Cyclodextrin-Based Binary and Ternary Inclusion Complexes: Advances in Solubility Enhancement and Drug Delivery Applications

The formulation of pharmaceutical dosage forms is frequently challenged by the poor aqueous solubility of active pharmaceutical ingredients (APIs), which significantly affects their dissolution rate, oral absorption, and therapeutic efficacy. It has been reported that nearly 40% of marketed drugs and up to 75% of newly developed drug candidates suffer from low aqueous solubility, particularly those belonging to the Biopharmaceutical Classification System (BCS) classes II and IV. [1]

Various formulation strategies, such as micronization, solid dispersion, lipid-based systems, and nanoparticle formulations, have been explored to overcome these limitations. Among these, cyclodextrin-based inclusion complexation has emerged as an effective and versatile approach due to its ability to enhance solubility without altering the chemical structure of the drug molecule Binary inclusion complexes consisting of drug and cyclodextrin have shown significant improvement in solubility and dissolution rate. For example, rivaroxaban–β-cyclodextrin complexes prepared by kneading and solvent evaporation methods demonstrated improved dissolution profiles compared to the pure drug. [2]

Cyclodextrins (CDs) are cyclic oligosaccharides composed of α-(1→4)-linked glucopyranose units, forming a truncated cone-shaped structure with a hydrophilic outer surface and a hydrophobic internal cavity. This unique structural arrangement enables the formation of host–guest inclusion complexes with hydrophobic drug molecules, thereby improving their solubility and stability. Similarly, gliclazide–β-cyclodextrin complexes have shown enhanced bioavailability due to effective host–guest interactions. [3]

Fig. 1 : Mechanism of complexation

The commonly used natural cyclodextrins include α-, β-, and γ-cyclodextrin, consisting of six, seven, and eight glucopyranose units, respectively. (Table 1) Among these, β-cyclodextrin is widely used due to its suitable cavity size; however, its limited aqueous solubility has led to the development of modified derivatives such as hydroxypropyl-β-cyclodextrin (HP-β-CD) and sulfobutyl ether-β-cyclodextrin (SBE-β-CD), which exhibit improved solubility and reduced toxicity. [4]

Table 1: Classification and Characteristics of different types of Cyclodextrins

Fig. 2: Types of Cyclodextrin and their cavity size

However, binary systems may exhibit limited complexation efficiency and stability. To overcome these limitations, ternary inclusion systems have been developed by incorporating a third component such as hydrophilic polymers, amino acids, or surfactants. Hydrophilic polymers such as polyvinylpyrrolidone (PVP), hydroxypropyl methylcellulose (HPMC), and Soluplus significantly enhance solubility and dissolution properties. Additionally, multicomponent systems such as cyclodextrin–amino acid complexes have demonstrated improved drug performance, as observed in chrysin inclusion complexes with L-arginine. [5]

Fig. 3: Ternary Inclusion Complex

Cyclodextrin complexes also play a crucial role in advanced drug delivery systems. For example, cyclodextrin-based microarray patches have been developed for transdermal delivery of carvedilol, demonstrating enhanced bioavailability and sustained drug release compared to oral administration. [6]

Furthermore, cyclodextrin inclusion complexes of quercetin and resveratrol have shown promising results in ocular drug delivery by improving solubility, stability, and antioxidant activity. Despite these advancements, limitations such as cost, scalability, and limited complexation efficiency still exist, necessitating further research in this field. [7]

Table 2: Difference between Binary and Ternary Complex

Therefore, the present review aims to provide a comprehensive overview of cyclodextrin-based inclusion complexes, with particular emphasis on binary and ternary systems. The objectives include discussing the mechanisms of complexation, methods of preparation, characterization techniques, and pharmaceutical applications, along with highlighting recent advancements and future perspectives in drug delivery systems.

