Drug Nanocrystals: From Formulation Design to Clinical Translation

INTRODUCTION

Poor aqueous solubility remains one of the most significant challenges in contemporary pharmaceutical development and is a major contributor to formulation failure and inconsistent therapeutic outcomes. It is estimated that nearly 70-90% of Newly Discovered Chemical Entities (NCEs) fall under Biopharmaceutics Classification System (BCS) class II or IV, where drug absorption is primarily limited by dissolution rather than permeability (Rathod et al., 2025; Reza Pahlevi et al., 2023). As a result, many promising drug candidates exhibit low and variable oral bioavailability, ultimately affecting clinical efficacy and increasing the risk of attrition during development (Deshmukh et al., 2017).

To address solubility-limited absorption, a variety of formulation strategies have been investigated, including salt formation, pH modification, co-solvent systems, cyclodextrin complexation, amorphous solid dispersions, and lipid-based delivery systems. While these approaches have proven effective in specific circumstances, their broader application is often restricted by intrinsic limitations. For example, salt formation and prodrug strategies are highly dependent on molecular structure and may introduce chemical instability or regulatory complexity (Noreen et al., 2016; Varma et al., 2026). Amorphous solid dispersions, while capable of improving dissolution, are thermodynamically unstable and prone to recrystallization during storage or after administration, which can compromise long-term performance. Similarly, lipid-based and polymeric nanocarrier systems often require high excipient concentrations, leading to reduced drug loading, increased formulation complexity, and variability in vivo performance. These challenges highlight the need for formulation approaches that can improve bioavailability while maintaining stability, scalability, and regulatory feasibility (B and Ghosh, 2025).

In this context, drug nanocrystals have emerged as a potential and clinically confirmed approach for increasing the bioavailability of poorly soluble drugs while preserving their chemical structure. Nanocrystals are submicron-sized crystalline particles composed predominantly of the drug, typically stabilized by a small amount of surfactant or polymer to prevent aggregation (Eckert et al., 2020; Gigliobianco et al., 2018). This carrier-free nature distinguishes nanocrystals from conventional nanosystems and enables high drug loading, reduced excipient burden, and simplified formulation. Importantly, several nanocrystal-based products have been successfully translated into clinical use, showing improved pharmacokinetic profiles and enhanced therapeutic consistency across different routes of administration (Zhou et al., 2016).

The improved performance of nanocrystals (Figure 1) is mostly due to particle size reduction to the nanometer range, which alters drug behavior using well-established physicochemical principles. Nanosizing increases surface area and apparent saturation solubility, thereby accelerating dissolution and generating higher concentration gradients at absorption sites (Pinarbasli and Doganay, 2021). In addition, nanocrystals exhibit improved adhesion to biological membranes and prolonged residence time at absorption interfaces, which can further enhance drug uptake. Unlike amorphous systems, nanocrystals retain their crystalline structure, offering improved physical stability and lowering the possibility of performance loss due to recrystallization (Sun et al., 2018) .

Click to enlarge

From a translational perspective, nanocrystals represent a practical balance between conventional formulations and complex nanocarrier systems. They can be produced using scalable and well-established techniques such as wet media milling and high-pressure homogenization, which are compatible with existing pharmaceutical manufacturing infrastructure (Srivalli and Mishra, 2015). Furthermore, regulatory evaluation of nanocrystal-based products is generally based on established quality attributes, including particle size distribution, crystallinity, and dissolution performance, facilitating their clinical translation (Dhaval et al., 2020).

Despite these advantages, the successful development of nanocrystal formulations requires a comprehensive understanding of multiple interrelated factors, including fabrication technologies, stabilizer selection, surface characteristics, biological interactions, and safety considerations. These aspects collectively determine the in vivo fate and therapeutic performance of nanocrystal systems (Dhibar et al., 2023).

In this context, the present review provides a systematic and critical overview of drug nanocrystals as a strategy for enhancing bioavailability. Emphasis is placed on fabrication technologies, mechanistic basis of dissolution and absorption enhancement, biological fate, regulatory considerations, and clinical translation. By integrating fundamental principles with practical and translational insights, this review provides a comprehensive framework for the rational design and development of nanocrystal-based drug delivery systems.

Search Strategy

A systematic search was conducted across primary electronic databases, including PubMed, Scopus, and Web of Science, to identify relevant literature published up to 2025. Search terms included "drug nanocrystals," "nanosuspensions," "bioavailability enhancement," and "clinical translation." Articles were selected based on their relevance to fabrication technologies, stability mechanisms, and route-specific applications. Preference was given to peer-reviewed research papers and recent systematic reviews that provided quantitative outcomes and translational insights.

Limitations of Conventional Bioavailability Enhancement Techniques

In pharmaceutical development, increasing the bioavailability of weakly water-soluble drugs has long been a problem, especially for substances with dissolution-limited absorption. Several formulation approaches have been studied, including molecular modification, solid-state manipulation, and colloidal delivery systems. Despite the fact that these methods have proven effective in certain situations, their wider applicability is still limited by intrinsic physicochemical and practical constraints (Verma, 2022).

Molecular-level methods such as salt creation, prodrug design, and cyclodextrin complexation try to improve apparent solubility by altering drug-solvent interactions (Wankhade and Dudhe, 2024). However, their efficacy is dependent on drug chemistry. For example, salt formation is restricted to ionizable drugs, but co-solvent systems may precipitate upon dilution, resulting in variable in vivo efficacy and potential toxicity. Prodrug methods, while effective, frequently increase regulatory complication (Gamsiz et al., 2010; Jornada et al., 2015).

