1. INTRODUCTION:

NSAIDs are the pillar of topical therapies for symptomatic relief in various conditions such as osteoarthritis, joint pain¹. The most common musculoskeletal disorder is osteoarthritis (OA) and increasing enormously at an alarming rate. Recent studies suggest that a huge proportion of adults are affected by OA within the last decade. OA commonly affects large weight-bearing joints such as knee and hips. NSAIDs are given to the affected population for symptomatic relief to OA. Diclofenac is a well-established first-line drug among NSAIDs².

Among various diclofenac salts, diethylamine salt is generally used for the topical drug delivery system due to high lipophilic nature³. The conversion of arachidonic acid to prostaglandin is due to catalytic activity of enzymes cyclooxygenase-1 and cyclooxygenase-2 and inhibited by NSAIDs mainly diclofenac, this is known as anti-inflammatory activity⁴.

For over five decades, a myriad of topical products was developed and introduced in public domain. Topical products are available in a variety of dosage forms such as cream, gel, lotion and ointment to treat various diseases. For local action, various formulations such as antiseptics, antifungal agent and skin emollients, topical formulations applied to the skin. The core advantage of this drug administration includes avoidance of the first-pass metabolism, better patient compliance and site-specific for their action. Topical drug administration ensures the localized or systemic effect when applied anywhere on the skin⁵. Topical drug delivery has been the backbone in the treatment of local infections such as fungal infection and has been an attractive option for site-specific drug delivery and circumvention of side effects related with the oral route.

A comparatively newer class of dosage form is gels composed of either small inorganic particles or large organic molecule interpenetrated and enclosing by a liquid. The major disadvantage of the gel dosage form is the incorporation of drugs which have hydrophobic nature. Hence, to overcome this obstacle, emulsion dosage form was incorporated in gels called emulgel. Emulgels are the unique formulation that contains emulsion that either o/w or w/o type that is incorporated in gel formulation⁶. Hydrophobic drugs cannot be directly incorporated in the gel so this novel transdermal drug delivery technique is employed for the hydrophobic drug delivery via skin⁷.

Emulgel formulation mainly consist of three phases that is oil phase, aqueous phase and gel phase and selection of suitable excipients is important aspect for better and stable formulation. Oils extracted from plants have medicinal value that can be used to formulate effective formulation now in the practice of a recently established emulgel drug delivery system⁸. Emulsifying agents promote emulsification and play a dynamic role in stability of the emulsion. Emulsions are thermodynamically unstable system but the steadiness of the emulsion rely on the selection of appropriate emulsifier and to decrease the interfacial tension⁹. Vehicles are designed in such a way that the ability of the delivery system to overcome the obstacle present by the stratum corneum for the delivery of drugs to the deeper layers of skin is enhanced¹⁰. Currently, for the topical drug delivery most widely used approach to enhance the drug permeation across the skin is chemical penetration enhancers¹¹. Penetration enhancers must be non-irritant and in the specified limit. The polymer used to form the gel matrix influences the release rate of the incorporated drugs. Polymer with higher concentration or have higher molecular weight causes the slower discharge of the entrapped ingredient out of the formulation¹².

The performances of the topical products are demonstrated using a release study. The Franz diffusion cells play an important role for in vitro release studies. The key step is selection of synthetic membrane for in vitro release method that provides inert support to the formulation¹³. During the experiment, the main goals are to maintain the sink condition and prevent the formation of an unstirred boundary layer that will affect the diffusion rate¹⁴. A critical parameter for the drug release experiment is the selection of the proper receptor medium to maintain sink conditions¹⁵.

Quality by design (QbD) is a logical methodology for developing a product with predefined goals and highlights product and process considerate and control based on sound science and quality risk management^16-17.

To get the best outcomes from the available sources and prioritized criteria is termed as optimization. In case of pharmaceuticals, the statistical optimization is designing a formulation with optimum characteristics using various techniques¹⁸. The central composite design can be used to optimize conditions by adding some more experimental studies to full fractional design. The variable studies in this design are of continuous nature (quantitative nature) such as temperature, concentration, etc.