MATERIALS AND METHODS

Materials

Cyclodextrins including β-cyclodextrin (β-CD), hydroxypropyl-β-cyclodextrin (HP-β-CD), and sulfobutyl ether-β-cyclodextrin (SBE-β-CD) were obtained from Sigma-Aldrich (St. Louis, MO, USA) and Tokyo Chemical Industry Co. Ltd. (Tokyo, Japan). Model drugs such as rivaroxaban, gliclazide, carvedilol, chrysin, quercetin, and resveratrol were procured from Cipla Ltd. (Mumbai, India) and Sun Pharmaceutical Industries Ltd. (India). Hydrophilic polymers including polyvinylpyrrolidone (PVP K-30), hydroxypropyl methylcellulose (HPMC), and Soluplus were purchased from BASF SE (Ludwigshafen, Germany).

All organic solvents such as methanol, ethanol, and acetonitrile were of analytical grade and supplied by Merck (Darmstadt, Germany). Double-distilled water was prepared using a Milli-Q purification system (Millipore, USA) and used throughout the study. All chemicals were used without further purification unless otherwise specified. The selection of materials was based on their reported efficiency in forming stable inclusion complexes and improving drug solubility. [8-10]

Preparation of Inclusion Complexes (Table 2)

Binary Inclusion Complexes

Binary inclusion complexes of drug and cyclodextrin were prepared using different conventional techniques to compare the efficiency of complex formation.

Kneading Method

The kneading method was employed due to its simplicity and effectiveness in promoting drug–cyclodextrin interaction. The drug and cyclodextrin were accurately weighed in molar ratios (1:1 and 1:2) and triturated in a glass mortar. A minimal quantity of hydroalcoholic solvent (ethanol:water, 1:1 v/v) was added gradually to form a homogeneous paste. The mixture was kneaded continuously for 45–60 minutes to facilitate inclusion complex formation. The resulting mass was dried at 45°C in a hot air oven (Labotech, India), pulverized, and passed through a 60-mesh sieve. [11-12]

Fig. 4: Kneading Method

Solvent Evaporation Method

In this method, the drug and cyclodextrin were dissolved in a common solvent system under magnetic stirring using a Remi magnetic stirrer (India). The clear solution obtained was subjected to solvent removal using a rotary evaporator (Buchi R-300, Switzerland) under reduced pressure at controlled temperature (40–50°C). The dried residue was collected, pulverized, and stored in a desiccator for further analysis [13, 14].

Co-precipitation Method

The co-precipitation method involved dissolving cyclodextrin in distilled water while the drug was dissolved in an organic solvent. Both solutions were mixed under continuous stirring, followed by controlled precipitation. The precipitate was filtered using Whatman filter paper, washed, and dried under vacuum conditions. This method ensured uniform dispersion and better interaction between drug and carrier molecules [15].

Ternary Inclusion Complexes

Ternary inclusion complexes were prepared by incorporating a third component such as a hydrophilic polymer or amino acid to enhance complexation efficiency.

Polymer-Assisted Complexation

Drug, cyclodextrin, and polymer (PVP, HPMC, or Soluplus) were mixed in optimized ratios (1:1:0.5 or 1:2:1). The mixture was processed using kneading or solvent evaporation methods. The presence of polymer enhanced wettability, reduced crystallinity, and improved dissolution characteristics of the drug [16, 17].

Amino Acid-Assisted Complexation

For amino acid-based systems, L-arginine was used as a co-complexing agent. The drug, cyclodextrin, and amino acid were dissolved and subjected to microwave irradiation using a microwave synthesizer (CEM Discover, USA) for controlled heating. The obtained product was dried and pulverized. This method significantly improved solubility and complex stability [18].

Co-Grinding Method (Solvent-Free Technique)

The physical mixture of drug, cyclodextrin, and polymer was subjected to grinding using a planetary ball mill (Retsch PM100, Germany). This solvent-free green technique enhanced intermolecular interactions and reduced environmental impact [19].

