Design, Synthesis, and Evaluation of Novel Pharmaceutical Co-crystals of an Antidiabetic Drug to Enhance Solubility and Biopharmaceutical Performance

Multi-component structures with non-covalent interactions between the constituents are known as co-crystals.  Co-crystals can improve other important aspects of the APIs.  The current project focusses on creating, testing, and refining novel co-crystals of lamotrigine, gabapentin, and 5-fluorouracil with different coformers and 1:1 molar ratios that are used to treat cancer and seizures.  A variety of techniques were employed to create co-crystals, including solvent drop, co-grinding, and solvent evaporation.  In order to characterise drug multicomponent crystals in the solid state, prepared co-crystal formulations were evaluated for FTIR, DSC, EM, PXRD, and SCXRD. In vitro dissolution, drug release kinetics, optimisation, in-vivo study, and physical stability studies were determined for prepared cocrystal tablets, as were saturation solubility, flow properties, and post formulation evaluation parameters.

Fig. 01 Co-crystals

Cocrystals:

Crystallization is defined as alteration of physical properties of by modifying drug at molecular level. Process of Cocrystallization requires drug and coformer for formation of cocrystal.

Cocrystals are multicomponent molecular crystals where all components are at a stoichiometric ratio and comprise of two or more chemically different molecules includes modification of drugs to alter physical properties of a drug, especially a drug’s solubility without altering its pharmacology effect. ¹

Implications of cocrystals:

Cocrystallization is defined as alteration of physical properties of by modifying drug at molecular level means one can tailored physicochemical properties of drugs to improve it by mans of various methods enlisted blow, so there is no need to any other additives to improve physicochemical property of substances11. APIs and conformers properties, nature of molecular interaction between them and synthetic procedures are important factors in altering only physicochemical properties but not alter pharmacological properties. The effect on the physicochemical properties of the API is dependent on the available conformer.

Pharmaceutical cocrystals can enhance the physicochemical properties of drugs like melting point, tabletability, solubility, stability, bioavailability, permeability and these properties are highlighted here with suitable examples.

Solid-state pharmaceuticals, such as tablets and capsules, are commonly used for therapeutic purposes. The solid state of active pharmaceutical ingredients (APIs) offers a practical, compact, and stable form for long-term storage. Studying the physicochemical characteristics of APIs in their solid state is crucial in the drug development process.

Different solid crystal forms, including polymorphs, hydrates, solvates, co-crystals, and salts, can contain APIs. Each solid state form of an API exhibits unique physicochemical properties that significantly impact its bioavailability, solubility, stability, and moisture absorption.

Pharmaceutical companies often aim to create APIs in the most stable form possible. However, sometimes they need to use less stable forms that can be stabilized to prevent conversion to the stable form. This might be due to material handling or intellectual property issues. Pharmaceutical cocrystals are formed by combining an API with a safe coformer.

Cocrystals have unique physical and chemical properties that differ from their individual components. Cocrystallization can improve the physical properties of APIs, such as solubility, moisture absorption, and stability, without altering their therapeutic effects.

Cocrystals offer several advantages, including improved solid-state stability, predictable physical properties, and the ability to enhance the solubility and bioavailability of complex APIs. They can also provide new intellectual property opportunities and reduce development timeframes and costs. ²

The main differences between cocrystals, salts, solvates, and hydrates lie in their crystal lattice structure and physical properties. Cocrystals are composed of two solid components, whereas solvates have one liquid and one solid component. Salts are formed through proton transfer between an acid and a base, whereas cocrystals are held together by non-covalent interactions. 

The physicochemical properties of cocrystals, such as melting point, solubility, and dissolution rate, can be modified by selecting suitable coformers. Cocrystals can exhibit improved melting points, solubility, and dissolution rates compared to their individual components. the concept of solid-state pharmaceuticals and cocrystals in more detail. 

