*Review* **Advances of Combinative Nanocrystal Preparation Technology for Improving the Insoluble Drug Solubility and Bioavailability**

**Qiuyan Ran 1,2, Mengwei Wang 1,2, Wenjie Kuang 1,2, Jinbo Ouyang 3, Dandan Han 1,2,\*, Zhenguo Gao 1,2,\* and Junbo Gong 1,2,4**


**Abstract:** The low solubility and bioavailability of aqueous insoluble drugs are critical challenges in the field of pharmaceuticals that need to be overcome. Nanocrystal technology, a novel pharmacological route to address the poor aqueous solubility problem of many poorly soluble drugs, has recently demonstrated great potential for industrial applications and developments. This review focuses on today's preparation technologies, containing top-down, bottom-up, and combinative technology. Among them, the highlighted combinative technology can improve the efficiency of particle size reduction and overcome the shortcomings of a single technology. Then, the characterization methods of nanocrystal production are presented in terms of particle size, morphology, structural state, and surface property. After that, we introduced performance evaluations on the stability, safety, and the *in vitro*/*in vivo* dissolution of drug nanocrystals. Finally, the applications and prospects of nanocrystals in drug development are presented. This review may provide some references for the further development and optimization of poorly soluble drug nanocrystals.

**Keywords:** nanocrystals; combinative technology; aqueous solubility; stability; dissolution rate

### **1. Introduction**

One of the most challenging problems in pharmaceutical science is the bioavailability limitations of drugs with poor solubility. About 40% of the drugs currently on the market are struggling with poor aqueous solubility [1,2] and approximately 90% of drugs in development are classified as poorly soluble drugs [3] based on the definition of the biopharmaceutical classification system (BCS). In particular, BCS II drugs with low solubility and high permeability (Figure 1a) account for approximately 70% [4]. The Developability Classification System (DCS) for oral administered drugs was proposed based on the BCS classification system. According to the DCS, DCS II drugs can be divided into two categories, one is dissolving rate limiting DCS IIa, which is insoluble in water and organic phases, and the other is solubility limiting DCS IIb, which is usually soluble in at least some lipids (Figure 1b) [5,6]. Until now, several techniques have been proposed to solve the problems of drug insolubility, mainly involving two approaches: (i) modification of morphological properties of raw drug particles, i.e., improving the surface area to volume ratio by preparing a fine powder or promoting the porosity; and (ii) modification of some physicochemical and structural properties of insoluble active pharmaceutical ingredients (APIs), such as preparation of polymorphic forms, cocrystal, solid dispersions, etc. [7–9]. However, the second approach usually requires large screening efforts (e.g., the selection of solvents and ligands) when it comes to DCS IIa drugs. Therefore, reducing drug particle size is the best option for DCS IIa drugs [10].

**Citation:** Ran, Q.; Wang, M.; Kuang, W.; Ouyang, J.; Han, D.; Gao, Z.; Gong, J. Advances of Combinative Nanocrystal Preparation Technology for Improving the Insoluble Drug Solubility and Bioavailability. *Crystals* **2022**, *12*, 1200. https:// doi.org/10.3390/cryst12091200

Academic Editor: Waldemar Maniukiewicz

Received: 29 July 2022 Accepted: 22 August 2022 Published: 25 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Nanocrystal technology brings a new dawn for improving the solubility and bioavailability of insoluble drugs [11]. Nanocrystals (usually 1–1000 nm) are pure drug particles stabilized by suitable surfactants/polymers [2,11,12]. Nanocrystals have the following features. (i) The surface area of nanocrystals increase with decreasing particle size. According to the Noyes-Whitney equation [13], the dissolution rate of nanocrystals increase with improving the surface area. (ii) According to the Ostwald-Freundlich equation [14], downsizing the size to the nanometer range (Figure 1c) significantly enhances the solubility of a drug [15]. (iii) A mucus layer with a porous structure is present on the surface of the gastrointestinal tract. The nanograined size is small, which can rapidly permeate into the pore channels of the mucus layer and tightly adhere to them [2,16,17], so it can prolong the effective range and time of drugs in the gastrointestinal tract (Figure 1d). All of these properties contribute to enhancing the absorption and bioavailability of drugs [17]. In fact, nanocrystals were originally invented to improve the oral bioavailability of insoluble drugs [18]. With advanced research in drug nanocrystals, other advantages have also been explored, such as loading high active pharmaceutical ingredients (APIs) [4,16], improving the metabolism behavior of drugs [19], reducing toxic and side effects, promoting patient compliance [20], and so on. Consequently, the superior properties of drug nanocrystals have attracted more and more attention from pharmaceutical enterprises.

**Figure 1.** (**a**) description of BCS, (**b**) description of DCS, (**c**) size boundaries of nanoparticles, (**d**) features of nanocrystals: increased saturation solubility (upper), increased dissolution velocity (middle), and increased adhesiveness of nanomaterial, for surface: calculations were performed as cubes. Figure 1a was reprinted from ref. [4] with permission from Elsevier, Copyright® 2019. Figure 1b was reprinted from ref. [5] with permission from Elsevier, Copyright® 2010. Figure 1d was reprinted from ref. [21] with permission from Elsevier, Copyright® 2011.