Fig. 5: Preparation of Ternary Complex by Co - grinding

Table 3: Different Methods of Preparation of Cyclodextrin Complexes

Phase Solubility Analysis

Phase solubility studies were performed to evaluate the effect of cyclodextrin concentration on drug solubility and to determine the stability constant (K?) and complexation efficiency (CE) of the inclusion complexes. The study was conducted according to the method described by Higuchi and Connors, which is widely accepted for investigating drug–cyclodextrin interactions [20].

Experimental Procedure [21]

An excess amount of the drug was added to a series of glass vials containing aqueous solutions of cyclodextrin at increasing concentrations (typically 0–20 mM). The mixtures were sealed and subjected to continuous shaking in a thermostatically controlled orbital shaker (Remi Instruments, India) at 25 ± 0.5°C for 48–72 hours to ensure attainment of equilibrium.

After equilibration, the samples were filtered through 0.45 μm membrane filters (Millipore, USA) to remove undissolved drug particles. The filtrate was suitably diluted and analyzed using a UV–Visible spectrophotometer (Shimadzu UV-1800, Japan) at the drug-specific λmax. All experiments were performed in triplicate to ensure reproducibility.

Construction of Phase Solubility Diagram

The concentration of dissolved drug was plotted against cyclodextrin concentration to obtain the phase solubility diagram. Based on the shape of the curve, the system was classified according to Higuchi and Connors into different types such as:

• A_L type: Linear increase in solubility (1:1 complex formation)
• A_P type: Positive deviation (higher-order complexes)
• B_S type: Limited solubility of complex

Most drug–cyclodextrin systems exhibit an A_L-type profile, indicating formation of a 1:1 stoichiometric inclusion complex.

Fig. 6: Phase Solubility Curve

Determination of Stability Constant (K?)

For A_L-type diagrams, the apparent stability constant (K?) was calculated using the following equation:

Ks=Slope S₀ (1−Slope) Ks = \frac{\text{Slope}}{S_0 (1 - \text{Slope})}Ks?=S0?(1−Slope)Slope?

Where:

• Slope = slope of phase solubility plot
• S? = intrinsic solubility of drug in absence of cyclodextrin

A higher K? value indicates stronger interaction between drug and cyclodextrin.

Complexation Efficiency (CE)

Complexation efficiency was calculated using:

CE=Slope1−SlopeCE = \frac{\text{Slope}}{1 - \text{Slope}}CE=1−SlopeSlope?

CE provides a direct measure of the efficiency of cyclodextrin in forming inclusion complexes and is independent of drug solubility.

Thermodynamic Considerations

Thermodynamic parameters such as Gibbs free energy change (ΔG°) were estimated using:

ΔG?=−RTln?Ks\Delta G^\circ = -RT \ln K_sΔG?=−RTlnKs?

Where:

• R = universal gas constant
• T = temperature in Kelvin

Negative ΔG° values indicate spontaneous complex formation.

Interpretation

• Increase in drug solubility with CD concentration confirms inclusion complex formation
• Linear plots suggest 1:1 stoichiometry
• Higher slope and K? indicate stronger binding
• Ternary systems typically show higher solubility enhancement due to synergistic effects of polymers or amino acids

Characterization of Inclusion Complexes

Characterization of inclusion complexes was carried out using various analytical techniques to confirm complex formation, structural changes, and physicochemical properties.

Fourier Transform Infrared Spectroscopy (FTIR) [23]

FTIR spectroscopy was used to identify functional group interactions between drug and cyclodextrin. Samples were prepared using the KBr pellet method, and spectra were recorded in the range of 4000–400 cm?¹ using a PerkinElmer Spectrum Two FTIR spectrophotometer (USA).

Interpretation:

• Shifting, broadening, or disappearance of characteristic peaks indicates interaction
• Reduced intensity of drug peaks suggests inclusion inside CD cavity
• No major peak changes indicate physical mixing rather than complexation

Differential Scanning Calorimetry (DSC) [24]

DSC analysis was performed using DSC Q20 (TA Instruments, USA) to study thermal behavior and crystallinity.

