Impact of stabilizers on particle size and dispersion behavior in biorelevant media in solid nanocrystal formulations

Introduction

Nanosizing is a common approach to formulate poorly water-soluble API [1] for two main reasons. First, reduction in particle size increases the particle’s surface area and therefore the dissolution rate according to Noyes-Whitney/Nernst-Brunner equation [2], [3], [4], [5]. Second, nanosizing can increase the saturation solubility of the API as described by the Ostwald-Freundlich equation [2], [6]. As the increase in saturation solubility is only significant for particle sizes below 1–2  µm [7], [8], nanosizing is considered superior to micronization with respect to oral formulations of poorly water-soluble APIs.

These advantages depend on the particle size of the API, the average particle size and the particle size distribution (PSD) and this why these are the most relevant critical quality attributes (CQA) for nanocrystalline formulations [9], [10]. However, controlling the particle size reduction and maintaining a submicron particle size makes manufacturing of nanocrystalline formulations extremely challenging and requires specific methods, equipment, and excipients.
The most common approach for manufacturing nanocrystalline formulations is wet media milling, a “top-down” approach. In this context, wet media milling is often referred to as nanomilling [11], [12]. API, stabilizers, and dispersant, commonly water, are combined with a milling medium, for example, zirconium or polystyrene beads, which are rotated or shaken at high speeds [6], [11], [13]. For nanomilling, the API concentration is usually between 1 and 400 mg/mL and the API to stabilizer ratio is between 20:1 and 2:1 [13].

For the manufacturing of conventional oral dosage forms like capsules or tablets, the liquid nanosuspensions must be converted into solid nanocrystals. For this purpose, the nanosuspensions are usually spray-dried or spray-granulated [14], [15]. Instead of dosing a liquid formulation in early development phases, lyophilization followed by reconstitution can be applied to increase the shelf life of the nanocrystal system. Independently from the applied drying techniques, protectants must be added to prevent agglomeration and particle growth during water removal and to ensure redispersibility while maintaining the nanoparticulate structure. Commonly used protectants are sugars or sugar alcohols such as mannitol, sucrose, lactose, or trehalose [2], [11]. The selection of type and amount of protectants can influence redispersibility [16].

As the particle size distribution is one of the most relevant QAs in nanocrystalline formulations, the methodology for PSD characterization in nanosuspensions or after the redispersion of solid nanocrystalline formulations is particularly important. The most common techniques for PSD characterization in nanocrystalline formulations are dynamic light scattering (DLS), laser diffraction (LD), and microscopy [2], [17].

DLS is a technique that relies on the Brownian motion experienced by nanocrystals in suspension. This motion causes variations in the intensity of scattered laser light passing through the dispersion, allowing the determination of the hydrodynamic diameter [18]. Although DLS measurements are fast and simple, characterization of particles larger than 6 µm is not possible, and results may not be accurate for polydisperse nanosuspensions [2], [17], [18].
In contrast, the LD method can detect larger particles up to approximately 2000 µm, but the Mie formula must be used to determine the size of particles smaller than 3.5 µm, which requires knowledge of the optical properties of the material. However, sometimes optical properties of the API may not be available for early-stage development candidates [17].

In contrast, optical microscopy provides clear and unambiguous information about particle sizes and potential agglomeration. However, optical microscopy and also video imaging technologies are limited to the optical resolution and therefore, nanoparticles are not detected by standard techniques [17].
In addition to PSD determination, the particle surface charge is another important property of nanocrystals. The surface charge, mostly characterized by Zeta potential measurements, provides information about the particle’s ionization or adsorption of ions (e.g., ionic surfactants) onto the particle surface. Nanocrystals with good electrostatic stabilization typically have a positive or negative Zeta potential of 15  mV to 30  mV [2].

One of the advantages of nanosuspensions is that the particles do not sediment as they experience Brownian motion [6], [19]. However, due to the high surface energy of nanocrystals, agglomeration is likely to occur [2], [20]. Agglomeration in nanosuspensions can be explained by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. This describes the interplay of attractive Van Der Waals and repulsive, electrostatic forces between dispersed particles [21], [22], [23]. When the distance between particles falls below a certain energy barrier, particles will agglomerate [22], [24]. Next to agglomeration, Ostwald ripening may occur in nanosuspensions. During Ostwald ripening, nanocrystals grow because API redistributes from smaller to larger particles through enthalpy-driven dissolution and precipitation processes [20], [25].