Solid-state approaches, such as amorphous solid dispersions and cocrystals, improve solubility by enhancing the drug's energy state. Despite their advantages, these complexes are thermodynamically unstable and susceptible to recrystallization, which could risk long-term stability and bioavailability. This requires tight control over formulation and storage conditions, which limits their industrial robustness (Bookwala and Wildfong, 2023).

Colloidal and lipid-based systems, such as emulsions and polymeric nanoparticles, improve solubilization through carrier-mediated delivery. However, these systems typically require high excipient concentrations, resulting in low drug loading and increased formulation complexity. Additionally, variability in in vivo results and challenges related to scale-up and stability further restrict their widespread application (Losada-Barreiro et al., 2024).

Overall, the limitations of conventional approaches highlight (Table 1) the need for alternative strategies that offer improved stability, scalability, and drug loading. Drug nanocrystals solve many of these issues by enhancing dissolution and absorption through particle size reduction without altering the drug’s chemical structure.

Fundamentals of Drug Nanocrystals and Mechanistic Basis of Bioavailability Enhancement

Poor aqueous solubility remains a critical limitation in drug development, particularly for Biopharmaceutics Classification System (BCS) class II and IV compounds, where dissolution governs absorption. Drug nanocrystals address this limitation by reducing particle size to the nanometer scale while preserving the crystalline structure of the active pharmaceutical ingredient. Unlike carrier-based nanosystems, nanocrystals consist predominantly of the drug itself, enabling high drug loading with minimal excipient requirement (Deljavan Ghodrati and Turk, 2024).

Nanocrystals are generally defined as particles in the size range of 50-500 nm. Reduction to this scale alters drug behavior through both thermodynamic and kinetic mechanisms. According to the Ostwald-Freundlich equation, decreasing particle size increases surface tension and surface free energy, resulting in improving apparent saturation solubility, especially below ~200 nm. In parallel, the Noyes-Whitney equation explains the increase in dissolution velocity due to more surface area and reduced diffusion layer thickness. Together, these effects provide high concentration gradients at the absorption site, facilitating drug diffusion across biological membranes (Sivanathan et al., 2024).

Beyond dissolution, nanosizing influences cellular uptake and biodistribution. Wang et al. (2023) demonstrated a clear size-dependent uptake, where ~70 nm nanocrystals showed significantly higher internalization compared to ~200 nm particles, with rapid cellular entry observed within minutes in both in vitro and zebrafish models (Wang et al., 2023). Similarly, Hollis et al. reported that paclitaxel nanocrystals (~200 nm) accumulated in tumor tissue within 1-6 hr, although substantial distribution in liver and kidneys highlighted limitations of passive targeting mechanisms such as the EPR effect (Hollis et al., 2013).

Formulation-based studies further support the performance advantages of nanocrystals. Mauludin et al. showed improved dissolution of Rutin nanocrystal tablets compared to conventional formulations (Mauludin et al., 2008). Dolenc et al. reported enhanced dissolution of celecoxib nanocrystals stabilized with surfactants (Dolenc et al., 2009), while Kocbek et al. reported a marked increase in drug release (>65% within 10 min) compared to micronized drug (<15%), confirming the impact of nanosizing on dissolution kinetics (Kocbek et al., 2006). In addition, long et al. developed functionalized cellulose nanocrystals for doxorubicin delivery, achieving elevated cellular uptake and improved anticancer activity (Long et al., 2021).

Collectively, these findings indicate that nanocrystals improve drug performance through integrated mechanisms involving enhanced solubility, faster dissolution, and improved cellular uptake. Representative quantitative outcomes are summarized in Table 2.

Fabrication Technologies for Drug Nanocrystals

Nanocrystal fabrication involves a variety of approaches to modifying particle size, morphology, and surface properties, all of which have a direct impact on drug solubility, dissolution rate, and bioavailability. These approaches have changed dramatically in response to advances in pharmaceutical engineering and nanotechnology, allowing for the creation of robust and scalable drug delivery systems. Nanocrystal fabrication approaches are broadly classified into top-down, bottom-up, and hybrid techniques, each offering distinct advantages and limitations depending on drug characteristics and process requirements (Bhaskar et al., 2025).

Top-down approaches include mechanical size reduction of bulk drug particles, whereas bottom-up techniques rely on controlled nucleation and crystal growth from supersaturated solutions. More recently, emerging and hybrid technologies have been developed to combine the benefits of both approaches while resolving their individual limitations (Chen et al., 2025). A comparative overview of major nanocrystal preparation methods is summarized in Table 3 and Figure 2.

Click to enlarge

Microfluidization

Microfluidization is a top-down method that uses high-pressure fluid dynamics to reduce particle size via shear, cavitation, and collision in microchannels. This approach produces nanocrystals with a narrow size distribution and good repeatability; however, its performance is highly dependent on process parameters like as pressure and cycle count. Liu et al. (2021) reported the production of siRNA-loaded lipid nanocarriers with an average size of ~164.2 nm and PDI of 0.23, demonstrating uniformity and stability for up to one month (Liu et al., 2021). Similarly, Chen et al. achieved curcumin nanoliposomes of 68.1 nm with a PDI of 0.246, exhibiting enhanced stability under acidic conditions (Chen et al., 2015; Khadke et al., 2020). Further showed that increasing pressure from 15,000 to 20,000 psi and multiple processing cycles significantly reduced particle size and PDI, highlighting the sensitivity of this method to operational conditions. While microfluidization is scalable and suitable for lipid-based systems, its dependence on precise process optimization remains a limitation (Khadke et al., 2020).

