Formulation And Evaluation of Mefenamic Acid Cocrystals for Taste Masking and Solubility Enhancement

Oral drug delivery remains the most preferred route for pharmaceutical administration due to its convenience, patient-friendly nature, cost-effectiveness, ease of manufacturing, and favourable stability during storage. Despite these advantages, oral dosage forms often face challenges such as difficulty in swallowing and unpleasant taste, which significantly affect patient compliance particularly among paediatric and geriatric populations. The bitter taste of active pharmaceutical ingredients (APIs) is primarily detected by taste buds located on the tongue, making taste masking a crucial step in formulation development to ensure better patient acceptability.¹ A variety of taste-masking techniques have been employed over the years, including the use of flavouring and sweetening agents, microencapsulation, ion-exchange resins, prodrug strategies, inclusion complexation, granulation, multiple emulsions, and gel formation. However, limited studies have explored co-crystallization as a viable approach for taste masking. This novel technique not only mitigates bitterness but also enhances other physicochemical properties of the drug, such as solubility, dissolution, and compressibility.² Co-crystallization is grounded in the principles of crystal engineering, which involves the rational design of crystalline materials by manipulating intermolecular forces and supramolecular assemblies. Co-crystallization has recently gained significant attention in the pharmaceutical field, particularly due to increased regulatory emphasis on solid-state characterization of drug substances.³ Mefenamic acid (MA), a commonly used non-steroidal anti-inflammatory drug (NSAID), is known for its potent analgesic, antipyretic, and anti-inflammatory effects. However, it suffers from poor aqueous solubility and an intensely bitter taste, leading to its classification as a BCS Class II drug. These drawbacks contribute to limited oral bioavailability and poor compliance among patients, especially in sensitive populations.⁴ The present research work focuses on the application of co-crystallization to improve the solubility and palatability of mefenamic acid. By selecting pharmaceutically acceptable sweeteners as coformers, this study aims to form cocrystals through non-covalent interactions that retain the therapeutic efficacy of the drug while significantly enhancing its organoleptic and dissolution properties.

MATERIALS AND METHODS

MATERIALS

Mefenamic acid (MA) was a gift sample obtained from Blue cross, Nashik. Mannitol, citric acid, sucralose, aspartame and saccharine was received as gift samples from Fullife Health Care Pvt. Ltd, Khopoli.

METHODS

Mefenamic acid co-crystals were formulated using the liquid assisted grinding method. Mannitol, citric acid, sucralose, aspartame and saccharine used as a co-former in their equimolar ratios.⁵

Figure 1 Liquid Assisted Grinding method

Preliminary Characterization of Drug and Conformer

Organoleptic properties

Drug and coformers were evaluated for its colour, odour, taste and appearance. These sensory characteristics play a vital role in ensuring patient acceptability and compliance, especially for oral dosage forms, where an unpleasant taste or Odor can lead to poor patient compliance.

Melting point

Melting points of the Selected drug and co-former were taken by melting point apparatus. Melting point determination is important for identification of drug and coformer. It is primary indication for formation of cocrystals. It provides valuable information about the thermal behavior and stability of the cocrystal compared to the pure drug and coformer. A change in the melting point typically different from that of either the parent drug or the conformer indicates the formation of a new solid phase with distinct physicochemical properties, confirming successful cocrystal formation.

UV Spectroscopy

UV spectrometric technique was based on the Lambert-Beer law. UV spectrophotometer was used for the identification of given chemical substance as each of them has concentration of known substance. To prepare the stock solution, accurately weighed 10 mg of Mefenamic acid transfer into a 100 ml volumetric flask. Then, add 50 ml of distilled water to dissolve the compound. Afterward, the volume was adjusted to 100 ml using distilled water, resulting in a solution concentration of 100 μg/ml. To generate a calibration curve, aliquots of 0.5, 1, 1.5, 2, 2.5 and 3 ml were taken from the 100 μg/ml stock solution. Each aliquot was then diluted to a final volume of 10 ml with distilled water, resulting in concentrations of 5, 10, 15, 20, 25 and 30 μg/ml, respectively. The absorbance of each dilution was measured at a wavelength of 286 nm using the UV-Visible spectroscopy instrument (SHIMADZU UV-2450). Finally, a graph was plotted with absorbance on the y-axis and concentration on the x-axis using the collected data from the calibration curve.

