Julie Suman is President at NextBreath LLC, USA.

Julie Suman

The way in which a suspension nasal spray product interacts with the body depends not only on the droplet size of the delivered droplet, but also on the particle size of the suspended active pharmaceutical ingredient (API). These dependencies are clearly recognised in the FDA draft guidance document '

Bioequivalance (BE) and bioavailability (BA) studies for nasal sprays and nasal aerosols for local action

  which also recommends appropriate analytical techniques to quantify these two parameters. 

For suspensions, the guidance states that 'drug particle size may be important for rate of dissolution and availability to sites of action within the nose'. For 

 bioequivalence studies, it recommends measurement of the API particle size distribution within the product formulation prior to actuation, and in the spray following actuation in order to characterise the influence of the device on de-agglomeration. In highlighting the presence of insoluble suspending agents/excipients as a complicating factor in this measurement, the guidance recommends the use of light microscopy, or alternative related techniques. However, it can be difficult to differentiate these excipients from the API using only light microscopy.  

With respect to droplet size and particle size distribution, the regulatory guidance states, that this "

is an important property influencing the nasal deposition of aerosols and sprays

". Droplets in the sub-10 µm region may be drawn into the lung rather than remaining in the nasal cavity, while excessively large droplets may be deposited primarily in the front of the nose and not at the intended site of deposition. Most prescription nasal sprays achieve the maximum therapeutic effect when the droplets deposit beyond the nasal valve in the posterior two thirds of the nasal cavity. Laser diffraction is recommended for droplet size measurement.  

Continuing high interest in nasal drug delivery intensifies the need for fast and detailed nasal spray characterisation that meets the demands of the regulators. In the study described here, laser diffraction is used to measure droplet size and to investigate the dynamics of spray formation in a commercial device. The API particle size distribution of the formulation, before and after dispersion, is measured using an automated imaging system with Raman spectroscopy capabilities. Much faster than manual light microscopy, automated imaging techniques provide statistically relevant analysis of size, shape and, in this case composition for precise API characterisation. 

Investigating the dynamics of spray formation

Laser diffraction is an ensemble particle sizing technique that measures size distributions in the range 0.1 to 3000 µm, comfortably spanning the area of interest for nasal sprays. Fast and non-destructive, laser diffraction is suitable for measuring both dry and wet samples, with instruments designed for spray analysis measuring at rates of up to 10 kHz. There are now several suppliers of laser diffraction equipment. Such systems capture the evolution of droplet size in real-time, throughout the duration of a spray event, providing detailed information on which to base decisions about a formulation or device.

 shows laser diffraction (Spraytec, Malvern Instruments) droplet size data for a commercial nasal spray product actuated at different velocities. Investigating the impact of actuation profile on droplet size is essential since in-use operation will vary from patient to patient. Here, tests were carried out at actuation velocities of 40, 70 and 100 mm/s. Samples were measured at a distance of 30 mm from the laser beam at a frequency of one measurement every 400 µs. Measuring over a period of 400 ms the spray event is tracked through to completion. The results are presented in the form of a particle size profile — a plot of Dv50 (the particle size below which 50% of the droplet population lies) with time. 

Figure 1: Particle size profiles and averaged particle size distributions (from the stable phase), for a commercial nasal spray product actuated at 40, 70 and 100 mm/s.  

Nasal spray events can be divided into three phases: formation, fully developed/stable, and dissipation. In the formation phase, flow through the spray pump nozzle is relatively low, droplet size is large, and the output of the nasal spray product is not yet stable. Flow is also low during dissipation at the end of the spray event when the metering chamber is empty. The FDA therefore recommends that data from the fully developed phase is used for statistically valid comparisons of the performance of the product under different conditions. 

The results show that as actuation velocity increases, the duration of the fully developed phase decreases; the dose is delivered more rapidly. In terms of atomisation behaviour, actuation at 70 and 100 mm/s produces similar results, generating a stable Dv50 of 35.4 µm and 32.2 µm, respectively. In contrast, the slowest actuation profile (40 mm/s), as well as producing a longer fully developed phase, atomises droplets to a much larger diameter, a Dv50 of 72.1 µm. This means that the performance of this product will be markedly different if the actuation profile applied by the patient is below a certain velocity, some way between 40 and 70 mm/s. The regulatory guidance states that pumps should be tested at velocities that are typical of hand actuation by patients, so care should be taken to select the appropriate settings for automated actuation during laser diffraction studies.  

Figure 2: Example particle images of API (left) and excipient (right) from a commercially available corticosteroid nasal spray formulation. The individual Circular Equivalent Diameter is indicated below each image (images are not to same scale).  

