To Study the Impact of Ultrasound-Assisted Antisolvent Crystallization on Aprepitant Crystal Habits

Abstract

Aprepitant (APT), an antiemetic drug used for chemotherapy-induced nausea and vomiting (CINV), exhibits poor compressibility, solubility, and micromeritic properties. Crystal habit modification was studied using solvent evaporation, conventional antisolvent crystallization (APT_AS), cooling crystallization (APT_CC), and the advanced sonocrystallization technique (APT_SN). Morphological analysis of the sonocrystallized crystals revealed small, platy crystals exhibiting an aspect ratio of 1.35 ± 0.04 and a span value of 1.06. The APT_SN showed improved micromeritics as compared to APT_AS (1.59 ± 0.03) and APT_CC (1.48 ± 0.04) (antisolvent-crystallized APT and cooling crystallized APT, respectively). All modified crystals exhibited a plate-shaped crystal habit with no agglomeration. The angle of repose, Carr’s index, and Hausner’s ratio exhibit that the APT_SN showed improvement in powder properties. Solid-state characterization using differential scanning calorimetry (DSC), Powder X-ray Diffraction Spectroscopy (PXRD), and Thermogravimetric Analysis (TGA) proved no change in polymorph. Contact angle-driven wettability was as follows: APT > APT_AS > APT_SN > APT_CC, and the results were corroborated by X-ray photon spectroscopy (XPS) and intrinsic dissolution profiles. The XPS studies revealed a decrease in the surface polar component of APT_SN, resulting in reduced wettability. APT_SN showed the highest tensile strength at 20 kg/cm² among all other crystals. All the modified crystals exhibited a reduced IDR profile, resulting from a reduction in the polar component at the surface.

1. Introduction

Chemotherapy-induced nausea and vomiting (CINV) is a critical problem after cancer therapy and has remarkable adverse effects on the health of the patient, which hampers the patient’s compliance with chemotherapeutic agents [1,2,3,4]. CINV causes dehydration, malnutrition, and electrolytic imbalance, leading to complications like esophageal tears, declining mental status, and wound impairment, finally leading to reduced willingness [5,6]. Aprepitant (APT) is a Biopharmaceutics Classification System (BCS) Class II, exhibiting low solubility in aqueous environments that leads to inconsistent absorption, poor bioavailability, and reduced therapeutic effectiveness [7]. The flow properties of a drug are a crucial parameter for the processing and formulation of a robust dosage form (i.e., one devoid of content uniformity and weight variation problems). In the case of APT, its crystalline form significantly affects its powder flowability, making it challenging to handle during the manufacturing of solid dosage forms.

APT is available in several pharmaceutical products, including EMEND, which is commonly used in CINV. It blocks the action of substance P, a neuropeptide involved in the vomiting reflex, by targeting and inhibiting the neurokinin 1 (NK1) receptor. The chemical structure of APT is described as 5-[[(2R,3S)-2-[(1R)-1-[3,5-bis(trifluoromethyl) phenyl]ethoxy]-3-(4-fluorophenyl)-4-morpholinyl]methyl]-1,2-dihydro-3H-1,2,4-triazol-3-one. APT is commonly administered as part of a combination therapy with other antiemetic agents, such as corticosteroids and 5-HT3 antagonists, to achieve enhanced anti-nausea effects [1,5,8,9]. Common approaches to improve solubility include the amorphous solid dispersion [10], micronization [11], salt formation [12], cosolvency [13], etc. However, these methods face several challenges, including devitrification [14], the development of static charge [14], uncontrolled particle morphology [15], and phase separation in solid dispersions [14]. Crystal engineering is an industrially feasible approach to improve micromeritic properties and the solubility of particles, thereby producing directly compressible powders.

Sonocrystallization is an emerging crystal engineering approach that utilizes ultrasound to influence nucleation and crystal growth. This technique leverages cavitation-induced phenomena, including the formation and collapse of bubbles that generate localized hotspots with extremely high temperatures, pressures, and cooling rates. These transient microenvironments create zones of elevated supersaturation and reduced crystallization temperature, significantly lowering the energy barrier for primary nucleation, even in nominally particle-free solutions. Consequently, ultrasound shortens induction times, enhances nucleation reproducibility, and promotes uniform distribution of nuclei via intense micromixing and shock waves. Additionally, cavitation-induced microjets and turbulent flows disrupt crystal–crystal contacts, suppress agglomeration, and favor secondary nucleation through crystal fragmentation. Collectively, these effects enable precise control over particle size distribution, crystal habit, and surface properties without necessarily altering polymorphism, allowing for the formation of smaller, purer, and more uniform crystals [15]. The present study aims to apply the sonocrystallization technique to APT for optimization of its crystal properties and improve its solubility and dissolution profile. The experiment was conducted using the antisolvent crystallization technique, considering the factors such as crystallization temperature, antisolvent flow rate, and sonication parameters (i.e., frequency, power rates, etc.) [16,17,18]. In sonocrystallization, induction time refers to the interval between the establishment of a supersaturated state and the first detectable formation of crystals. This period encompasses molecular clustering, stable nucleus formation, and initial crystal growth. Applying ultrasonic irradiation significantly reduces induction time by enhancing micromixing and cavitation effects, thereby promoting faster nucleation and crystal formation. Factors such as solution supersaturation, viscosity, and impurities can influence induction time, making it a critical parameter in optimizing crystallization processes and controlling final crystal properties [19]. The induction time measurements are significantly affected by factors such as impurities, agitation, solution viscosity, cooling rate, and the level of supersaturation. Ultrasonic irradiation is suggested to provide the energy required for primary nucleation [20]. The metastable zone width (MSZW) is defined as the range between the equilibrium saturation curve and the practically determined supersaturation point where spontaneous nucleation and crystallization occur [21,22]. Sonocrystallization significantly reduces the metastable zone width (MSZW) by promoting rapid and uniform nucleation through cavitation effects. This leads to more controlled crystallization, thereby minimizing the risk of uncontrolled nucleation events.

