Many medicines in routine use are particulate suspensions of an active pharmaceutical ingredient in a continuous liquid phase. Homogeneous dispersion of the key ingredient in such products is essential for uniform dosing. Understanding and controlling suspension instability is therefore crucial, to ensure that a product is both efficacious and safe.
The flocculating and settling behavior of a suspension is a function of the size of its suspended particles, the forces of attraction/repulsion between them, and the viscosity of the continuous liquid phase. In this article we assess strategies for inducing stability in a pharmaceutical suspension, or alternatively, rapid redispersion in the event of settling, and explain how measurements of particle size, zeta potential and rheology support the associated formulation process. A key focus is the interplay between different components of a pharmaceutical suspension and how their properties can be controlled to ensure consistent therapeutic activity.
Stability – A Critical Quality Attribute?
Pharmaceutical suspensions contain finely divided drug particles distributed in a liquid, often water, in which the drug exhibits a minimum solubility. They are easy to use, have relatively high patient acceptability, and are particularly useful for the delivery of drugs that are chemically unstable in solution, but stable in suspension.
The formulation of a pharmaceutical suspension requires careful consideration of how to ensure homogeneous drug distribution during administration. Settling or sedimentation is problematic because it introduces the risk of non-homogeneity and an unknown dosage. An ideal solution is therefore to maintain the dispersed phase in a uniformly-suspended state for the lifetime of the product, under all relevant conditions. However, in some instances this is not feasible, in which case easy redispersion is vital.
A formulator typically has some scope to modify suspended particle size, the strength of particle-particle interactions and/or the viscosity of the continuous phase, in order to achieve the goal of uniform dosing. These parameters can be manipulated to achieve acceptable performance via three distinct strategies:
Ensure That Particles Remain Discrete and Uniformly Suspended
A number of factors contribute to the maintenance of particles in a stably dispersed state, and these may be classified as either kinetic or thermodynamic in origin. In a suspension, particles move as a result of Brownian motion or under the influence of gravity. Kinetic stability is facilitated by slowing down such movement, inhibiting aggregation and decreasing sedimentation. Thermodynamic sta-bility, in contrast, relates to steric and electrostatic effects and can be induced by changing the size or shape of a particle or the electrostatic charge it carries.
Promote the Formation of Loose Flocculants That Are Easily Redispersed With Shaking
Where particles cannot be maintained in a discrete suspended state, it may be preferable to prevent the tight cohesion of small particles by intentionally inducing flocculation. In a floc, particles are held together by weak van der Waals forces, producing a loose porous structure that can entrain a large amount of liquid. This means that the volume of any sediment will be large and that the drug will easily be redispersed by moderate agitation or shaking, to reinstate uniformity. Although flocs settle more rapidly than individual discrete particles, flocculated particles form a lattice that resists complete settling and thus are less prone to compaction and cake formation.
Induce a Network Gel In The Continuous Phase
An alternative way of inducing stability in suspensions where gravitational forces dominate is to introduce a network structure into the continuous phase to give the system a yield stress. Systems with a yield stress remain stationary and do not flow until the applied shear exceeds a certain value. Such suspensions therefore exhibit kinetic stability with particles remaining stationary and suspended within the network, provided that any applied stress does not exceed the yield stress. One way to achieve such structure is to gel the continuous phase via the introduction of suitable additives.
Measurements That Support the Formulation Process
The successful implementation of one of these outlined strategies relies on measuring a number of physical and chemical properties of the suspension. A combination of particle size, rheology and zeta potential data is especially helpful in driving the formulation process.
Particle Size
Suspended particle size is one of the easiest properties of a suspension to change; however, formulators may not be free to do this because of the potential influence on other aspects of performance, such as bioavailability.
In terms of stability, particle size has a direct effect on the ease of maintenance of a uniform suspended phase. In a submicron suspension, Brownian motion helps to keep the particles in a dispersed state. However, for larger particles, the effect of gravity becomes significant, especially if there is a sizeable difference in density between the dispersed and continuous phases. The ratio of gravitational to Brownian forces therefore correlates with the likelihood of sedimentation which can be predicted from equation 1.1
a^4 Δρg/kB T
[where a is the particle radius, Δρ is the density difference between the dispersed and continuous phases, g is acceleration due to gravity, kB is the Boltzmann constant and T is the temperature]
According to Stokes’ law, the velocity of a suspended particle falling under gravity is directly proportional to the particle’s size.2 Reducing the size of suspended drug particles therefore reduces the rate and likelihood of sedimentation, helping to maintain the dispersion. However, if particles still settle although they are fine, the result may be rigid aggregates and a compact cake that resists breakup and is not easily dispersed.
