in vitro tool. 3.5. Multivariate statistical analyses
3.4. Aerodynamic particle size distribution Shear stress and the formulation carrier lactose influenced the APSD. Measurements with a SET of lower shear stress led, in general, to higher emitted doses while the fine particle fraction decreased concurrently due to a lower dispersion and deagglomeration force during air flow. Higher values for mass median aerodynamic diameter (MMAD) and geometrical standard deviation (GSD) were obtained for powder formulations with milled lactose as carrier compared to lower values for formulations with sieved lactose as carrier (Table 1). Particle deagglomeration from the milled carrier surface was hindered due to
Table 1 Performance of batches of albuterol sulfate (AS) and budesonide (B) prepared in sieved (SV) or milled (ML) lactose and delivered from standardized entrainment tubes (SETs) under defined airflow conditions (N/m2) as measured in terms of delivered dose (% and μg), mass median aerodynamic diameter (MMAD) and geometric standard deviation (GSD). Drug
SET, N/m2
Carrier
DD, %
DD, μg
MMAD, μm
GSD
AS AS AS AS B B B B
1.41 1.41 4.34 4.34 1.41 1.41 4.34 4.34
SV ML SV ML SV ML SV ML
96.4 94.1 97.2 91.6 95.3 84.7 86.6 58.4
103.0 157.5 159.1 179.9 347.5 305.0 279.4 415.2
1.57 1.85 1.84 1.99 2.77 3.31 2.09 3.31
1.78 1.81 1.81 1.80 1.92 2.09 1.67 1.91
Table 2 contains the output data derived from the obscuration study, which was conducted as a 23 full factorial design experiment. All the output variable were assessed in this multivariate statistical design including, from Table 2, onset of obscuration (ms), time of powder to transit (period of obscuration) (ms), time to exit (ms), amplitude of response (V) (ms), area under the response curve (Vms), and from Table 1, delivered dose (%), mass median aerodynamic diameter (MMAD, μm) and geometric standard deviation. Multivariate statistical analyses of the data demonstrated that only four outcomes were affected by the input variables. These were time of transit (onset to exit time), amplitude (V), area under the response curve (Vms) and MMAD. Table 2 Delivery of batches of albuterol sulfate (AS) and budesonide (B) prepared in sieved (SV) or milled (ML) lactose and delivered from standardized entrainment tubes (SETs) under defined airflow conditions (N/m2) as measured in terms of obscuration for which parameters are onset time (ms), transit time (ms), exit time (ms), amplitude (V) and area under the effect curve (AUC, Vms). Drug
SET, N/m2
Carrier
Onset time, ms
Transit time, ms
Exit time, ms
Amplitude, V
AUC, Vms
AS AS AS AS B B B B
1.41 1.41 4.34 4.34 1.41 1.41 4.34 4.34
SV ML SV ML SV ML SV ML
40.3 40.3 43.1 41.7 38.6 41.0 41.0 38.6
48.6 97.2 104.2 163.9 84.1 134.1 111.4 172.8
90.3 136.1 145.8 204.2 125.0 175.0 152.3 211.0
1.95 2.21 0.58 0.74 1.87 2.17 0.56 0.96
−35.8 −59.0 −29.2 −53.0 −33.4 −63.3 −29.3 −52.1
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Time to transit as evaluated by light obscuration was influenced most significantly by carrier and to a lesser, but still significant, extent by the shear stress, as shown in Fig. 7a below. These observations are both consistent with expectations. However, it was clear that time to transit was more rapid at low shear than at high shear which seems counter intuitive. This observation supports the notion that measurement of this phenomenon is important to extrapolation to clinical use if only because it might not otherwise be inferred. Since time of onset was essentially the same for all powders time of exit will not be discussed as it corresponds with time to transit. Amplitude of response, corresponding to extent of obscuration, was also significantly affected by shear stress and to a smaller extent by the carrier employed, which is a reversal of the observation for time to transit as shown in Fig. 7b. One interpretation of this observation in conjunction with that of the transit time is that the increased shear has greater capacity to carry particles of larger sizes or boluses of powder rather than stripping from the surface but the influence of increased turbulence may be give rise to extended residence time in entrainment tube leading to the optical volume. This is a relevant observation for inhaler performance. The area under the response curve observed was influenced by the shear conditions and carrier type as shown in Fig. 7c. This observation may be viewed as a single descriptor of obscuration for the period of transit of the powder. The carrier effect seems to be much larger than the shear effect for this measure and is consistent with, but more significant than, the time to transit observation. Mass median aerodynamic diameter (MMAD) was influenced to a small but significant extent only by the drug as shown in Fig. 7d. Consequently, the performance of the various carrier formulations under the range of shear stresses each drug, taken independently, would be considered equivalent if only based on the conventional measures of APSD and delivered dose. Clearly they would not be considered