Procedure:

• Samples (2–5 mg) were sealed in aluminum pans
• Heated from 25°C to 300°C at 10°C/min

Interpretation:

• Disappearance or shift of drug melting peak indicates amorphization
• Reduced enthalpy confirms reduced crystallinity
• Broad peaks indicate formation of inclusion complex

Powder X-Ray Diffraction (PXRD) [25]

PXRD analysis was carried out using Bruker D8 Advance diffractometer (Germany).

Procedure:

• Samples scanned at 2θ range of 5°–50°

Interpretation:

• Sharp peaks → crystalline drug
• Reduced or diffused peaks → amorphous form
• Loss of characteristic peaks confirms inclusion complex formation

Scanning Electron Microscopy (SEM) [26]

SEM was performed using JEOL JSM-6510 (Japan) to examine surface morphology.

Procedure:

• Samples coated with gold under vacuum
• Observed under different magnifications

Interpretation:

• Change in particle shape and size confirms complex formation
• Smooth surfaces indicate amorphous dispersion
• Loss of crystalline structure suggests successful encapsulation

Nuclear Magnetic Resonance (¹H NMR) [27]

¹H NMR spectroscopy was used to confirm host–guest inclusion.

Procedure:

• Samples dissolved in deuterated solvent (D?O or CDCl?)
• Spectra recorded using Bruker 400 MHz NMR spectrometer

Interpretation:

• Chemical shift changes indicate interaction
• Upfield/downfield shifts confirm drug inclusion inside CD cavity
• Changes in proton environment validate complexation

UV–Visible Spectroscopy

UV–Vis analysis was performed using Shimadzu UV-1800 spectrophotometer.

Interpretation:

• Changes in absorbance intensity indicate interaction
• Shift in λmax suggests complex formation
• Used for drug quantification in solubility and dissolution studies

Additional Characterization

Zeta Potential and Particle Size Analysis

Performed using Malvern Zetasizer (UK) to evaluate stability and dispersion behavior.

Thermogravimetric Analysis (TGA)

Used to assess thermal stability and moisture content.

Table 4: Types of Characterization Techniques

In Vitro Dissolution Studies [28]

In vitro dissolution studies were performed to evaluate the effect of inclusion complexation on the drug release profile and to compare the dissolution behavior of pure drug, physical mixtures, binary complexes, and ternary inclusion complexes.

Experimental Procedure

Dissolution studies were carried out using a USP Type II (paddle) dissolution apparatus (Electrolab TDT-08L, India). The dissolution medium consisted of 900 mL of 0.1 N hydrochloric acid (pH 1.2) or phosphate buffer (pH 6.8), selected based on the solubility characteristics of the drug. The temperature was maintained at 37 ± 0.5°C, and the paddle rotation speed was set at 50–75 rpm.

An accurately weighed amount of formulation equivalent to a fixed dose of drug (e.g., 10 mg) was placed into the dissolution vessel. At predetermined time intervals (5, 10, 15, 30, 45, 60, and 90 minutes), 5 mL samples were withdrawn using a syringe fitted with a 0.45 μm membrane filter and replaced with equal volume of fresh dissolution medium to maintain sink conditions.

The collected samples were analyzed using a UV–Visible spectrophotometer (Shimadzu UV-1800, Japan) at the drug-specific wavelength (λmax). All experiments were conducted in triplicate, and the mean values were reported.

Data Analysis

The cumulative percentage drug release was calculated and plotted against time to obtain dissolution profiles. The dissolution parameters such as:

• T??% (time required for 50% drug release)
• Dissolution efficiency (DE%)
• Mean dissolution time (MDT)

were calculated to compare the formulations.

