Febuxostat (FBX) is a xanthine oxidoreductase inhibitor recommended for the treatment of gout that lowers serum urate concentrations. Its approval by the FDA in 2009 was a significant advancement in the management of hyperuricemia.^1,2 FBX is administered orally once daily and is rapidly and well absorbed, with 99.2% plasma protein binding. Metabolism of the drug occurs via oxidation through the Cytochrome P450 (CYP) system and conjugation through the uridine diphosphate glucuronosyltransferase enzyme system. It is eliminated by the renal and hepatic systems. However, FBX is a BCS class II drug,³ which is poorly soluble in water and has low solubility and wettability, leading to poor bioavailability.⁴ To overcome this restriction, an emulsion-based topically applied formulation is developed, which incorporate hydrophobic therapeutic moieties and improve patient compliance. The half-life of FBX is 5 to 8 hr, and the topical route of administration avoids the first-pass metabolism, enhancing its bioavailability.⁵ Topical medications like creams, lotions, and ointments have several drawbacks, including stickiness, instability, and low spreading coefficient, making the use of transparent gels popular in cosmetic and pharmaceutical preparations. Emulgels, which are a combination of gels and emulsions, have significant potential as drug delivery vehicles, improving permeation efficacy, reducing side effects, increasing drug absorption, and preventing enzymatic breakdown in the stomach. Therefore, the pharmaceutical industry is currently focusing on emulgel systems for drug delivery.^6,7 The present study focused on development of topical in situ gel of FBX in view to efficacious treatment of gout.

FBX was obtained from Zydus Cadila as a gift sample. Cinnamon Oil, Castor Oil, PVA, Cremophor RH40 were purchased from Himedia. Poloxamer 407 was obtained from BSA as a gift sample. All other chemicals used were of analytical grade.

The drug was dissolved in fatty oils and nonaqueous oils and it was mixed using a vortex mixer. The solubility study was performed in different oils.

Initially, emulsions were formed using FBX in DMSO or Transcutol or a combination of Transcutol and Cremophor RH 40 in the internal phase and 1% Tween 80 (1:2 ratio) solution in the external phase. Preliminary screening of formulation of emulsion was done using cinnamon oil, Cremophor RH and Span 80 in internal phase and Tween 80 and water as external phase. Trials were taken to check the effect of changes in the volume of the external phase using different ratios (1:1.5, 1:2, 1:2.5) of internal to external phase (1% Tween 80 solution) on drug precipitation and emulsion stability. Table 1 (Batch F1-F14) shows the preliminary screening of formulation parameters and the development of coarse emulsion.

Trials were taken to check the effect of benzyl alcohol and castor oil (in the internal phase) on drug precipitation and emulsion stability. Trials were taken using different concentrations of coconut oil as a carrier oil to reduce the volatility of cinnamon oil. Results for these trials are discussed in results and discussion section. Several batches of emulsions were prepared with different internal and external phase combinations and varying drug concentrations to find a suitable emulsifier that would ensure emulsion stabilization and prevent drug precipitation. The batches were labeled with internal and external phase compositions (% w/w) and drug concentration (% w/w) were recorded in Tables 2 and 3 with different ratios (Batch F15-F20).

The stability of an emulsion and the occurrence of drug precipitation are not significantly affected by altering the proportion of the internal and external phases. However, to achieve overall stability and avoid drug precipitation, it is necessary to use an optimal concentration of castor oil to reduce volatility and include cremophor RH 40 as a co-solubilizer in the internal phase. As a result, subsequent batches were designed using the optimal concentrations of castor oil and cremophor RH 40 in the internal phase, along with an aqueous PVA solution in the external phase which is shown in Table 4.

FBX emulgel formulations and the preparation method involve cold mixing of the drug cinnamon oil, cremophor RH 40, and castor oil using a mortar and pestle. The water, Polyvinyl Alcohol (PVA), and poloxamer solution was kept to dissolve overnight in a freezer before adding the internal phase to the external phase in a controlled manner. The resulting mixture was thoroughly triturated and mixed at room temperature to obtain a milky white emulgel. Table 5 shows different batches with varying concentrations.

The experimental design i.e. a 3²-level factorial design to has been employed to investigate the impact of two independent variables, i.e. concentration of Cremophor RH40 (% w/w) and the concentration of Poloxamer 407 (% w/w), on two dependent variables, namely % cumulative drug release at 2 hr (Q2) and % cumulative drug release at 6 hr (Q6). The concentrations that were used in the design were 2.5% w/w for the drug, 12% w/w for cinnamon oil, 6% w/w for castor oil, 0.1% w/w for PVA, 5%-15% w/w for Cremophor RH40, and 15%-25% w/w for Poloxamer 407.