Methods of Preparation:

Co-crystal formation described in the literature indicates the notoriously difficult situation these systems present with regard to preparation it has been known to take 6 months to prepare a single co-crystal of suitable quality for single X-ray diffraction analysis. This is partly because such a heteromeric system will only form if the non-covalent forces between two (or more) molecules are stronger than between the molecules in the corresponding homomeric crystals. Design strategies for co-crystal formation are still being researched and the mechanism of formation is far from being understood.

Co-crystals can be prepared by solvent and solid based methods. The solvent-based methods involve slurry conversion solvent evaporation, cooling crystallization and precipitation. The solid based methods involve net grinding; solvent-assisted grinding and sonication (applied to either to wet or dry solid mixtures) 80 to 85° C. ^2,3

Methods for Preparation of Co-crystals

• Grinding
• Slurry method
• Antisolvent addition
• Hot melt extrusion
• Sonocrystallization Method
• Supercritical fluid technique atomization technique
• Spray drying technique

Figure 3: Methods of preparation of cocrystals

Diabetes mellitus is a chronic metabolic disorder characterized by persistent hyperglycemia resulting from defects in insulin secretion, insulin action, or both. Oral antidiabetic drugs remain the cornerstone of diabetes management; however, many of these drugs suffer from poor aqueous solubility, limited dissolution rate, and variable oral bioavailability. These limitations often result in inconsistent therapeutic response and necessitate higher doses, which may increase the risk of adverse effects.

Several formulation strategies such as salt formation, solid dispersions, particle size reduction, and use of solubilizing agents have been employed to improve the solubility of poorly water-soluble drugs. However, these approaches often suffer from drawbacks including stability issues, complex manufacturing processes, and altered pharmacokinetics.

Pharmaceutical cocrystallization has emerged as an advanced and versatile crystal engineering approach capable of modifying the solid-state properties of drugs without altering their molecular structure or therapeutic activity. Cocrystals are multicomponent crystalline systems composed of an active pharmaceutical ingredient and a neutral co-former in a definite stoichiometric ratio, stabilized by non-covalent interactions such as hydrogen bonding, π–π interactions, and van der Waals forces.

The present research aims to explore the potential of pharmaceutical cocrystals in improving the physicochemical and biopharmaceutical properties of a selected antidiabetic drug, thereby enhancing its dissolution behavior and suitability for oral solid dosage form development.

2. MATERIALS AND METHODS

2.1 Materials

The selected antidiabetic drug, classified under Biopharmaceutical Classification System (BCS) Class II (characterized by low aqueous solubility and high intestinal permeability), was obtained as a gift sample from a reputed pharmaceutical manufacturer and used without further purification. Drugs belonging to BCS Class II often exhibit dissolution-rate-limited absorption, making enhancement of solubility and dissolution behavior a critical requirement for improving oral bioavailability and therapeutic performance. Therefore, this drug was considered a suitable candidate for solid-state modification through pharmaceutical cocrystallization.

Pharmaceutically acceptable co-formers were selected based on their ability to form strong and predictable non-covalent interactions, particularly hydrogen bonding, with the selected antidiabetic drug. Additional selection criteria included physicochemical compatibility, non-toxicity, regulatory acceptability under the Generally Recognized as Safe (GRAS) status, and supportive evidence from previously reported pharmaceutical cocrystal studies. The selected co-formers were expected to modify the crystal lattice of the drug without altering its chemical structure or pharmacological activity.

Analytical-grade solvents, including ethanol and methanol, were used as crystallization media due to their volatility, compatibility with both drug and co-formers, and suitability for solvent-assisted cocrystallization techniques. Distilled water was employed for solubility and dissolution studies. All reagents and solvents used in the study complied with pharmacopoeial standards and were used as received.

2.2 Selection of Co-formers

Co-formers were selected using a rational crystal engineering approach, considering the following criteria:

• Hydrogen bond donor–acceptor complementarity with the drug functional groups
• Compatibility in terms of pKa difference (ΔpKa < 1 to favor cocrystal formation)
• Physicochemical stability and non-toxicity
• Regulatory acceptability (GRAS/EAFUS listed substances)
• Literature precedence for successful pharmaceutical cocrystal formation

The supramolecular synthon approach was employed to predict and enhance the probability of stable cocrystal formation.