Multiple studies have reported that they investigated the use of sonocrystallization to modify the crystal habit of dapagliflozin [23], rosiglitazone [24], nimesulide [25], and ketoprofen [26] pharmaceutical compounds. This research demonstrated that applying ultrasound during crystallization could alter the crystal morphology, leading to improved flow properties and dissolution rates. Bučar et al. demonstrated that sonocrystallization of paracetamol produces a mixture of nano- and microsized monoclinic crystals, which significantly enhance compaction behavior compared to conventional macrosized crystals. The resulting powders exhibited higher elastic moduli and bulk cohesion, resulting in improved tabletability without the need for excipients, particle coating, or salt/cocrystal formation. Experimental compaction tests and finite element analysis confirmed that the enhanced mechanical properties arise from the controlled crystal size distribution achieved via ultrasound [27]. Studies have also investigated the effects of ultrasound on crystallization beyond pharmaceutical systems. For example, Dewes et al. conducted an experimental and numerical investigation on the growth kinetics of zeolite A, demonstrating that sonication can reduce synthesis time by up to ~40% and narrow the particle size distribution by enhancing nucleation and crystal growth under cavitation conditions [28]. This approach highlights the effectiveness of sonocrystallization in modulating the properties of crystalline materials for specific applications.

In this research, we applied sonication by a probe sonicator during antisolvent crystallization; hence, it is termed the antisolvent sonocrystallization technique. In addition, the study includes a comparison with traditional crystallization approaches, such as solvent evaporation, cooling crystallization, and antisolvent crystallization. SEM was employed to investigate the solid-state characteristics of APT, with emphasis on particle morphology and surface characteristics; PXRD to determine crystallinity and polymorphic structure; differential scanning calorimetry (DSC) to identify thermal effects such as melting points and heat of fusion; TGA to examine the thermal stability and mass loss events; Fourier-transform infrared spectroscopy (FTIR) to identify functional group integrity and detect any intermolecular interactions; XPS to analyze element composition on the surface Brunauer–Emmett–Teller (BET) for surface area analysis; and particle size distribution for the span value and aspect ratio. Micromeritic evaluations were carried out by measuring the angle of repose, Hausner’s ratio, and Carr’s index to evaluate the flow behavior of the powders. Wettability was evaluated by measuring contact angles and determining the surface free energy. Intrinsic dissolution studies were performed to check the effect of the modified crystal habit on the dissolution. The compressibility, tabletability, and compactability studies aim to understand mechanical properties. The crystal size distribution (CSD) profile is crucial in the development of pharmaceutical solids, including conventional tablets and multilayered tablets. Evaluating the CTC profile helps predict how a formulation might behave during tablet manufacturing, particularly in relation to common defects such as capping, lamination, sticking, and picking [29].

2. Materials and Methodology

2.1. Materials

Pure Aprepitant (APT, purity 99.8%; CAS No. 170729-80-3) was obtained as a gift sample from Glenmark Pharmaceuticals, Mumbai, India. Methanol (≥99.9% purity; CAS No. 67-56-1), acetone (≥99.5% purity; CAS No. 67-64-1), cyclohexane (≥99% purity; CAS No. 110-82-7), acetonitrile (≥99.5% purity; CAS No. 75-05-8), and hexane (≥99% purity; CAS No. 110-54-3) were procured from Merck Limited, Mumbai, India. Additionally, ethanol (96% purity; CAS No. 64-17-5) was obtained from Thermo Fisher Scientific, Mumbai, India. Sonocrystallization was carried out using a probe sonicator (Labman Scientific Instruments Pvt. Ltd., Chennai, India).

2.2. Methodology

2.2.1. Solvent Screening and Solubility Studies

The apparent solubility was determined across a series of solvents arranged in order of increasing polarity, namely, hexane, cyclohexane, acetone, acetonitrile, ethanol, and methanol. For each experiment, 1 mL of the solvent was added to a glass vial and equilibrated at 25 °C in a thermostatic water bath. The drug (approximately 2 mg) was added gradually with gentle manual mixing until a visible undissolved solid remained, which was taken as the point of saturation. The vials were then allowed to stand for 1 h to ensure equilibrium. After this time, the saturated solution was visually inspected to confirm the presence of any residual undissolved material. Solubility was therefore determined based on the maximum amount of drug that dissolved before the appearance of a persistent undissolved solid. All experiments were performed in triplicate (n = 3), and the results are reported as the mean ± standard deviation (SD).

2.2.2. Crystallization with Screened Solvents

Depending upon the drug solubility data, recrystallization of APT was first carried out in selected solvents by performing the following:

Solvent evaporation technique: For the solvent evaporation experiments, 50 mg of the drug substance was precisely weighed and transferred into a 10 mL glass vial. Thereafter, 10 mL of the chosen solvent was introduced, and the solution was stirred at 25 °C until the drug was fully dissolved. The resulting clear solution was then subjected to solvent evaporation by placing the vial at room temperature. Following solvent evaporation, the resulting dried material was collected and preserved in a desiccator until subsequent characterization. Each experiment was carried out three times to confirm reproducibility.

Antisolvent crystallization: Antisolvent crystallization was performed using the selected solvent–antisolvent system. Initially, 460 mg of the drug was added to 10 mL of the selected solvent and heated to 60 °C with constant stirring until complete dissolution was achieved. Afterwards, the solution was filtered through a syringe filter. The antisolvent (water) was then added dropwise to the drug solution at controlled rates of 1 mL/min, 2 mL/min, and 4 mL/min using a syringe connected to a burette to ensure uniform mixing and controlled supersaturation at 800 rpm, thereby maintaining homogeneity. The addition of the antisolvent continued until the predetermined solvent–antisolvent ratio (e.g., 1:5) was reached. After the process was complete, the mixture was stirred for 30 min to promote crystal growth and attain equilibrium. The formed crystals were then collected by vacuum filtration, rinsed with a minimal amount of chilled antisolvent, and dried under vacuum at ambient temperature for 24 h prior to subsequent analysis.