The concentration of formulation ingredients for optimization was fixed based on the preliminary trials and drug solubility study. Nine FBX emulgel formulations (D1 to D9) were prepared based on the independent and dependent variables listed in Table 7, using Cremophor RH40 and Poloxamer 407. The independent variables were set at three levels (-1, 0, and 1), while the dependent factors were % drug release in 2 hr (Q2) and % drug release in 6 hr (Q6). The levels of the independent variables were 5%, 10%, and 15% for the concentration of Cremophor RH40 and 15%, 20%, and 25% for the concentration of Poloxamer 407. The goal was to optimize the formulation for % drug release at both 2 and 6 hr. The dependent and independent variables are listed in Table 6. The in vitro drug release of D1-D9 is listed in Table 7.

FBX content in the emulsion is determined using High-Performance Liquid Chromatography (Agilent 1260 Infinity II Diode Array Detector) of ODS column using mobile phase 2.8 gm of sodium acetate buffer pH 3 in 500 mL milliQ water and methanol in the ratio of 30:60. Injection volume was 10μL. The drug content was determined at wavelength 254 nm. Experiments were carried out in triplicates.^6,8

Dynamic Light Scattering Particle Size Analyzer was used to measure globule size distribution. Operating range was 0.02 nm to 2.8 μm. Readings were collected after the emulsion was diluted from 1 mL to 250 mL with distilled water.⁹

The viscosity of the prepared batches was measured by Anton Parr, MCR 102c rheometer. The liquid was added to the beaker and allowed to settle for 30 min at the room temperature (25±1°C) before the measurement. The parallel plate shear rate was 0.1 to 100 1/s and temperature 25°C and 35°C. The viscosity value was recorded. The viscosity of Emulgel was taken in triplicates.¹⁰

In vitro drug release study was performed using a modified Franz diffusion cell apparatus. The dialysis membrane with 2.4 nm pore size, and molecular cut of 12KD-14KD were used. 1 gm of emulgel was packed in a dialysis membrane which is equivalent to 25 mg of the drug. 1 mL of buffer was withdrawn at predetermined time points and an equivalent amount was replaced with fresh buffer to maintain sink conditions. The study was done in PBS pH 7.4 at 32±0.5°C temperature and 25 mL buffer volume.

Real-time and accelerated stability studies were performed at 5°C ± 3°C and 30°C ± 2°C/65% RH ± 5% RH for 6 months. After 6 months, batches were assessed for drug content, viscosity, and drug release.

FBX shows solubility in oils like Dimethyl sulfoxide (DMSO), Cinnamon Oil, Transcutol, Cremophor RH 40, Isopropyl alcohol, Abratil M 1944 CS, Corn Oil, Capryol 90, Lemon Oil, Cremophor EL. The results showed that the 10 mg of the drug was able to dissolve in 0.1 mL DMSO, 0.1 mL Transcutol, 0.1 mL Cinnamon Oil, and 0.2 mL Cremophor RH 40, while it showed little or no solubility in other oils. These findings suggest that the solubility of the drug can be enhanced by using specific oils as solvents. While Triacetin, Olive Oil, Camphor Oil, IPM, labrafac lipophile WL, 1349 Medium chain Triglyceride, Kollisolv P 124 (Poloxamer 124), IMWITOR 780K (Make: Cremer Oleo Division), Plurol Oleique CC 947 (Polyglyceryl-3 Dioleate), Lauroglycol 90 (Propylene Glycol Monolaurate), Maisine 35-1 (Glycerol Monolinoleate), caprylic acid, ethyl oleate, Castor Oil, Rose Oil, Dill Oil, Coconut Oil, Peppermint Oil FBX show very little or no solubility.

Drug precipitation was observed due to the miscibility of Transcutol and DMSO with the internal phase. Solubility trials revealed that the drug showed good solubility in Cinnamon oil and Cremophor RH 40. Cinnamon oil was incorporated into the oil phase for further optimization of the formulation due to its non-miscibility with the aqueous phase. However, the resultant emulsions were unstable and cracked immediately. Further trials were taken using different combinations of FBX, Cinnamon oil, Cremophor RH 40, and Span 80 in the internal phase and aqueous Tween 80 solution in the external phase. However, the resultant emulsion was initially yellowish opaque and became translucent after some time. Drug crystals were seen after 24 hr, indicating that the emulsion was unstable. Higher concentrations of Span 80 in combination with Tween 80 were also ineffective in addressing the issue of drug crystallization. Labrasol in place of Cremophor RH 40 was also ineffective. Trials taken with benzyl alcohol and castor oil (in the internal phase) on drug precipitation and emulsion stability resulted in cracked emulsion immediately. Trials were also taken using different concentrations of coconut oil as a carrier oil to reduce the volatility of cinnamon oil but the resultant emulsion was unstable, and drug crystallization was observed. As a result, subsequent batches were designed using the optimal concentrations of castor oil and cremophor RH 40 in the internal phase, along with an aqueous PVA solution in the external phase which is shown in Table 4. By incorporating castor oil and cremophor RH 40 in the internal phase, and using an aqueous PVA solution in the external phase, the stability of the emulsion is improved and the issue of drug precipitation is resolved. As the concentration of castor oil increases concerning cremophor RH 40, the viscosity of the emulsion also increases.