To reach small particle sizes during milling and to prevent agglomeration and particle growth during further processing and storage, stabilizing excipients are needed [2]. For nanocrystalline formulations, there are two main stabilizing mechanisms, steric stabilization and electrostatic stabilization. Steric stabilization is achieved via a mechanical barrier between the particles, which prevents the energy barrier derived from the DLVO theory from being crossed. Electrostatic stabilization works by increasing the electrostatic repulsive forces and thus increasing the energy that would be necessary to achieve irreversible particle agglomeration according to the DLVO theory [2], [20]. Often, a combination of both steric and electrostatic stabilization, called electrostatic stabilization, is used and results in the smallest achievable and physically most stable nanosuspensions [20], [26].
Stabilizers for nanocrystalline formulations can be divided into three main groups, steric stabilizers as well as nonionic and anionic surfactants (Table 1).

First, polymeric stabilizers, which are steric stabilizers with a typical molecular weight between 50  kDa and 100  kDa can be applied. They must be big enough to provide steric hindrance, but short enough to not hinder dissolution [2]. Typical polymeric stabilizers are celluloses like hydroxypropyl cellulose (HPC) or hydroxypropyl methyl cellulose (HPMC), and polyvinylpyrrolidone (PVP) [2], [6], [20]. Higher concentrations of polymeric stabilizer typically lead to better results in terms of particle size and stability. However, a higher concentration of polymer also increases the viscosity of the nanosuspension which may hamper the comminution process of API crystals during milling [12].
Nonionic surfactants are also considered to be steric stabilizers. These increase the wettability and dispersibility of nanocrystalline formulations. Typical nonionic surfactants in nanocrystalline formulations are polysorbates like Tween® 20 or 80 and D-α-Tocopherol polyethylene glycol 1000 succinate (Vitamin E-TPGS) [2], [12], [20].

In contrast, ionic surfactants act as electrostatic stabilizers inducing electric repulsion forces between particles. Although it is possible to use cationic stabilizers in nanocrystalline formulations, mainly anionic surfactants are used in pharmaceutical formulations [2], [27]. Anionic surfactants increase the wettability and the electrostatic repulsion between particles. Sodium dodecyl sulfate (SDS) and docusate sodium (DOSS) are commonly used anionic surfactants in nanocrystalline formulations and have been used in several commercial drug products [2], [12], [20].
Although anionic surfactants are regularly used for the electrostatic stabilization of nanocrystalline formulations, little is known the stabilization after redispersion in the luminal fluids of the human GI tract. For most nanocrystalline formulations administered orally, dissolution will start in the acidic environment of the stomach. As electrostatic stabilization may be sensitive against changes in pH or ionic strength [20], the pH of gastric fluids may have great impact on particle sizes after redispersion and thus, on the in vivo performance of the nanocrystalline formulation. Kesisoglou and co-workers also mentioned that charge-based interactions are possible for nanocrystalline formulations containing ionizable APIs with a pKa within the physiological pH range [2]. This statement is further supported by Donoso et al., who described the agglomeration of anionically stabilized ketoconazole and itraconazole nanosuspensions after reducing the pH to gastric levels. This agglomeration was attributed to a salt formation between the basic APIs, ketoconazole and itraconazole, and the anionic surfactant DOSS [28].

The main objective of this work was to investigate and understand the behavior of anionic surfactants in solid nanocrystalline formulations and their effects on redispersibility under biorelevant conditions. As the present study focused on examining the isolated effect of surfactants, other factors that can have an influence on redispersibility such as the protectant type and level used were not varied.

First, a systematic nano stabilizer screening was performed, in which nanosuspensions with combinations of different API, polymeric stabilizers, and either nonionic or anionic surfactants were manufactured by wet media milling. BCS class II drugs with neutral (fenofibrate, FNB), basic (itraconazole, ITZ, and ritonavir, RTV) and acidic (naproxen, NPX) properties were selected as model APIs. They reflected a broad range of APIs with different physicochemical properties that could potentially affect the physical stability after redispersion in aqueous media. The comminution effectiveness of nonionic and anionic surfactants in each case was assessed based on particle size determinations.
Second, larger amounts of selected prototype formulations were re-manufactured to convert the nanosuspensions into solid nanocrystals by lyophilization. The redispersibility in water was tested as the most important quality attribute to evaluate whether the drying process did induce any agglomeration or particle growth. In addition, the dispersibility and particle size of the solid nanocrystals after redispersion were tested systematically in biorelevant media simulating both the gastric and the intestinal environment.

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Nils Christian Böck, Julius Sundermann, Mirko Koziolek, Benjamin-Luca Keller, Karsten Mäder, Impact of stabilizers on particle size and dispersion behavior in biorelevant media in solid nanocrystal formulations, European Journal of Pharmaceutics and Biopharmaceutics, 2025, 114651, ISSN 0939-6411, https://doi.org/10.1016/j.ejpb.2025.114651.

See also the interesting video on Vitamin E TPGS: