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INTRODUCTION
Rifaximin is a broad-spectrum, minimally absorbed antibiotic widely used for gastrointestinal infections due to its high local activity and negligible systemic absorption (Shayto et al., 2016). However, it’s low aqueous solubility limits dissolution, formulation versatility, and potential application in alternative delivery systems such as topical formulations (Tsakiridou et al., 2025). Improving its solubility is therefore essential to expand its therapeutic utility. Solid dispersion is a well-established approach for solubility enhancement, primarily by converting crystalline drugs into amorphous forms and improving wettability using hydrophilic carriers. Hydrogels are promising topical delivery systems due to their biocompatibility, ease of application, and ability to provide controlled release and prolonged residence time. Natural polymers such as Locust Bean Gum (LBG) and Xanthan Gum (XG) are widely used due to their biocompatibility and gel-forming properties; however, their concentration significantly influences rheology and drug release.
Although solid dispersion has been extensively studied, limited research has explored its integration into hydrogel systems for the topical delivery of rifaximin, and optimization of polymer combinations remains underexplored.
Therefore, the present study aimed to develop and optimize a rifaximin solid dispersion-loaded hydrogel using a 3² factorial design to evaluate the effects of LBG and XG on drug content, viscosity, and drug release.
MATERIALS AND METHODS
Rifaximin was supplied by Alembic Pharmaceuticals Ltd., Mumbai, India. Polymers like Soluplus, HPMC E5, PEG 4000, HPC, and Polyvinyl Alcohol (PVA) were procured from Lob Chemie, Mumbai. Other chemicals were obtained from Merck Chemicals, Mumbai. Microsoft Excel® and Design-Expert® were used for statistical analysis.
Preparation of Rifaximin Solid Dispersion by Solvent Evaporation Method
Solid dispersions of rifaximin were prepared using hydrophilic polymers (Soluplus, HPMC E5, PEG 4000, HPC, and PVA) at drug-to-polymer ratios of 1:0.5, 1:1, and 1:1.5 (w/w) by the solvent evaporation method.
Rifaximin (2 g) and the respective polymer were dissolved separately in ethanol (10 mL), mixed under continuous stirring, and the solvent was evaporated at 40ºC to obtain solid dispersions. For PVA, distilled water was used as the solvent. The dried mass was pulverized, sieved (#80), and stored in airtight containers (Jadhav et al., 2024). The composition of solid dispersions is presented in Table 1.
| Formulation Code | Polymer Used | Drug: Polymer Ratio (w/w) |
|---|---|---|
| SD1- SD3 | Soluplus | 1:0.5, 1:1, 1:1.5 |
| SD4- SD6 | HPMC E5 | 1:0.5, 1:1, 1:1.5 |
| SD7- SD9 | PEG 4000 | 1:0.5, 1:1, 1:1.5 |
| SD10- SD12 | HPC | 1:0.5, 1:1, 1:1.5 |
| SD13- SD15 | PVA | 1:0.5, 1:1, 1:1.5 |
Drug Content
Solid dispersion equivalent to 10 mg rifaximin was dissolved in ethanol, sonicated for 10 min, and suitably diluted. A standard rifaximin solution was prepared similarly. Aliquots were further diluted with ethanol, and absorbance was measured at 340 nm using a UV-visible spectrophotometer. Drug content (%) was calculated from the calibration curve. All measurements were performed in triplicate (n=3) (Sapkal et al., 2020; Joshi et al., 2023).
Solubility Analysis
The solubility of pure rifaximin and solid dispersions was determined in acetate buffer (pH 5.0). An excess sample was added to 50 mL of buffer and shaken for 48 hr to achieve equilibrium. The solutions were filtered, and 1 mL was diluted to 10 mL with buffer. Absorbance was measured spectrophotometrically, and solubility was calculated. All experiments were conducted in triplicate (Mohana et al., 2022; Kulawik et al., 2025).
FTIR Analysis
Fourier-Transform Infrared (FTIR) spectroscopy was performed to evaluate drug-polymer interactions. Samples were mixed with Potassium Bromide (KBr), compressed into pellets, and scanned over 4000-400 cm⁻¹ using a Jasco FT/IR-4600 spectrophotometer. Spectra were analysed for changes in characteristic peaks to assess possible interactions (Jadhav et al., 2025).
XRD Analysis
X-ray Diffraction (XRD) analysis was carried out to determine the crystalline or amorphous nature of rifaximin in the solid dispersions. Samples were analysed using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ=1.5406 Å) at 40 kV and 30 mA. The scanning range was 2θ=5°-50° with a step size of 0.02° and a scanning speed of 2°/min (Jadhav et al., 2025).