Using the above-mentioned ingredients, the next step involved incorporating gelling agents like Cremophor RH 40 and Poloxamer 407 to form an emulgel formulation.

The study analyzed the results of a 3² full factorial design, where multiple regression analysis was used to model the response (Y1 and Y2) obtained from different levels of two independent variables (X1 and X2). Table 8 presents the results of the multiple regression analysis for Y1 and Y2, while Tables 9 and 10 show the analysis of variance tables for Q2 and Q6 factors, respectively. The model F-value and Prob>F values indicate that the model terms were significant. The lack of fit F-value and Prob>F values suggest that the lack of fit was not significant, which is desirable.

Figure 1 presents the contour plot and response surface plot for Q2 and Q6 factors, which suggest that drug release is improved at optimal concentrations of Cremophor RH 40 and Poloxamer 407. Higher concentrations of these variables have a detrimental effect on drug release and increase viscosity. However, these parameters have a negligible effect on average globule size.

Figure 1:
Contour plot and response surface plot of Q2 and Q6 factor.

To select the optimized batch, certain criteria were established, a Q2 of 20-30% release and a Q6 of 70-80% release. From a total of 30 suggested solutions, the solution with a desirability of 1% (D5) was chosen as the optimized batch. Figure 2 shows the desirability plot with points ranging from 0 to 1, and the variables X1 (concentration of Cremophor RH 40) and X2 (concentration of Poloxamer 407) were used. Figure 3 shows an overlay plot with the same variables X1 and X2.

Figure 2:
Desiribility Plot.

Figure 3:
Overlay plot.

The results of the Globule size of the optimized batch D5 emulgel is shown in Figure 4. The average globule size of the formulation is 13.95 μ.

Figure 4:
Globule size of optimized batch.

The viscosity of the emulgel was measured at 12 rpm. The viscosity of the optimized batch D5 was found to be 6890.2 cps. Figure 5 shows the viscosity curve of optimized batch D5. The data indicates that the formulation is stable.

Figure 5:
Rheocompass of optimized batch D5.

Based on the design criteria of dependent factors Q2 and Q6, the optimum and linear release is obtained in batch number D5 (Figure 6) which is in accordance with the results of the response surface plot and contour plot.

Figure 6:
Comparative drug release study of batches D1-D9.

Drug content, viscosity and drug release was checked after 6 months of stability study at 5°C ± 3°C and 30°C ± 2°C/65% RH ± 5% RH which is shown in Tables 11 and 12. The data indicates that the formulation is stable.

The primary objective of the current research study was to formulate a skin-permeating emulgel for FBX, a xanthine oxidase inhibitor utilized in the treatment of gout. The poor solubility of FBX in water necessitated the strategic development of a formulation aimed at enhancing its bioavailability. Emulgels, distinguished by their amalgamation of gel and emulsion properties, have emerged as promising candidates for drug delivery due to their potential to enhance permeation efficacy, mitigate side effects, enhance drug absorption, and prevent enzymatic breakdown in the gastrointestinal tract. In the initial stages of the study, screening experiments were conducted to identify specific oils capable of increasing the solubility of FBX. Cinnamon oil emerged as the most efficacious oil solvent of all. Cremophor RH 40 and Poloxamer 407 were incorporated as co-surfactant/ solubilizer and emulsifier, respectively, in the formulation. After preliminary screening, a systematic optimization approach utilizing a 3²-factorial design was used to optimize the emulgel formulation. Formulation optimization using a 3²-factorial design resulted in an optimized emulgel formulation containing 10% Cremophor RH 40 and 20% Poloxamer 407. The prepared emulgel is also effective in enhancing the solubility and bioavailability of FBX for the management of gout. Notably, this formulation exhibited desirable globule size, enhanced drug release, and maintained stability throughout 6 months of stability studies. The prepared emulgel not only effectively increased the solubility of FBX but also demonstrated enhanced bioavailability, thereby showcasing its potential as an efficacious carrier for topical formulations in the management of gout. Furthermore, the development of a cost-effective and industrially applicable skin-permeating emulsion-based gel presents an opportunity to further improve drug penetration and permeation through the skin. This study underscores the significant potential of emulgel systems as versatile drug delivery vehicles, providing a robust platform for the topical administration of hydrophobic drugs. The findings herein contribute valuable insights into the optimization and efficacy of emulgel formulations, paving the way for advancements in therapeutic strategies for gout.

The authors would like to thank Nirma University for providing support to carry out the project.