Milling Methods (Wet Media Milling)

Wet Media Milling (WBM), widely known as NanoCrystal technology, is one of the most established top-down techniques for nanocrystal production. It relies on mechanical attrition in the presence of stabilizers and is applicable to a broad range of poorly soluble drugs.

Mura et al. demonstrated that wet milling produced glyburide nanocrystals (~180 nm) with superior dissolution compared to dry milling (~120 nm), achieving 28% drug release within 10 min versus 14% for dry milling (Mura et al., 2023). This indicates that wettability and stabilizer interaction can outweigh particle size alone in determining performance. Similarly, Toziopoulou et al. (2017) reported that stabilizer such as HPMC and HPC significantly improved size reduction compared to PVP K30 (Toziopoulou et al., 2017), underscoring the importance of formulation components. Despite its industrial stability and scalability, milling is energy-intensive process that can introduce contamination from milling media.

Hot-Melt Extrusion (HME)

Hot-Melt Extrusion (HME) is a hybrid technique that combines thermal and mechanical processing to create nanocrystal-based solid dispersions. Its solvent-free and continuous nature makes it attractive for industrial applications.

Baumgartner et al. reported that phenytoin nanocrystals remained uniformly dispersed within polymer matrices without loss of crystallinity, leading to improved dissolution. Patil et al. further reported 92% drug release within 5 hr for fenofibrate-loaded systems, compared to 62% from marketed formulations and 41% from pure drug (Baumgartner et al., 2014), showing the efficiency of extrusion-based approaches. However (Ye et al., 2015), showed that process parameters such as feed rate significantly influence energy input and product quality (Ye et al., 2015). However, these advantages, thermal degradation and polymorphic transitions remain major limitations, particularly for heat-sensitive drugs.

Nanoprecipitation

Nanoprecipitation is a bottom-up technique based on rapid supersaturation and nucleation following solvent-antisolvent mixing. It offers precise control over particle size but requires strict control of process conditions.

Kumar et al. (2022) developed niclosamide nanocrystals using nanoprecipitation, stabilized with hyaluronic acid and PVP/PVA. The optimized formulation (173.3±10.2 nm; PDI 0.221±0.09) was incorporated into Carbopol 940 hydrogels for topical delivery. Using a quality-by-design approach, three formulations were optimized, exhibiting shear-thinning behavior and Higuchi-model drug release (≈71-86% over 12 hr). Nanocrystal conversion significantly improved efficacy, reducing the IC₅₀ from 1.25 μM to ~0.017-0.023 μM in HaCaT cells, indicating improved bioavailability and anticancer potential (Kumar et al., 2022). Similarly, Panwar et al. (2023) developed Hyaluronic Acid (HA)-modified Venetoclax Nanocrystals (HVNCs) using the nanoprecipitation method, enabling CD44-targeted delivery in breast cancer. HA coating increased particle size (~132 nm to ~173 nm) and improved targeting. The formulation showed Higuchi-type drug release and good hemocompatibility. HVNCs significantly enhanced anticancer activity, reducing IC₅₀ ~9.7-fold compared to free drug and promoting higher apoptosis and cell cycle arrest (Panwar et al., 2023). While nanoprecipitation is versatile and efficient, issues such as aggregation, solvent removal, and scale-up remain major challenges.

Spray Drying

Spray drying is a scalable bottom-up technique that converts liquid dispersions into dry powders via rapid solvent evaporation. It is particularly suitable for heat-sensitive compounds due to short exposure times. Shete et al. (2015) developed Hesperetin nanocrystalline solid dispersions using a bottom-up spray drying method with mannitol. The formulation significantly increased dissolution and oral bioavailability (≈1.8-fold C_max, 2.25-fold AUC). Improved intestinal uptake and cellular permeability were observed, along with elevated antioxidant and anti-inflammatory effects, highlighting its therapeutic potential (Shete et al., 2015). This method also decreases the need for stabilizers, which improves formulation simplicity. However, particle aggregation and process control remain critical concerns.

Supercritical Fluid Technology (SCF)

Supercritical Fluid (SCF) technology is a sophisticated and ecologically friendly technique to nanocrystal production. Supercritical CO₂ is commonly employed for its non-toxic properties and moderate critical conditions. Lu et al. created paclitaxel-loaded chitosan and HTCC nanoparticles using SCF-assisted processing and an O/W/O double emulsion with temperature-induced solidification. HTCC-NPs surpassed other nanoparticles (~130 nm, PDI<0.1) in terms of permeability, cell uptake, and tumor accumulation. Bioavailability increased by up to ~9.6%, with reduced hypersensitivity and higher cytotoxicity, suggesting promise for targeted cancer delivery (Lv et al., 2011). Despite advantages such as solvent-free processing and high purity, limitations related to high equipment cost and limited drug solubility restrict widespread industrial application.