equivalent in performance with respect to the time of delivery as indicated by the obscuration measures. 4. Discussion The impact of delivered dose, dose uniformity and APSD on lung deposition has been described thoroughly in the occupational and environmental medicine literature and was established decades ago [32]. The data employed to correlate these parameters with lung disposition were derived from exposure to ambient aerosols generally under steady state conditions. The same models were adopted for pharmaceutical aerosol delivery despite the unique situation in which dry powder inhalers deliver boluses to the patient as a function of inspiratory effort [2]. Consequently, the rate of delivery with respect to inspiratory flow cycle may dictate dose and site of deposition in the lungs in a manner not usually discussed in the context of steady state aerosols. Fig. 8(a) shows two hypothetical mass deliveries as a function of time with respect to inspiratory flow as schematic illustrations. As shown in Fig. 8(b) the potential impact of delivering early in the inspiratory flow is to increase the probability of peripheral lung deposition. However, late delivery may limit the penetration of drug to the lungs since the initial airflow traveling to the periphery cannot convey drug. When the data in Figs. 5 and 6 are considered with respect to the simple illustration in Fig. 8 the importance of rate of delivery and variations that might occur from one formulation to another are clear. Simply knowing the delivered dose uniformity and APSD may not be sufficient to predict lung deposition and by inference other biological performance measures such as pharmacokinetics of disposition or therapeutic effect. The distribution of the receptors of the sympathetic and parasympathetic branches of the autonomic nervous system in the lungs has been measured. These are the receptors that are targeted to achieve bronchodilatation to relieve symptoms in asthma and COPD. It is also
Fig. 7. Half normal probability plots with respect to standardized effect for (a) transit time, (b) amplitude, (c) mass median aerodynamic diameter and, (d) area under the response curve. Points deviating from the line of best fit represent statistically significant effects.
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formulations should be considered when bioequivalence of dry powder inhalers is an objective. In vitro testing is an important element of quality and performance testing of DPIs. The current desire to adopt predictive methods for in vitro aerosol testing that might be sufficient to allow reduced or selective clinical studies presents both an opportunity and a challenge [31]. The opportunity is to identify the key parameters required for predictive modeling and, in the most desirable outcome allow in vitro in vivo correlation. The challenge is that great control can be exerted over the product and test methodologies but the range of biological variation occurring as a result of age, gender and disease state reduce the accuracy and precision of predictions of estimated lung dose and deposition characteristics. The rewards for success in terms of cost, time and resource savings in product development and regulatory oversight are sufficient to justify continued efforts to grasp the opportunities and overcome the challenges. Acknowledgments
Fig. 8. (a) Differences in point in inspiratory flow on which the aerosol is delivered (A. early, B. late) give rise to, (b) differences in lung deposition (A. good peripheral, whole lung, B. poor peripheral, central airways). Solid shading indicates high deposition, hatched shading low, or no, deposition. Reproduced with permission from [18].
acknowledged that they are not uniformly distributed. Thus, the site of deposition of agonists or antagonists will play a significant role in viable therapy. β-adrenergic receptors are in their highest density in the periphery of the lungs [33,34] whereas cholinergic receptors have their highest density in the upper and central airways [35–38]. The latter are essentially absent in the periphery [34,37]. Differences in deposition of drugs intended to target these receptors, i.e. short or long acting β2adrenergic agonists (e.g. albuterol or salmeterol) or anticholinergic (ipratropium or tiotropium) drugs might be expected to result in different therapeutic outcomes depending on the capacity to exceed a dose threshold for local therapeutic effect. For those drugs that require systemic delivery, such as insulin, it should be noted that surface area for absorption, blood supply into which drug will be transported from the airways, metabolizing enzymes and the cells in which they reside, and clearance mechanisms all differ throughout the lungs [39,40]. The point at which the aerosol particles are delivered on the inspiratory flow and its effect on regional deposition of drugs may influence the capacity to employ or circumvent particular mechanisms or pathways to gain entry to the systemic circulation. The experimental observations from the in vitro studies described may have important therapeutic implications. However, a coordinated evaluation of in vitro DPI performance with human lung deposition, pharmacokinetic and pharmacodynamic studies is required to probe the impact of variation in delivery time on drug deposition, clearance and efficacy under controlled conditions.
5. Conclusions A new tool for the determination of delivery time and rate of dry powder inhaler formulations has been developed. Preliminary results show a significant influence of powder formulation on powder delivery time, time-dependent delivery rate and aerodynamic particle size distribution. However, powder formulations with similar aerodynamic particle size distributions can exhibit substantially different powder delivery times and rates, which may influence the area of impaction in the lungs. Therefore, delivery time and delivery rate of dry powder
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