2. MATERIALS AND METHODS:

2.1 Materials:

Diclofenac diethylamine as active pharmaceutical ingredient was a kind gift from Sun Pharmaceutical Industries Ltd., Gurugram. The materials and reagents used for this study were potassium dihydrogen phosphate and potassium chloride provided by Merck Pharmaceutical, India. Light liquid paraffin, isopropanol, benzyl alcohol were purchased from Loba chemie. Kolliphor CS20, Kollicream 3C were purchased from BASF. HPMC K4M CR was purchased from Dow chemical USA, synthetic membrane “Supor” was purchased from ultipore.

2.2 Methods:

2.2.1 Identification of critical material attributes:

The inactive ingredients of various marketed emulgel formulation includes thickening agent, emulsifiers and solvent and oil that forms the oil phase. The key excipients like thickening agent (HPMC K4M), emulsifier (Kolliphor CS20) and light liquid paraffin were chosen as critical material variables¹⁹.

2.2.2 Identification of critical quality attributes:

Accordance with QbD approaches, the development and optimization of diclofenac diethylamine were carried out. Based on the literatures, in vitro release rate was identified as the critical quality attribute. In vitro drug release study was conducted with Franz diffusion cell that was maintained at maximum sink conditions²⁰.

2.2.3 Development and optimization of formulation:

The preparation of diclofenac diethylamine emulgel was performed using HPMC K4M as gelling agent, light liquid paraffin as fatty alcohol that forms the oil base, Kollicream 3C and Kolliphor CS20 as emulsifiers, isopropanol and propylene glycol as solvent with benzyl alcohol as preservative. The oil phase comprises of light liquid paraffin, Kollicream 3C and Kolliphor CS20. The oil phase was heated upto 70˚C and added to aqueous phase with continuous stirring. The drug solution contains diclofenac diethylamine, dissolved in isopropanol, propylene glycol and added to former solution to form the emulsion. The gel phase was prepared using HPMC K4M by dissolving in distilled water and left overnight. The emulsion phase is incorporated to gel phase with continuous stirring until a homogenous mixture was not formed. Based on the literature search, the compositions are illustrated in Table 1.

Table 1: Composition of the formulations.

Ingredients	Quantity (g)/100g
Diclofenac diethylamine	1.16
Kolliphor CS20	1-4
Isopropanol	10
Propylene glycol	10
Liquid paraffin light	2-4
HPMC K4M	3-6
Benzyl alcohol	1
Kollicream 3C	2.5
Distilled water	Upto 100 g

The impact of material variables on the products CQAs can easily be interpreted by DoE (Design of Experiment). Using Design-Expert software the experimental data were analysed. A face centred central composite design with 16 experiments was used for in vitro drug release study. The Design space criteria were based on the reference product.

The relationship between CQAs in CCD and the factors is expressed by Equation 1.

Y=β₀+β₁A+β₂B+β₃C+β₁₂AB+β₁₃AC+β₂₃BC+β₁₁A²+β₂₂B²+β₃₃C²Equation 1

Where, A is the amount of HPMC,

B is the amount of Kolliphor CS20,

C is the amount of light liquid paraffin,

β₀ is the intercept and

β₁, β₂, β₃, β₁₂, β₁₁, β₂₂ and β₃₃ are coefficients computed from observed experimental values of the response Y that is release rate.

2.2.4 Physiochemical evaluation:

The appearance and homogeneity of the optimized emulgel was checked visually. With digital pH meter, the pH of the emulgel formulation was determined in triplicate²¹.

2.2.5 Drug content determination:

The drug content of the formulation was determined by dissolving them in phosphate buffer saline (PBS) pH 7.4 and filtered through PTEE filter. The filtered solutions were analysed using UV spectrophotometer at wavelength of 276nm against PBS of pH 7.4 as blank^22-23.

2.2.6 Determination of viscosity:

The viscosity of the optimized batch was determined using analogue viscometer (Brookfield Engineering Laboratories, USA) attached with spindle 6. Spindle was allowed to move freely into emulgel formulations and reading on dial was noted. All the measurements were done triplicates at 25⁰C²⁴.