Comparative Characterization of Prepared Cocrystals

Screening of optimized cocrystal done on the basis of following parameters, 1. Melting point, 2. Saturation solubility study, 3. FTIR. From that Cocrystals of Mefenamic acid: aspartame (1:2) shows highest solubility and strongest hydrogen bonding.

Melting point

Melting point of prepared cocrystals of mefenamic acid were determined by using melting point apparatus.

Saturation solubility study

Saturated solubility was performed to determine the actual solubility of co-crystals as follows: An excess amount of co-crystals was taken in a 10 ml volumetric flask containing 5 ml water then the sample was sonicated at 90 rpm (CITIZEN LAB CD 4820) for 4 min. After an equilibrium period of time (24hr) supernatant was collected then filtered the solution using Whatman filter paper (0.42 µm). resultant filtrate absorbance was taken using UV-visible spectroscopy at selected wavelength (286 nm) with proper dilution water as a diluent.

FTIR Spectroscopy

FTIR spectra gives Information about the functional group present in the compound. FTIR spectral analysis was performed to evaluate the chemical stability of the drug as well as to confirm if there any interaction in the drug and co former by using FTIR spectrometer (Bruker Alpha Eco-ATR). The scanning range was 4000 cm^-1 to 400 cm^-1  and the resolution was 1cm^-1.

Characterization Of Screened Cocrystals

Visual Morphology

The visual appearance of plain Mefenamic acid, cocrystals was recorded to observed changes in crystal habit. The study was carried using Light microscope and crystal shapes based on visual morphology were compared.

Differential Scanning Calorimetry (DSC)

DSC help to determine melting behaviour and enthalpy of fusion. A DSC study was carried out to detect possible polymorphic transition during the cocrystallization process. The thermal behaviour of pure drug was determined using differential scanning calorimeter (DSC 60Plus, Shimadzu, Japan). Samples (2−5 mg) were crimped in aluminium pans and scanned from 30 – 300°C at heating rate of 10 °C/min under constant purging of dry nitrogen at 50 mL/min. TA Explorer software was used to manage the data.

Powder X-Ray Diffraction Study (PXRD)

Every new crystalline material exhibits unique peaks indicative of reflections from specific atomic planes. Powerful technique for determining the presence of polymorphs, crystal habit modifications in drug crystals, and/or generation of new crystal form. X-ray diffractogram of the pure drug was recorded by diffractogram using Bruker AXS D8 Advance Diffractometer system. The samples were radiated using a Monochromatized Cu Kα radiation (WL= 1.5406 Å), then analyzed between the 5 and 40° range at a step size of 0.020° and a step time 31.2 s. PXRD patterns were refined using Diffrac plus software. All samples were measured in the 2θ angle range between 0°C and 50°C.

In-Vitro Dissolution Studies

The solubility advantage of cocrystals has been shown to correlate with increased dissolution. Cocrystals of mefenamic acid has solubility greater than pure mefenamic acid. Therefore, in order to assess whether this solubility advantage correlates with increased dissolution, in vitro release study was performed in biorelevant media at PBS pH 7.4.

Following data shows the dissolution data of pure mefenamic acid and its cocrystals:

• Apparatus: USP Type II (Paddle) apparatus.
• Medium: 900 ml pH 7.4 phosphate buffer
• Temperature: 37± 0.5°C
• Speed: 50 rpm
• Sample withdrawal: 10 ml sample was removed at a time interval of 10 min to the 60 min.

Concentration was determined by UV spectrophotometer at a wavelength of 286 nm. The dissolution was significantly improved after cocrystallization.

Taste evaluation

In this method, taste evaluation was done on 11 healthy human volunteers selected for taste evaluation. The volunteers were made to sign a written consent form before the test. They were told to keep 1 mg of mefenamic acid in the pure form in the mouth for 15-30 seconds and then were asked to spit it out. Similarly, they were told to keep the co-crystals (1 mg) in the mouth until they were completely soluble. The degree of bitterness was immediately scored as per the bitterness intensity scale from 0-5 where 0, 1, 2, 3, 4 and 5 indicate no bitterness, very slight bitterness, slight bitterness, moderate bitterness, strong bitterness and very strong bitterness respectively.

