Autoclaving behavior of trimyristin nanoemulsions stabilized with different poloxamers

Abstract

Lipid nanoemulsions are being investigated as carrier systems for the parenteral administration of poorly soluble drugs. Defined particle sizes, narrow particle size distributions, and sterility are prerequisites for the safe administration of such formulations to patients. In the current study, autoclaving of such formulations was performed to achieve both, sterility and a narrow particle size distribution. The high temperatures applied might, however, have an impact on emulsion stability. The influence of various types of poloxamer (Pol) on the formation of trimyristin (TM) nanoemulsions with around 100 nm (± 2 nm) mean particle size, as well as on the physical stability, particle size and particle size distribution changes during autoclaving of the emulsions was investigated. Higher homogenization pressures were required to achieve the target particle size in emulsion formulations stabilized with poloxamers of larger molecular size. A correlation between the autoclavability of the formulations and the cloud point of the respective poloxamer used for emulsion stabilization was observed. A PEO content of 70 % or above within the poloxamer molecule was needed to achieve stable nanoemulsions after autoclaving. In stable emulsions, Ostwald ripening occurred during autoclaving, indicated by particle size growth and narrowing of the particle size distribution, which was accompanied by changes in the melting behavior of the recrystallized emulsion droplets. Autoclaving of TM nanoemulsions stabilized with Pol 108, 188, 237, 238, 338 and 407 yielded systems with particularly well-defined particle sizes and narrow particle size distributions.

Highlights

• For emulsions stabilized with poloxamers of larger molecular size, higher homogenization pressures are required to achieve a certain target mean droplet size.
• Trimyristin nanoemulsions stabilized with poloxamer 108, 188, 237, 238, 338, and 407 remain physically stable under standard autoclaving conditions.
• The polyethylene oxide content of the poloxamer and its cloud point are key parameters for the physical stability of trimyristin nanoemulsions during autoclaving.
• In stable trimyristin nanoemulsions there is an Ostwald ripening-driven increase in mean droplet size and decrease in droplet size distribution width upon autoclaving.

Introduction

The development of colloidal drug carrier systems has attracted significant interest in pharmaceutical technology for many years. One option is using colloidal lipid emulsions, or lipid nanoemulsions, that are particularly interesting for the delivery of poorly water-soluble, lipophilic drugs (Floyd, 1999, Hörmann and Zimmer, 2016). The mean droplet size of such emulsions is usually below 500  nm, which allows intravenous administration of the formulations.
One of the most common preparation methods for lipid nanoemulsions is high-pressure homogenization. In this process, a coarse pre-emulsion is first prepared from the aqueous emulsifier solution and the lipid phase, e.g. by rotor–stator treatment. For homogenization, the pre-emulsion is then forced through an narrow gap or an interaction chamber under pressure (Floyd A. G., 1999; Grumbach et al., 2022). Although the latter method (microfluidization), in particular, enables a relatively good control of the resulting mean particle size and the width of the particle size distribution, it is extremely difficult to achieve very narrow particle size distributions.

In the manufacturing process of parenteral lipid emulsions, the emulsions are usually autoclaved after homogenization to ensure sterility (Floyd A. G., 1999). For trimyristin (TM) nanoemulsions stabilized with Pol 188, a previous study demonstrated a positive effect of autoclaving on the uniformity of the particle size distribution (Göke et al., 2016). Subsequent research further explored this phenomenon to explain the underlying principle and mechanism. It was assumed that the effect was caused by accelerated Ostwald ripening during autoclaving with the formation of Pol 188 micelles and an increased solubility of TM during heat treatment playing a central role in the observed process (Göke et al., 2018).
A beneficial effect on particle size homogeneity was also observed upon autoclaving dispersions of supercooled smectic nanoparticles stabilized with Pol 188 (Kuntsche and Bunjes, 2007). Monoolein dispersions stabilized with Pol 407 did not only show a narrowing of the particle size distribution upon autoclaving but also a structural transformation: Monoolein particles that initially lacked a liquid crystalline cubic structure could be transformed into cubic phase particles by autoclaving (Wörle et al., 2006).

TM is a saturated triglyceride of myristic acid with a melting point of around 56 °C. As a special feature, it is known to form supercooled droplets when formulated in colloidal lipid nanoemulsions by melt-homogenization. This means that the emulsified lipid does not crystallize after cooling below the melting temperature of the bulk material (e.g., to room temperature in the case of TM), but remains in the liquid state (Bunjes et al., 1996).