2.2.7 Determination of in vitro release rate:

DDA is an amphiphilic drug that is compatible with hydrophilic membrane. The in vitro release study was carried out using hydrophilic synthetic membrane. The synthetic membrane was comprised of polyethersulfone (Supor) having pore size of 0.22µm. The Franz diffusion (FD) cell having cross sectional area of 4.90cm² with capacity of 20ml was used for the study. A PBS of pH 7.4 was used as dissolution medium to maintain the sink conditions. Accurately weighed 800mg of emulgel, equivalent to 9.28mg of diclofenac diethylamine was applied on pre-soaked membrane. The receptor compartment was maintained at 32⁰C with constant stirring at 600 rpm. Sample of 1ml were collected at 1h, 2h, 4h, 6h and 8h. It was replaced with fresh media of pH 7.4 for maintaining sink conditions. Samples were analysed using UV spectrophotometer wavelength of 276nm for determination of drug release rate²⁵.

2.2.8 Model fitting on in vitro release tests:

The various release kinetic models such as first order, zero order, Korsmeyer-peppas and Higuchi model were used for model fitting of in vitro release testing. The regression coefficients on each model demonstrate that highest value of regression coefficient (R²) in the Higuchi model²⁶.

2.2.9 Comparative drug release study of optimized emulgel with reference product:

The comparative study was conducted with optimized formulation and the reference formulation in PBS of pH 7.4 to study the drug release behaviour of both formulations. Samples were analysed using UV spectrophotometer wavelength of 276nm for determination of drug release rate. The results of drug release rate (µg/cm².h^-1/2) were noted down^27-28.

2.2.10 Stability study:

An accelerated stability study at 40±2°C/75±5% RH in stability chamber (Thermolab, India) was carried out for three months. At periodic time, test samples were expelled and evaluated for physical appearances and content drug²⁹.

3. RESULTS AND DISCUSSION:

3.1 Identification of critical material attributes:

The key excipients like thickening agent (HPMC K4M), emulsifier (Kolliphor CS20) and light liquid paraffin were chosen as critical material variables. The quantities of critical material variables were fixed using the trials in lab development as shown in Table 2.

Table 2: Details of experimental conditions.

Factor	Low level	Centre point	High level
HPMC K4M	3	4.5	6
Kolliphor CS20	1	2.5	4
Light, liquid paraffin	2	3	4

3.2 Identification of critical quality attributes:

Based on the literatures, in vitro drug release rate was identified as the critical quality attribute. The in vitro release study was conducted Franz diffusion cell maintained at maximum sink conditions.

3.3 Preparation and optimization of formulation:

Table 3: Composition and of different formulations suggested by Design Expert software®.

Formulation code	HPMC K4M (g)	Kolliphor CS20 (g)	Liquid paraffin (g)	Release rate (µgcm^-1.t^-1/2)
F-1	3	1	2	509.00
F-2	6	1	2	424.14
F-3	3	4	2	537.72
F-4	6	4	2	458.44
F-5	3	1	4	502.81
F-6	6	1	4	411.24
F-7	3	4	4	556.33
F-8	6	4	4	470.22
F-9	3	2.5	3	521.87
F-10	6	2.5	3	448.37
F-11	4.5	1	3	456.63
F-12	4.5	4	3	487.46
F-13	4.5	2.5	2	461.16
F-14	4.5	2.5	4	465.60
F-15	4.5	2.5	3	450.52
F-16	4.5	2.5	3	455.77

Implementation of design:

Using Design expert^®software the experimental data was analysed. The relationship between the variables and the response can be easily interpreted by contour and 3D surface graphs as shown in Figure 1.

Figure 1: Effect of HPMC and Kolliphor CS20 on release rate (a) Contour plot and (b) 3-D surface plot

The model for in vitro release was significant as p value found less than 0.05. Lack of fit was non-significant for the model. The high value of regression coefficient for the model signifies the goodness of fit as shown in Table 4. The high value to regression coefficient explained the strong relationship between the factors with the response.

Table 4: Results of ANOVA analysis.

Observations	16
Mean of responses	476.12
p value	0.0001
Lack of fit	0.3113
R²	0.9835
Adjusted R²	0.9589

The relationship established between the response and variables was by the given in Equation 2.