Automated particle imaging for API measurement 

Reliable measurement of the API of a suspension nasal spray formulation relies on the successful differentiation of API particles from any other suspended solids present, such as insoluble excipients for example. Traditionally, manual microscopy is employed for this application, with the operator visually discriminating between particles on the basis of their appearance. However, this approach is both operator dependent and extremely labour intensive, and the results tend to be relatively subjective with low statistical significance, pragmatism limiting the number of particles that can be measured. 

Figure 3: Overlay of the API CE diameter distribution before and after spraying.  

Using automated particle image analysis, tens of thousands of particles can be characterised in minutes. Individual images of every detected particle are captured, and analysed to generate statistically relevant descriptors of size, shape and transparency. For nasal sprays the formulation is simply dispersed on to a microscope slide prior to measurement. 

 shows example images, captured using automated image analysis, of an API and insoluble excipient present in the nasal spray formulation previously studied, as measured prior to actuation of the device. 

Figure 4: Size classification of the API particles before and after spraying.  

In this case, the active and excipient particles are quite different in terms of their morphological characteristics. Automatic classification of the particle images on the basis of a relevant shape parameter therefore enables identification of the API population and the gathering of data for this discrete group. 

show particle size distribution data, specifically for the API, measured before and after spraying. A slight shift in the distribution towards a finer particle size suggests that some shear-induced de-agglomeration occurs within the device during pump actuation. 

Figure 5: The Raman spectra of two individual particles, showing the ability of Raman todifferentiate API and excipients particles. Reference spectra for the API and excipient are shown in the green boxes.  

Although automated image analysis offers significant practical advantage relative to manual microscopy, it shares the limitation of being unable to discriminate between API and excipient particles that are visually identical. The addition of a Raman microprobe and spectrometer to an automated imaging system (Morphologi G3-ID, Malvern Instruments) overcomes this problem, enabling the chemical identification of API particles that are morphologically similar to insoluble excipients. Following acquisition of the images of individual particles, via routine automated imaging, the Raman spectra of selected particles can be measured and correlated with reference spectra to chemically differentiate the population of interest (

). By using the imaging data in this way to target the acquisition of Raman spectra — so called Morphologically Directed Raman spectroscopy — the time for measurements can be significantly reduced compared with standard Raman mapping methods. In addition, the measurement set up is simplified compared with standard methods, as the position of the particles is automatically determined prior to analysis, removing any operator subjectivity. 

To illustrate the benefits of this approach, the nasal spray product previously investigated was analysed. Following dispersion of the sample on to a slide, chemical identification data were gathered for around 9000 particles from a single scan area. 

 shows scatter-plots of the correlation scores for individual particles, referenced to the Raman spectra for the API and excipient. These scatter-plots chemically differentiate populations of API and excipient particles. Of the approximately 9000 particles analysed, around 450 were classified as API, this is consistent with the stated API : excipient ratio of the formulation which is 1:20 by weight. 

Figure 6: Scatterplots for Raman correlation scores of API and excipient particles and examples of associated particle images for the two chemical classes defined by the yellow regions.  

On the basis of this chemical classification it is possible to measure the API particle size distribution (

). Additionally, it is possible to compare the ingredient-specific particle size and shape distributions for the API and excipient (

). These data suggest that, in fact, the bulk of the excipient population can be excluded from chemical analysis on the basis of shape alone, by automatic classification using the shape parameter of elongation. Particles with an elongation greater than 0.4 are not API, and so do not need to be chemically identified using Raman. This finding opens up a route to improved measurement times. 

Figure 7: Particle size distribution of API in a nasal spray based upon Raman chemical classification.  

For this product, applying a morphological filter reduces the volume of spectral data which has to be generated, and would ease the analytical burden. In this particular example, classification on the basis of elongation would mean that only around 3000 of the original 9000 particles (approx.) would need to be chemically identified, a time saving of 66%.  

Figure 8: Comparison of the particle shape distributions for API and excipient in a nasal spray based upon chemical classification. Shaded area shows particles which can be excluded as being API based upon their elongation alone.  

Using rapid automated imaging in this way, as a pre-selection tool for chemical analysis of selected particles, is an efficient and powerful way of streamlining measurement towards routine quality control. It also allows for a more objective and statistically sound measurement to compare innovator and generic nasal spray products as required by the FDA draft BE guidance. In addition, of course, the detailed information provided by these techniques allows the insightful investigation of nasal spray products, and indeed other pharmaceuticals, in support of product development goals. 