Rate the bitterness intensity using the scales below:

4: Strong bitterness, 3: Moderately bitterness, 2: Slight bitterness, 1: Acceptable bitterness, 0: No bitterness.

RESULTS AND DISCUSSION

Preliminary Characterization of Drug and Conformer

Organoleptic Properties

Table 2 Melting point of API and coformers

The serial dilutions 5 to 30 µg/ml was made from a 100 µg/ml solution, and absorbance was recorded at wavelength 286 nm.

Figure 2 UV Callibration Curve

Comparative Characterization of Prepared Cocrystals

Melting point

Saturation Solubility

Based on the solubility data mentioned above, the co-crystal of Mefenamic acid: aspartame with a stoichiometric ratio of 1:2 was found to have an optimized ratio, indicating improved solubility properties. Therefore, this specific ratio of the co-crystal was selected for further studies.

Characterization of Screened Cocrystals

FTIR Spectroscopy

Figure 3 FTIR spectra of Mefenamic acid

Figure 4 FTIR spectra of Aspartame

Figure 5 FTIR spectra of Cocrystals of Mefenamic acid: Aspartame (1:2)

Morphology

Figure 6 Morphology Study of (a) pure mefenamic acid (b) aspartame and (c) mefenamic acid: aspartame (1:2)

Mefenamic acid – Plate shape

Aspartame – Needle shape

Mefenamic acid: Aspartame (1:2) – Flake shape

Differential Scanning Calorimetry (DSC)

Figure 7 DSC of Mefenamic acid

Figure 8 DSC of Aspartame

Figure 9 DSC OF Cocrystals of Mefenamic acid: Aspartame (1:2)

The Differential Scanning Calorimetry (DSC) analysis provided clear evidence of cocrystal formation between mefenamic acid and aspartame. The thermogram of pure mefenamic acid showed a sharp endothermic peak at 232°C corresponding to its melting point, while aspartame exhibited a distinct endothermic peak at 248°C of its own. In contrast, the DSC thermogram of the cocrystal displayed a new, single sharp endothermic peak at a temperature 230°C different from that of either of the pure components. The disappearance of the original melting points and the emergence of a new thermal event indicate the formation of a novel solid phase. His distinct thermal behaviour confirms successful cocrystallization and suggests a change in the molecular arrangement resulting from intermolecular interactions between the drug and the conformer.

Powder X-Ray Diffraction Study (PXRD)

Figure 10 PXRD of Mefenamic acid

Figure 11 PXRD OF Aspartame

Figure 12 PXRD OF Cocrystals of Mefenamic acid: Aspartame (1:2)

PXRD was performed to confirm the formation of cocrystals of mefenamic acid with aspartame in a 1:2 molar ratio. The diffraction pattern of pure mefenamic acid exhibited sharp and intense peaks at characteristic 2θ values such as 6.5°, 13.5°, 15.5°, and 22.3°, confirming its crystalline nature. Similarly, pure aspartame displayed multiple peaks at 7.0°, 10.5°, 12.8°, 14.9°, and 19.6°, indicating its own distinct crystalline profile. In contrast, the PXRD spectrum of the mefenamic acid: aspartame cocrystal showed a new set of diffraction peaks at 6.7°, 16.5°, 18.3°, 22.7°, and 25.1°, which were absent in the individual components. Additionally, several original peaks of both the drug and coformer either disappeared or were significantly reduced in intensity. These observations indicate the formation of a new crystalline phase, confirming successful cocrystallization. The appearance of new peaks, along with the disappearance or shifting of characteristic peaks of the parent compounds, strongly supports the formation of a mefenamic acid–aspartame cocrystal with altered lattice arrangement due to intermolecular interactions, such as hydrogen bonding.

In-Vitro Dissolution study

Mefenamic acid cocrystal solubility was greater than mefenamic acid. Therefore, in order to assess whether this solubility advantage correlates with increased dissolution, in vitro release study was performed in biorelevant media at pH 7.4.

Figure 13 In-Vitro dissolution profile of Mefenamic acid and Co-crystal of Mefenamic acid and Aspartame (1:2)