Sonocrystallization: Sonocrystallization experiments were conducted using a probe sonicator PRO-650 (Labman Scientific Instruments Pvt. Ltd., Chennai, India) equipped with a 6 mm titanium probe and a maximum ultrasonic output power of 650 W. The sonicator was operated at 10%, 15%, and 20% of its maximum power, corresponding to 65 W, 97.5 W, and 130 W, respectively. Distilled water was used as the antisolvent, with an addition rate of 1 mL/min, a solvent-to-antisolvent ratio of 1:5, and a controlled temperature of 15–17 °C. An on/off sonication cycle of 2s/1s was used throughout the experiments. Prior to sonication, the APT was dissolved in the selected solvent and heated to 60 °C to achieve supersaturation. For each experiment, 460 mg of APT was dissolved in 10 mL of the selected solvent at 60 °C to obtain a saturated solution prior to ultrasound application. The saturated APT solution was placed under the sonicator, and distilled water was introduced as antisolvent at the predetermined rate using a syringe for precise flow control. Ultrasound was applied continuously during the antisolvent addition to promote nucleation and control crystal size. After crystallization, the former crystals were allowed to sediment, the supernatant was decanted, and the crystals were collected. The crystals were then allowed to dry at 60 °C for 24 h.

Cooling crystallization: Cooling crystallization experiments were conducted in a jacketed glass crystallizer equipped with a mechanical stirrer, temperature probe, and circulating chiller. For each experiment, 460 mg of APT was dissolved in 10 mL of the selected solvent at 60 °C under constant stirring until a homogeneous solution was obtained. The solution was then passed through a 0.25 µm syringe filter to eliminate any visible particulates. Crystallization was initiated by cooling the solution from 60 °C to −5 °C at a controlled cooling rate of 1 °C/min while maintaining a stirring speed of 50 rpm. The temperature was decreased linearly to ensure uniform cooling. Once the system reached −5 °C, it was held isothermally for 1 h to allow for complete crystal growth and equilibration. Following the holding step, the crystals were recovered by vacuum filtration, rinsed with cold solvent maintained at −5 °C, and dried in a vacuum oven for 24 h. The crystallization data obtained from antisolvent crystallization, sonocrystallization and cooling crystallization are summarized in Tables S2–S4.

2.2.3. Particle Size Distribution and Shape Analysis

Crystal images were acquired using a digital microscope (DIGI1000, Dewinter Excel, Delhi, India). Samples were gently spread onto a glass slide using a soft brush to minimize particle overlap. The captured images were analyzed using ImageJ software (version is 1.54g) to determine particle size distribution, aspect ratio, and span. Crystal size distribution parameters (D10, D50, and D90) were obtained from image-based particle size analysis, representing the particle sizes below which 10%, 50%, and 90% of the analyzed particles fall, respectively. The aspect ratio was calculated from optical microscopy images as the ratio of the major (long) to the minor (short) axis of individual crystals. Approximately 250 randomly selected particles per sample were analyzed for quantitative evaluation.

$$Span value={\displaystyle \frac{D_{\left(90\right)}-D_{\left(10\right)}}{D_{\left(50\right)}}}$$

(1)

$$Aspect ratios={\displaystyle \frac{Avg. partice lengths}{Avg. particle widths}}$$

(2)

Crystal shape uniformity and CSD were assessed using the aspect ratio and span value, respectively.

2.2.4. Scanning Electron Microscopy

Morphological characterization of APT crystals was performed using scanning electron microscopy (EVO SEM MA15/18, Carl Zeiss Microscopy Ltd., Jena, Germany). SEM was performed at accelerating voltages of 5–30 kV and a beam current up to 2.1 A. Prior to imaging, samples were mounted on aluminum stubs using carbon tape and coated with a ~10 nm conductive layer to minimize charging effects. Micrographs were recorded under identical imaging conditions for all samples, enabling a reliable morphological comparison.

2.2.5. Fourier-Transform Infrared Spectroscopy

FTIR spectra were recorded using a JASCO FT/IR-4X spectrometer (JASCO Corporation, Tokyo, Japan) equipped with a zinc selenide crystal, at a resolution of 4 cm⁻¹ over a wavenumber range of 600–4000 cm⁻¹. Approximately 2 mg of APT crystals were directly placed on the ATR crystal without any additional sample preparation. The background spectrum was collected before each sample measurement. The acquired data were processed with Origin-2024 software and analyzed to identify characteristic functional groups by comparing the absorption bands with known reference spectra.

2.2.6. Thermal Characterization

The TGA (Model: TGA-50; Manufacturer: Shimadzu (Asia Pacific) Pte Ltd., Singapore, Singapore) was employed to investigate the % weight loss of the materials as a function of temperature, providing information on thermal stability, composition, and decomposition characteristics. Measurements were performed under nitrogen purge (100 mL/min) with platinum sample pans. Samples were heated from 20 °C to 400 °C at 20 °C/min. During the process, changes in sample weight were monitored over the temperature range, and the resulting thermograms were analyzed to identify significant thermal events [30]. The thermal behavior of APT was also examined using DSC (Shimadzu DSC-60 Plus Instrument, Singapore, Singapore). Around 5 mg of the sample was hermetically sealed in an aluminum pan, while an empty aluminum pan served as the reference. The samples were then heated from 20 °C to 400 °C at a uniform rate of 10 °C/min. The resulting DSC thermograms were analyzed to evaluate the melting temperature, enthalpy of fusion, and other thermal events, such as crystallization and thermal degradation.