In this review, the latest applications of combinative drug nanocrystal preparation technology are highlighted. Firstly, the current technologies for the fabrication of drug nanocrystals are introduced. Subsequently, the characterization and evaluation methods are summarized. Finally, applications and future prospects are briefly mentioned.

#### **2. Preparation Technology**

Nanocrystals are generated by reducing particle size (top-down technology), growing particles in the nanometer size range (bottom-up technology), and combining these two technologies [22].

#### *2.1. Top-Down Technology*

Top-down technology mainly includes wet bead milling and high pressure homogenization [23], which is easily industrialized. Drug particles decrease by mechanically generated shear and collision forces [24], accompanied by the fragmentation of crystalline species and the appearance of secondary nucleation nuclei. The formation rate of top-down technology is independent of supersaturation. Most previous reported anticancer drugs have been prepared by this technology because they do not require organic solvents and are relatively easy to scale up production [25]. In summary, top-down technology can be used for drugs that are insoluble in both the aqueous and organic phases. It processes quickly and is widely used for marketed drug nanocrystals.

#### 2.1.1. Wet Bead Milling

Wet bead milling involves crushing the drug itself into nanoparticles by high intensity mechanical force with stabilizers and water [26]. The particle size of nanocrystals is mainly relevant to the size of the milling beads [27] (usually 0.1–20 nm), the property parameters of the drug, and the setting parameters [24]. Since the temperature can be controlled in the preparation process, wet bead milling is especially suitable for preparing thermally unstable drug nanocrystals [16]. It operates easily to obtain a uniform product. However, stabilizers and wetting agents still need to be added, and several cycles are required to reach the specific particle size range. Meanwhile, the obtained product has disadvantages in the contamination caused by grinding beads [28] and poor physical storage stability due to agglomeration. Funahashi et al. [29] found that ice beads melted after the milling process, which could avoid contamination. Most of the drug nanocrystals that have been successfully translated industrially are prepared using milling, including the earliest marketed pentoxifylline capsule Verelan®PM, a fenofibrate tablet for the treatment of hypercholesterolemia Tricor®, and an anti-inflammatory drug Naprelan® [30].

#### 2.1.2. High Pressure Homogenization

High pressure homogenization (HPH) utilizes violent shearing, collision, and cavitation generated in a high pressure homogenization chamber to break down drug particles. Depending on the instrumentation and solution used, it can be divided into microfluidization, IDD-P™, Dissocubes®, and Nanopure®. Microfluidization has the 'Z' or 'Y' type chamber based on the jet stream principle. IDD-P™ uses a jet homogenizer for the homogenization of suspensions. Dissocubes® uses a piston gap homogenizer for homogenization in aqueous media. Nanopure® is suitable for the production of easily hydrolyzed drugs in reduced/non-aqueous media [31]. In general, the setting parameters of the homogenization process and the hardness of the drug mainly affect the properties of the product [32]. Through these efficient methods, the obtained product has a small particle size with narrow distribution and is not contaminated by the grinding medium. Most importantly, the method can be better combined with other methods to reduce the cycle number of homogenizations and the requirement of homogenization pressure. However, expensive equipment and demanding techniques hinder the transferability to larger scales [33]. In addition, high pressure may unintentionally lead to crystal structure changes, increase the content of amorphous states, and affect the stability of some amorphous nanosuspensions [32]. The currently marketed paliperidone palmitate intramuscular suspension Invega Sustenna, fenofibrate tablets Triglide®, and Luteolin nanocrystals [34] are prepared by the HPH technique [30].

#### 2.1.3. Laser Ablation

Laser ablation is a new technique developed in recent years for nanocrystal preparation. During laser ablation, the solid target is irradiated and the ejected material forms nanoparticles in the surrounding liquid. Then, stirred suspensions of microparticles are broken into nanoparticles by laser-mediated fragmentation [35]. According to the laser processing time, it is divided into nanosecond, picosecond, and femtosecond laser irradiation, among which more nanoscale particles can be produced [36]. The parameters affecting the particle size include the laser intensity, scanning speed, and the properties of the suspension, etc. In this process, no organic solvent is involved, but a small fraction of the drug may undergo oxidative degradation and crystal state changes due to excessive power. This method has been successfully used to prepare paclitaxel, megestrol acetate, and curcumin nanosuspensions [37].

#### 2.1.4. Ultrasound

Ultrasound is an efficient method to break drug particles into smaller particles through the vibration of acoustic waves. Ultrasound has been shown to enhance nucleation by creating acoustic cavitation in solution and rapidly dispersing the drug solution [38]. Because it is easily operated in the laboratory and is highly reproducible, it is also usually combined with other techniques [39]. Ultrasound-assisted precipitation of nanoparticles mainly alters the mixing process, nucleation, growth, and agglomeration [40]. The size of the nanocrystal depends on the intensity of the ultrasound treatment, the horn length, the horn immersion depth, and the cavitation depth [12].