Drug Nanocrystal Stability

Drug Nanocrystals (NCs), due to their nanoscale size and increased surface area, have rapid dissolving rates and higher bioavailability than bulk drug forms. However, these benefits are coupled by thermodynamic instability, as increased surface free energy favors aggregation and crystal development. Ostwald ripening is an important instability mechanism in which smaller particles aggregated under supersaturated conditions, resulting in particle growth and loss of nanoscale advantages. This phenomenon may also raise safety concerns, particularly for intravenous formulations, where aggregation could potentially result in vascular complications (Shahiwala, 2025; Yang et al., 2018).

To overcome these challenges, the incorporation of stabilizers is essential. Stabilizers reduce interfacial free energy and prevent aggregation through electrostatic or steric mechanisms. Ionic surfactants such as Sodium Dodecyl Sulfate (SDS) (Kojima et al., 2018) provide electrostatic repulsion, whereas non-ionic surfactants (e.g., Tween, TPGS) (Rachmawati et al., 2013; S. Sharma et al., 2015) and polymeric stabilizers (e.g., Polyvinyl Alcohol (PVA), Polyvinylpyrrolidone (PVP) (Kumar and Siril, 2017), Hydroxypropyl Methylcellulose (HPMC) (Ueda et al., 2019), Polyethylene Glycol (PEG)) (Wang et al., 2020; Z. Zhang et al., 2020) offer steric stabilization by forming protective layers around nanocrystals. Apart from physical stabilization, several excipients offer additional functional benefits. For example, TPGS has been shown to hinder P-glycoprotein (P-gp) efflux, increasing intracellular drug accumulation and overcoming multidrug resistance. Similarly, PEGylation lowers absorption by the Reticuloendothelial System (RES), extending systemic circulation; nevertheless, repeated exposure may result in Accelerated Blood Clearance (ABC), limiting long-term therapeutic efficacy. The Molecular Weight (MW) of polymeric stabilizers is particularly important, as lower MW polymers promote mucosal diffusion and oral absorption while higher MW polymers give stronger steric stabilization at lower doses. Additionally, stabilizers that improve both solubility and dissolution are wanted for maximum bioavailability. The growing interest in natural stabilizers like saponins indicates a trend toward safer and more biocompatible options. Table 4 summarizes the classification, processes, and functional functions of commonly employed stabilizers in nanocrystal formulations, emphasizing their importance in preserving stability as well as impacting drug delivery performance.

Surface Engineering for Functional and Targeted Nanocrystals

Surface engineering of drug nanocrystals has emerged as an important technique for overcoming conventional systems' disadvantages, such as fast dissolution, non-specific distribution, and clearance by the mononuclear phagocyte system (MPS). While conventional stabilizers (surfactants and polymers) primarily maintain colloidal stability, they lack biological functionality. Thus, the incorporation of targeted ligands onto nanocrystal surfaces has become vital for improving pharmacokinetics and therapeutic specificity.

Because of their high surface energy, nanocrystals are intrinsically unstable and must be stabilized using steric or electrostatic methods. These interactions, guided by DLVO theory, keep the dispersion stable by balancing attractive and repulsive forces. However, effective surface engineering goes beyond stabilization to include rational ligand design, such as strong surface stabilization, resistance to protein adsorption, and controlled release of drug for targeting.

Folic acid (FA) is a frequently targeted ligand due to the activation of folate receptors in cancer cells. FA-functionalized nanocrystals have increased tumor accumulation and receptor-mediated uptake, as seen in sorafenib and paclitaxel systems, allowing for better intracellular drug release and therapeutic efficacy (Liang et al., 2021; Setua et al., 2009; Zhao et al., 2021). Similarly, Hyaluronic Acid (HA), targeting CD44 receptors, has found to improve cellular internalization and tumor inhibition, particularly in lapatinib-based formulations (Agrawal et al., 2017; Seo et al., 2020).

Other polysaccharides such as chondroitin sulfate also use CD44-mediated uptake to achieve selective tumor targeting (Liang et al., 2020; et al., 2022). Natural polymers like alginate and chitosan provide additional benefits by allowing pH-responsive behavior and protecting nanocrystals in adverse physiological environments, hence increase oral bioavailability (Lin et al., 2011; Ryu et al., 2020).

Protein-based ligands, such as Transferrin (Tf), promote receptor-mediated uptake in proliferating cells while also delaying systemic circulation (Han et al., 2020; Yong et al., 2019). Triphenylphosphonium (TPP) allows for mitochondrial targeting, which is very useful in decreasing multidrug resistance by interrupting cellular energy pathways. Emerging techniques, such as entropic ligands, broaden functionality by modifying thermodynamic features to increase nanocrystal solubility (Yang et al., 2016; Yang et al., 2016).

Despite these improvements, obstacles remain, such as ligand detachment, protein corona formation, and heterogeneity in receptor expression, all of which can limit targeting efficacy in vivo conditions. Furthermore, repeatability and large-scale production of surface-functionalized nanocrystals remain major obstacles. Addressing these obstacles will necessitate the creation of multifunctional, stimuli-responsive systems that combine stability, targeting, and controlled drug release. As shown in Table 5, diverse ligand systems enable the functionalization of nanocrystals for targeted and responsive drug delivery.

Biological Fate and in vivo Performance of Drug Nanocrystals

The in vivo behavior of drug nanocrystals has traditionally been interpreted based on rapid dissolution at the site of administration, leading to enhanced concentration gradients and improved drug absorption. Early pharmacokinetic studies supported this assumption, reporting comparable profiles between nanocrystal formulations and drug solutions, suggesting near-complete dissolution in biological environments (Liu et al., 2012). However, recent advances in imaging and tracking techniques have significantly revised this understanding.