2.2.7. Powder X-Ray Diffraction Spectroscopy

The crystalline structure of APT was examined using a Rigaku Miniflex 600 Desktop X-ray Diffraction System (Rigaku Corporation, Tokyo, Japan), operated at 40 kV and 15 mA, with Cu Kα radiation (λ = 1.5406 Å). The sample was prepared by evenly spreading the APT crystal onto the sample holder. Measurements were performed over a 2θ range of 5° to 40° with a 0.02° step size and a scan rate of 5° per minute. The divergence slit, receiving slit, and detector settings were adjusted to optimize resolution and improve the signal-to-noise ratio.

2.2.8. Micromeritics

Precisely weighed samples were placed in a graduated cylinder to record both the bulk and tapped volumes, which were then used to compute the bulk and tapped densities. Tapped density measurements were carried out using a Digital Bulk Density Apparatus (Ikon Instruments, Delhi, India). Tapping was performed for 10, 500, and 1250 cycles in accordance with USP <616> [31]. Based on these measurements, powder flow properties were evaluated using Hausner’s ratio (Equation (3)).

$$Hausner’s ratio={\rho }_t/{\rho }_b$$

(3)

The packing behavior of APT crystals was evaluated using Carr’s index [32] (Equation (4)),

$$Carr’s Index\left(\%\right)={\displaystyle \frac{{\rho }_t-{\rho }_b}{{\rho }_t}}\times 100$$

(4)

where ${\rho }_t$ and ${\rho }_b$ are the tapped and bulk density in g/mL, respectively.

The flow characteristics of the samples were evaluated using the angle of repose method with a fixed-funnel system. The samples were weighed and transferred into the funnel, which was positioned 20 mm above the bottom of the funnel. The pile height was determined with a digital caliper (Model DF2690, Archeral Pvt. Ltd., Mumbai, India), and the pile diameter was determined using a measuring scale. The angle of repose $\left(\theta \right)$ for each sample was then obtained using the equation below:

$$\theta ={\mathit{tan}}^{-1}\left({\displaystyle \frac{h}{r}}\right)$$

(5)

where h is the height and r is the radius of the pile in mm. All measurements were conducted in triplicate, and the corresponding SDs were calculated.

2.2.9. Contact Angle Measurement and Surface Free Energy Analysis

The wetting behavior and surface free energy (SFE) of APT and habit-modified crystals were evaluated using a contact angle goniometer [33]. Contact angles were measured using a Dura Vision (Dura Vision CAM-02 instrument, Pune, India) via the sessile drop method, with diiodomethane (DM), ethylene glycol (EG), and double-distilled water (DW) as non-polar, semi-polar, and polar probe liquids, respectively [34]. The compact was positioned on the goniometer stage, where droplets of approximately 1 μL (water), 0.6 μL (ethylene glycol), and 0.2 μL (diiodomethane) were dispensed onto their surfaces. After allowing the droplets to stabilize for 35 s, a high-speed camera captured images of the droplet and pellet interfaces. These measurements, performed using probe liquids with different surface tensions, enable the estimation of both polar and dispersive components [23]. The surface free energy (γ^T) was calculated using the Owens–Wendt–Kaelble method [35], which accounts for both the polar and dispersive components of the probe liquids. The surface acid (γ⁺) and base (γ⁻) polar content were assessed using the van Oss–Chaudhury–Good theory [36]. For each probe liquid, the measurements were performed in triplicate to confirm reproducibility and expressed as mean values along with the corresponding SD. Dispersive energy (γ^d) is related to Van der Waals forces, while polar energy (γ^p) represents the polar interactions between molecules. Acidic (γ⁺) and basic (γ⁻) components characterize the Lewis acid and base contributions of the surface, respectively. Polarity is the ratio of polar to total surface energy components. The total surface free energy (γ^T) is obtained by combining the dispersive and polar components.

The van Oss–Chaudhari–Good equation is as follows:

$$\left(1+cos\theta \right){\gamma }_{LV}=2 [\sqrt{\left\{{\gamma }_s^{\left\{LW\right\}}{\gamma }_l^{\left\{LW\right\}}\right\}}+ \sqrt{\left\{{\gamma }_S^{\left\{A\right\} }{\gamma }_S^{\left\{B\right\}}\right\} }+\sqrt{\left\{{\gamma }_L^{\left\{A\right\}}{\gamma }_S^{\left\{B\right\}}\right\}} ]$$

(6)

where ${\gamma }_{LV}$ is the surface tension; ${\gamma }_S^{LW}$ and ${\gamma }_L^{LW}$ stand for the dispersive surface energy of solid and liquid, respectively.

Total surface free energy (${\gamma }_T)$ is obtained as follows:

$${\gamma }_T={\gamma }^d+{\gamma }^p$$

(7)

$\mathrm{w}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{e} \left({\gamma }^p\right)$ and (${\gamma }^{\mathit{d}}$) are the polar component and dispersive component, respectively.

Polarity (p) is obtained as follows:

$$P={\displaystyle \frac{{\gamma }^p}{{\gamma }_T}}$$

(8)

Acidic and basic components are obtained as follows:

$${\gamma }_s^p=2\sqrt{{\gamma }_s^{+}{\gamma }_s^{-}}$$

(9)

2.2.10. X-Ray Photon Spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS; K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA) was employed to analyze the surface chemistry of the pellets, probing depths below 10 nm. Pellets compressed at 2 tons were used for the study.

2.2.11. Compressibility, Tabletability, and Compactability (CTC) Profiling

APT and habit-modified crystals were compressed under progressively increasing pressures, ranging from 10 kg/cm² to 50 kg/cm², at a gap of 10 kg/cm² to determine the CTC profiling. Compressibility describes the ability of a powder to undergo volume reduction when pressure is applied. This is represented by a compressibility plot of the solid fraction (SF) versus compression force, as defined by the following equation:

$$\mathrm{C}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{b}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{y}={\displaystyle \frac{Solid Fraction}{Compression force}}$$

(10)

The solid fraction (SF) is obtained using the following equation:

$$Solid Fraction= {\displaystyle \frac{Tablet density}{True density}}$$

(11)

where the tablet density is determined by

$$Tablet density={\displaystyle \frac{mass\left(\mathrm{m}\mathrm{g}\right)}{\pi \ast r2\ast thickness}}$$

(12)

This equation is applicable for calculating the density of tablets with a flat, round shape.