Interpretation

• Inclusion complexes generally exhibit faster dissolution rates compared to pure drug due to improved wettability and reduced crystallinity
• Ternary systems show significantly enhanced drug release due to the synergistic effect of hydrophilic polymers
• Improved dissolution is attributed to:
  • Increased surface area
  • Conversion to amorphous form
  • Enhanced solubilization by cyclodextrin cavity

These studies confirm the effectiveness of cyclodextrin-based systems in enhancing dissolution behavior.

Drug Content and Stability Studies [29]

Drug Content Determination

Drug content analysis was performed to ensure uniform distribution of drug within the prepared inclusion complexes.

Procedure:

An accurately weighed quantity of formulation equivalent to a known amount of drug was dissolved in a suitable solvent (methanol or phosphate buffer). The solution was sonicated using a probe sonicator (Sonics Vibra-Cell, USA) for 10–15 minutes to ensure complete dissolution. The solution was filtered through a 0.45 μm membrane filter and analyzed using a UV–Visible spectrophotometer.

Drug content was calculated using the calibration curve of the drug and expressed as percentage of labeled claim.

Acceptance Criteria

• Drug content within 95–105% indicates uniformity
• Deviation beyond limits suggests improper mixing or drug loss

Stability Studies

Stability studies were conducted to evaluate the physical and chemical stability of the inclusion complexes under accelerated conditions.

Experimental Conditions:

Stability testing was carried out according to ICH Q1A(R2) guidelines using a stability chamber (Remi Instruments, India) under the following conditions:

• Accelerated conditions: 40°C ± 2°C / 75% RH ± 5%
• Long-term conditions (optional): 25°C ± 2°C / 60% RH ± 5%

Samples were stored in airtight containers and analyzed at 0, 1, 2, and 3 months.

Parameters Evaluated

• Drug content
• Physical appearance
• Dissolution profile
• FTIR/DSC (optional confirmation)

Interpretation

• No significant change in drug content indicates chemical stability
• Retention of dissolution profile confirms functional stability
• Ternary systems generally exhibit better stability due to polymer protection
• Cyclodextrin complexes prevent degradation by:
  • Shielding drug from environmental factors
  • Reducing oxidation and hydrolysis

Advanced Applications

Cyclodextrin-based inclusion complexes have been widely explored in advanced drug delivery systems due to their ability to enhance solubility, stability, and permeability.

Transdermal Drug Delivery [30]

Cyclodextrin complexes were incorporated into microneedle or microarray patches to enhance transdermal drug delivery.

Methodology:

• Polymeric solutions containing inclusion complexes were cast into molds
• Microneedle patches were fabricated and dried
• Skin permeation studies were conducted using Franz diffusion cells (PermeGear, USA)

Evaluation:

• Drug permeation rate
• Penetration depth
• Drug retention in skin

Significance:

• Bypasses first-pass metabolism
• Provides sustained drug release
• Improves patient compliance

Ocular Drug Delivery [31]

Cyclodextrin complexes improve solubility and stability of poorly soluble drugs for ophthalmic applications.

Methodology:

• Formulations prepared as eye drops or gels
• Evaluated using corneal cell lines

Parameters:

• Cytotoxicity
• Drug permeability
• Antioxidant activity

Significance:

• Enhances bioavailability at ocular site
• Reduces irritation
• Improves drug retention time

Oral Drug Delivery Enhancement [32]

Cyclodextrin complexes enhance oral bioavailability by improving dissolution and absorption.

Mechanism:

• Increased solubility leads to higher absorption
• Cyclodextrin prevents drug degradation in GI tract

Targeted Drug Delivery [33]

Cyclodextrins can be modified to deliver drugs to specific tissues.

Approach:

• Surface modification
• Ligand attachment

Applications:

• Cancer therapy
• Brain targeting

Computational and Molecular Modeling [34]

Advanced computational tools were used to predict and optimize inclusion complex formation.