A variety of instruments and techniques exist for monitoring particle size, including laser diffraction and dynamic light scattering (DLS).1 Both are fully automated and offer rapid measurement with high repeatability and reproducibility. Laser diffraction is most suitable for materials ranging from several hundreds of nanometers up to several millimeters in size, while DLS is most widely used for samples containing particles in the submicron region, potentially down to 0.3 nm. Together these techniques therefore span the size range of interest for the majority of pharmaceutical suspensions.
Rheology
For a dilute suspension, Stokes’ law states that the sedimentation velocity is inversely proportional to the viscosity of the continuous phase,2 as well as being proportional to particle size. For particles of a given size, doubling the suspension viscosity will therefore halve the rate of sedimentation. However, settling behavior is more complex in more concentrated suspensions, due to interactions between neighboring particles and the fact that high particle loading leads to an increase in overall density and viscosity.
Viscosity can be measured using a viscometer or preferably a rotational rheometer. Unlike viscometers, rotational rheometers are able to measure across a very wide range of conditions, for example to characterize behavior at low shear, corresponding to the suspension at rest, and at high shear, which occurs when the suspension is shaken.
Rotational rheometers can also be used to measure yield stress, and more generally, to investigate the underlying structure of suspensions and further support the development of stability.
Zeta Potential
Zeta potential provides a measure of the magnitude of the electrostatic or charge repulsion/attraction between particles at the slipping plane, between the particle and its associated double layer, and the surrounding solvent (see Figure 1). For systems that are subject to thermodynamic, as opposed to kinetic, instability, zeta potential can therefore provide insight as to how to enhance the properties of the formulation.
Figure 1. Zeta potential quantifies the forces of attraction/ repulsion between particles, supporting the development of systems with thermodynamic stability.If a suspension has a large negative or positive zeta potential, the particles within it tend to successfully repel each other, whereas smaller negative or positive zeta potential values increase the likelihood of flocculation. The dividing line between stable and unstable suspensions is generally taken as ±30 mV, with systems with zeta potentials which are respectively more positive or negative than this conferring suspension stability.3
Zeta potential measurements are made using the technique of Electrophoretic Light Scattering (ELS). When an electric field is applied across a dispersion, particles with a net charge, or more accurately a net zeta potential, will migrate towards the oppositely-charged electrode with a velocity, or mobility, which is directly related to their zeta potential. This velocity can be measured using ELS to determine an overall zeta potential for the sample. Highly-specified DLS systems such as the Zetasizer Nano from Malvern Instruments contain all the components required for ELS analysis and can therefore provide both size and zeta potential measurements, boosting their value for formulation studies.
The following case study illustrates the principles of the integrated application of particle size, rheology and zeta potential measurements to support the optimization of a stable suspension.
Case Study: Using Size, Rheology and Zeta Potential Data Together to Achieve Suspension Stability
Measurements were made to assess the effect of pH on the zeta potential of a suspension containing particles with an average size of 3.7 μm as determined by laser diffraction measurements. An autotitrator was used to steadily increase the acidity of the suspension down to a pH of 1.0 using standard solutions of HCl, and zeta potential measurements were made at ten equally-spaced pH intervals across the range.
With the suspension pH above 2.0, the measured zeta potential was in excess of -30 mV (see Figure 2). Despite this, the suspension was found to be unstable, forming a compact sedimented layer upon standing. This suggests that suspension instability in this instance was being driven by gravitational forces rather than by electrostatic interactions, which are insufficient to provide stability and kinetic stabilization in a size/density dominated system. In this case, the options for achieving dose uniformity would be to gel the continuous phase, or otherwise substantially increase its viscosity, or to encourage the suspended particles to flocculate.