The compression of powder under applied force produces tablets with specific tensile strength, represented by a tabletability plot. The plot illustrates how the tensile strength of tablets varies with the applied compression force. The tensile strength (σ) is determined using the following equation [32]:

$$Tensile strength (\sigma )= {\displaystyle \frac{2\chi }{\pi dt}}$$

(13)

In this equation, χ denotes the pellet hardness in newtons (N); d and t correspond to the diameter and the thickness of the pellet in millimeters (mm).

Compactability describes a powder’s capacity to be compressed into a tablet with a defined tensile strength. It is assessed by plotting the solid fraction against the pellet’s tensile strength. This analysis helps us understand how densification affects the mechanical strength of the compact [23,37].

$$Compactability={\displaystyle \frac{Solid fraction}{Temsile strength}}$$

(14)

2.2.12. Surface Area Analysis

The surface area of 200 mg samples of APT and APT_SN was determined using the BET method with a BELLSORP-MAX-II and BELCAT-II instrument (Microtrac-BEL Corp, Osaka, Japan) via nitrogen adsorption. The samples were degassed under vacuum (100 Pa) for 4 h at 50 °C. Isotherms were measured at −196 °C using a fully automated BET instrument with nitrogen gas at a pressure of 5 kg/cm².

2.2.13. Intrinsic Dissolution Rate

Intrinsic dissolution rate (IDR) measurements were conducted using an intrinsic dissolution apparatus following the USP ⟨1087⟩ guideline [38]. For this, 125 mg of pellets was prepared by compacting each sample using a hydraulic press (Polyhydron Pvt. Ltd., Belgaum, India) at an applied pressure of 20 kg/cm² for 1 min. The dissolution tests were performed in 900 mL of 2.2% w/v SLS at 37 °C with a stirring speed of 100 rpm [32,39,40]. At regular intervals, 5 mL samples were taken and replaced with fresh medium to ensure sink conditions. The samples were passed through 0.2 µm nylon filters and analyzed using a validated UV spectroscopic method at 210 nm using a Cary 60 spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) [41]. The surface area exposed during dissolution was 0.502 cm².

2.2.14. Statistical Analysis

Data obtained from the habit-modified samples were compared with those of APT. The results are expressed as mean ± SD, and statistical analysis was performed using ANOVA in GraphPad Prism 10.

3. Results and Discussion

3.1. Solubility Analysis

APT exhibited high solubility in acetone, methanol, and ethanol, with values of 60.45 ± 1.73, 55.58 ± 1.93, and 28.43 ± 1.40 mg/mL, respectively. In contrast, the solubilities in hexane (2.66 ± 0.57 mg/mL), cyclohexane (1.16 ± 0.28 mg/mL), and acetonitrile (3.5 ± 0.50 mg/mL) were substantially lower, indicating their limited ability to dissolve APT (

Table 1. Apparent solubility of APT in various solvents, measured in triplicate and expressed as mean ± SD.

Sr. No.	Solvent	Average Solubility (mg/mL)
1	Ethanol	28.43 ± 1.40
2	Methanol	55.58 ± 1.93
3	Acetone	60.45 ±1.73
4	Hexane	2.66 ± 0.57
5	Cyclohexane	1.16 ± 0.28
6	Acetonitrile	3.5 ± 0.50

). Based on these solubility results, acetone, methanol, and ethanol were initially selected for crystallization studies. However, in the case of acetone, a milky suspension consistently formed during the study, indicating poor crystallization behavior and hindering crystal recovery. This phenomenon ultimately resulted in very low yield, and therefore, it was excluded from further experiments. Between methanol and ethanol, methanol produced higher crystallization efficiency and better recovery and was thus chosen as the solvent for subsequent processing. Water, due to its negligible solubility for APT, was selected as the antisolvent.

3.2. Crystallization for Screened Solvents

Samples obtained from solvent evaporation are not used for further characterization because of the difficulty in sample extraction. The cooling crystallization revealed plate-like crystal habits with large particle sizes. In the case of antisolvent crystallization, the impact of increasing the antisolvent addition rate at a constant stirring rate (800 rpm) was observed (Table S2). A sample with an antisolvent addition of 1 mL/min was considered for further evaluation, having a span value of 1.62 and an aspect ratio of 1.59 ± 0.03 for crystals with plate-like and rectangular morphologies. The utilization of ultrasound during antisolvent crystallization changed the morphology of the resulting crystals. Sonocrystallization induced a modest shift toward more uniformly small plate-like crystals, as evidenced by the reduced aspect ratio and corresponding SEM images (Table S3). The melting point of APT is 253 °C; hence, there is no melting or fusion of crystals during sonication. APT_SN stands for solvent methanol, with a saturation of 80% w/v, a water addition rate of 1 mL/min, a sonication power of 15%, a solvent-to-antisolvent ratio of 1:5, a span value of 1.06, and an aspect ratio of 1.35 ± 0.04, which shows improved morphology compared to APT, APT_CC, and APP_AS.

3.3. Crystal Size Distribution Analysis (CSD)

Crystal size and shape analysis were conducted using the Dewinter digital microscope, Figure S1, and surface analysis was supported by SEM images, as shown in Figure 1.

Table 2. Particle size distribution, span values, and aspect ratios of APT and habit-modified crystals. Values are presented as mean ± SD (n = 3).