Laser diffraction and automated imaging combined with Raman spectroscopy are powerful, complementary techniques for nasal spray product characterisation. Laser diffraction measurements relate to the formulation in its entirety and support the development of formulation and device towards optimal dispersion. Laser diffraction studies are also required for routine quality control and to determine

 bioequivalence. In contrast, automated image analysis in combination with Raman spectroscopy enables particle size analysis of the API alone, an important variable with respect to bioavailiabilty. Both techniques are fast and efficient, with automated imaging offering substantial advantage — quicker measurement, better reproducibility — compared with manual microscopy. Together they promote the efficient commercialisation of new nasal spray products in accordance with the regulatory guidances. 

is Product Group Manager – Micrometrics, Malvern Instruments. 

is Product Technical Specialist – Morphological Imaging Systems, Malvern Instruments. 

is Sales Specialist – Analytical Imaging Systems, Malvern Instruments. 

is Product Technical Specialist – Diffraction, Malvern Instruments. 

1. FDA Guidance — Bioequivalance (BE) and bioavailability (BA) studies for nasal sprays and nasal aerosols for local action, April 2003. 

 the authors discuss in more detail the value of laser diffraction in helping formulators achieve optimal nasal spray product performance by enabling the capture of nasal droplet size data in real time. 

Articles

The way in which a suspension nasal spray product interacts with the body depends not only on the droplet size of the delivered droplet, but also on the particle size of the suspended API.

Characterizing A Nasal Spray Formulation From Droplet To API Particle Size

Nasal sprays are widely used for locally acting preparations, such as decongestants and allergy treatments, as well as for the rapid delivery of systemic therapies, hormones and migraine treatments. In each case, the effectiveness of drug delivery depends on both the delivered dose and the size of the droplets produced by the spray pump during actuation. The latter influences both deposition and, potentially, bioavailability. In turn, droplet size derives from the characteristics of both the device and formulation, in combination, as recognised in the testing protocols.

  To achieve the desired performance of a product, developers may choose to manipulate the properties of the formulation or the design of the device, or both; however, developers require clear and relevant data on which to base their decisions. 

Laser diffraction-based particle size analysis is a powerful tool for nasal spray characterisation primarily because it is fast enough to capture droplet size evolution in real time. This article examines its value, and describes studies that explore the effect of formulation viscosity on droplet size, as well as device characteristics, such as orifice diameter of the nasal actuator and spray pump mechanism. Such studies support the faster and more efficient development of nasal spray products. 

Regulatory guidance recommends that nasal sprays are tested as combined products (device and formulation together) to determine reproducible delivery.

  Achieving clinical efficacy and good product consistency relies on understanding and controlling the interactions between device and formulation, which together dictate performance. 

testing is specified for a range of variables, with droplet size being one of the most important parameters. Droplet size measurements are used to assess: 

Quality — the consistency of performance; for example, over the lifetime of the product, from batch-to-batch, or after storage. 

Safety — droplets of 10 µm and below may pass through the nasal passages and penetrate into the lungs.

  Consequently, APIs in this fraction will enter the body by pulmonary absorption, rather than by the intended route. Quantifying the extent of this risk and assessing the associated clinical effect is essential. 

Efficacy — droplet size influences the site of deposition within the nasal passages, which may affect bioavailability. 

The FDA recommends droplet size measurement at several stages of the product life cycle, including: development/optimisation and preclinical/IND trials; stability, robustness and in vitro bioequivalence testing; and batch release.

  The European Medicines Evaluation Agency (EMA) sets out similar guidance for droplet sizing as part of product applications,

 bioequivalence testing is not as clearly stated. Such testing allows developers to successfully optimise controllable parameters to meet performance targets and provides quality control for the manufactured product.  

The two droplet sizing techniques for nasal spray testing highlighted by the regulators are cascade impaction and laser diffraction. Cascade impaction is a relatively time and labour-intensive technique that allows measurement of the amount of API in the sub -µm aerodynamic fraction, which is important for assessing the potential for pulmonary absorption (as noted above). Laser diffraction, in contrast, is sufficiently rapid to provide real-time size measurement of the whole droplet rather than just the API. Because of this, laser diffraction is an excellent tool to use during the development cycle when the aim is to target a defined droplet size and obtain an early assessment of the percentage of droplets less than 10 µm in size. 

Developers of nasal spray products tune an array of variables, some relating to the formulation others to the device, to achieve a target droplet size profile. Nasal spray devices usually incorporate a manually driven spray pump, which when actuated by the patient, pushes the liquid formulation through an orifice, thereby applying energy for atomisation (

). Aspects of spray pump design, such as the precompression ratio, and the geometry, length and orifice size of the actuator determine the shear force applied during use, which, in turn, influences the size of the droplets produced. The number and size of doses for which the product is intended will also be taken into account when determining the design of the device.  

