Particle size distribution in drug development

Particle size distribution in drug development

In pharmaceutical development, the measurement of particle size distributions has become an indispensable analytical method in the characterisation of starting materials and finished products. The size of active ingredient particles can have a decisive influence on the efficacy, safety and stability of drugs. For example, if an active ingredient is present in dispersed form as part of a crystal suspension, the size of the dispersed particles can have a decisive influence on the physical stability of the suspension. In addition, it can also affect the release of the active ingredient in vitro and in vivo. Conversely, the release behaviour and bioavailability of dispersed dosage forms can be influenced by knowing and modulating the particle size distribution (PSD). In this context, not only undissolved solid particles can be measured, but also emulsified droplets in the case of emulsions. The size distribution of emulsified droplets is just as important a parameter for emulsions as the PSD of dispersed particles is for suspensions.
In the manufacture of solid dosage forms, the particle size of an excipient can have a significant influence on the behaviour within a powder mixture and consequently on the properties of the resulting dosage form (e.g. the tablet). The particle size distribution can therefore be an important factor in the development and manufacture of finished products. Valid PSD data can accordingly help to develop and manufacture products in a wide variety of dosage forms.

Another field of application is the determination of the size distribution of liquid particles (droplets). In this context, the term droplet size distribution (DSD) is also used. This type of size distribution measurement is primarily used in the characterisation of drugs that are delivered via spray applicators (e.g. nasal sprays). According to regulatory requirements (e.g. the EMA Guideline on the Pharmaceutical Quality of Inhalation and Nasal Products), the droplet size distribution of nasal spray products must be determined as part of both release and stability testing. The methods used to measure particle size distributions for different dosage forms and their strengths and limitations will be discussed below.

Methods for particle size determination

Analytical methods for determining the particle size distribution range from simple methods (e.g. light microscopy, sieve analysis) to highly technical methods with statistical evaluation (e.g. laser diffraction, dynamic light scattering) of large particle populations. Several analytical methods are often combined. Microscopic methods - both classic light microscopy and high-resolution electron microscopy methods - can provide further important information on shape and morphology in addition to the size of the particles. Modern high-resolution electron microscopy methods achieve resolutions of less than 1 nm and make it possible to characterise even the smallest dispersed particles. The combination of several analytical methods can offer decisive advantages for pharmaceutical development - as will be shown using a practical example in the following section.

Laser diffraction
Laser diffraction (LD) is a method for measuring particle size distributions based on the deflection of light waves from a laser beam when they hit particles (e.g. dispersed active ingredient particles in a suspension). This deflection generates different diffraction patterns depending on the size of the illuminated particles. The detected diffraction patterns in turn allow the particle size or particle size distributions of the measured sample to be calculated. Laser diffraction methods use models to calculate the particle size distributions. The so-called Mie theory is particularly useful here. This assumes a spherical shape of the particles to calculate the particle size distribution. This must be taken into account when evaluating the results of such measurement data. If the particle shape deviates from these assumptions, this can impair the quality of the generated results. The calculation also includes material-specific properties of the sample, such as the refractive index. When comparing particle size distributions, measurement and evaluation parameters as well as the measuring instrument used must therefore always be taken into account. Different evaluation methods and/or input differences in the optical sample properties can otherwise lead to incorrect results. Nowadays, laser diffraction is one of the most common methods for analysing particle size distributions. Apparatus-based systems offer simple feasibility and fast, reproducible measurement results. In pharmaceutical development, laser diffraction has become indispensable as an in-process control and release measure parameter in both development and routine production.
In addition to measuring dispersed particles, it is also possible to measure dry powders and aerosols directly using laser diffraction. On the one hand, this enables the direct characterization of solids, e.g. as part of the initial testing of active ingredients and excipients, and at the same time opens up a further field of characterization of inhaled and intranasal dosage forms. The direct solid characterization of powders plays an important role in the development of solid oral dosage forms. It makes it possible to understand the flow properties of powders and to make predictions about the compaction of solids (e.g. in direct tableting). In the development of intranasal dosage forms, the drop size distribution (DSD) - i.e. the size distribution of the drops that form the spray mist after actuation of the nasal spray - is an important parameter. The DSD allows conclusions to be drawn about other properties of a liquid dosage form, such as viscosity, and is therefore of great importance in the quality control of intranasal liquid products.