Following oral administration, nanosizing has been found to significantly increase bioavailability. For example, danazol nanocrystals (~169 nm) showed an approximately 16-fold increase in oral bioavailability, due to improved dissolution (Liversidge & Cundy, 1995). Despite this traditional rationale, more recent research suggests that nanocrystals can also be absorbed in their intact form. Imaging data has identified nanocrystals in intestinal villi and mesenteric lymph, implying transcellular transport via endocytosis processes such as caveolae-mediated uptake (Fu et al., 2013; G. Zhang et al., 2021). These findings showing that oral absorption is governed by a dual mechanism involving both dissolution and particle-mediated transport.

In the case of intravenous administration, early interpretations similarly emphasized rapid dissolution. However, subsequent studies have demonstrated prolonged persistence of intact nanocrystals in vivo. For instance, itraconazole nanocrystals were detected in splenic macrophages up to 7 days post-administration, highlighting the dominant role of the Mononuclear Phagocyte System (MPS) in determining biodistribution (Rabinow et al., 2007). Particle size further influences pharmacokinetics, where larger nanocrystals exhibit increased Area Under the Curve (AUC), Mean Residence Time (MRT), and half-life due to slower dissolution and enhanced phagocytic uptake (Liu et al., 2012). Moreover, it has been estimated that nanocrystals require 5-10 min for complete dissolution, whereas systemic circulation occurs within seconds, allowing repeated circulation before dissolution. Consequently, up to ~95% of the administered dose can accumulate in liver and spleen within minutes, indicating rapid clearance by the MPS (Manjunath and Venkateswarlu, 2006).

For dermal delivery, nanocrystals predominantly accumulate within hair follicles rather than diffusing directly through the skin. Studies have demonstrated follicular penetration depths of approximately 196 μm, where nanocrystals act as drug reservoirs and release the drug gradually (Pelikh and Keck, 2020). Similarly, in ocular delivery, nanocrystals exhibit enhanced adhesion and prolonged retention, resulting in approximately two-fold higher bioavailability and extended therapeutic duration (up to 9 hr compared to 5 hr) (Donia et al., 2020). At the cellular level, nanocrystals undergo endocytosis-mediated uptake and intracellular trafficking through organelles such as the endoplasmic reticulum and Golgi apparatus, enabling controlled intracellular drug release (Deng et al., 2017).

Overall, the biological fate of nanocrystals is determined by a complicated interaction of dissolution kinetics and particle-mediated transport mechanisms. This dual-purpose sets nanocrystals apart from conventional formulations and points out their potential for controlled and targeted drug delivery. The important mechanistic discoveries from various administration methods are given in Table 6 and Figure 3.

Click to enlarge

Drug Nanocrystals: Translational Performance and Clinical Utility

Drug nanocrystals have been effectively converted into clinically useful dosage forms via a variety of modes of administration, including oral and parenteral delivery. Their primary advantage is improved bioavailability, reduced pharmacokinetic variability, and constant drug administration without affecting intrinsic pharmacological efficacy. Nanocrystals in oral delivery reduce food effects and improve dissolution-limited absorption, whilst in parenteral systems they eliminate the requirement for hazardous solubilizing excipients and allow for controlled drug release. Collectively, these characteristics help to increase therapeutic performance and patient compliance (Chary et al., 2024). Several marketed products illustrate flexibility and translated success of nanocrystal technology across various administration routes, as shown in Table 7.

Applications of Drug Nanocrystals

Drug nanocrystals have emerged as a clinically important strategy (Figure 4) for increasing the therapeutic effectiveness of poorly water-soluble drugs in a variety of delivery routes. Unlike mechanistic explanations that focus on in vivo fate, their applications are generally defined by increases in pharmacokinetic uniformity, therapeutic efficacy, dose reduction, and patient compliance (Dhibar et al., 2023). Table 8 summarizes the route-specific advantages of nanocrystal-based systems, allowing them to be translated into realistic drug delivery solutions.

Click to enlarge

Nanocrystals in oral delivery of drugs have significantly boosted the efficiency of poorly soluble drugs by increasing dissolution-limited absorption and decreasing gastrointestinal variability. For example, baicalein nano co-crystals showed a 6.09-fold increase in bioavailability compared to pure drug and 3.32-fold compared to conventional nanocrystals, highlighting the advantage of combining crystal engineering with nanosizing (Pi et al., 2018). Similarly, Silybin nanocrystals (HM40) produced by wet milling showed improved solubility, prolong stability over 24 months, and significantly improved oral bioavailability (2.61-fold in rats; 1.51-fold in humans), demonstrating effective clinical translation potential (Seo et al., 2024). Lutein nanocrystals (~110.7 nm) produced by antisolvent precipitation significantly enhanced solubility (7.3 to 215.7 μg/mL) and dissolution, resulting in improved oral bioavailability in rat models, with ~3.24-fold increase in C_max and ~2.28-fold increase in AUC. These findings highlight nanosizing as an effective strategy for overcoming solubility-limited absorption (Chang et al., 2017). Lutein nanocrystals (~230 nm) significantly improved dissolution and oral bioavailability (~15.2-fold) in Sprague-Dawley rats, with enhanced ocular accumulation (~3.4-fold), this suggesting nanosizing as an effective strategy for improving systemic and ocular delivery of poorly soluble nutraceuticals (Seto et al., 2019). Collectively, these findings show that oral nanocrystals provide constant drug release and reduce interpatient variability while maintaining pharmacological activity.