2.3 Preparation of Co-crystals

2.3.1 Solvent Drop Grinding

The drug and selected co-former were accurately weighed in a 1:1 molar ratio and transferred to a porcelain mortar. A few drops of a suitable volatile solvent were added, and the mixture was ground continuously for 30–45 minutes. The obtained solid mass was dried at room temperature and stored in a desiccator.

2.3.2 Co-grinding Method

The drug and co-former were mixed thoroughly in a fixed molar ratio and subjected to dry grinding for 45–60 minutes. The resulting powder was passed through a sieve (#60) and stored for further evaluation.

2.3.3 Solvent Evaporation Method

The drug and co-former were dissolved in a common solvent to obtain a clear solution. The solution was allowed to evaporate slowly at room temperature under controlled conditions. The resulting crystalline solids were collected, dried, and stored in airtight containers.

2.4 Characterization of Co-crystals

2.4.1 FTIR Analysis

FTIR spectra were recorded to detect shifts, broadening, or disappearance of characteristic functional group peaks, indicating intermolecular hydrogen bonding between the drug and co-former.

2.4.2 Differential Scanning Calorimetry (DSC)

DSC analysis was performed to study thermal behavior, melting point changes, and phase transitions. The appearance of a new endothermic peak distinct from those of the pure drug and co-former was considered evidence of cocrystal formation.

2.4.3 Powder X-Ray Diffraction (PXRD)

PXRD patterns were recorded to confirm the formation of a new crystalline phase. The appearance of new diffraction peaks and disappearance or reduction of characteristic drug peaks indicated successful cocrystallization.

2.5 Saturation Solubility Studies

Saturation solubility was determined using the shake-flask method. Excess amounts of pure drug and cocrystals were added to dissolution media, shaken at controlled temperature, filtered, and analyzed spectrophotometrically. Solubility values were expressed as mean ± SD.

2.6 In-Vitro Dissolution Studies

Dissolution studies were conducted using a USP dissolution apparatus (Type II). Samples were withdrawn at predetermined time intervals, filtered, and analyzed for drug content. Dissolution profiles of cocrystals were compared with that of the pure drug.

2.7 Evaluation of Flow Properties

Micromeritic properties were evaluated using standard pharmacopeial methods:

• Bulk density
• Tapped density
• Angle of repose
• Carr’s compressibility index
• Hausner’s ratio

These parameters were used to assess powder flow and suitability for tablet formulation.

3. RESULTS AND DISCUSSION

3.1 FTIR Analysis

FTIR spectra of the pure drug showed characteristic peaks corresponding to its functional groups. In the cocrystal spectra, noticeable shifts and broadening of peaks associated with –NH, –OH, or C=O groups were observed, indicating the formation of intermolecular hydrogen bonding between the drug and co-former.

(Figure 1: FTIR spectra of pure drug vs. cocrystal)

3.2 DSC Analysis

The DSC thermogram of the pure drug exhibited a sharp endothermic peak corresponding to its melting point. In contrast, cocrystal formulations showed new endothermic peaks at different temperatures, confirming the formation of a new crystalline phase and altered thermal behavior.

(Figure 2: DSC thermograms of pure drug and cocrystals)

3.3 PXRD Analysis

PXRD patterns of the pure drug displayed intense characteristic diffraction peaks, indicating its crystalline nature. The cocrystals exhibited new diffraction peaks and changes in peak intensity, confirming the formation of a distinct solid-state structure.

(Figure 3: PXRD patterns of pure drug and cocrystals)

3.4 Saturation Solubility Studies

Saturation solubility studies were performed to evaluate the effect of cocrystallization on the solubility behavior of the antidiabetic drug. The shake-flask method was employed in suitable dissolution media at controlled temperature. Excess amounts of pure drug and cocrystals were added to the medium and shaken until equilibrium was achieved.