Dynamic light scattering
Dynamic light scattering (DLS), often referred to as photon correlation spectroscopy (PCS) or quasi-elastic light scattering (QELS), is another method of particle analysis. The DLS method is based on the principle of the scattering of light when it strikes particles. If a time-resolved measurement of the scattering intensity is carried out, it fluctuates due to the molecular motion (Brownian motion) of the measured particles. The fluctuation of the scattering intensity depends on the particle size: it is higher for small particles than for large particles. The time-resolved analysis can be used to determine the particle velocity and, in turn, the diffusion coefficient. If the temperature and viscosity of the sample are known, the hydrodynamic radius of the measured particles can be calculated using the Stokes-Einstein relation. The primary result of such a DLS measurement is the so-called intensity distribution - i.e. the size distribution according to scattering intensity - of the measured size populations. For certain applications, this can in turn be converted into a distribution by volume or number.
The intensity of the light scattered by the sample also depends on the angle between the incident light beam and the detector (the so-called scattering angle). DLS measurements are often only carried out at one scattering angle. When analysing and comparing DLS data, the scattering angle, at which the measurement was taken, is therefore important information. An innovation in DLS is the measurement at several scattering angles. By measuring at different scattering angles and analysing the resulting data in combination, a higher resolution of the measured samples can be achieved.
The zeta potential is another important parameter that should be mentioned when characterising disperse dosage forms. It is a measure of the electrostatic attraction or repulsion of dispersed particles. For suspensions and emulsions, it can be used to estimate the electrostatic repulsion and consequently make profound assumptions about the physical stability of the finished product. The zeta potential is not a property of the dispersed particle itself, but a result of the overall system of dispersed particles and dispersion medium. Changes in the dispersion medium, e.g. the choice or concentration of a buffer, can therefore have a major influence on the zeta potential. The zeta potential is measured using electrophoretic light scattering (ELS). An indirect measurement is carried out via the movement of the particles depending on their surface potential by applying an electric field and subsequent conversion into the zeta potential. In practice, the sample to be measured is placed in a cuvette with two electrodes. After a voltage is applied, the dispersed particles move to the opposite pole at a speed proportional to their surface charge. The zeta potential is calculated accordingly via the particle mobility. In the pharmaceutical environment, laser diffraction and dynamic light scattering are considered the industry standard for the applications described above. The comparatively simple sample preparation and carrying out of measurements make both methods indispensable not only in the context of development but also in routine analysis.

Particle characterisation in drug development

Particle characterisation plays an important role in the development of finished medicinal products. A central element here is the measurement of the particle size distribution of active ingredients and excipients. Depending on the dosage form and application, the aim of the measurement can be different. Some applications for different dosage forms will be explained below.
In the development and production of emulsions (and suspensions), the determination of the emulsion droplet size distribution (or the determination of the size distribution of dispersed particles) serves as an in-process control as well as release and stability parameter. A target size range is defined in the Quality Target Product Profile (QTPP) of a product to be developed, as well as in the release and runtime specification. If a process for particle reduction (e.g. wet grinding or high-pressure homogenisation) is used, the quality of the manufacturing process can be monitored by sampling and measuring the particle size distribution. As a rule, the D10, D50 and D90, i.e. the 10, 50 and 90 % of the measured particles of a sample that are smaller than or equal to the specified value, are indicated. In addition to the size distribution itself, the width of the distribution is also of decisive importance. This is known as the span and is calculated from the values for D10, D50 and D90 as follows: Span = (D90-D10)/D50. In practice, a low scattering width and a unimodal distribution of the particle size is usually favoured. In certain cases, however, a multimodal and/or broad distribution may be the aim of development. Nonetheless, these are technologically complex and difficult to generate and stabilise.
As described above, it often makes sense to use complementary measurement methods for particle characterisation. This will be illustrated below using a practical example. Figure 1 shows electron micrographs of an active ingredient from two different manufacturers.

Figure 1: Electron micrograph of a drug substance from two different manufacturers.

As part of the finished product development of a suspension, both active ingredient qualities were compared and, based on corresponding PSD data (measured using laser diffraction), were considered comparable for the production of the suspension concerned. Formulation tests showed very different behaviour of the two active ingredient sources in terms of wettability and stabilisation in dispersion. Further characterisation using electron microscopy revealed clear differences in the morphology and shape of the particles. The differences shown could therefore also explain the different behaviour of the two active ingredient sources. As a result, two different analytical methods were combined to solve the issue, which could not be solved by laser diffraction measurements alone.

Particle characterisation at HWI

HWI offers size distribution measurements of particles and droplets as part of the characterisation of active ingredients and the development of finished products using various apparatus-based methods. The most important of these include laser diffraction, dynamic and electrophoretic light scattering as well as light and electron microscopy. In addition to particle size distributions of powders, dispersions and emulsions, measurements of droplet size distributions for nasal, throat and other sprays are also carried out. Out team of experts has many years of experience in method development and validation. Thefollowing equipment is available:

• Malvern Mastersizer 2000 and 3000 zfor measuring dry powders, dispersions and emulsions using laser diffraction and dispersions in aqueous or organic environments.
• Malvern Zetasizer Advance Pro for particle size determination and measurement of the zeta potential using dynamic light scattering.
• HELOS BR Sprayer Rotor for measuring the droplet size distribution of spray products using laser diffraction.
• COXEM EM-30 electron microscope with energy dispersive X-ray detector (EDX).

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