Figure 2. Measurements of zeta potential as a function of pH show that higher pH values are associated with greater thermodynamic stability.Promoting Flocculation
The data displayed in Figure 2 suggests that more strongly acidic systems may be prone to flocculation. While flocculation is likely to lead to settling, and is therefore clearly linked to suspension instability, in this case it offers a potential route to formulation success (see second strategy outlined above) when the primary particles cannot be stabilized, as is the case here. Rheological measurements were carried out to assess the impact such flocculation may have on system viscosity. The shear viscosity of the suspension was measured as a function of shear rate, using an equilibrium step shear rate test at varying pH. (See Figure 3).
Figure 3. Flow curves show that at higher pH, the suspension exhibits a zero shear viscosity plateau, suggestive of solid-like behavior.At a pH of 3.9, suspension viscosity attains a constant value at low shear rates, i.e. the system exhibits a zero shear viscosity plateau. This suggests liquid-like behavior under very low shear conditions. The curves measured at more strongly acidic pHs do not exhibit this feature, indicating that the suspension may be more solid-like or gellike at low shear under these conditions.
A three-step shear rate test was implemented to further investigate the behavior of the suspension under strongly acidic conditions (data not shown). This involved measuring viscosity at a low shear rate, at a high shear rate, and finally under the original conditions. These data indicated that the suspension regains its low shear viscosity very rapidly once shear rate is reduced. In combination with the zeta potential data, these results suggest that the flocculation that occurs at a strongly acidic pH brings structure to the sample which rapidly breaks down and/or reforms following a perturbation. These are attractive characteristics for easy redispersion of the particles.
Investigating Yield Stress
In a final series of rheological tests, the yield stress of each of the suspensions was measured using a shear stress sweep test to quantify the impact of structure in the fluid. The results are shown in Figure 4.
Figure 4. Yield stress measurements provide further evidence that, for this suspension structure, stability can be induced by increasing acidityThe two samples that are more strongly acidic show a peak in viscosity that is indicative of a yield stress. This peak occurs at a much higher shear stress for the pH 2.42 sample than for the pH 3.52 sample, indicating that the structural strength developed is higher in the more acidic solution, when zeta potential is lower. For the sample with a pH of 3.97 there is no peak in viscosity, confirming the lack of a yield stress and the absence of a network structure, as observed in the flow curve data. In this example, making the suspension more acidic clearly improves stability, although further tests would be required to confirm that stability levels were sufficiently high for a specific application.
Conclusion
In the formulation of pharmaceutical suspensions, achieving product stability is critical, but can be demanding and time-consuming. The principles and case study presented here show how this task can be made easier through the application of particle size and zeta potential measurements, together with rotational rheometry. Together, zeta potential measurement and particle size data provide essential insight into the mechanisms that are controlling stability in the suspension, whether kinetic or thermodynamic. This provides an essential foundation for systematic and efficient development of a strategy to induce the stability needed to fulfil inuse requirements, with rheology providing vital information about the structural characteristics of the suspension and their ability to support particles or promote rapid redispersion. Complementary application of the three techniques outlined here therefore leads to robust and efficient formulation to meet the critical quality attribute of uniform dosing.
References
- Larson, R.G (1999), The Structure and Rheology of Complex Fluids, Oxford University Press, New York.
- Barnes, H.A (2000), A Handbook of Elementary Rheology, University of Wales, Institute of Non-Newtonian Fluid Mechanics.
- Zeta Potential of Colloids in Water and Waste Water, ASTM Standard D 4187-82, American Society for Testing and Materials, 1985.
Author Biographies
Lisa Newey-Keane is the Life Sciences Sector Marketing Manager for Malvern Instruments, based at Malvern’s head office in the UK. She holds a PhD in Microbiology/Protein Biochemistry from the University of Birmingham, and her industry background is within contract manufacture and research, primarily for biopharmaceuticals and anti-infectives.
Dr Steve Carrington is Product Manager - Innovations for Malvern Instruments, based at the company’s headquarters in Malvern, Worcestershire. He joined Malvern in September 2004 from MicroRheology, a spin out company from Bristol University in the UK. Steve gained his PhD in the extensional flow of polymer solutions from Bristol University, where he also held a variety of post-Doctoral research posts working with novel micromechanical and microfluidic-based techniques.