(W1030-11-62) Production of Drug Nanocrystals Using a Cost-Effective Miniaturised Media Milling Approach

Purpose: Nanocrystals (NCs), nanoparticles made of up 100% of drug¹, have become a commonly used strategy for the formulation of poorly soluble actives. NCs present unique features when compared to other nanotechnologies, such as high drug loading, long-term stability, and ease of scaling-up². Wet milling technology is the most used technique for the production of most of marketed nanocrystalline products³, and is a top-down manufacturing method that exploits shear, impact and hitting forces generated between drug particles and micrometric beads within a high-energy milling chamber, converting drug coarse powders into NCs⁴. Since it possesses flexibility in terms of batch size, the screening of critical parameters for the development of an optimised formulation is normally performed on the lab-scale and then further transferred to the industrial-scale machinery. Most commercially available laboratory-scale media mills typically require suspension volumes of 50-100 mL and considerable amounts of active (typically 5-20 g per run). This is particularly challenging at early development stages, when multiple experiments are required for process optimisation². This leads to excessive costs and presents considerable challenges when working with new chemical entities that are expensive and scarcely available. The aim of this work was to design an affordable, mini scale milling system (mini mill) to produce drug nanocrystals with reduced amounts of drug ( < 10 mg) and equipment commonly available in formulation laboratories.

Methods: Itraconazole (ITZ), Acyclovir (ACV) and Amphotericin B (AmB) were chosen as model drugs and processed into a miniaturised system consisting of a 2-mL Eppendorf used as the milling chamber, where the drug and 0.75 mL of stabiliser solution (1% w/w) were placed (Fig.1A). Two magnetic stirring bars (20 x 3 mm) agitated by a magnetic stirrer and a milling time of 24 hours were kept constant during all phases of the screening process. The effect of variations in type of stabiliser, milling media quantity, milling speed and drug amount (7.5-22.5 mg range) on NCs properties was assessed. The characterisation of the NCs was carried out by DLS used to evaluate the particle size, polydispersity index (PDI), and charge of NCs. Transferability of data for each experiment was determined by comparing the miniaturised method to a larger-scale wet bead milling device involving the use of 50-150 mg of drug and consisting of a 7 mL glass vial and two stirring bars (25 x 8 mm) previously reported by our group (micro mill)⁵.

Results: AmB, ACV and AmB NCs were obtained using the novel media milling device. The type of stabiliser resulted the most critical parameter, as the differences between the NCs’ particle size decreased when a strong surfactant such as SDS was used, with a similar trend being observed at the larger scale (Fig. 2A). Variation in drug amount showed different effects depending on the drug (Fig. 1B). For AmB, it exhibited a negligible difference between the two scales, while increasing ACV amounts led to a decreased in mean particle size when the milling was performed in the mini mills (237.98 nm), rather than the micro mills (322.91 nm). For ITZ, the increment of drug ratio produced NCs slightly bigger than the corresponding micro mill batch (140.31 nm - 125.66 nm, respectively) but all the other parameters produced transferable information. The milling beads ratio did not show noticeable difference between the different scales for exception of AmB, where it seemed to have greater affection on the mini scale (Fig. 2B). The same could be affirmed for the influence of the milling speed, that did not show significant influence when milling ACV and IT. However, for AmB, 1200 rpm achieved the smallest size among the other speeds for both mini and micro scale (125.58 - 113.01 nm, respectively). The evaluation of the Z-potential was mainly used to determinate the influence of the different surfactants on the NCs charge as well as giving an indication about the risk of aggregation, showing values between -5 mV and -15 mV in all cases. All the screening phases produced monodispersed nanosuspensions with a PDI lower than 0.2 and converted all tested actives into small NCs (size < 200 nm), with minor differences in particle size between the optimised formulations of both scales.

Conclusion: This work describes the production of drug NCs by a simple and low-cost wet bead milling method that could be easily performed by using equipment commonly available in any laboratory. This could represent an ideal solution for the early screening phases during the optimization process when producing NCs of poorly soluble drugs. All the model drugs employed in the study were converted into small NCs with acceptable PDI while reducing the use of active by up to 85%. For all the experiments the results of the minimum achievable size and most suitable stabilizer type produced transferable information from mini to maxi scale highlighting the potential of this technique.

References: 1- Keck, C. M.; Müller, R. H. Drug Nanocrystals of Poorly Soluble Drugs Produced by High Pressure Homogenisation. Eur. J. Pharm. Biopharm. 2006, 62 (1), 3–16.
2 - Romero, G. B.; Keck, C. M.; Müller, R. H. Simple Low-Cost Miniaturization Approach for Pharmaceutical Nanocrystals Production. Int. J. Pharm. 2016, 501 (1), 236–244.
3 - Thipparaboina, R.; Chavan, R. B.; Shastri, N. R. Nanocrystals for Delivery of Therapeutic Agents. Part. Technol. Deliv. Ther. 2017, 291–316.
4 - Merisko-Liversidge, E.; Liversidge, G. G.; Cooper, E. R. Nanosizing: A Formulation Approach for Poorly-Water-Soluble Compounds. Eur. J. Pharm. Sci. 2003, 18 (2), 113–120.
5 - Permana, A. D.; Paredes, A. J.; Volpe-Zanutto, F.; Anjani, Q. K.; Utomo, E.; Donnelly, R. F. Dissolving Microneedle-Mediated Dermal Delivery of Itraconazole Nanocrystals for Improved Treatment of Cutaneous Candidiasis. Eur. J. Pharm. Biopharm. 2020, 154, 50–61.

Acknowledgements: The authors declare no conflicts of interest related to this work.


Figure 1. A- Miniaturized wet bead milling process schematised. B- Size and PDI of ITZ (green), ACV (purple) and AmB (blue) NCs obtained by micro mills and mini mills when varying the drug to surfactant mass ratio.


Figure 2. A- Size and PDI of ITZ (green), ACV (purple) and AmB (blue) NCs obtained by micro mills and mini mills when varying stabiliser type and after the optimisation process. B- Size and PDI of AmB NCs obtained by micro and mini mills when varying beads to stabilizer volume ratio.