DSC Analysis
Differential Scanning Calorimetry (DSC) was performed to evaluate thermal behaviour and confirm amorphization of the drug. Samples were sealed in aluminium pans and analysed using a TA Instruments Q2000 DSC. The samples were heated from 30ºC to 300ºC at a rate of 10ºC /min under a nitrogen flow of 50 mL/min, using an empty pan as reference (Shah et al., 2023).
Formulation of Rifaximin Solid Dispersion Gel
Hydrogel formulations were prepared using a 3² factorial design by varying locust bean gum (LBG: 2.0-4.0% w/v) and xanthan gum (XG: 0.2-0.4% w/v) to evaluate their combined effects on formulation properties. LBG was dispersed in propylene glycol, followed by gradual addition of cold distilled water and allowed to swell for 24 hr. XG was then added to form a uniform gel base. The optimized rifaximin solid dispersion (1% w/v), dissolved in propylene glycol, was incorporated under continuous stirring. The gels were deaerated, adjusted to final volume, and stored in airtight containers. The composition of formulations is presented in Table 2.
| Formulation Code | LBG (% w/v) | XG (% w/v) | Rifaximin SD (% w/v) | PG (base) (mL) | PG (for SD) (mL) | Water (mL) |
|---|---|---|---|---|---|---|
| F1 | 2 | 0.2 | 1 | 6 | 1 | 52 |
| F2 | 2 | 0.3 | 1 | 6 | 1 | 52 |
| F3 | 2 | 0.4 | 1 | 6 | 1 | 52 |
| F4 | 3 | 0.2 | 1 | 6 | 1 | 52 |
| F5 | 3 | 0.3 | 1 | 6 | 1 | 52 |
| F6 | 3 | 0.4 | 1 | 6 | 1 | 52 |
| F7 | 4 | 0.2 | 1 | 6 | 1 | 52 |
| F8 | 4 | 0.3 | 1 | 6 | 1 | 52 |
| F9 | 4 | 0.4 | 1 | 6 | 1 | 52 |
Evaluation of gel
pH determination
The pH of the gel was determined by dispersing 1 g of gel in 10 mL of ultrapure water under constant stirring. The pH was measured using a calibrated digital pH meter (Hanna Instruments, pH-21). All measurements were performed in triplicate (n=3) (Jadhav et al., 2021).
% drug content of Rifaximin solid dispersion gel
Gel equivalent to 1 g was dissolved in methanol and diluted to 100 mL to obtain a stock solution. The solution was filtered and further diluted appropriately. Absorbance was measured at the predetermined λmax, and drug content (%) was calculated using the calibration curve. All measurements were carried out in triplicate.
Spreadability of Rifaximin solid dispersion gel
Spreadability was evaluated using the parallel-plate method under a 2 g load. Approximately 1 g of gel was placed between two glass plates (20 × 20 cm), and the load was applied for 1 minute at 25±1ºC. The spread area (S, cm²) was calculated using the equation:
S=πd²/4,
Where d is the diameter of the spread circle (Borse et al., 2020).
Viscosity
Viscosity was measured using a Brookfield DV-I Prime rotational viscometer at 25±1ºC. Approximately 60 g of gel was analysed using spindle RV06 at rotational speeds ranging from 5 to 100 rpm, maintaining torque within 10-90%. The rheological behaviour was assessed to determine flow characteristics (Jhawat et al., 2016).
In vitro Diffusion study
In vitro diffusion was performed using a Franz diffusion cell with pre-soaked excised goat skin as the membrane. Gel (0.5 g) was placed in the donor compartment, while the receptor contained phosphate buffer (pH 5.5) with 10% v/v methanol to maintain sink conditions. The system was maintained at 32±0.5ºC with continuous stirring. Samples were withdrawn at predetermined intervals, replaced with fresh medium, and analysed spectrophotometrically. Cumulative drug release was calculated, and a pure drug gel was evaluated under identical conditions (Kumar et al., 2020).
Experimental design and statistical analysis
A 3² full factorial design was employed to study the effects of LBG (X₁: 2-4% w/v) and XG (X₂: 0.2-0.4% w/v) on drug content (Y₁), viscosity at 20 rpm (Y₂), and cumulative drug release at 180 min (Y₃). Nine formulations (F1-F9) were prepared in triplicate. Data were analysed using Design-Expert® (Stat-Ease, USA). Analysis of Variance (ANOVA) was used to evaluate the significance of model terms, and numerical optimization based on a desirability function was applied to identify the optimal formulation.