Poloxamers are triblock copolymers consisting of a central hydrophobic poly(propylene oxide) (PPO) chain flanked by two hydrophilic poly(ethylene oxide) (PEO) chains. The molecular structure of these copolymers is denoted as PEOa-PPOb-PEOc, indicating a flexible composition where the lengths of chains a, b, and c can vary significantly, depending on the poloxamer type. Due to their amphiphilic properties in aqueous solution – which are attributable to the water-soluble PEO and water-insoluble PPO segments – these copolymers can form micellar structures. Such structures are able to encapsulate hydrophobic drugs in their core, thereby enhancing the solubility of drugs that are poorly soluble in physiological media (Chiappetta and Sosnik, 2007). Poloxamers are valued in pharmaceutical applications for their biocompatibility and generally low toxicity, which ensures safe use (Bodratti and Alexandridis, 2018).

In the monograph “Poloxamers”, the European Pharmacopoeia describes five different types of poloxamers, namely poloxamer 124, 188, 237, 338 and 407 (European Pharmacopoeia, 10th ed.). Dosage forms that may contain poloxamers include tablets (Pol 188, 407), soft capsules, gels (Pol 124, 407), hard capsules, powders for suspension preparation, creams (Pol 188, 407), granules, solutions for oral and parenteral application (Pol 188), ocular application (Pol 188, 407) and suspensions for oral use (Pol 188) (Bodratti and Alexandridis, 2018).

For parenteral application, poloxamers are employed in various products, including Fluosol®, an emulsion of perfluorocarbon oxygen carriers used as a blood substitute containing 2.72 % (m/v) Pol 188 as emulsifier (Lowe, 2006). Other products available on the market that have been approved by the European Medicines Agency (EMA), such as the gene vector product Luxturna® (EMA. 2024g) offered as a concentrate and solvent for solution for injection and Zolgensma® (EMA. 2024d), provided as a solution for infusion, also contain Pol 188. Further, several peptide-/protein-containing products such as Sogroya® (EMA. 2024e) and Bemfola® (EMA, 2024) solution for injection in prefilled pen as well as Orencia® (EMA. 2024f) and Mircera® (EMA. 2024b) solution for injection in prefilled syringe for subcutaneous application are formulated with Pol 188. In addition, the EMA has approved a prolonged-release suspension for injection of rilpivirine for the treatment of HIV infections which contains Pol 338 as a stabilizing agent (EMA. 2024c).

In the current study, a first goal was to develop TM nanoemulsions stabilized with different poloxamers with a target mean particle size of 100 nm (± 2 nm), focusing on the dependence of the particle size parameters on the average molecular weight of the poloxamers and the applied homogenization pressure. This involved adjusting the pressures employed during the homogenization process to achieve the target particle size for all formulations. After successful production of these nanoemulsions, the primary objective was to evaluate their physical stability under pharmacopeial reference conditions for steam sterilization (121 °C). Apart from observing the colloidal stability of the nanoemulsions, a further emphasis was placed on examining changes in particle size and particle size distribution after autoclaving. This comprehensive analysis aimed to provide insights into the suitability of various types of poloxamers for stabilizing TM nanoemulsions suitable for parenteral administration, with a major focus on the autoclavability of the emulsions.

Download the full article as PDF here Autoclaving behavior of trimyristin nanoemulsions stabilized with different poloxamers

or read it here

Materials

TM (Dynasan® 114, IOI Oleo GmbH, Hamburg, Germany) served as lipophilic phase. Pol 108, 124, 184, 188, 338, 407 (Pluronic F38, Kollisolv® P124, Pluracare® L64, Kolliphor® P188, Kolliphor® P338, Kolliphor® P407, BASF, Ludwigshafen, Germany), Pol 234, 235, 238, 333 (Adeka Nol P-84, Adeka Nol P-85, Adeka Nol F-88, Adeka Nol P-103, ADEKA Europe GmbH, Düsseldorf, Germany), Pol 237 (Synperonic PF/F 87-FL-(CQ), Croda Europe, Chocques, France), and Pol 403 (Pluronic® P123, Sigma-Aldrich Chemie GmbH, Schnelldorf, Germany) were used to stabilize the nanoemulsions (for more information on the different types of poloxamers see Table 1). TM and poloxamers were kind gifts by the respective manufacturer. For all nanoemulsions, sodium azide (Sigma-Aldrich Chemie, Steinheim, Germany) was used as preservative. Water was of bidistilled quality for the preparation of nanoemulsions and ultrapure water (EASYpureTM LF, Barnstead, Dubuque, IA, USA) for the dilution of nanoemulsions for particle size measurement. Cloud point experiments were carried out with sodium chloride (Carl Roth, Karlsruhe, Germany).

Oyunbileg Sukhbat, Denise Steiner, Heike Bunjes, Autoclaving behavior of trimyristin nanoemulsions stabilized with different poloxamers, International Journal of Pharmaceutics, 2025, 125376, ISSN 0378-5173, https://doi.org/10.1016/j.ijpharm.2025.125376.

See our next webinar:

“Next-Gen Coatings:Merging Sustainability with Superior Performance“

Date: 27th of March, Time: 3:00 PM (Amsterdam, Berlin)

WEBINAR REGISTRATION HERE