Figure 1: Schematic of a nasal spray device.			

In terms of the formulation, modifying properties by changing the composition alters the response to conditions imposed by the device. A product may be solution- or suspension-based, with excipients levels controlled to meet atomisation and stability requirements. Viscosity modifiers such as glycerin, polyvinylpyrrolidone (PVP) and various cellulose-derivatives are especially common because the viscosity of the formulation has a marked impact on behaviour, with more viscous liquids requiring greater energy for dispersion to the same droplet size. Surface tension is also an important physical property and may affect droplet size. 

Developing an optimal product relies on the effective manipulation of these influential variables, but this is only possible with reference to relevant droplet size data. Laser diffraction meets this need by providing real-time particle size analysis through a spray event.  

Using laser diffraction for nasal spray analysis

With laser diffraction, the size of droplets in a spray is determined from the scattering pattern produced as particles pass through a collimated beam of light. Smaller particles scatter light weakly at wider angles, while larger particles produce a stronger signal at narrower angles. The technique is non-destructive, requires no calibration and is suitable for even concentrated sprays, providing that appropriate mathematical algorithms are used to analyse the scattered light pattern. 

To be effective for spray measurement, laser diffraction systems must meet some essential criteria, including the ability to rapidly acquire data to track the droplet size of a spray as it evolves during a single actuation of the device. For nasal sprays, a wide dynamic range is needed so that the very large droplets (up to 600 µm in size) delivered at the beginning and end of a spray pump actuation can be detected, along with any fines. This enables researchers to understand each phase of atomisation during actuation, aiding the process of product optimisation. 

Understanding the impact of product viscosity

An example of the data that can be obtained using laser diffraction is shown in 

. Here, the output of a typical nasal pump spray has been measured during actuation for solutions containing increasing concentrations of PVP (PVP K90) up to and beyond the normal concentrations found in commercial products. PVP is commonly used to provide relatively high viscosity to improve product stability for suspension formulations. In this case, actuation of the spray pump has been controlled using a velocitycontrolled actuator (SprayVIEW NSx; Proveris Scientific, MA, USA), set to achieve a maximum velocity of 40 mm/s. Measurements were made 30 mm from the nozzle, at the centre of the spray plume, directly above the nozzle. This is within the range mentioned in the FDA guidance.

 are typical for a nasal spray event. During the initial formation phase, droplet size falls rapidly as flow rate through the spray pump increases. There is then a prolonged period of steady atomisation behaviour — the fully developed phase — during which droplet size remains relatively consistent; the FDA recommends that data from this period is used for statistically valid comparisons between different products. Towards the end of the spray event, flow rate through the spray pump decreases again to produce a corresponding increase in droplet size. This latter phase is referred to as the dissipation phase. The entire spray event is complete in less than 200 ms, underlining the importance of a rapid measurement technique. 

Figure 2: Using the technique of laser diffraction to investigate the impact of solution viscosity on atomisation behaviour and droplet size.			

) for 1.0 and 1.5% PVP concentrations, respectively, show the impact of increasing solution viscosity; there is an approximate doubling of viscosity as additive level is increased from 0.5 to 1.5%. These profiles indicate less than ideal behaviour and suggest that at higher viscosities the spray pump is unable to adequately atomise the formulation. With this particular nasal spray pump, flow rate through the device varies during actuation and reaches a peak towards the end of the profile. For the 1.0% solution, flow rate becomes high enough at around 100 ms after the start of actuation to achieve effective atomisation and a short-lived fully developed phase is observed. With the 1.5% solution, however, this point is never reached because the device is unable to supply sufficient energy to properly disperse the most viscous formulation. 

In the case studies below, which follow the principles of quality by design, the limits of pump performance are sought to fully understand the nasal spray performance. In the example above, just one parameter (formulation viscosity) is changing. More frequently, developers need to assess the impact of changes to both device and formulation to find a combination that delivers the required performance. One option may be to modify the way energy is released into the liquid during atomisation by selecting an alternative spray pump mechanism. 

Modifying nozzle spray mechanism to control atomisation

shows data for the same formulations as those in 

, but this time using a spray pump mechanism that incorporates an energy storage mechanism (Equadel; Aptar  Pharma Prescription Division, IL, USA). As the user depresses the spray pump, energy is stored within a spring and is released when the spray pump reaches a pre-determined hydraulic pressure. This has a pronounced impact on atomisation behaviour, with an obvious stable phase being observed even at high viscosities. In addition, the length of the stable phase is much longer, which may help to improve the efficiency of droplet deposition within the nasal passages. 