Parenteral nanocrystals represent an important milestone in systemic drug administration, particularly in oncology and long-acting drugs that require fast exposure and controlled pharmacokinetics. These systems, which typically vary in size from 100 to 300 nm, have prolonged circulation due to delayed dissolution, acting as particle reservoirs which allow controlled drug release. Surface functionalization with Polyethylene Glycol (PEG) and targeted ligands promotes tumor accumulation through Enhanced Permeability and Retention (EPR), while lowering premature clearance (Lu et al., 2016).

Nanocrystals show significant improvements in therapeutic efficiency and dose modification. Anlotinib nanocrystals (~200 nm) showed strong anticancer activity in a nude mouse hepatocellular carcinoma model, achieving tumor suppression at a significantly lower dose (50 μg/kg) compared to oral solution (1 mg/kg), resulting in a ~20-fold dose reduction (Luo et al., 2021). Similarly, paclitaxel nanocrystals stabilized with TPGS showed sustained release and enhanced cytotoxicity in resistant H460 cells, as well, improved tumor suppression in xenograft models (Liu et al., 2010), while camptothecin nanocrystals showed superior efficacy in MCF-7 tumor-bearing mice (Zhang et al., 2011).

The clinical potential of nanocrystals is further revealed by prolonged-acting uses. In mice and rats, cabotegravir nanocrystal prodrug (NM2CAB) maintained plasma levels over 90% inhibitory concentration for up to a year after a single dosage, with sustained retention in muscle and lymphoid tissues and no obvious toxicity (Gautam et al., 2021). Surface-engineered systems such as nevirapine nanocrystals also showed improved organ accumulation and reduced RES clearance, improving systemic retention (Shegokar and Singh, 2011). These findings highlight parenteral nanocrystals' ability to provide sustain release, reduce doses, and improve therapeutic effects.

Ocular drug delivery remains quite difficult due to physiological obstacles such as tear turnover, blinking, and nasolacrimal drainage, which result in rapid drug clearance and limited bioavailability. Conventional formulations sometimes need frequent dosing, which leads to reduced patient compliance and dose-related adverse effects. In this context, nanocrystals present an attractive approach for improving ocular retention and therapeutic efficacy by improving adhesion, controlling release, and increasing local drug concentration (Geng et al., 2024).

Recent research has shown the translational potential of nanocrystal-based ocular systems. In albino rabbits, hydrocortisone nanosuspensions (~300 nm) produced using nanoprecipitation and wet milling showed significantly improved bioavailability. The drug effect lasted up to 9 hr, compared to 5 hr for conventional solutions, and the AUC values were higher (~28-31 vs 15.86) (Ali et al., 2010). Similarly, ion-paired moxifloxacin nanocrystals allowed sustained intraocular drug release following a single subconjunctival injection, compared with conventional eye drops in both rat and rabbit infection models (Appell et al., 2025).

Furthermore, forskolin nanocrystal-based in situ gel systems showed prolonged intraocular pressure reduction (31% for 12 hr) compared to conventional formulations (18%, 4-6 hr), highlighting improved therapeutic outcomes (Gupta et al., 2010).

Collectively, these findings indicate that nanocrystals improved ocular drug delivery by prolonging residence time, improving bioavailability, and reducing dosing frequency, thereby offering a clinically alternative to conventional therapies.

Pulmonary drug delivery is a very efficient route for both local and systemic therapy because of its huge surface area and lack of first-pass metabolism, allowing for rapid absorption that may match intravenous pharmacokinetics. However, traditional inhalation methods are frequently hampered by low solubility, heterogeneous deposition, and fast mucociliary clearance (Kee et al., 2026; Peng et al., 2024). Nanocrystals solve these challenges by improving dissolution kinetics and allowing uniform distribution within the deep lung region, particularly when formulated as aerosolizable systems (Yue et al., 2022).

Recent studies showed that formulation engineering key role in pulmonary performance. Zheng et al. reported polysorbate-stabilized nintedanib nanocrystals with optimized nebulization behavior, effectively overcoming solubility and oral bioavailability limitations in pulmonary fibrosis therapy (Zheng et al., 2024). Similarly, Kumar et al. developed lipid-coated paclitaxel nanocrystals that surpassed uncoated systems in terms of aerosolization efficiency, sustained release, and lung retention, indicating the importance of surface modification in controlling release and clearance (Kumar et al., 2024).

Critically, whereas nanocrystals increase local drug concentration and lower systemic toxicity, their efficacy is still dependent on aerodynamic features and mucus interactions. As a result, rational design that incorporates carrier systems and particle engineering is crucial. Overall, pulmonary nanocrystals are a potential approach to targeted lung delivery, notably in cancer and fibrotic disorders.

Topical drug delivery provides localized therapeutic effect while avoiding first-pass metabolism, making it ideal for dermatological diseases. However, medication penetration and efficacy may be limited by the epidermal barrier and enzymatic activity (e.g., P450s, N-acetyltransferases) (Xue et al., 2024). Nanocrystals solve these issues by increasing drug solubility, preserving high concentration gradients, and improving penetration into deeper epidermal layers with smaller particles achieve superior permeation (Singh et al., 2022).