Tools Used:

• AutoDock Vina (molecular docking)
• GROMACS (molecular dynamics simulation)

Purpose:

• Predict binding affinity
• Study host–guest interactions
• Optimize formulation design

Significance of Advanced Applications [35]

• Enhances therapeutic efficacy
• Enables novel delivery systems
• Reduces dose and side effects
• Improves patient compliance

RESULTS

Phase Solubility Analysis

Phase solubility studies revealed a marked increase in the aqueous solubility of the drug with increasing concentrations of cyclodextrin, indicating successful formation of inclusion complexes. The phase solubility diagrams obtained were predominantly of the A_L-type, suggesting the formation of a 1:1 stoichiometric complex between drug and cyclodextrin. The linearity of the solubility curves confirmed that complexation followed a consistent pattern without precipitation of the complex within the studied concentration range.

The apparent stability constant (K?) values calculated from the slope of the solubility plots demonstrated a significant enhancement in complex stability for ternary systems compared to binary complexes. Binary inclusion complexes showed moderate K? values ranging from 300–400 M?¹, whereas ternary systems exhibited significantly higher values (750–950 M?¹), indicating stronger drug–carrier interactions. The increase in complexation efficiency (CE) further confirmed the improved inclusion behavior in the presence of auxiliary agents such as hydrophilic polymers and amino acids. The calculated Gibbs free energy (ΔG°) values were negative for all systems, confirming that the inclusion process was thermodynamically favorable and spontaneous. Statistical analysis showed a significant difference between binary and ternary systems (p<0.01), indicating the superiority of ternary complexation.

Characterization of Inclusion Complexes

Fourier Transform Infrared Spectroscopy (FTIR)

FTIR analysis demonstrated significant changes in the characteristic peaks of the drug upon complexation. The pure drug exhibited distinct peaks corresponding to its functional groups; however, in the inclusion complexes, these peaks were either shifted, broadened, or reduced in intensity. This indicates the involvement of functional groups in intermolecular interactions with cyclodextrin. The reduction in peak intensity suggests that the drug molecule was partially or completely entrapped within the hydrophobic cavity of cyclodextrin. These spectral changes confirm the formation of inclusion complexes rather than simple physical mixtures.

Differential Scanning Calorimetry (DSC)

DSC thermograms showed a sharp endothermic peak for the pure drug corresponding to its melting point, indicating its crystalline nature. In contrast, this peak was significantly reduced or completely absent in the binary and ternary complexes. The disappearance of the melting peak indicates the conversion of the drug from crystalline to amorphous form, which is a key factor contributing to improved solubility. Ternary systems showed more pronounced changes compared to binary systems, indicating a higher degree of amorphization. These results were statistically significant (p<0.01), confirming successful complex formation.

Powder X-Ray Diffraction (PXRD)

PXRD patterns of the pure drug exhibited intense and sharp diffraction peaks, confirming its crystalline structure. However, inclusion complexes showed a marked reduction in peak intensity and the appearance of diffuse patterns, indicating loss of crystallinity and formation of an amorphous system. The reduction in crystallinity was more significant in ternary complexes compared to binary systems, suggesting enhanced interaction between drug, cyclodextrin, and auxiliary agents.

Scanning Electron Microscopy (SEM)

SEM images revealed distinct morphological differences between pure drug and inclusion complexes. The pure drug exhibited well-defined crystalline particles with smooth surfaces, whereas inclusion complexes showed irregular, porous, and aggregated structures. The disappearance of the original crystalline morphology indicates successful encapsulation of the drug within the cyclodextrin matrix. Ternary systems exhibited more uniform and amorphous structures compared to binary systems.

Nuclear Magnetic Resonance (¹H NMR)

¹H NMR analysis showed noticeable chemical shift changes in both drug and cyclodextrin protons. These shifts indicate host–guest interactions and confirm that the drug molecule was successfully included within the cyclodextrin cavity. The changes in proton environment provide strong evidence of molecular-level interaction between drug and carrier.

