(T1430-03-18) Balancing Particle Stability and Permeability Advantages of High Drug Loading Amorphous Nanoparticle Formulations

Purpose: For amorphous solid dispersions (ASDs), low drug loadings are often necessary to maintain formulation stability and promote fast drug release. However, this will lead to unusually large pill size for high dose drugs. An alternative strategy to overcome the drug loading limit and achieve desired exposure is to directly form amorphous drug nanoparticles. Combining the amorphous solubility advantage with reduced particle size, directly engineered amorphous nanoparticles may act as drug reservoirs and reduce the diffusional resistance of the aqueous boundary layer, thus promoting drug absorption. However, the drying stability and re-dispersibility of amorphous nanoparticles remain significant challenges for most drugs. In this study, we evaluated the drying stability and permeability advantages of amorphous drug nanoparticles.

Methods: Clotrimazole was chosen as a low T_g model compound which demonstrated superior particle stability in solution and during drying. It is a weak base with a pK_a of 5.89¹. Lopinavir was chosen as a neutral low T_g model drug. Based on results from prior solution stability studies², Eudragit® L100 was selected as a particle stabilizer. For lopinavir, dried amorphous particles were also prepared through freeze drying. To avoid the use of organic solvents, a 10% drug loading ASD was prepared using hydroxypropyl methylcellulose (HPMC) to generate amorphous nanoparticles in solution. The nanoparticle suspension was then freeze dried using a bench top lyophilizer. Dried amorphous nanoparticles were prepared using a rotary evaporator following particle generation using a solvent-shift method. The dried samples were subsequently reconstituted in a physiological pH (6.5) phosphate buffer solution if not specified elsewhere. Re-dispersibility was assessed through particle size measurements using a dynamic light scattering instrument before and after drying. To evaluate the morphology and crystallinity of nanoparticles after drying, transmission electron microscopy (TEM) complemented with energy dispersive X-ray (EDX) was performed. The permeability advantages provided by nanoparticles were examined in vitro using artificial membrane-based diffusion experiments.

Results: Eudragit effectively stabilized clotrimazole amorphous nanoparticles throughout drying as shown in Figure 1. Conversely, for the neutral drug lopinavir, significant particle growth occurred. Clotrimazole was shown to form amorphous salts with Eudragit previously³, and enhanced particle stability are observed to be associated with such ionic interactions. Alternatively, lopinavir amorphous nanoparticles were prepared via freeze drying. Eudragit promoted lopinavir crystallization during lyophilization, and a different polymer stabilizer HPMC was used. Next, we evaluated the permeability advantages provided by these amorphous nanoparticles. The results are summarized in Figure 2. Amorphous nanoparticles are expected to increase the drug’s passive permeation rate beyond its solubility limit by acting as drug reservoirs in the diffusional boundary layer^4-8. Such permeability advantages provided by amorphous nanoparticles were confirmed in the case of particles formed in situ by antisolvent titration, mimicking those generated during ASD dissolution. However, for clotrimazole, as shown in Figure 2A, the dried amorphous particles showed minimum permeability advantages. TEM-based imaging, EDX, and diffraction analyses (Figure 3) confirmed the lack of aggregation or crystallization in the re-dispersed nanoparticles. Elemental analysis also confirmed the drug-rich nature of these nanoparticles, where chlorine was only detected in the discrete particulate features. If we switch the dissolution media from pH 6.5 to pH 9 where the drug becomes completely unionized and the hydrophobic ionic interaction between the drug and the polymer is suppressed, the dried amorphous nanoparticles released the drug quickly and demonstrated permeation rate on par with particles formed in situ. Therefore, such slow drug release was attributed to the strong ionic interaction between Eudragit and clotrimazole, which retarded drug release from amorphous particles. Hydrophobic ion pairing were also shown to result in poor drug release from ASDs, where polymer hydration was inhibited by binding with the hydrophobic drug through ionic interactions, leading to reduced polymer dissolution and poor drug release⁹. On the contrary, as shown in Figure 2B, for lopinavir nanoparticles where the hydrophobic ionic interactions are absent, dried nanoparticles exhibited comparable permeability advantages to amorphous nanoparticles generated in situ. TEM-EDX analyses (Figure 3) also confirmed the amorphous state, drug-rich nature, and lack of aggregation of the re-dispersed particles.

Conclusion: Ionic interactions between Eudragit and clotrimazole was found to provide superior stability for amorphous nanoparticles in solution and throughout drying. However, drug release from dried amorphous nanoparticles were found to be more rapid when ionic interactions are absent. Such retardation effects diminishes if the particles were freshly generated without drying and reconstitution. The formation of a hydrophobic clotrimazole-Eudragit ion pair is thought to reduce the hydration of the polymer, which prevents the polymer and the drug from being released into solution. This study highlights the importance of stabilizer selection in amorphous nanoparticle formulations, where a balance between maintaining particle stability and drug release must be achieved. Results from this study also suggest that comparable permeability advantages may be achieved by producing directly engineered amorphous nanoparticles compared to ASDs, providing an alternative strategy to overcome the drug loading limit of conventional ASDs.

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Figure 1. Particle size of A) clotrimazole and B) lopinavir amorphous nanoparticles before and after drying


Figure 2. Membrane flux of in situ and dried amorphous nanoparticles of A) clotrimazole and B) lopinavir, with dried clotrimazole nanoparticles showing minimal membrane flux at pH 6.5


Figure 3. TEM-based (1) imaging, (2) EDX, and (3) diffraction analyses showing non-aggregated, drug-rich, and amorphous nanoparticles of A) clotrimazole and B) lopinavir. Drug-rich regions are shown in purple by the presence of chlorine for clotrimazole and nitrogen for lopinavir. The web-like structures are lacey carbon films from the TEM grid.