Recent studies showed the translational potential of nanocrystal-based topical systems. Kumar et al. (2022) reported niclosamide nanocrystals (~173 nm) incorporated into a hyaluronic acid-modified hydrogel, achieving sustained release (~12 h) and significantly improved cytotoxic efficacy in HaCaT cells (IC₅₀ reduced from 1.25 μM to 0.017 μM), showing improved cellular uptake and targeting (Kumar et al., 2022). Curcumin nanocrystals incorporated into clay-based hydrogels enhanced skin penetration and antioxidant activity, demonstrating improved topical bioavailability through prolonged contact and optimized rheological properties (Ruggeri et al., 2022). Similarly, Vasanth (2024) found that ketoconazole nanocrystals (48-83 nm) in a gel system with high entrapment efficiency (~85-93%) and prolonged drug release (~12 hr), resulting in improved local bioavailability and reduced dosing frequency (Nayak et al., 2020).

Collectively, nanocrystal-based topical systems enable improved penetration, sustained release, and improved therapeutic outcomes in skin disorders.

Overall, drug nanocrystals are used to increase drug performance while retaining the drugs pharmacological characteristics. Their adaptability across various modes of administration showing their importance as a formulation-driven approach for overcoming solubility and delivery issues. As shown in Table 8, the integration of nanocrystal technology with sophisticated formulation methodologies is expanding its therapeutic value. Future development is intended to focus on optimizing formulation design, increasing targeting efficiency, and improving scalability to support wider industrial and clinical application.

Regulatory Considerations in the Development of Drug Nanocrystals

The expanding adoption of nanocrystal technology into pharmaceutical development is shown in the number of regulatory applications and approvals over the last several decades. Retrospective evaluations of regulatory submissions show that nanocrystal-based formulations account for a sizable number of nanotechnology-enabled drug items examined by the US Food and Drug Administration. From 1973 to 2015, a total of 82 nanocrystal-related submissions were reported, comprising 55% Investigational New Drug (IND), 32% Abbreviated New Drug Applications (ANDAs), and 13% New Drug Applications (NDAs), highlighting both innovation potential and reformulation opportunities (Chen et al., 2017; Kapoor et al., 2017).

Nanocrystal formulations are especially important for Biopharmaceutics Classification System (BCS) class II drugs, as poor water solubility limits absorption despite acceptable permeability. According to regulatory data, more than 60% of nanocrystal-based products are intended for oral administration, demonstrating their efficiency in enhancing dissolving and lowering food-dependent variability (Chen et al., 2017). Comparative studies show that nanocrystal formulations have better pharmacokinetic uniformity and less affinity to physiological variables than conventional micronized systems.

Nanocrystal drugs are regulated using recognized Chemistry, Manufacturing, and Controls (CMC) frameworks. Critical quality attributes include Particle Size Distribution (PSD), polymorphic form and surface features, which all have significant impact on dissolving behavior, bioavailability, and stability (Kapoor et al., 2017). According to regulatory guidelines, in order to ensure reliable efficacy in therapy, nanocrystal size should typically stay below 1000 nm with controlled distribution (Chen et al., 2017). In order to prevent Ostwald ripening, which causes instability and unpredictability in drug performance as smaller particles dissolve and accumulate onto greater sized particles, narrow PSD is very important.

One of most important regulatory issues is still stability. Nanocrystals are prone to aggregation and recrystallization because of their high surface energy. Formulation creation requires careful consideration of both chemical stability (degradation of labile drugs like omeprazole and paclitaxel) and physical stability (aggregation, crystal formation) (Müller et al., 2011). Stabilizers play vital role by providing steric and electrostatic repulsion, thereby maintaining dispersion stability and preventing particle growth (Goodman et al., 2021). In addition, the presence of amorphous fractions frequently produced during processing might improve dissolution but requires strict control to ensure long-term stability (Gao et al., 2008).

Regulatory approval is also influenced by manufacturing factors. While top-down techniques like milling and High-Pressure Homogenization (HPH) must deal with contamination and process reproducibility, bottom-up approaches like precipitation techniques require precise control over residual solvents and nucleation conditions (Hu et al., 2016; Malamatari et al., 2018). Among these, HPH is widely favoured due to its solvent-free and scalability, making it suitable for industrial production. The comparative regulatory and manufacturing considerations across different fabrication methods are summarized in Table 9.

Compatibility with the 505(b)(2) regulatory process, which permits reformulation of current drugs using already documented safety and efficacy data, is another benefit of nanocrystal technology. Compared to more complex nanocarrier-based systems, this approach facilitated quicker clinical translation (Chandra and C, 2021). Overall, as Table 9 summarizes, effective development necessitates strict control of particle size, stability, and manufacturing methods to assure consistent product quality and therapeutic performance, even if nanocrystals offer a relatively simplified path to regulatory approval.

Toxicological and Safety Considerations

The toxicological profile of drug nanocrystals is mostly affected by particle size, biodegradability, and formulation composition. Particle size is a key factor in determining cellular interaction and potential toxicity. Nanocrystals smaller than 100 nm have a higher cellular internalization rate, which may increase intracellular exposure and associated toxicity risks, whereas particles between 100 and 1000 nm are more likely to undergo controlled clearance via the mononuclear phagocyte system, resulting in a lower overall risk.