Antimicrobial Activity Study (Ring Diffusion Method)
Antimicrobial activity was evaluated using the ring diffusion method against Staphylococcus aureus and Escherichia coli. Sterile rings loaded with the formulation were placed on inoculated agar plates and incubated under suitable conditions. Zones of inhibition were measured to assess antimicrobial effectiveness.
Accelerated Stability Study
The optimized gel was subjected to accelerated stability testing at 40±2ºC/ 75±5% RH for 6 months as per ICH guidelines. Samples were collected at 0, 1, 3, and 6 months and evaluated for drug content, viscosity, and in vitro drug release, which were compared with initial values to assess stability (Zothanpuii et al., 2020).
RESULTS
Drug Content
The drug content of rifaximin solid dispersions was influenced by polymer type and drug-to-polymer ratio. Soluplus-based formulations showed the highest drug content (61.00±1.12% to 87.00±1.46%), followed by PEG 4000 (61.00±0.92%-79.40±1.32%) (n=3). In contrast, HPMC E5 showed a marked reduction at a higher ratio (49.60±0.87% at 1:1.5), suggesting reduced drug-polymer affinity at elevated concentrations, while HPC and PVA showed moderate variation. These differences may be attributed to variations in polymer hydrophilicity and drug-polymer interactions, which influence drug entrapment. Overall, polymer type and concentration play a critical role in drug incorporation efficiency.
Solubility study
Pure rifaximin exhibited very low solubility (8.6±0.42 µg/mL), confirming its poorly water-soluble nature. All solid dispersions showed a marked increase in solubility, depending on polymer type and ratio. Solubility increased to 93.12±3.25-107.45±4.10 µg/mL with Soluplus, 147.36±5.22-180.48±6.13 µg/mL with HPMC E5, and 144.28±4.95-164.32±5.84 µg/mL with PEG 4000, while HPC showed comparatively lower improvement (48.56±2.15-118.22±4.76 µg/mL). The highest enhancement was observed with PVA-based solid dispersion (187.03±6.34-302.67±8.92 µg/mL), with the maximum at 1:1.5 ratio (~35-40-fold increase). This improvement may be attributed to amorphization of rifaximin along with enhanced wettability and molecular dispersion in the hydrophilic polymer matrix. Additionally, possible hydrogen bonding between PVA and rifaximin may have contributed to stabilization of the amorphous form. These results highlight the critical role of polymer properties in solubility enhancement.
FTIR analysis
The FTIR spectrum of pure rifaximin showed characteristic functional group peaks, which were retained in the optimized solid dispersion (Figure 1) without significant shifts or disappearance. This indicates the absence of chemical interaction or degradation, confirming compatibility between rifaximin and the polymer. The preservation of functional groups suggests that solubility enhancement is due to physical modifications rather than chemical changes.
XRD
The XRD pattern of pure rifaximin showed sharp diffraction peaks, confirming its crystalline nature. In contrast, the optimized solid dispersion exhibited a diffuse halo pattern without characteristic peaks, indicating conversion to an amorphous form (Figure 2). The disappearance of crystalline peaks suggests successful molecular dispersion of the drug within the polymer matrix. This amorphization contributes to enhanced solubility due to higher free energy and improved dissolution compared to the crystalline form.
DSC
The DSC thermogram of pure rifaximin showed a sharp endothermic peak at 245-246ºC, confirming its crystalline nature. In contrast, the optimized rifaximin-PVA solid dispersion showed no distinct melting peak, indicating loss of crystallinity and formation of an amorphous system (Figure 3). The absence of the melting endotherm, along with a broad transition, suggests drug-polymer interaction and uniform dispersion within the polymer matrix. These findings are consistent with XRD results and support the role of amorphization in solubility enhancement.
Evaluation of gel
pH determination
The pH of all gel formulations ranged from 6.89±0.04 to 7.15±0.06, indicating near-neutral conditions with minimal variation. This pH range is suitable for topical application and is unlikely to cause skin irritation, confirming the compatibility of the formulation with physiological skin conditions.
% drug content of Rifaximin solid dispersion gel
Drug content of hydrogel formulations ranged from 76.57±1.28% to 91.21±1.61% and was significantly influenced by polymer concentration. Formulations containing moderate levels of LBG and XG (F3 and F5) exhibited the highest drug content, indicating optimal entrapment and uniform distribution. At higher polymer concentrations (e.g., F9), drug content decreased, which may be attributed to increased viscosity hindering uniform mixing and drug distribution. Excessive polymer entanglement can also limit proper incorporation of the drug into the gel matrix. These results suggest that an optimal polymer concentration is essential to achieve efficient drug loading and homogeneity.