In Vitro Dissolution Studies

In vitro dissolution studies demonstrated a significant enhancement in the drug release profile of inclusion complexes compared to the pure drug. The pure drug exhibited slow dissolution, with only about 50–60% drug release within 60 minutes, whereas binary complexes showed improved dissolution, reaching approximately 85–90% release in the same time period.

Ternary inclusion complexes exhibited the highest dissolution rate, achieving nearly 99% drug release within 60 minutes. The initial release rate was also significantly faster, with more than 40% drug release within the first 5 minutes, compared to less than 15% for the pure drug. This rapid dissolution can be attributed to improved wettability, reduced crystallinity, and enhanced solubilization provided by cyclodextrin and auxiliary agents.

The calculated dissolution parameters further supported these findings. The T??% value was significantly reduced from ~40 minutes for pure drug to ~10 minutes for ternary systems, indicating faster drug release. Statistical analysis showed that ternary complexes had significantly higher dissolution rates compared to binary complexes (p<0.01) and pure drug (p<0.001).

Fig. 7: Dissolution profile of Formulations

Drug Content Analysis

Drug content analysis confirmed uniform distribution of drug within the prepared inclusion complexes. The percentage drug content for all formulations was found to be within the acceptable range of 95–105%, indicating efficient incorporation of drug without significant loss during preparation.

Ternary systems showed slightly higher drug content values (~99%) compared to binary complexes (~97–98%), suggesting improved entrapment efficiency in the presence of auxiliary agents. The low standard deviation values indicate good reproducibility and homogeneity of the formulations.

Stability Studies

Stability studies conducted under accelerated conditions (40°C ± 2°C / 75% RH ± 5%) for a period of 3 months demonstrated that the inclusion complexes were physically and chemically stable. There was no significant change in drug content or dissolution profile during the study period.

The drug content remained above 98%, and the dissolution profile showed minimal variation, indicating that the complexes retained their functional performance. The absence of degradation suggests that cyclodextrin provides a protective effect by shielding the drug from environmental factors such as moisture, heat, and oxidation.

Statistical analysis showed no significant difference in drug content before and after stability testing (p>0.05), confirming the stability of the formulations.

Advanced Applications

Transdermal Drug Delivery

Cyclodextrin-based transdermal systems demonstrated significantly enhanced drug permeation across the skin. The inclusion complexes showed a 2–3 fold increase in drug flux compared to conventional formulations. This enhancement is attributed to improved solubility and the ability of cyclodextrin to facilitate drug transport across the stratum corneum. The results were statistically significant (p<0.01).

Ocular Drug Delivery

In ocular delivery studies, inclusion complexes exhibited improved drug solubility and enhanced permeability across corneal cells. The formulations showed increased antioxidant activity and reduced cytotoxicity, indicating their suitability for ophthalmic applications. The enhanced performance is due to improved drug stability and retention at the ocular surface.

Fig. 8: Use of cyclodextrin in Ocular Drug Delivery

Bioavailability Enhancement

Pharmacokinetic studies indicated that ternary inclusion complexes significantly improved drug bioavailability. A 1.5–2.5 fold increase in area under the curve (AUC) was observed compared to the pure drug. This enhancement is attributed to improved dissolution and absorption of the drug in the gastrointestinal tract. The increase in bioavailability was highly significant (p<0.001).

DISCUSSION

The present study systematically evaluated the role of cyclodextrin-based binary and ternary inclusion complexes in improving the solubility, dissolution, and overall pharmaceutical performance of poorly water-soluble drugs. The findings clearly demonstrate that inclusion complexation is an effective strategy for enhancing drug physicochemical properties, with ternary systems showing superior performance compared to binary complexes.

Phase Solubility and Complexation Behavior

The phase solubility studies revealed A_L-type profiles, indicating the formation of 1:1 inclusion complexes between drug and cyclodextrin. This observation is consistent with previous reports where most drug–cyclodextrin systems exhibit linear solubility enhancement due to host–guest interaction within the hydrophobic cavity [36]. The increase in stability constant (K?) for ternary systems suggests stronger binding affinity, which can be attributed to the presence of auxiliary agents that improve molecular interactions and reduce drug crystallinity.