Y₁ = +461.54 – 41.47A +20.69B +1.63C -1.46AB -1.62AC+ 6.26BC +19.38A² +6.30B²-2.36C²

Equation 2

Where, A = Conc. of HPMC

B = Conc. of Kolliphor CS20

C = Conc. of Light liquid paraffin

The pareto chart is showing the relationship between the response and variables as shown in Figure 2.

Figure 2: Pareto chart showing the relationship between the selected response and variables

The impact of gelling agent on release rate can be easily explained in terms of viscosity; decrease in amount of gelling agent decrease viscosity as a result release rate is increased. On the basis of interaction graphs, it can be predicted that the release rate is influenced by the variables in the order gel base ˃ surfactant ˃ oil base. The release rate of the drug is influenced by the amount of gel base significantly whereas the concentration of the surfactant also influences the release rate but less than that of the gel base. The interaction of gel base, oil base and surfactant with release rate is shown in Figure 3.

Figure 3: Interaction with release rate of (a) gel base, (b surfactant) and (c) oil base

The design space was verified by performing the experiment in the laboratory. The formulation is prepared using the quantity of excipients within the design space. The design space was established on basis of in vitro release testing of the reference product. The yellow region of the overlay plot shows the acceptance criteria for the in vitro release rate as shown in Figure 4.

Figure 4: Overlay plot showing the acceptance criteria for the in vitro release rate

Based on these results, a formulation with 3.8% w/w, 2.95% w/w, 2.2% w/w quantities of HPMC K4M, Kolliphor CS20 and light liquid paraffin respectively was found to be optimized batch.

3.4 Physiochemical evaluation of Diclofenac emulgel:

The optimized formulation was evaluated for colour, homogeneity and appearance. This formulations was milky white and homogenous in appearance. pH of this formulation was found to be 6.4±0.04 which is considered acceptable for the topical products.

3.5 Drug content:

The optimized formulation had satisfactory drug content of 99.98±1.2%. In accordance with the British Pharmacopoeia limits that is 95% to 105%.

3.6 Viscosity:

The viscosity of optimized formulation was found to be 460,000 cP that was measured at lower shear stress with spindle no. 6 at 25⁰C. The higher viscosity of some formulations was due to the high concentration of HPMC.

3.7 In vitro release rate:

The difference between the release rate of individually performed test for different formulations can reveal mutual impact of physical and chemical properties, including rheological properties, solubility, size of particle and can be used for “final quality control”³⁰. The straight line slope obtained from the graph of amount released per unit area (μg/cm²) VS the square root of time gives drug release rate for a semisolid product. The results of in vitro release rate are shown in Table 5.

Table 5: Result for in vitro release rate at different time intervals from optimized formulation.

Time	%CDR (%)
Drug release at 1h	20.96
Drug release at 2 h	30.79
Drug release at 4h	48.80
Drug release at 6h	60.01
Drug release at 8h	66.43
Release rate (µg/cm².h^-1/2)	487.77
Regression coefficient (R²)	0.9931

3.8 Model fitting on in vitro release tests:

All the data obtained from in vitro release experiments fitted on the various release kinetic equations. The regression coefficients on each model demonstrate that highest value of regression coefficient (R²) in the Higuchi model. Therefore, drug release is diffusion controlled from all formulations. The release kinetics data of selected formulations are demonstrated in Table 6.

Table 6: Model fitting data of in vitro release rate.

Formulation	Higuchi model (R²)	Korsmeyer-peppas (R²)	Zero order (R²)	First order (R²)
F14	0.988	0.5819	0.9381	0.9792

3.9 Comparative drug release study of optimized emulgel with reference product:

The results of this study showed the better drug release from the optimized emulgel with reference product. The results of comparative study of drug release rate (µg/cm².h^-1/2) between optimized formulation and the reference formulation are shown in Figure 5.

Figure 5: Comparative drug release study of optimized emulgel with reference formulation

3.10 Stability study:

It was observed that emulgel formulation was chemically and physically stable at accelerated stability study. No significant change was noted in appearance and drug content after 3 months of stability testing.