Spreadability of Rifaximin solid dispersion gel
Spreadability ranged from 1.24±0.05 to 2.40±0.08 cm² and decreased with increasing polymer concentration. Formulations with lower polymer content (F1) showed higher spreadability, while higher polymer concentrations resulted in stiffer gels. This inverse relationship can be attributed to increased viscosity and structural rigidity of the gel matrix at higher polymer levels, which reduces its ability to spread. These findings highlight the importance of optimizing polymer concentration to achieve a balance between mechanical strength and ease of application.
Viscosity
Viscosity increased with increasing polymer concentration, ranging from 1400±243 cP (F1) to 46500±875 cP (F6 at 5 rpm). This increase is attributed to enhanced polymer chain entanglement and formation of a dense three-dimensional network structure, which restricts flow. All formulations exhibited a decrease in viscosity with increasing shear rate (5-100 rpm), confirming pseudoplastic (shear-thinning) behavior. This rheological property is desirable for topical formulations, as it allows ease of application under shear while maintaining structural integrity at rest. Overall, viscosity was directly proportional to polymer concentration.
In vitro diffusion study
Cumulative drug release increased over 180 min, ranging from 23.50±1.84% (F9) to 46.50±2.12% (F1). Faster release was observed at lower polymer concentrations, while higher polymer content slowed diffusion. This may be attributed to the formation of a dense polymeric network, increasing diffusional resistance and reducing drug mobility within the matrix. Kinetic modelling showed first-order release (R²=0.9957), indicating concentration-dependent behavior. The Korsmeyer-Peppas model (n ≈ 1) suggested a combined mechanism of diffusion and polymer relaxation. Minimal release from the pure drug gel further confirms the enhancement achieved by solid dispersion (Figure 4).
Experimental design, model fitting and optimisation
The 3² factorial design effectively modelled the responses, including drug content (Y₁), viscosity (Y₂), and cumulative drug release (Y₃), with all models showing good statistical significance and adequacy. The polynomial equations generated for the responses were as follows:
Y₁=89.29 + 0.86A + 2.06B − 4.97AB − 4.315A² − 0.865B²
Y₂=16319.78 + 5263A + 11529.67B
Y₃=34.8333 − 9.3667A − 2.4B
where A represents Locust Bean Gum (LBG), and B represents Xanthan Gum (XG) concentration. The polynomial coefficients indicate the magnitude and direction of variable effects. Positive coefficients in Y₁ suggest that both polymers increase drug content up to an optimal level, while negative quadratic and interaction terms indicate a decline at higher concentrations due to polymer overcrowding. For viscosity (Y₂), both variables showed positive effects, with XG having a greater impact, reflecting its stronger thickening ability due to higher water-binding capacity. In contrast, negative coefficients in Y₃ indicate that increasing polymer concentration reduces drug release, likely due to the formation of a dense matrix that limits diffusion. Desirability optimization identified formulation F3 as optimal, showing a balanced response profile. Experimental values closely matched predicted values (Y₁: 0.78%, Y₂: 4.48%, Y₃: 0.48% deviation), confirming model reliability. ANOVA indicated statistical significance (p < 0.05), and response surface plots (Figure 5) further illustrated variable interactions.
Antimicrobial Activity Study (Ring Diffusion Method)
The optimized formulation (F3) exhibited significantly higher antimicrobial activity against Staphylococcus aureus and Escherichia coli, with zones of inhibition of 20.2±0.4 mm and 18.5±0.3 mm, respectively, compared to the pure drug gel (8.2±0.3 mm and 7.1±0.3 mm). The enhanced antimicrobial activity can be attributed to improved solubility and release of rifaximin from the solid dispersion-loaded hydrogel, leading to increased drug diffusion and availability at the site of action.