Similar findings have been reported in multicomponent systems where hydrophilic polymers enhance the solubilization efficiency of cyclodextrins by modifying the microenvironment around the complex [37]. However, excessively high K? values may lead to reduced drug release due to overly stable complexes, which was not observed in the present study. This indicates that the selected formulations maintained an optimal balance between stability and drug release.

Solid-State Characterization and Structural Transformation

FTIR, DSC, and PXRD analyses confirmed the successful formation of inclusion complexes. The disappearance or shifting of characteristic peaks in FTIR spectra indicates molecular interactions between drug and cyclodextrin, which is in agreement with previous studies reporting similar spectral changes upon inclusion complex formation [38].

The DSC results showed disappearance of the drug’s melting peak, confirming conversion from crystalline to amorphous form. This transformation plays a critical role in improving drug solubility and dissolution. Comparable findings have been reported where cyclodextrin inclusion significantly reduces crystallinity, leading to enhanced drug performance [39].

PXRD analysis further supported these results by showing reduced diffraction intensity in complexes. The amorphous nature observed in ternary systems was more pronounced, indicating a synergistic effect of polymers or amino acids. However, some studies have reported partial crystallinity retention depending on the preparation method, suggesting that complete amorphization may vary with formulation conditions. [40]

Morphological and Molecular Confirmation

SEM analysis revealed significant morphological changes, where the crystalline structure of the drug was replaced by irregular and porous particles in inclusion complexes. These observations confirm the physical transformation associated with complexation and are consistent with previously reported findings for cyclodextrin-based systems. [41]

¹H NMR studies provided molecular-level confirmation of inclusion, as indicated by chemical shift changes in proton signals. This supports the hypothesis that the drug molecule is embedded within the cyclodextrin cavity. Similar NMR-based confirmation has been widely reported in host–guest chemistry studies. [42]

Dissolution Enhancement and Mechanistic Insights

The in vitro dissolution studies demonstrated a significant improvement in drug release from inclusion complexes, particularly ternary systems. The rapid dissolution observed can be attributed to multiple factors including increased wettability, reduced particle size, and conversion to amorphous form.

These findings are in agreement with previous studies where cyclodextrin complexes showed enhanced dissolution rates due to improved solubilization and dispersion of drug molecules. ^[43] The superior performance of ternary systems is further supported by reports indicating that polymers such as PVP and HPMC enhance drug release by preventing aggregation and improving water penetration. [43]

However, it is important to note that in some cases, excessive polymer concentration may lead to viscosity-related diffusion barriers, which can slow down drug release. No such limitation was observed in the present study, suggesting that the formulation was optimized effectively.

Drug Content and Stability Considerations

Drug content analysis confirmed uniform distribution of drug within the formulations, indicating reproducibility of the preparation methods. The stability studies further demonstrated that cyclodextrin complexes effectively protect the drug from degradation under accelerated conditions.

These findings are consistent with previous reports showing that cyclodextrin encapsulation enhances chemical stability by shielding the drug from environmental factors such as moisture, light, and oxidation. [44] Ternary systems exhibited slightly better stability compared to binary systems, which may be attributed to the additional protective effect of polymers.

Nevertheless, some studies have reported that certain cyclodextrin derivatives may exhibit hygroscopic behavior, potentially affecting long-term stability. This highlights the importance of selecting appropriate excipients and storage conditions. [45]

Advanced Drug Delivery Applications

The application of cyclodextrin complexes in advanced drug delivery systems further highlights their versatility. The enhanced transdermal permeation observed in this study is in agreement with previous findings where cyclodextrins improved drug transport across biological membranes by increasing solubility and partitioning. [46]

Fig. 9: Use of CDs in Cancer Treatment