4. CONCLUSION:

Since topical formulations are widely utilized for better patient compliance and emulgel is helpful in improving loading of hydrophobic drugs in gel dosage form for control release with long term stability. Furthermore they enhance spreadability, extrusion, adhesion and viscosity. Similarly in this study diclofenac diethylamine emulgel formulation were formulated with HPMC as gelling agent using Qbd approach. The performance of the formulation was determined using in vitro release test using synthetic membrane. The formulation was also evaluated on the other parameters such as pH, drug content and drug content. The design space is verified by preparing formulation within design space quantities of critical material attributes and shows better release with reference product.

5. AVAILABILITY OF DATA AND MATERIAL:

The authors confirm that the data supporting the results and findings of this study are available within the article and its supplementary materials.

6. CONFLICT OF INTEREST:

Authors declare no conflict of interest financial or otherwise.

7. ACKNOWLEDGMENT:

The authors also thank Sun Pharmaceuticals laboratories Pvt. Ltd. Gurugram for their help in this research.

8. REFERENCES:

1. Haroutiunian S, Drennan DA, Lipman AG. Topical NSAID therapy for musculoskeletal pain. Pain Medicine. 2010; 11(4): 535-549.

2. Atzeni F, Masala IF, Sarzi-Puttini P. A review of chronic musculoskeletal pain: Central and peripheral effects of diclofenac. Pain and Therapy. 2018; 7(2): 163-177.

3. Hamed R, Basil M, AlBaraghthi T, Sunoqrot S, Tarawneh O. Nanoemulsion-based gel formulation of diclofenac diethylamine: design, optimization, rheological behavior and in vitro diffusion studies. Pharmaceutical Development and Technology. 2016; 21(8): 980-989.

4. Altman R, Bosch B, Brune K, Patrignani P, Young C. Advances in NSAID development: Evolution of diclofenac products using pharmaceutical technology. Drugs. 2015; 75(8): 859-877.

5. Khullar R, Kumar D, Seth N, Saini S. Formulation and evaluation of mefenamic acid emulgel for topical delivery. Saudi Pharmaceutical Journal. 2012; 20(1): 63-67.

6. Mohamed MI. Optimization of chlorphenesin emulgel formulation. AAPS Journal. 2004; 6(3): 81-87.

7. Hardenia A, Jayronia S, Jain S. Emulgel: An emergent tool in topical drug delivery. International Journal of Pharmaceutical Sciences and Research. 2014; 5(5): 1653.

8. Kaushik R, Budhwar V, Kaushik D. An overview on recent patents and technologies on solid dispersion. Recent Patents on Drug Delivery and Formulations. 2020;14(1):63-74. doi:10.2174/1872211314666200117094406.

9. Kumar D, Singh J, Antil M, Kumar V. Emulgel-novel topical drug delivery system-a comprehensive review. International Journal of Pharmaceutical Sciences and Research. 2016; 7(12): 4733.

10. Foldvari M. Non-invasive administration of drugs through the skin: Challenges in delivery system design. Pharmaceutical Science and Technology Today. 2000; 3(12): 417-425.

11. Pandey A, Mittal A, Chauhan N, Alam S. Role of surfactants as penetration enhancer in transdermal drug delivery system. Journal of Molecular Pharmaceutics and Organic Process Research. 2014; 2(113): 2-7.

12. Shahin M, Hady SA, Hammad M, Mortada N. Novel jojoba oil-based emulsion gel formulations for clotrimazole delivery. AAPS PharmSciTech. 2011; 12(1): 239-247.

13. Thakker KD, Chern WH. Development and validation of in vitro release tests for semisolid dosage forms-case study. Dissolution Technologies. 2003; 10(2): 10-16.

14. Miller MA, Kasting G. A measurement of the unstirred aqueous boundary layer in a Franz diffusion cell. Pharmaceutical Development and Technology. 2012; 17(6): 705-711.

15. Shah VP, Elkins JS, Williams RL. Evaluation of the test system used for in vitro release of drugs for topical dermatological drug products. Pharmaceutical Development and Technology. 1999; 4(3): 377-385.

16. Verma R, Mittal V, Kaushik D. Quality based design approach for improving oral bioavailability of valsartan loaded SMEDDS and study of impact of lipolysis on the drug diffusion. Drug Delivery Letters. 2018; 8(2): 130–139.