Sample	D10 (µm)	D50 (µm)	D90 (µm)	Span Value	Aspect Ratio Mean ± SD
APT	0.112	0.155	0.213	0.65	1.41 ± 0.08
APT_CC	0.340	1.143	3.502	2.76	1.48 ± 0.04
APT_AS	0.258	0.737	1.456	1.62	1.59 ± 0.03
APT_SN	0.162	0.264	0.441	1.06	1.35 ± 0.04

represents morphological data for APT, APT_CC, APT_AS, and APT_SN, each performed in triplicate. APT_CC shows large, elongated plate crystals with a span value of 2.76 and a mean aspect ratio of 1.48 ± 0.04, indicating poor distribution characteristics [42]. However, the span value of APT_AS (1.62) suggests better CSD than APT_CC. In contrast, the application of ultrasound has caused the fragmentation of primary nuclei, leading to an improved CSD and aspect ratio for APT_SN. Breaking of primary nuclei during the antisolvent sonocrystallization crystallization yields a better impact on the span value of 1.06 and the aspect ratio of 1.35 ± 0.04. The D90 values of APT_CC and APT_AS are 3.502 µm and 1.456 µm, respectively, while the application of ultrasound produced smaller crystals compared to APT_CC and APT_AS. Fragmentation of primary nuclei leads to an increased number of secondary nuclei and smaller crystals. Ultrasound improves the micromixing and power dissipation throughout the solution [43,44,45]. The particle size distribution of different batches of APT, APT_CC, APT_AS, and APT_SN is shown in Figure S2 and Tables S1–S4.

3.4. Scanning Electron Microscopy

SEM images (Figure 1a–d) clearly illustrate the evolution of crystal habit across the different crystallization methods. APT crystals (Figure 1a) display an irregular morphology, which was successfully transformed into flat, platy forms in APT_CC (1b), APT_AS (1c), and APT_SN (1d) with no agglomeration. Each crystal remained distinct and well-resolved. Notably, APT_SN yielded the smallest particles, surpassing both APT_AS and APT_CC, and achieved a markedly improved crystal size distribution and aspect ratio of 1.35 ± 0.046 (n = 3). This refinement underscores the ability of sonocrystallization to finely tune crystal habit and size via enhanced nucleation and microjet-driven fragmentation. In essence, the ultrasonic process not only optimized particle dimensions and uniformity but also rendered a more processable crystal form.

3.5. Solid-State Characterization

The solid-state properties of APT and its sonocrystallized and conventionally crystallized variants were investigated to assess molecular integrity and stability under ultrasound exposure. FTIR was utilized to confirm preservation of functional groups and identify any shifts indicative of ultrasound-induced molecular interactions. DSC analyzed thermal transitions such as melting point and heat of fusion to detect any polymorphic changes. The TGA provided insights into thermal stability and weight loss events. Finally, PXRD was used to compare the crystalline patterns obtained after sonocrystallization.

FTIR revealed the characteristic peak for APT (Figure 2a), such as the peak at 1703 cm⁻¹ for C=O, 3168 cm⁻¹ for N-H, 1127 cm⁻¹ for C-O stretching alcohol, 1029 cm⁻¹ for C-O stretching for ether, 831 cm⁻¹ for C=C bending, 706 cm⁻¹ for C=C bending, and 1279 cm⁻¹ for C-O bending alkyl aryl ether. An FTIR peak of APT matches with the APT_CC, APT_AS, and APT_SN peaks. These identical spectra suggest that no molecular-level alterations occurred in APT following ultrasound treatment.

The thermal properties of the samples were evaluated using DSC and TGA. In TGA, APT exhibited a single weight loss step of approximately 75. 25%, beginning at ~250 °C, which indicates thermal decomposition of APT. The TGA profile of habit-modified crystals (Figure 2b) exhibited a weight loss pattern similar to that of APT, indicating a comparable thermal decomposition pattern. The DSC results for APT, APT_CC, APT_AS, and APT_SN (Figure 2c) demonstrated no significant changes in their thermal properties. The melting points were recorded as 255.59 °C for APT, 254.82 °C for APT_CC, 254.60 °C for APT_AS, and 254.80 °C for APT_SN. The presence of sharp melting peaks confirms the crystalline nature of APT and the modified crystals, which is favorable for physical stability. Overall, the DSC data indicate that the melting points of the habit-modified crystals remain unchanged.

PXRD patterns are used to study the variations in polymorphism and solvation state of the molecule. The identical PXRD patterns of the samples indicate that they share the same internal structure, confirming they are the same polymorph (Figure 2d). The PXRD analysis of APT yields characteristic peaks at 12.1°, 15.3°, 16.6°, 17°, 17.6°, 20.6°, 21.95°, 23.6°, and 24.75 °. All peaks are sharp, indicating a complete crystalline state. The PXRD analysis of APT_CC, APT_AS, and APT_SN yielded the same diffraction patterns as APT, giving an idea of no polymorphic change [23,32].

3.6. Micromeritic Properties

Micromeritics studies are performed to analyze the powder’s flow characteristics, including Carr’s compressibility index and Hausner’s ratio. A change in the crystal habit led to a variation in the angle of repose, i.e., flowability. Powders with high flowability are most desired for pharmaceutical manufacturing. Irregular crystals of APT are a major concern for its flowability. The compressibility of powder depends on crystal facets. As shown in Figure 3, APT displayed a Carr’s index of 37.41 ± 2.14, a Hausner’s ratio of 1.6 ± 0.05, and an angle of repose of 47.1 ± 1.85, reflecting poor flowability and compressibility. Micromeritic properties of APT and all modified formulations are given in

Table 3. Micromeritic properties of APT, APT_CC, APT_AS, and APT_SN. All measurements were carried out in triplicate, and the results are expressed as mean ± SD.