Biodegradability is a significant advantage of drug nanocrystals. Unlike non-degradable nanomaterials, drug nanocrystals are mostly made up of the active pharmaceutical ingredient and dissolve in vivo, reducing long-term accumulation and associated toxicity risks (Müller et al., 2011). However, temporary interactions with immune cells and excipient-related effects must be carefully assessed because stabilizers and surfactants may affect biocompatibility.

Experimental and translational examinations support nanocrystals' excellent safety profile. For example, PX-13 and PX-18 nanocrystals (130-280 nm) had equivalent or lower cytotoxicity than traditional formulations, with no irritation seen in EPISKIN and HET-CAM models (Pardeike, 2009). Similarly, hesperetin nanocrystals (~100 nm) reduced inflammatory markers (TNF-α, IL-6) and oxidative stress while promoting cellular compatibility (Stahr et al., 2021). In vivo, rutin nanocrystals (270-500 nm) demonstrated improved therapeutic effectiveness without evidence of nanocrystal-induced toxicity (Hassan and Soliman, 2022). Comparable findings in cosmetic nanocrystal systems support their safety in human-relevant environments (Hassan and Soliman, 2022). Comparable findings in cosmetic nanocrystal systems further confirm their safety in human-relevant conditions (Sharma and Medhi, 2025).

Overall, these findings show that pharmacological nanocrystals have a predictable and tolerable safety profile when important issues like size, stability, and excipient composition are well regulated. Table 10 summarizes the important toxicological factors.

CHALLENGES AND FUTURE OUTLOOK

Despite significant advancements, various formulation and translational hurdles continue to restrict the clinical use of therapeutic nanocrystals. Physical instability, particularly aggregation and Ostwald ripening caused by high surface energy, is still a major hurdle for stability and efficiency. The variability in particle size distribution delays drug dissolution and the reproducibility of in vivo consequences. In surface-engineered systems, additional restrictions such as ligand detachment, protein corona formation, and heterogeneity receptor expression may impair targeting efficacy under physiological environments.

Manufacturing and regulatory problems, including as scalability, process reproducibility, and polymorphic transition control, persist to be major obstacles to industrial translation. Furthermore, consistent critical quality features and strong in vitro-in vivo correlations are required for regulatory approval. Although nanocrystals are typically regarded as safe, long-term toxicity, excipient-related effects, and immunological interactions must be further investigated.

Future research should focus on rational design strategies that combine stability, targeting, and controlled drug release on a single platform. Green manufacturing, continuous processing, and AI-assisted formulation optimization are examples of emerging technologies that show promise for enhancing scalability and repeatability. Furthermore, the development of predictive models that link physicochemical characteristics to in vivo performance will be essential to accelerating clinical translation.

CONCLUSION

Drug nanocrystals have emerged as one of the most effective formulation-based techniques for increasing the effectiveness of drugs that are poorly water soluble. Their carrier-free nature, high drug loading, and potential to improve dissolution without affecting pharmacological efficacy set them apart from typical nanocarriers. Importantly, clinically authorized products exhibit translational feasibility via oral and parenteral routes.

Advances in fabrication methods, surface engineering, and understanding of in vivo behavior have increased their therapeutic utility. However, obstacles like as stability, production control, and regulatory standards remain important to broad implementation.

Overall, drug nanocrystals are a clinically established and scalable technology with significant potential for future drug development, especially when combined with increased process control and mechanistic understanding.

ACKNOWLEDGEMENT

The authors are thankful to the Principal of R.C. Patel Institute of Pharmaceutical Education and Research and R.C. Patel Institute of Pharmacy, Shirpur, Dist.: Dhule (MS) India- 425 405, for providing necessary library facilities.

ABBREVIATIONS

ABC

Accelerated Blood Clearance

API

Active Pharmaceutical Ingredient

AUC

Area Under the Curve

BCS

Biopharmaceutics Classification System

CMC

Chemistry, Manufacturing, and Controls

CNTs

Carbon Nanotubes

DLVO

Derjaguin-Landau-Verwey-Overbeek Theory

EPR

Enhanced Permeability and Retention

ER

Endoplasmic Reticulum

FA

Folic Acid

HA

Hyaluronic Acid

HET-CAM

Hen’s Egg Test-Chorioallantoic Membrane

HME

Hot-Melt Extrusion

HPC

Hydroxypropyl Cellulose

HPMC

Hydroxypropyl Methylcellulose

HPH

High-Pressure Homogenization

HVNCs

Hyaluronic Acid-Modified Venetoclax Nanocrystals

IC₅₀

Half Maximal Inhibitory Concentration

IND

Investigational New Drug

IOP

Intraocular Pressure

MDR

Multidrug Resistance

MPS

Mononuclear Phagocyte System

MRT

Mean Residence Time

MW

Molecular Weight

NCs

Nanocrystals

NCEs

New Chemical Entities

NDAs

New Drug Applications

ANDAs

Abbreviated New Drug Applications

PEG

Polyethylene Glycol

PDI

Polydispersity Index

P-gp

P-glycoprotein

PSD

Particle Size Distribution

PVA

Polyvinyl Alcohol

PVP

Polyvinylpyrrolidone

RES

Reticuloendothelial System

SCF

Supercritical Fluid

SDS

Sodium Dodecyl Sulfate

Tf

Transferrin

TPGS

D-α-Tocopheryl Polyethylene Glycol Succinate

TPP

Triphenylphosphonium

WBM

Wet Bead Milling.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interest.