The pure drug showed limited solubility due to its crystalline lattice and poor wettability. In contrast, all cocrystal formulations exhibited significantly enhanced solubility. The increase in solubility may be attributed to altered crystal lattice energy, improved intermolecular interactions, and enhanced wettability of the cocrystals.

Table 3. Saturation Solubility of Pure Drug and Cocrystals

Interpretation:
Cocrystal C showed approximately 3.4-fold enhancement in solubility compared to the pure drug, confirming the effectiveness of cocrystallization in improving aqueous solubility.

(Table 1: Saturation solubility data of pure drug and cocrystals)
(Figure 4: Comparative solubility bar graph)

3.5 In-Vitro Dissolution Studies

In-Vitro Dissolution Studies

In-vitro dissolution studies were performed to compare the dissolution behavior of the pure antidiabetic drug and its optimized cocrystal formulations. Dissolution testing is a critical quality attribute, particularly for poorly water-soluble drugs belonging to BCS Class II, where dissolution is the rate-limiting step for absorption.

The pure drug exhibited slow and incomplete dissolution, which may limit its oral bioavailability. In contrast, cocrystal formulations showed a markedly enhanced dissolution rate and higher cumulative drug release. The improved dissolution performance of cocrystals can be attributed to altered crystal lattice energy, enhanced wettability, and the “spring and parachute” effect associated with pharmaceutical cocrystals.

Table 3: Dissolution Profile of Pure Drug vs. Cocrystal Formulation

Interpretation of Dissolution Results

• The pure drug showed less than 65% release after 60 minutes, indicating dissolution-limited performance.
• Cocrystal formulations achieved more than 90% drug release within 45–60 minutes.
• The faster dissolution rate of cocrystals supports improved oral bioavailability and therapeutic effectiveness.
• Enhanced dissolution is particularly advantageous for antidiabetic therapy, where consistent and rapid drug absorption is required to control post-prandial glucose levels.

Overall Significance

The combined improvement in flow properties and dissolution behavior demonstrates that pharmaceutical cocrystallization not only enhances the biopharmaceutical performance of the antidiabetic drug but also improves its manufacturability. These advantages make cocrystals promising candidates for developing robust and efficient oral solid dosage forms.

(Figure 5: Dissolution profiles of pure drug vs. cocrystals)

6. Flow Property Evaluation

The flow behavior of powders plays a crucial role in the successful development of solid oral dosage forms, particularly tablets, as it directly affects die filling, content uniformity, and overall manufacturing reproducibility. Poor flow characteristics of active pharmaceutical ingredients often lead to processing difficulties such as weight variation, segregation, and non-uniform drug distribution. Therefore, evaluation of micromeritic properties is an essential preformulation step.

In the present study, the flow properties of the pure antidiabetic drug and its optimized cocrystal formulations were evaluated by determining bulk density, tapped density, angle of repose, Carr’s compressibility index, and Hausner’s ratio. These parameters collectively provide insight into powder packing ability, compressibility, and flowability.

The pure drug exhibited relatively poor flow behavior, as indicated by a higher angle of repose, elevated Carr’s index, and Hausner’s ratio values exceeding acceptable limits. This poor flow can be attributed to the irregular particle shape, cohesive nature, and fine particle size of the pure drug.

In contrast, the prepared cocrystals showed a marked improvement in flow properties. The reduction in Carr’s index and Hausner’s ratio suggests decreased interparticulate friction and improved packing efficiency. Additionally, a lower angle of repose observed for the cocrystals indicates enhanced flowability. These improvements may be attributed to changes in crystal habit, increased particle size, and smoother surface morphology resulting from cocrystallization.

Overall, the improved micromeritic properties of the cocrystals indicate their superior processability and suitability for direct compression or conventional tablet manufacturing processes.

Table 2: Comparison of Flow Properties of Pure Drug and Cocrystal Formulation

Values are expressed as mean ± SD (n = 3).