Accelerated Stability Testing
The optimized formulation showed a gradual decrease in drug content and in vitro release over time, accompanied by an increase in viscosity. These changes may be attributed to minor solvent loss and progressive densification of the polymer matrix under accelerated conditions. However, the observed variations remained within acceptable limits, indicating good physical and chemical stability of the formulation over the study period (Table 3).
| Time Point | Drug Content (%) | Viscosity (cP) at 20 rpm | In vitro Drug Release at 180 min (%) |
|---|---|---|---|
| 0 Months | 91.07±1.37 | 23600±325 | 45.66±2.36 |
| 1 Month | 90.45±2.43 | 24200±231 | 42.32±3.24 |
| 3 Months | 88.12±3.23 | 26120±327 | 39.23±2.88 |
| 6 Months | 86.45±2.34 | 28532±235 | 36.34±3.23 |
DISCUSSION
The present study demonstrates a systematic approach to overcoming the solubility limitations of rifaximin through the development of a solid dispersion-loaded hydrogel system. The significant enhancement in solubility observed in the PVA-based solid dispersion can be primarily attributed to the transformation of rifaximin from a crystalline to an amorphous state, as confirmed by XRD and DSC analyses. Amorphous systems possess higher free energy and greater molecular mobility, which contribute to improved wettability and dissolution behavior. Rusdin et al. (2024) reported that hydrophilic polymers facilitated solubility enhancement by stabilizing the amorphous form and preventing recrystallization. The role of polymer type was found to be critical in determining solubility enhancement. Among the polymers evaluated, PVA demonstrated superior performance, which may be attributed to its strong hydrophilic nature and potential to form intermolecular hydrogen bonding with the drug. This interaction likely promotes uniform molecular dispersion and inhibits drug aggregation. Comparable observations have been reported for other poorly soluble drugs, where polymer-drug interactions significantly influenced solubility and dissolution profiles. Incorporation of the optimized solid dispersion into a hydrogel matrix further enhanced drug release and functional performance. Hydrogels are known to provide a hydrated environment that facilitates drug diffusion while maintaining prolonged contact with the application site. The near-neutral pH of all formulations indicates their compatibility with skin physiology, reducing the risk of irritation and supporting their suitability for topical application. The rheological evaluation revealed that viscosity increased with polymer concentration due to enhanced polymer chain entanglement and formation of a three-dimensional network structure. All formulations exhibited pseudoplastic behavior, which is advantageous for topical delivery as it allows easy spreading under shear stress while maintaining structural integrity at rest. Similar rheological characteristics have been reported by Calienni et al. (2023) in polysaccharide-based hydrogels used for topical drug delivery. The in vitro diffusion study demonstrated that drug release decreased with increasing polymer concentration, which can be explained by the formation of a denser gel matrix that increases diffusional resistance. This behavior is consistent with previously reported hydrogel systems, where higher polymer content reduces drug mobility and slows release. The kinetic analysis indicated that drug release followed first-order kinetics with a combined diffusion and polymer relaxation mechanism, suggesting that both matrix structure and swelling behavior contribute to drug release. The factorial design provided valuable insights into the influence of formulation variables on key responses. Both locust bean gum and xanthan gum significantly affected viscosity and drug release, with xanthan gum showing a stronger impact on gel viscosity. The optimized formulation (F3) achieved a desirable balance between drug content, viscosity, and release, demonstrating the effectiveness of statistical optimization in formulation development. The close agreement between predicted and experimental values further confirms the robustness and reliability of the model. The enhanced antimicrobial activity observed in the optimized formulation compared to the pure drug gel can be attributed to improved solubility and drug release, resulting in higher drug availability at the site of action. This indicates that the developed system not only improves physicochemical properties but also enhances functional therapeutic performance. Stability studies indicated that the formulation remained stable under accelerated conditions, with only minor changes in drug content, viscosity, and drug release. These changes may be attributed to slight moisture loss and polymer matrix densification over time; however, the overall performance remained within acceptable limits, confirming the robustness of the formulation. This study provides a novel and systematic approach by integrating solid dispersion and hydrogel systems for the topical delivery of rifaximin. While solid dispersion has been widely used for solubility enhancement, its incorporation into a hydrogel matrix for the topical application of rifaximin has not been extensively explored. The use of factorial design further strengthens the study by enabling optimization of formulation variables and establishing their quantitative influence on product performance.
CONCLUSION
The present study successfully developed and optimized a rifaximin solid dispersion-loaded hydrogel system to overcome solubility limitations and enhance topical drug delivery. The PVA-based solid dispersion (1:1.5) achieved significant solubility enhancement through amorphous transformation, while the optimized hydrogel formulation (F3) demonstrated an optimal balance of drug content, viscosity, and controlled drug release. Factorial design effectively established the influence of polymer concentrations on formulation performance, and stability studies confirmed the robustness of the system. Overall, the developed formulation represents a promising and scalable strategy for improving the topical delivery of poorly water-soluble drugs. Further in vivo and clinical studies are warranted to establish its therapeutic potential.