Sample	Bulk Density (g/mL)	Tapped Density (g/mL)	Carr’s Index (%)	Housner’s Ratio	Angle of Repose (°) (g/mL)
APT	0.273 ± 0.020	0.436 ± 0.020	37.41 ± 2.14	1.60 ± 0.05	47.10 ± 1.85
APT_CC	0.172 ± 0.011	0.379 ± 0.008	54.65 ± 3.03	2.21 ± 0.14	51.92 ± 1.12
APT_AS	0.142 ± 0.002	0.300 ± 0.008	51.32 ± 1.07	2.06 ± 0.04	50.43 ± 2.12
APT_SN	0.260 ± 0.010	0.361 ± 0.001	27.40 ± 1.42	1.37 ± 0.03	35.65 ± 0.30

.

The APT_CC and APT_AS have poor micromeritics properties, as indicated by Carr’s index (54.65 ± 3.03 and 51.32 ± 1.07), Hausner’s ratio (2.21 ± 0.147 and 2.06 ± 0.04), and angle of repose (51.92 ± 1.12 and 50.43 ± 2.12), respectively. The APT_CC exhibited the highest compressibility, while APT_AS showed poor compressibility profiles, as confirmed by further CTC profiling studies. Application of ultrasound during crystallization enhanced the flow and compressibility properties, as evidenced by a lower Carr’s index, Hausner’s ratio, and angle of repose relative to APT. The APT_SN, with a high bulk density of 0.260 ± 0.010, exhibits better flow properties compared to APT_CC (0.172 ± 0.011) and APT_AS (0.142 ± 0.002), as supported by its lower aspect ratio. The powder bed attained a stable volume after 1250 tappings, which was taken as the tapped volume, indicating that the packing had reached equilibrium. The difference between the tapped density and bulk density plays a crucial role in the flow properties, viz., the smaller the difference, the better the flow properties and a higher difference gives poor flow characteristics of the powder, as expressed by Hausner’s ratio for APT, which is significantly higher than APT_SN. This represents that APT_SN has better flow characteristics, supported by the angle of repose. However, for APT_CC crystals and APT_AS crystals, the angle of repose, Carr’s index, and Hausner’s ratio replicated the negative value, giving the importance of sonication power in antisolvent crystallization for APT. Application of sonication power led to reduced particle size, increased number of crystals, and increased bulk density for APT_SN (0.260 ± 0.010) crystals as compared to APT_CC (0.172 ± 0.011) and APT_AS (0.142 ± 0.002), suggesting its dense nature, as APT_CC and APT_AS show a fluffy nature, which hinders their flow properties.

3.7. Contact Angle Measurement and Surface Free Energy Analysis

After the crystal habit modification, the functional group on the facets of the crystal tends to change their orientation, which alters its wetting properties. The contact angle value of 0° with water means perfect wetting (hydrophilic), and 180° means absence of wetting (hydrophobic). The wettability studies of APT and modified crystals were evaluated using the contact angle method with DW, EG, and DM (Figure S3).

The contact angle values for DW (polar solvent) were 57.97° ± 2.26, 58.15° ± 2.31, 67.11° ± 2.00, and 63.28° ± 2.37 for APT, APT_CC, APT_AS, and APT_SN, respectively (Table S5). Two-way ANOVA with multiple comparisons indicated no significant alteration in the surface characteristics of APT_CC in water, as shown in Figure 4. In contrast, APT_AS and APT_SN showed a statistically significant increase in contact angle values (p < 0.0001), reflecting decreased wettability. Similarly, contact angle values for EG (a semi-polar medium) are significantly increased (p < 0.0001), indicating that APT has a better wetting tendency in both DW and EG media than all modified crystals. The contact angle values for diiodomethane (nonpolar and dispersive medium) showed a relative decrease in contact angle values for APT_CC (ns), APT_AS (p < 0.0001), and APT_SN (p < 0.01). DW consistently shows higher contact angles compared to EG and DM, especially for modified samples, indicating greater hydrophobicity. Surface free energies were calculated using the van Oss–Chaudhari method for APT and modified crystals to assess the change in surface energy after crystal habit modification. As correlated with contact angle values, the dispersive energy of modified crystals has increased, while the polar energy has decreased, indicating that modified crystals have reduced wettability (Table S6). The total surface energy is highest for APT_AS (38.18 ± 2.33 mJ/m²), while the application of sonication power has reduced the total surface free energy. Polar energy is correlated with the solubility of the crystals, as APT_SN has poor polar energy; its dissolution profiles might be reduced. The acidic component provides insight into the electron-accepting tendency of the molecule, which is used to assess the wettability and adhesion of the surface. The acidic energy of sonocrystals is increased as compared to APT_AS but decreased as compared to APT. APT_CC, being the highest acidic support, has a high tendency to bind with the basic surface (Figure 5).

3.8. X-Ray Photoelectron Spectroscopy Analysis

XPS is a highly surface-sensitive analytical method that analyzes just the top 1–10 nm of the sample to determine which elements are present and their precise chemical states, based on binding-energy shifts. It is extensively used to examine surface elemental composition, distinguish between API and excipient presence, and detect chemical modifications (e.g., oxidation or coating) that can affect critical properties such as wettability, dissolution, adhesion, and stability. XPS was employed to study the surface elements for APT and APT_SN (Figure 6). The elemental compositions of carbon (C), oxygen (O), nitrogen (N), and fluorine (F) were determined. Carbon and fluorine are the elements with hydrophobic characteristics, while oxygen is hydrophilic in nature. The atomic concentration (%) of all the surface elements for APT and APT_SN is given in

Table 4. Surface elemental composition (%) of APT and APT_SN.