17. Yu LX, Amidon G, Khan MA, Hoag SW, Polli J, Raju GK, Woodcock J. Understanding pharmaceutical quality by design. AAPS Journal. 2014; 16(4): 771–783.

18. Gujral G, Kapoor D, Jaimini M. An updated review on design of experiment (DoE) in pharmaceuticals. Journal of Drug Delivery and Therapeutics. 2018; 8(3): 147-152.

19. Bolla PK, Clark BA, Juluri A, Cheruvu HS, Renukuntla. Journal Evaluation of formulation parameters on permeation of ibuprofen from topical formulations using Strat-M® membrane. Pharmaceutics, 2020; 12(2): 151.

20. ICH. Guidance for Industry: Q8 (R2) Pharmaceutical Development; U.S. Department of Health and Human Services: Washington, DC, USA. 2009; pp. 1–29.

21. De Oliveira Neto AS, Souza I, Amorim M, de Freitas Souza T, Rocha VN, do Couto RO, Fabri RL, de Freitas Araújo MG. Antifungal efficacy of atorvastatin-containing emulgel in the treatment of oral and vulvovaginal candidiasis. Med. Mycol. 2020; myaa071. Advance online publication. https://doi.org/10.1093/mmy/myaa071

22. Yassin GE. Formulation and evaluation of optimized clotrimazole emulgel formulations. Journal of Pharmaceutical Research International. 2014; 1014-1030.

23. Gul R, Ahmed N, Ullah N, Khan MI, Elaissari A, Rehman AU. Biodegradable ingredient-based emulgel loaded with Ketoprofen nanoparticles. AAPS PharmSciTech. 2018; 19(4): 1869–1881.

24. Burki IK, Khan MK, Khan BA, Uzair B, Braga VA, Jamil QA. Formulation development, characterization, and evaluation of a novel dexibuprofen-capsaicin skin emulgel with improved in vivo anti-inflammatory and analgesic effects. AAPS PharmSciTech. 2020; 21(6): 211.

25. Verma R, Kaushik A, Almeer R, Rahman MH, Abdel-Daim MM, Kaushik D. Improved pharmacodynamic potential of rosuvastatin by self-nanoemulsifying drug delivery system: An in vitro and in vivo evaluation. International Journal of Nanomedicine. 2021; 16:905-924. Published 2021 Feb 9. doi:10.2147/IJN.S287665

26. Kapoor D, Vyas RB, Lad C, Patel M, Lal B, Parmar R. Formulation, development and characterization of emulgel of an NSAID’S. Pharma Chem. Journal. 2014; 1: 9-16.

27. Rawooth M, Qureshi D, Hoque M, Prasad M, Mohanty B, Alam MA, Anis A, Sarkar P, Pal K. Synthesis and characterization of novel tamarind gum and rice bran oil-based emulgels for the ocular delivery of antibiotics. International Journal of Biological Macromolecules. 2020; S0141-8130(20): 33993-339113. Advance online publication. https://doi.org/10.1016/j.ijbiomac.2020.07.231

28. Shanmugarajan TS, Selvan NK, Uppuluri V. Development and characterization of squalene-loaded topical agar-based emulgel scaffold: Wound healing potential in full-thickness burn model. Int. Journal of Low. Extrem. Wounds. 2020, 1534734620921629. Advance online publication.

29. Verma R, Kaushik D. Design and optimization of candesartan loaded self-nanoemulsifying drug delivery system for improving its dissolution rate and pharmacodynamic potential. Drug Delivery. 2020; 1: 756-771.

30. Food and drug administration (FDA). Guidance for industry: SUPAC-SS nonsterile semisolid dosage forms, scale-up and post-approval changes, chemistry, manufacturing, and controls, in vitro release testing and in vivo bioequivalence documentation. Rockville: Centre of Drug Evaluation and Research, 1997; pp. 26.

Received on 10.11.2020 Modified on 15.07.2021

Research J. Pharm. and Tech. 2022; 15(7):3260-3266.

DOI: 10.52711/0974-360X.2022.00547