Figure 3: Droplet size profiles obtained using the Equadel pump mechanism for the formulations shown in figure 2.			

Modifying actuator orifice diameter to improve nasal spray performance

Although changing the spray pump mechanism offers some advantages,

 shows that the droplet size delivered for high viscosity solutions, although more controlled, is still large. While the FDA provides no upper size limit for nasal sprays, a Dv90 of up to 150 µm is typical. Droplets sized towards the upper end of this range and beyond increase the risk of product dripping out of the nasal cavity and an associated reduction in drug delivery efficiency to the posterior nasal cavity. Since switching to the Equadel spray pump leaves droplet size largely unchanged, it is useful to identify additional levers to tune droplet size. Dv50, one of the defined parameters of interest in the FDA guidance,

  provides a simple representative measure for comparative studies of spray pump output. The experimental measurements of Dv10 and Dv90 indicate similar trends, although the data are not shown. 

If atomisation to a finer droplet size is considered important then changes to the geometry of the nasal spray pump actuator may deliver desirable performance. Important parameters include the geometry of the swirl chamber within the actuator and the diameter of the actuator orifice. This case study involves investigating the impact of actuator orifice diameter and actuation velocity on droplet size for different solutions.  

 shows the data reported using a smaller actuator orifice diameter than that used in 

. These data indicate that with the smaller orifice, effective atomisation to a finer droplet size is achieved with all three solutions — a rapidly established fully developed phase is observed in each case. Droplet size increases with increasing viscosity, as expected, and is unaffected by actuation velocity.

  For this system then, moving to a small diameter may provide one means of ensuring a more effective spray deposition pattern in the nose.  

Figure 4: Size profiles obtained using the Equadel pump mechanism with a small and large actuator orifice diameter.			

It is also possible to determine the effect of increasing the actuator orifice size. In this case, a very large orifice size (approximately 0.2 mm larger than the small orifice tested above) was used. Although the diameter of this orifice was considered unrealistic in terms of standard product design, these measurements allowed the robustness of the pump to actuator design changes to be understood. 

 shows the data recorded for actuation velocities of 40, 70 and 100 mm/s with water only. The results suggest that a stable phase is achieved at each actuation velocity when using the larger actuator orifice. At the lowest actuation speed (40 mm/s), however, the pump is seen to produce larger droplets. This trend holds for the 0.5% PVP and 1.0% PVP solutions.

Analysis of the atomisation mechanism at work in the device provides a rationalisation of the observations made. As actuator orifice diameter decreases, the pressure drop across it (at any given flow rate) increases, thereby improving the atomisation characteristic. This explains the improvement in atomisation performance observed when using the smaller orifice diameter. However, the Equadel spray pump energy storage mechanism of actuation, which is triggered by the back pressure developed during the initial stages, is also affected. Because a smaller nozzle diameter induces a greater back pressure, the trigger mechanism works more effectively. When the actuator orifice diameter is increased, the back pressure is reduced causing the spray pump to actuate differently at low velocities. This is why the results obtained at 40 mm/s are not the same as those obtained at high actuation velocities. 

The conclusion from this study is that reducing actuator orifice diameter makes it easier to access the optimal performance of the spray pump and actuation independent drug delivery. With a smaller diameter, solution viscosity is not as critical for effective drug delivery, which gives the formulator greater flexibility — although there is clearly a link between viscosity and droplet size.  

To achieve optimal product performance, nasal spray developers need to manipulate device and formulation parameters on the basis of detailed understanding. Laser diffraction is a valuable technique for accessing relevant information because it captures droplet size data in real time, enabling researchers to evaluate the impact of any change. The influence of formulation properties such as viscosity and device parameters, including actuation profile, spray pump mechanism and actuator orifice size, can be systematically studied to facilitate a knowledge-based approach, as the presented data show. The recommendation by the regulators of laser diffraction as a measurement technique, relevant across the complete product lifecycle, from development through to quality control, underlines its value.

is Product Manager — Diffraction Products at Malvern Instruments Ltd., UK. Tel. +44 (0)1684 892 456 

is Director, R&D Laboratory Services at AptarGroup SAS, France.   

1. Nasal Spray and Inhalation Solution, Suspension, and Spray Drug Products — Chemistry, Manufacturing, and Controls Documentation

2. Bioavailability and Bioequivalence Studies for Nasal Aerosols and Nasal Sprays for Local Action

3. Guideline on the Pharmaceutical Quality of Inhalation and Nasal Products 

(European Medicines Agency, London, UK, June 2006). 

One particularly crucial parameter for nasal sprays is the size of the droplets produced during actuation, which can potentially impact bioavailability.