Tested Pellets	Atomic Concentrations (%)
	C1s	O1s	N1s	F1s	(O/C + F)
APT	63.31	9.91	9.33	17.44	12.27
APT_SN	62.03	9.67	9.57	18.73	11.97

. The oxygen concentration decreased from 9.91% to 9.67%, corresponding to an approximate 0.24% reduction after sonocrystallization, indicating a decrease in polar surface components and a relative increase in the non-polar fluorine content. The polarity of the surface was determined using the formula (O/(C + F)), which utilizes the ratio of polar components to total non-polar components, resulting in a reduced surface polarity to 11.97%, i.e., approximately a 0.30% reduction in surface polarity in APT_SN compared to APT. The XPS elemental results were well aligned with the surface free energy results derived from contact angle studies, where APT_SN showed a high total surface free energy and a low polar component. The reductions in carbon and oxygen indicate a loss or modification of surface-bound carbonyl or hydroxyl groups, which typically contribute to oxygen peaks from C–O, C=O, OH, or H₂O species. The depletion of these polar oxygen-containing groups lowers the polar component of surface energy, consistent with the observed decrease in polar surface energy. This reduction in polarity decreases wettability, increasing the water contact angle. The resulting surface is enriched in nitrogen and fluorine-containing facets and depleted in polar oxygen-bearing groups, becoming more hydrophobic, which leads to reduced wettability and slower dissolution rates, in agreement with the observed dissolution behavior. The surface elemental composition of APT and APT_SN is shown in the following table.

3.9. Compressibility, Tabletability, and Compactability Profiling

CTC profiling plays a vital role in the development of solid oral dosage forms, including conventional and multilayer tablets. Compressibility refers to the capacity of a powder to undergo volume reduction when subjected to applied pressure. APT and habit-modified crystals were compressed under progressively increasing pressures, ranging from 10 kg/cm² to 50 kg/cm², at a gap of 10 kg/cm², and changes in tensile strength (MPa) with a solid fraction (%) were noted. The graph (Figure 7a) shows that APT_SN had the highest compressibility at low compression forces of 10 and 20 kg/cm². This maximum compressibility can be attributed to the uniformity of the particles in APT_SN. The solid fraction values are ideal between 90% and 95%, which accounts for better tablets. Lower solid fraction values yield weaker tablets, while higher values give rise to defects such as capping and lamination [46]. Compressibility for all samples increased as the compression force increased, which is attributed to the physical deformation of crystals at higher pressure. The compressibility for APT_CC and APT_AS was reduced compared to APT, while the application of sonication power during antisolvent crystallization improved the compressibility of APT_SN. The APT_CC and APT_AS were found to have a decreased solid fraction, resulting in weaker tablets. Crystals with slip plane or plate habits, as depicted in SEM images, deform plastically more easily and yield stronger tablets [47,48]. The compactability plot (Figure 7c), which shows the relationship between tensile strength (MPa) and solid fraction (%), for APT and modified crystals, provides an indication of the tablet’s strength. Overall, APT_CC had the lowest compactability, while the trend increased as APT_AS and then APT_SN were the highest. Compactability defines the higher bonding strength for powder. In some cases, the plate crystal habit exhibited a good tabletability profile [49]. The improvement in the compactability profile for APT_SN is attributed to a reduction in crystal size, an increase in contact degree, and CSD. APT_SN did not exhibit capping or lamination even at compression pressures up to 50 kg/cm². The tabletability profile (Figure 7b) for APT and modified crystals is the plot of tensile strength (MPa) versus compression force (kg/cm²), showing that the tensile strength for APT is up to 20 kg/cm² and for APT_SN is up to 30 kg/cm², while it is discontinuous for APT_CC and APT_AS. The APT_SN showed improved tensile strength up to 30 kg/cm² and further increases in compression pressure resulted in tablets with weak tensile strength. APT_SN has the highest tensile strength (MPa) compared to the others due to its smaller crystal size and improved CSD [23]. Figure 7d presents a comparison of tensile strength between APT and all modified crystals, with APT_SN exhibiting a significantly higher tensile strength (**** p < 0.0001) than APT.

3.10. Surface Area Analysis

The BET analysis revealed a change in surface characteristics upon sonocrystallization. The parent APT crystals exhibited a surface area of 6.22 m²/g, whereas APT_SN exhibited a reduced surface area of 5.19 m²/g, consistent with morphological observations observed by SEM. These results correlate with the contact angle measurements, indicating altered surface wettability, and are further supported by the IDR, where APT_SN exhibited a lower dissolution rate compared to APT.

3.11. Intrinsic Dissolution Rate

Modification of crystal habit can influence the dissolution behavior of an API in an aqueous environment. The intrinsic dissolution rate studies were carried out under controlled conditions [38]. The intrinsic dissolution profile of APT and the modified crystal is shown in Figure 8a. The IDR results align with previously described contact angle values, surface free energy, polar energy, and XPS studies. As expected, APT_CC, with the lowest polar energy (0.7 mJ/m²), exhibited the lowest dissolution rate (0.05063 ± 0.0024 mg/cm²/min). In contrast, APT exhibited the highest dissolution rate (0.0757 ± 0.0043 mg/cm²/min), which is consistent with its highest polar energy (1.87 mJ/m²) and polar component (9.91% oxygen) at the surface. The IDR profile of APT_SN decreased to 0.068 ± 0.004 mg/cm²/min, correlating with wettability studies (polar energy: 0.77 mJ/m²) and XPS analysis (polar component: 9.67% oxygen).

4. Conclusions

We have successfully studied the impact of sonication during antisolvent crystallization on the crystal morphology and particle size distribution of APT. SEM analysis demonstrated that the APT crystal habit was modified from irregular crystals to plate-shaped crystals without any change in polymorphic form. APT_SN exhibited a narrow crystal size distribution, as reflected by its low span value. APT_SN also showed improved flow properties, as indicated by a lower angle of repose, Carr’s index, and Hausner’s ratio. Furthermore, APT_SN exhibited the highest compressibility, compactability, and tabletability compared to APT, together with excellent tensile strength and solid fraction values. Sonication reduced the wettability of APT_SN, as evidenced by increased contact angle values for distilled water, suggesting a decrease in the polar component of surface oxygen. This conclusion is supported by X-ray photoelectron spectroscopy and is consistent with the observed reduction in intrinsic dissolution. Overall, this work demonstrates that sonication-assisted habit modification improves micromeritic and mechanical properties, although the wettability of the modified crystals is affected.